Trophic Cascade Research Papers - Academia.edu (original) (raw)

Large mammalian carnivores are ecologically important because relatively few individuals can cause strong predation-driven direct effects or feardriven indirect effects that can ripple through communities and, ultimately, influence... more

Large mammalian carnivores are ecologically important because relatively few individuals can cause strong predation-driven direct effects or feardriven indirect effects that can ripple through communities and, ultimately, influence ecosystem structure and function. Most mammalian carnivores are not large, however, but are small to midsized species collectively termed "mesocarnivores." Mesocarnivores are more numerous and more diverse than larger carnivores, and often reside in closer proximity to humans, yet we know little about how they influence communities and ecosystems. In this article we review the ecological role of the mesocarnivore and present examples where mesocarnivores drive community structure and function in roles similar to, or altogether different from, their larger brethren. Together, these examples substantiate the need for an assessment of the ecological role of mammalian carnivores beyond an examination of only the largest species. In particular, we emphasize the need to study the trophic penetrance of mesocarnivores and examine how ecological context modulates their functional role.

The suppression of agricultural pests has often been proposed as an important service of natural enemy diversity, but few experiments have tested this assertion. In this study we present empirical evidence that increasing the richness of... more

The suppression of agricultural pests has often been proposed as an important service of natural enemy diversity, but few experiments have tested this assertion. In this study we present empirical evidence that increasing the richness of a particular guild of natural enemies can reduce the density of a widespread group of herbivorous pests and, in turn, increase the yield of an economically important crop. We performed an experiment in large field enclosures where we manipulated the presence/absence of three of the most important natural enemies (the coccinellid beetle Harmonia axyridis, the damsel bug Nabis sp., and the parasitic wasp Aphidius ervi) of pea aphids (Acyrthosiphon pisum) that feed on alfalfa (Medicago sativa). When all three enemy species were together, the population density of the pea aphid was suppressed more than could be predicted from the summed impact of each enemy species alone. As crop yield was negatively related to pea aphid density, there was a concomitant non-additive increase in the production of alfalfa in enclosures containing the more diverse enemy guild. This trophic cascade appeared to be influenced by an indirect interaction involving a second herbivore inhabiting the systemthe cowpea aphid, Aphis craccivora. Data suggest that high relative densities of cowpea aphids inhibited parasitism of pea aphids by the specialist parasitoid, A. ervi. Therefore, when natural enemies were together and densities of cowpea aphids were reduced by generalist predators, parasitism of pea aphids increased. This interaction modification is similar to other types of indirect interactions among enemy species (e.g. predatorpredator facilitation) that can enhance the suppression of agricultural pests. Results of our study, and those of others performed in agroecosystems, complement the broader debate over how biodiversity influences ecosystem functioning by specifically focusing on systems that produce goods of immediate relevance to human society.

The indirect effects of climate on species interactions were initially surprising, but ecological models that account for ecosystem decline have long underestimated their ubiquity and strength. Indirect effects not only yield "unexpected... more

The indirect effects of climate on species interactions were initially surprising, but ecological models that account for ecosystem decline have long underestimated their ubiquity and strength. Indirect effects not only yield "unexpected results", but also some of the strongest ecological effects (i.e. phase-shifts) that have been regarded as "catastrophes" on coral reefs, "collapses" of kelp forests and "crises" in seagrass meadows. Such effects went unanticipated because the impact of one species on another required knowledge of a third element that was inadequately understood. Subsequent debate over the causes of habitat loss has often been polarised by two extreme points of view, i.e. consumer versus producer effects. It is our perspective that these debates will persist unless we clarify the context-dependency of two kinds of indirect effect; those driven by strong consumer effects and those driven by strong producer effects. On human-dominated coasts, loss of coral, kelp and seagrass can occur as a function of change in trophic cascades (i.e. consumer effects) as well as change to competitive hierarchies (i.e. producer competition for resources). Because production and consumption are under strong physiological control by climate (providing predictable responses), there is merit in recognising the type and context of indirect effects to reduce errors associated with model-based forecasting. Indeed, forecasts of how global (e.g. elevated temperature and CO 2 ) and local drivers (e.g. fishing and pollution) combine to drive ecological change will often depend on the relative strength of different kinds of indirect effects (i.e. consumer effects vs producer effects). By recognising the context-dependency of the indirect effects under investigation, the information content of forecasts may not only increase, but also provide an improved understanding of indirect effects and community ecology in general.

Top-order predators often have positive effects on biological diversity owing to their key functional roles in regulating trophic cascades and other ecological processes. Their loss has been identified as a major factor contributing to... more

Top-order predators often have positive effects on biological diversity owing to their key functional roles in regulating trophic cascades and other ecological processes. Their loss has been identified as a major factor contributing to the decline of biodiversity in both aquatic and terrestrial systems. Consequently, restoring and maintaining the ecological function of top predators is a critical global imperative. Here we review studies of the ecological effects of the dingo Canis lupus dingo, Australia's largest land predator, using this as a case study to explore the influence of a top predator on biodiversity at a continental scale. The dingo was introduced to Australia by people at least 3500 years ago and has an ambiguous status owing to its brief history on the continent, its adverse impacts on livestock production and its role as an ecosystem architect. A large body of research now indicates that dingoes regulate ecological cascades, particularly in arid Australia, and that the removal of dingoes results in an increase in the abundances and impacts of herbivores and invasive mesopredators, most notably the red fox Vulpes vulpes. The loss of dingoes has been linked to widespread losses of small and medium-sized native mammals, the depletion of plant biomass due to the effects of irrupting herbivore populations and increased predation rates by red foxes. We outline a suite of conceptual models to describe the effects of dingoes on vertebrate populations across different Australian environments. Finally, we discuss key issues that require consideration or warrant research before the ecological effects of dingoes can be incorporated formally into biodiversity conservation programs.

