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Papers by Bradley Cardinale

Bioscience, 2004

Empirical studies investigating the role of species diversity in sustaining ecosystem processes h... more Empirical studies investigating the role of species diversity in sustaining ecosystem processes have focused primarily on terrestrial plant and soil communities. Eighteen representative studies drawn from post-1999 literature specifically examined how changes in biodiversity affect benthic ecosystem processes. Results from these small-scale, low-diversity manipulative studies indicate that the effects of changes in biodiversity (mostly synonymous with local species richness) are highly variable over space and time and frequently depend on specific biological traits or functional roles of individual species. Future studies of freshwater and marine ecosystems will require the development of new experimental designs at larger spatial and temporal scales. Furthermore, to