From 1990-1993 juvenile pike (Esox lucius) were stocked each spring in the eutrophic Lake Lyng (9.9 ha, max. depth 7.6 m, mean depth 2.4 m) in densities between 515 and 3616 pike ha 1 . In 1989-90 the fish population consisted mainly of... more

From 1990-1993 juvenile pike (Esox lucius) were stocked each spring in the eutrophic Lake Lyng (9.9 ha, max. depth 7.6 m, mean depth 2.4 m) in densities between 515 and 3616 pike ha 1 . In 1989-90 the fish population consisted mainly of roach (Rutilus rutilus), rudd (Scardinius erythrophthalmus), perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus), and total fish biomass was estimated at 477 kg ha 1 . Prior to stocking pike was not present in the lake. Following the first year of stocking, the density of roach, rudd and ruffe fry expressed as catch per unit effort decreased significantly by 64 to 97%. In 1991 ruffe disappeared completely. The pike stocking did not affect the density of perch significantly. The growth of pike was high and also the growth of perch increased significantly from 1990 to 1991 (p<0.001) and from 1991 to 1994 (p<0.001). We found a linear negative relationship between stocking density of pike in May or June and the abundance of juvenile planktivorous fish (r 2 = 0.85, p<0.05) in the littoral zone in August. No relationship was found in the pelagic zone (r 2 = 0.21, p>0.4). Pike survival was low in late August/early September. We suggest that growth of the piscivorous perch increased due to increased Secchi depth and a continuous high density of 0+ planktivores in the pelagic zone of the lake during the years of pike stocking, possibly caused by behaviourial changes and the forcing of the 0+ planktivores into the pelagic zone.

As climate-driven environmental changes and anthropogenic perturbations increasingly affect ecological systems, the number of abrupt phase shifts in ecosystem dynamics is rising, with farreaching ecological, economic and social effects.... more

As climate-driven environmental changes and anthropogenic perturbations increasingly affect ecological systems, the number of abrupt phase shifts in ecosystem dynamics is rising, with farreaching ecological, economic and social effects. These shifts are notoriously difficult to study, anticipate and manage. Although indicators of impending phase shifts in ecosystems have been described theoretically, they have only been observed empirically either after the fact or under controlled experiments. Here we demonstrate the usefulness of case-specific simulation models to estimate tipping points in the dynamics of real ecological systems, characterise how these thresh-olds may vary depending on local conditions and derive safe management targets associated with low risk of undesirable phase shifts. Under the combined effects of ocean changes and fishing, inshore rocky reefs in eastern Tasmania can transition from dense seaweed beds to sea urchin 'barrens' habitat, realising severe local loss of habitat, productivity and valuable fisheries. Using Monte-Carlo simulations with a validated model that realistically captures reef dynamics, we characterise the hysteresis in community dynamics and the variability in ecological thresholds along the gradient of environmental conditions. Simulation suggests that prevention of ongoing sea urchin destructive grazing of macroalgal beds is achievable but the yet-to-be-observed restoration of seaweed beds from extensive sea urchin barrens is highly unlikely. To guide management against undesirable phase shifts, we define target points associated with low risk of widespread barrens formation and show that, along with sea urchin culling, recognising the role of lobsters in mitigating sea urchin destructive grazing through predation is key to maintain reef productivity. ) contains supplementary material, which is available to authorized users. Author contributions All authors conceived and contributed to the development of the project. MPM developed the TRITON model, designed and ran the simulations, analysed the results and elaborated the manuscript. CRJ and LRL provided guidance and feedback on model development and the elaboration of the ideas presented in this paper, and contributed to writing the manuscript.

There is global interest in restoring populations of apex predators, both to conserve them and to harness their ecological services. In Australia, reintroduction of dingoes (Canis dingo) has been proposed to help restore degraded... more

There is global interest in restoring populations of apex predators, both to conserve them and to harness their ecological services. In Australia, reintroduction of dingoes (Canis dingo) has been proposed to help restore degraded rangelands. This proposal is based on theories and the results of studies suggesting that dingoes can suppress populations of prey (especially medium- and large-sized herbivores) and invasive predators such as red foxes (Vulpes vulpes) and feral cats (Felis catus) that prey on threatened native species. However, the idea of dingo reintroduction has met opposition, especially from scientists who query the dingo’s positive effects for some species or in some environments. Here, we ask ‘what is a feasible experimental design for assessing the role of dingoes in ecological restoration?’ We outline and propose a dingo reintroduction experiment — one that draws upon the existing dingo-proof fence—and identify an area suitable for this (Sturt National Park, western New South Wales). Although challenging, this initiative would test whether dingoes can help restore Australia’s rangeland biodiversity, and potentially provide proof-of-concept for apex predator reintroductions globally.

The tritrophic interactions between plants, herbivores and avian predators are complex and prone to trophic cascades. We conducted a meta-analysis of original articles that have studied birds as predators of invertebrate herbivores, to... more

The tritrophic interactions between plants, herbivores and avian predators are complex and prone to trophic cascades. We conducted a meta-analysis of original articles that have studied birds as predators of invertebrate herbivores, to compare top-down trophic cascades with different plant responses from different environments and climatic areas. Our search found 29 suitable articles, with a total of 81 separate experimental study set-ups. The meta-analysis revealed that plants benefited from the presence of birds. A significant reduction was observed in the level of leaf damage and plant mortality. The presence of birds also positively affected the amount of plant biomass, whereas effects on plant growth were negligible. There were no differences in the effects between agricultural and natural environments. Similarly, plants performed better in all climatic areas (tropical, temperate and boreal) when birds were present. Moreover, both mature plants and saplings gained benefits from the presence of birds. Our results show that birds cause top-down trophic cascades and thus they play an integral role in ecosystems.

The ecosystem response to the 1989 spill of oil from the Exxon Valdez into Prince William Sound, Alaska, shows that current practices for assessing ecological risks of oil in the oceans and, by extension, other toxic sources should be... more

The ecosystem response to the 1989 spill of oil from the Exxon Valdez into Prince William Sound, Alaska, shows that current practices for assessing ecological risks of oil in the oceans and, by extension, other toxic sources should be changed. Previously, it was assumed that impacts to populations derive almost exclusively from acute mortality. However, in the Alaskan coastal ecosystem, unexpected persistence of toxic subsurface oil and chronic exposures, even at sublethal levels, have continued to affect wildlife. Delayed population reductions and cascades of indirect effects postponed recovery. Development of ecosystem-based toxicology is required to understand and ultimately predict chronic, delayed, and indirect long-term risks and impacts.

Recent studies document unprecedented declines in marine top predators that can initiate trophic cascades. Predicting the wider ecological consequences of these declines requires understanding how predators influence communities by... more

Recent studies document unprecedented declines in marine top predators that can initiate trophic cascades. Predicting the wider ecological consequences of these declines requires understanding how predators influence communities by inflicting mortality on prey and inducing behavioral modifications (risk effects). Both mechanisms are important in marine communities, and a sole focus on the effects of predator-inflicted mortality might severely underestimate the importance of predators. We outline direct and indirect consequences of marine predator declines and propose an integrated predictive framework that includes risk effects, which appear to be strongest for long-lived prey species and when resources are abundant. We conclude that marine predators should be managed for the maintenance of both density-and risk-driven ecological processes, and not demographic persistence alone.

Extensive regions around the world are influenced by the El Niño Southern Oscillation (ENSO). El Niño events occur irregularly but typically once every three to six years 1 . Although the effects of global warming on ENSO oscillations are... more

Extensive regions around the world are influenced by the El Niño Southern Oscillation (ENSO). El Niño events occur irregularly but typically once every three to six years 1 . Although the effects of global warming on ENSO oscillations are difficult to predict, recent high resolution climatic models suggest that the frequency of El Niño-like conditions are expected to increase over the coming decades 2 . During an El Niño episode, rainfall dramatically increases in certain areas of the world, whereas severe droughts occur in other regions . The rainfall during El Niño years can be four to ten times higher than average, and the phenomenon lasts approximately one year. The next phase, known as La Niña, produces roughly the opposite climate patterns to those found during an El Niño episode.

Ungulates can profoundly alter the structure and composition of forest communities via both direct and indirect mechanisms. Individual plant species often respond in a unique way to the direct effect of herbivory as a function of their... more

Ungulates can profoundly alter the structure and composition of forest communities via both direct and indirect mechanisms. Individual plant species often respond in a unique way to the direct effect of herbivory as a function of their sensitivity to browse damage, ungulate food preferences, and the density of ungulates present. Sustained browsing pressure can limit the regeneration of favored and susceptible woody plants and eliminate populations of favored or susceptible herbaceous plants. These losses, in turn, give rise to indirect effects via trophic cascades or physical habitat modification. These indirect effects affect many other plant and animal populations. In the mixed conifer-hardwood forests around the Great Lakes in North America, widespread habitat modification and the extirpation of native predators and other ungulates have acted to boost populations of white-tailed deer (Odocoileus virginianus) to historically high densities. Such densities have curtailed regeneration of several important conifers (e.g. Tsuga canadensis and Thuja occidentalis) as evidenced by demographic analysis. Deer also appear to limit regeneration of Quercus and Betula in many areas. Impacts on understory herbs are harder to assess, but baseline data from 50 years ago indicate that these communities are changing in a pattern that implicates deer: grasses, sedges, and some ferns are increasing while overall herb diversity is declining. Thus, deer are playing a keystone role in these communities. We are currently assessing an additional set of questions, including: How best can we measure and represent ungulate impacts? At which densities do deer threaten forest diversity? How do impacts depend on initial plant and ungulate densities? Which species emerge as 'winners' or 'losers' in heavily-browsed landscapes? What characteristics or traits make a species susceptible to ungulate herbivory? How do ungulates affect patterns of diversity and relative abundance in ecological communities? What are the pathways by which ungulates exert indirect effects on species? and How significant are indirect effects? #

Marine biodiversity worldwide is under increasing threat, primarily as a result of over-harvesting, pollution and climate change. Chondrichthyan fishes (sharks, rays and chimaeras) have a perceived higher intrinsic risk of extinction... more

Marine biodiversity worldwide is under increasing threat, primarily as a result of over-harvesting, pollution and climate change. Chondrichthyan fishes (sharks, rays and chimaeras) have a perceived higher intrinsic risk of extinction compared to other fish. Direct fishing mortality has driven many declines, even though some smaller fisheries persist without associated declines. Mixedspecies fisheries are of particular concern, as is illegal, unreported and unregulated (IUU) fishing. The lack of specific management and reporting mechanisms for the latter means that many chondrichthyans might already be susceptible to extinction from stochastic processes entirely unrelated to fishing pressure itself. Chondrichthyans might also suffer relatively more than other marine taxa from the effects of fishing and habitat loss and degradation given coastal habitat use for specific life stages. The effects of invasive species and pollution are as yet too poorly understood to predict their long-term role in affecting chondrichthyan population sizes. The spatial distribution of threatened chondrichthyan species under World Conservation Union (IUCN) Red List criteria are clustered mainly in (1) south-eastern South America;

Large ‘apex’ predators influence ecosystems in profound ways, by limiting the density of their prey and controlling smaller ‘mesopredators’. The loss of apex predators from much of their range has lead to a global outbreak of... more

Large ‘apex’ predators influence ecosystems in profound ways, by limiting the density of their prey and controlling smaller ‘mesopredators’. The loss of apex predators from much of their range has lead to a global outbreak of mesopredators, a process known as ‘mesopredator release’ that increases predation pressure and diminishes biodiversity. While the classifica- tions apex- and meso-predator are fundamental to current ecological thinking, their definition has remained ambiguous. Trophic cascades theory has shown the importance of predation as a limit to population size for a variety of taxa (top–down control). The largest of predators however are unlikely to be limited in this fashion, and their densities are commonly assumed to be determined by the availability of their prey (bottom–up control). However, bottom–up regulation of apex predators is contradicted by many studies, particularly of non-hunted populations. We offer an alternative view that apex predators are distinguishable by a capacity to limit their own population densities (self-regulation). We tested this idea using a set of life-history traits that could contribute to self-regulation in the Carnivora, and found that an upper limit body mass of 34 kg (corresponding with an average mass of 13–16 kg) marks a transition between extrinsically- and self- regulated carnivores. Small carnivores share fast reproductive rates and development and higher densities. Large carnivores share slow reproductive rates and development, extended parental care, sparsely populated territories, and a propensity towards infanticide, reproductive suppression, alloparental care and cooperative hunting. We discuss how the expression of traits that contribute to self-regulation (e.g. reproductive suppression) depends on social stability, and highlight the importance of studying predator–prey dynamics in the absence of predator persecution. Self-regulation in large carnivores may ensure that the largest and the fiercest do not overexploit their resources.

This work, illustrates the development of 2 GHz Low Noise Amplifier (LNA) interfaced with square truncated edge-fed right circularly polarized patch antenna. The LNA is simulated on Agilent ADS platform with TSMC 130nm RF CMOS process.... more

This work, illustrates the development of 2 GHz Low Noise Amplifier (LNA) interfaced with square truncated edge-fed
right circularly polarized patch antenna. The LNA is simulated on Agilent ADS platform with TSMC 130nm RF CMOS
process. The development of cascode amplifier and its optimization has been further exemplified. The developed LNA is
tuned for 2 GHz and the performance is tuned for high stability factor of 4, Gain of 19 dB which is essential for any
mobile device, Noise Figure (NF) of 1.15 dB with a P1dB point at -9 dBm. Further a truncated patch antenna with
right circular polarization has been simulated on EMpro. The antenna has a gain of 6.1 dB in the azimuth plane. The
simulated system can be further integrated to form the RF front end of TDD2000 LTE standard mobile device.

Foodwebs are nowadays an interesting and challenging subject of study for network analysis. In this study real food webs are compared one to each other in terms of how authors collected data, in order to build a coherent dataset. Emergent... more

Foodwebs are nowadays an interesting and challenging subject of study for network analysis. In this study real food webs are compared one to each other in terms of how authors collected data, in order to build a coherent dataset. Emergent patterns of these food webs are then quantified and three different generative models are used to try to reproduce them: cascade model, niche model and nested hierarchy model. A detailed comparison shows how niche model is the most suited of the three in accord with the data. Finally, an original clustering algorithm based on the idea of pollution spread is proposed and studied over food webs generated with the niche model. This algorithm needs no parameters, is suited for directed networks and provides information on which species may be screened to understand if the considered food web is polluted.

Lake Hayq, a highland lake in Ethiopia, was stocked with Tilapia fish (Oreochromis niloticus) in late 1970s, offering an opportunity to study the effect of fish predation in a natural lake. Since 1930s, some limnological surveys have been... more

Lake Hayq, a highland lake in Ethiopia, was stocked with Tilapia fish (Oreochromis niloticus) in late 1970s, offering an opportunity to study the effect of fish predation in a natural lake. Since 1930s, some limnological surveys have been done sporadically documenting a change in zooplankton composition including the disappearance of cladocerans, hypothesizing the stocked planktivorous fish could be a cause. Nevertheless, no detailed research was conducted to identify potential effects of fish stocking predominantly due to its remote location. The article presents data about zooplankton composition, abundance and biomass done between October 2007 and January 2009 on short-time intervals including the underlying limnological variables. The zooplankton community was depauperate comprising two copepods, three cladocerans, and six rotifers taxa, as typical for tropical lakes. Total mean standing biomass of all crustacean zooplankton was 237 mg dry mass m −3 , which gave Lake Hayq an intermediate position when compared with other tropical lakes. Of copepods, Thermocyclops ethiopiensis was almost an exclusive species, and its temporal variation was influenced by food supply and water temperature. We refute the hypothesis that Tilapia was the cause for the seasonal disappearance of cladocerans, and attribute it to the adverse effect of episodic mixing. Nevertheless, the planktivorous fish probably plays a key role in structuring the cladocerans in particular the large-sized Daphnia magna. In January 2008, we observed a massive planktivorous fish mortality that triggered high algal biomass, which was later grazed by large-sized D. magna demonstrating the trophic cascade hypothesis in a natural ecosystem.

The thylacine (Thylacinus cynocephalus) was one of Australia's largest predators, but became extinct in mainland Australia soon after the arrival of a new predator, the dingo (Canis lupus dingo) around 3500 bp. Evidence implicating the... more

The thylacine (Thylacinus cynocephalus) was one of Australia's largest predators, but became extinct in mainland Australia soon after the arrival of a new predator, the dingo (Canis lupus dingo) around 3500 bp. Evidence implicating the dingo in the thylacine's extinction has been equivocal, largely because thylacines are thought to be considerably larger than dingoes. Thus, other concurrent factors, such as shifts in human technology and population increase as well as climate change, have been cited to explain their extinction. Here we present new morphological evidence that female mainland Holocene thylacines were actually smaller than dingoes. We discuss these findings against archaeological and contemporary ecological evidence concerning dingoes' environmental impacts, and provide evidence that, as novel predators, dingoes induced a trophic cascade that had dramatic impacts on the fauna and economy of Holocene Australia. We suggest that dingoes, owing to their larger brains and body size, were likely a primary agent for the extinction of the thylacine from mainland Australia.

Novel assemblages of native and introduced species characterize a growing proportion of ecosystems worldwide. Some introduced species have contributed to extinctions, even extinction waves, spurring widespread efforts to eradicate or... more

Novel assemblages of native and introduced species characterize a growing proportion of ecosystems worldwide. Some introduced species have contributed to extinctions, even extinction waves, spurring widespread efforts to eradicate or control them. We propose that trophic cascade theory offers insights into why introduced species sometimes become harmful, but in other cases stably coexist with natives and offer net benefits. Large predators commonly limit populations of potentially irruptive prey and mesopredators, both native and introduced. This top-down force influences a wide range of ecosystem processes that often enhance biodiversity. We argue that many species, regardless of their origin or priors, are allies for the retention and restoration of biodiversity in top-down regulated ecosystems.

Apex predators have experienced catastrophic declines throughout the world as a result of human persecution and habitat loss. These collapses in top predator populations are commonly associated with dramatic increases in the abundance of... more

Apex predators have experienced catastrophic declines throughout the world as a result of human persecution and habitat loss. These collapses in top predator populations are commonly associated with dramatic increases in the abundance of smaller predators. Known as "mesopredator release," this trophic interaction has been recorded across a range of communities and ecosystems. Mesopredator outbreaks often lead to declining prey populations, sometimes destabilizing communities and driving local extinctions. We present an overview of mesopredator release and illustrate how its underlying concepts can be used to improve predator management in an increasingly fragmented world. We also examine shifts in North American carnivore ranges during the past 200 years and show that 60% of mesopredator ranges have expanded, whereas all apex predator ranges have contracted. The need to understand how best to predict and manage mesopredator release is urgent-mesopredator outbreaks are causing high ecological, economic, and social costs around the world.

Based on data from 233 Danish lakes, enclosure experiments, full-scale experiments and published empirical models we present evidence that top-down control is more important in shallow lakes than in deep lakes, excepting lakes with a high... more

Based on data from 233 Danish lakes, enclosure experiments, full-scale experiments and published empirical models we present evidence that top-down control is more important in shallow lakes than in deep lakes, excepting lakes with a high abundance of submerged macrophytes. The evidence in support is: (1) That at a given epilimnion total phosphorus concentration (TP) the biomass of fish per m 2 is independent of depth, which means that biomass per m 3 is markedly higher in shallow lakes. (2) That the biomass of benthic invertebrates is higher in shallow lakes, which means that the benthi-planktivorous fish are less dependent on zooplankton prey than in deep lakes. By their ability to shift to zooplankton predation their density can remain high even in periods when zooplankton is scarce and they can thereby maintain a potentially high predation pressure on zooplankton. (3) That the possibilities of cladocerans to escape predation by vertical migration are less. (4) That the zooplankton:phytoplankton mass ratio per m 2 is lower and presumably then also the grazing pressure on phytoplankton. (5) That nutrient constraints appear to be weaker, as evidenced by the fact that at a given annual mean TP, summer TP is considerably higher in shallow lakes, especially in eutrophic lakes lacking submerged macrophytes. (6) That negative feedback on cladocerans by cyanobacteria is lower as cyanobacterial dominance is less frequent in shallow lakes and more easily broken (at least in Northern temperate lakes), and (7) That top-down control by benthi-planktivorous fish is markedly reduced in lakes rich in submerged macrophytes because the plants serve as a refuge for pelagic cladocerans and encourage predatory fish at the expense of prey fish. We conclude that manipulation of fish and submerged macrophytes may have substantial impact on lake ecosystems, in particular in shallow eutrophic lakes. On the contrary, if the conditions for more permanent changes in plant abundance or fish community structure are lacking the feed-back mechanisms that endeavour a return to the original turbid state will be particularly strong in shallow lakes.

... Our understanding of eutrophication and its management has evolved from simple control of nutrient sources to recognition that it is often a cumulative effects problem that will require protection and restoration of many features of a... more

... Our understanding of eutrophication and its management has evolved from simple control of nutrient sources to recognition that it is often a cumulative effects problem that will require protection and restoration of many features of a lake&#x27;s community and its catchment. ...

Rewilding is a strategy for the conservation of complete, self-sustaining ecosystems, primarily involving the protection and, where necessary, reintroduction, of populations of keystone species in large, connected reserve networks. A... more

Rewilding is a strategy for the conservation of complete, self-sustaining ecosystems, primarily involving the protection and, where necessary, reintroduction, of populations of keystone species in large, connected reserve networks. A potential method of preserving ecosystem functions and biodiversity, it is now receiving a great deal of practical and political attention, particularly in North America. In Scotland, where many native species

Kelp forests are phyletically diverse, structurally complex and highly productive components of coldwater rocky marine coastlines. This paper reviews the conditions in which kelp forests develop globally and where, why and at what rate... more

Kelp forests are phyletically diverse, structurally complex and highly productive components of coldwater rocky marine coastlines. This paper reviews the conditions in which kelp forests develop globally and where, why and at what rate they become deforested. The ecology and long archaeological history of kelp forests are examined through case studies from southern California, the Aleutian Islands and the western North Atlantic, well-studied locations that represent the widest possible range in kelp forest biodiversity. Global distribution of kelp forests is physiologically constrained by light at high latitudes and by nutrients, warm temperatures and other macrophytes at low latitudes. Within mid-latitude belts (roughly 40-60°latitude in both hemispheres) well-developed kelp forests are most threatened by herbivory, usually from sea urchins. Overfishing and extirpation of highly valued vertebrate apex predators often triggered herbivore population increases, leading to widespread kelp deforestation. Such deforestations have the most profound and lasting impacts on species-depauperate systems, such as those in Alaska and the western North Atlantic. Globally urchininduced deforestation has been increasing over the past 2-3 decades. Continued fishing down of coastal food webs has resulted in shifting harvesting targets from apex predators to their invertebrate prey, including kelp-grazing herbivores. The recent global expansion of sea urchin harvesting has led to the widespread extirpation of this herbivore, and kelp forests have returned in some locations but, for the first time, these forests are devoid of vertebrate apex predators. In the western North Atlantic, large predatory crabs have recently filled this void and they have become the new apex predator in this system. Similar shifts from fish-to crab-dominance may have occurred in coastal zones of the United Kingdom and Japan, where large predatory finfish were extirpated long ago. Three North American case studies of kelp forests were examined to determine their long history with humans and project the status of future kelp forests to the year 2025. Fishing impacts on kelp forest systems have been both profound and much longer in duration than previously thought. Archaeological data suggest that coastal peoples exploited kelp forest organisms for thousands of years, occasionally resulting in localized losses of apex predators, outbreaks of sea urchin populations and probably small-scale deforestation. Over the past two centuries, commercial exploitation for export led to the extirpation of sea urchin predators, such as the sea otter in the North Pacific and predatory fishes like the cod in the North Atlantic. The largescale removal of predators for export markets increased sea urchin abundances and promoted the decline of kelp forests over vast areas. Despite southern California having one of the longest known associations with coastal kelp forests, widespread deforestation is rare. It is possible that functional redundancies among predators and herbivores make this most diverse system most stable. Such biodiverse kelp forests may also resist invasion from non-native species. In the species-depauperate western North Atlantic, introduced algal competitors carpet the benthos and threaten future kelp dominance. There, other non-native herbivores and predators have become established and dominant components of this system. Climate changes have had measurable impacts on kelp forest ecosystems and efforts to control the emission of greenhouse gasses should be a global priority. However, overfishing appears to be the greatest manageable threat to kelp forest ecosystems over the 2025 time horizon. Management should focus on minimizing fishing impacts and restoring populations of functionally important species in these systems.

... Blaire Van Valkenburgh is a paleobiologist and functional morphologist in the Department of Ecology and Evolutionary Biology at the University of ... predation-driven direct effects or fear-driven indirect effects on communities and... more

... Blaire Van Valkenburgh is a paleobiologist and functional morphologist in the Department of Ecology and Evolutionary Biology at the University of ... predation-driven direct effects or fear-driven indirect effects on communities and ecosystems (Ripple and Beschta 2004, Ray et al. ...

During the 1990s, ecologists such as Gary Polis catalysed a renaissance in food-web research by focusing on trophic processes occurring among habitats at the landscape scale. Examples include prey transported across ecotones to subsidize... more

During the 1990s, ecologists such as Gary Polis catalysed a renaissance in food-web research by focusing on trophic processes occurring among habitats at the landscape scale. Examples include prey transported across ecotones to subsidize predators in adjacent habitats, which, in turn, can have strong indirect effects such as initiating trophic cascades. Recent work in Japan by Shigeru Nakano and his colleagues has set new standards of holism and rigor in foodweb research by demonstrating complementary seasonal shifts in prey fluxes across a stream-forest ecotone that sustain higher densities and diversities of consumers in both habitats than would otherwise be supported in either alone. Although Nakano died in a tragic accident at sea with Polis and three other Japanese and American ecologists in March 2000, his work has left an indelible legacy that gives direction and purpose to further research on the significance of complex interrelationships in food webs across landscape scales.

NATURE CLIMATE CHANGE | VOL 1 | NOVEMBER 2011 | www.nature.com/natureclimatechange 401 C limate change is a serious threat to Earth's ecosystems. Increased greenhouse gases and the associated rise in temperature have been implicated in... more

NATURE CLIMATE CHANGE | VOL 1 | NOVEMBER 2011 | www.nature.com/natureclimatechange 401 C limate change is a serious threat to Earth's ecosystems. Increased greenhouse gases and the associated rise in temperature have been implicated in melting glaciers, rising sea levels, acidification of marine and freshwater systems, increased ultraviolet-B radiation and increased fire frequency. The past century has seen a nearly 1 °C rise in global average temperature 1 , with up to 7 °C of warming predicted by 2100 (ref. 2). Global warming has already led to significant shifts in the distribution, phenology and behaviour of organisms 3-5 . Of 1,700 plant, insect, amphibian and bird species examined in a review by Parmesan and Yohe 5 , 80% had a poleward range shift of 6.1 km per decade, and 87% had an advancement in the timing of phenological events, such as breeding or flowering, of 2.3 days per decade. Such changes in distribution and phenology in response to climate change have received much attention in the literature, but only recently have studies begun to address the effects of climate change on development and growth. For many organisms, development and growth, and thus organism size, are affected by temperature and water availability 6-8 . The increased temperatures and variability of precipitation associated with climate change are likely to influence the size of organisms, from primary producers to top predators 9-11 . However, as we discuss below, the degree to which organism size is affected by temperature or precipitation variability is likely to vary within and between taxa, which could disrupt ecosystem functioning. Here, we briefly summarize the changes in organism size that are most likely a result of climate change and increased carbon dioxide levels, and theorize on reasons for the observed patterns of size declines. We present evidence from fossil records, experimental and geographic comparisons, and recent studies implicating current climate change in the shrinking size of organisms. We discuss the mechanisms that are most likely to be contributing to the observed patterns, exceptions to these trends and implications for biodiversity. Ultimately, our opinion is that this effect will become much more pervasive, and that research should focus on quantifying size trends more broadly, and identifying proximate and ultimate drivers of size declines.

For elk (Cervus elaphus) in the Gallatin drainage of the Greater Yellowstone Ecosystem, Montana, USA, wolf movements caused local predation risk to vary substantially on a time scale of days. Spatially and temporally fine-scaled data from... more

For elk (Cervus elaphus) in the Gallatin drainage of the Greater Yellowstone Ecosystem, Montana, USA, wolf movements caused local predation risk to vary substantially on a time scale of days. Spatially and temporally fine-scaled data from GPS radio collars show that elk moved into the protective cover of wooded areas when wolves were present, reducing their use of preferred grassland foraging habitats that had high predation risk. By constraining habitat selection, wolves may have greater effects on elk dynamics than would be predicted on the basis of direct predation alone. Based on changes in the woody vegetation following the reintroduction of wolves, it has been suggested that antipredator responses by elk may be driving a trophic cascade in the Yellowstone ecosystem. However, studies to date have been hampered by a lack of direct data on spatial variation in predation risk, and the ways in which elk respond to variation in risk. Our data support a central portion of the hypothesis that elk antipredator behavior could drive a trophic cascade, but changes in elk numbers are also likely to have affected elk-plant interactions.

Human presence and activity in tropical forest is thought to exert top-down regulation over the various 'green-world' pathways of plant-based foodwebs. However, these effects have never been explored for the 'brown-world' pathways of... more

Human presence and activity in tropical forest is thought to exert top-down regulation over the various 'green-world' pathways of plant-based foodwebs. However, these effects have never been explored for the 'brown-world' pathways of fecal-detritus webs. The strong effects of humans on tropical game mammals are likely to indirectly influence fecal detritivores (including Scarabaeine dung beetles), with subsequent indirect impacts on detrivore-mediated and plantfacilitating detrital processes. Across a 380-km gradient of human influence in the western Brazilian Amazon, we conducted the first landscape-level assessment of human-induced cascade effects on the fecal detritus pathway, by coupling data on human impact, game mammal and detritivore community structure, and rate measurements of a key detritus process (i.e. dung beetle-mediated secondary seed dispersal). We found evidence that human impact indirectly influences both the diversity and biomass of fecal detritivores, but not detritivore-mediated processes. Cascade strength varied across detritivore groups defined by species' traits. We found smaller-bodied dung beetles were at higher risk of local decline in areas of human presence, and that body size was a better predictor of cascade structure than fecal resource manipulation strategy. Cascade strength was also stronger in upland, unflooded forests, than in seasonally flooded forests. Our results suggest that the impact of human activity in tropical forest on fecal-detritus food web structure is mediated by both species' traits and habitat type. Further research will be required to determine the conditions under which these cascade effects influence fecal-detritus web function. Citation: Nichols E, Uriarte M, Peres CA, Louzada J, Braga RF, et al. (2013) Human-Induced Trophic Cascades along the Fecal Detritus Pathway. PLoS ONE 8(10): e75819.

In the relatively simple Baltic Sea ecosystem, zooplankton-feeding sprat Sprattus sprattus is a major food source for breeding seabirds and piscivorous fish, and an important resource for commercial fisheries. Large-scale and long-term... more

In the relatively simple Baltic Sea ecosystem, zooplankton-feeding sprat Sprattus sprattus is a major food source for breeding seabirds and piscivorous fish, and an important resource for commercial fisheries. Large-scale and long-term ecosystem changes resulting mainly from over fishing and recruitment failure of cod Gadus morhua, which is the main fish predator of sprat, have affected natural-history patterns in a piscivorous seabird, the common guillemot Uria aalge, in a complex way. As the sprat stock increased, leading to lower energy content of fish, common guillemot chick body mass at fledging decreased. However, chick fledging body mass recovered in recent years as the sprat stock diminished, which brought about corresponding increases in sprat weight-at-age and energy content. The cod and sprat fishery affect the common guillemots in the Baltic Sea, but the effects differ depending on the management strategy.

Experimental evidence of trophic cascades initiated by large vertebrate predators is rare in terrestrial ecosystems. A serendipitous natural experiment provided an opportunity to test the trophic cascade hypothesis for wolves (Canis... more

Experimental evidence of trophic cascades initiated by large vertebrate predators is rare in terrestrial ecosystems. A serendipitous natural experiment provided an opportunity to test the trophic cascade hypothesis for wolves (Canis lupus) in Banff National Park, Canada. The first wolf pack recolonized the Bow Valley of Banff National Park in 1986. High human activity partially excluded wolves from one area of the Bow Valley (low-wolf area), whereas wolves made full use of an adjacent area (high-wolf area). We investigated the effects of differential wolf predation between these two areas on elk (Cervus elaphus) population density, adult female survival, and calf recruitment; aspen (Populus tremuloides) recruitment and browse intensity; willow (Salix spp.) production, browsing intensity, and net growth; beaver (Castor canadensis) density; and riparian songbird diversity, evenness, and abundance. We compared effects of recolonizing wolves on these response variables using the log response ratio between the low-wolf and high-wolf treatments. Elk population density diverged over time in the two treatments, such that elk were an order of magnitude more numerous in the low-wolf area compared to the high-wolf area at the end of the study. Annual survival of adult female elk was 62% in the high-wolf area vs. 89% in the low-wolf area. Annual recruitment of calves was 15% in the high-wolf area vs. 27% without wolves. Wolf exclusion decreased aspen recruitment, willow production, and increased willow and aspen browsing intensity. Beaver lodge density was negatively correlated to elk density, and elk herbivory had an indirect negative effect on riparian songbird diversity and abundance. These alternating patterns across trophic levels support the wolf-caused trophic cascade hypothesis. Human activity strongly mediated these cascade effects, through a depressing effect on habitat use by wolves. Thus, conservation strategies based on the trophic importance of large carnivores have increased support in terrestrial ecosystems.

A trophic cascade recently has been reported among wolves, elk, and aspen on the northern winter range of Yellowstone National Park, Wyoming, USA, but the mechanisms of indirect interactions within this food chain have yet to be... more

A trophic cascade recently has been reported among wolves, elk, and aspen on the northern winter range of Yellowstone National Park, Wyoming, USA, but the mechanisms of indirect interactions within this food chain have yet to be established. We investigated whether the observed trophic cascade might have a behavioral basis by exploring environmental factors influencing the movements of 13 female elk equipped with GPS radio collars. We developed a simple statistical approach that can unveil the concurrent influence of several environmental features on animal movements. Paths of elk traveling on their winter range were broken down into steps, which correspond to the straight-line segment between successive locations at 5-hour intervals. Each observed step was paired with 200 random steps having the same starting point, but differing in length and/or direction. Comparisons between the characteristics of observed and random steps using conditional logistic regression were used to model environmental features influencing movement patterns. We found that elk movements were influenced by multiple factors, such as the distance from roads, the presence of a steep slope along the step, and the cover type in which they ended. The influence of cover type on elk movements depended on the spatial distribution of wolves across the northern winter range of the park. In low wolf-use areas, the relative preference for end point locations of steps followed: aspen stands Ͼ open areas Ͼ conifer forests. As the risks of wolf encounter increased, the preference of elk for aspen stands gradually decreased, and selection became strongest for steps ending in conifer forests in high wolf-use areas. Our study clarifies the behavioral mechanisms involved in the trophic cascade of Yellowstone's wolf-elk-aspen system: elk respond to wolves on their winter range by a shift in habitat selection, which leads to local reductions in the use of aspen by elk.

We assessed the long-term (16 years) effects of introducing piscivores (northern pike) into a small, boreal lake (Lake 221, Experimental Lakes Area) containing abundant populations of two planktivorous fish species. After the... more

We assessed the long-term (16 years) effects of introducing piscivores (northern pike) into a small, boreal lake (Lake 221, Experimental Lakes Area) containing abundant populations of two planktivorous fish species. After the introduction, pearl dace were extirpated and yellow perch abundance was greatly reduced. Daphnia species shifted from D. galeata mendota to larger bodied Daphnia catawba, but the total zooplankton biomass did not increase, nor did the biomass of large grazers such as Daphnia. Phytoplankton biomass decreased after the northern pike introduction, but increased when northern pike were partially removed from the lake. Phosphorus (P) excretion by fish was $0.18 mg P m )2 d )1 before pike addition, declined rapidly to approximately 0.03-0.10 as planktivorous perch and dace populations were reduced by pike, and increased back to premanipulation levels after the pike were partially removed and the perch population recovered. When perch were abundant, P excretion by fish supported about 30% of the P demand by primary producers, decreasing to 6-14% when pike were abundant. Changes in phytoplankton abundance in Lake 221 appear to be driven by changes in P cycling by yellow perch, whose abundance was controlled by the addition and removal of pike. These results confirm the role of nutrient cycling in mediating trophic cascades and are consistent with previous enclosure experiments conducted in the same lake.

The spatial insurance hypothesis indicates that connectivity is an important attribute of natural ecosystems that sustains both biodiversity and ecosystem function. We tested the hypothesis by measuring the impact of manipulating... more

The spatial insurance hypothesis indicates that connectivity is an important attribute of natural ecosystems that sustains both biodiversity and ecosystem function. We tested the hypothesis by measuring the impact of manipulating connectivity in experimental metacommunties of a natural and diverse microecosystem. Isolation led to the extinction of large-bodied apex predators, subsequently followed by increases in prey species abundance. This trophic cascade was associated with significantly altered carbon and nitrogen fluxes in fragmented treatments. The ecosystem impacts were characteristic of a function debt because they persisted for several generations after the initial loss of connectivity. Local extinctions and disruption of ecosystem processes were mitigated, and even reversed, by the presence of corridors in the connected metacommunities, although these beneficial effects were unexpectedly delayed. We hypothesized that corridors maintained grazer movement between fragments, which enhanced microbial activity, and decomposition in comparison to isolated fragments. Our results indicate that knowledge of habitat connectivity and spatial processes is essential to understand the magnitude and timing of ecosystem perturbation in fragmented landscapes.

In this work, we evaluated the food web structure associated with a Patagonian scallop Zygochlamys patagonica fishing bed using stable isotopes as an indirect technique. Scallops showed ␦ 13 C isotope signatures between −17.9‰ and −18.8‰... more

In this work, we evaluated the food web structure associated with a Patagonian scallop Zygochlamys patagonica fishing bed using stable isotopes as an indirect technique. Scallops showed ␦ 13 C isotope signatures between −17.9‰ and −18.8‰ in muscle and −20.9‰ and −22.6‰ in gonads, which may indicate additional fractionation during metabolic transfers among organs. The isotopic signatures also suggest that larger scallops feed on organisms higher in the food web. Coexisting species, such as sponges, the ascidia Paramolgula gregaria, the anemone Actinostola crassicornis, colonial tunicates and Anthozoa, as well as the epibiont polychaete Idanthyrsus armatus, showed much lighter values of C than scallops, which indicates that they are using different food sources. Isotopic signatures suggest that species whose diet includes scallops are the gastropods Fusitriton magellanicus magellanicus, Odontocymbiola magellanica, Adelomelon ancilla and the sun starfish Labidiaster radiosus. The starfish Calyptraster sp. appears to be a top predator in this area, most likely preying on the gastropods previously mentioned but not directly on scallops. Our results suggest that there are at least three consumer trophic levels. Therefore, any strategy toward managing predators in this system should take into account potential trophic cascade effects given that extracting starfishes could enhance gastropods with an undesirable negative effect on the target species, the Patagonian scallop.

In many ecosystems, browsing of large mammals can aff ect plant species compositions. However, much less is known about potential above-and below-ground trophic interactions of large browsing mammals. Th is study focused on the direct and... more

In many ecosystems, browsing of large mammals can aff ect plant species compositions. However, much less is known about potential above-and below-ground trophic interactions of large browsing mammals. Th is study focused on the direct and indirect eff ects of browsing on trophic and abiotic interactions within forest ecosystems. To quantify above-and belowground cascade eff ects, white-tailed deer have been excluded for over 18 years from three 4-ha plots, which were paired with same sized deer access plots. Our results demonstrate complex direct and indirect cascade eff ects on forest food webs. Deer exclusion directly altered woody species composition and signifi cantly increased shrub and sapling density. Above-ground cascade eff ects include greater leaf litter accumulation and higher arthropod density and biomass within the exclosures. Below-ground indirect eff ects include signifi cant decrease in soil nutrients, and higher arbuscular mycorrhizal fungal inoculum potential in the exclosures. Because ecosystems have fi nite resource availability, high deer density may imbalance the system by redirecting resources toward maintaining deer biomass at the expense of multiple trophic levels throughout the forest community. Both complex bottom-up and top-down trophic cascade eff ects demonstrated largely unidirectional negative responses suggesting that high deer density has reduced the biodiversity of the forest community.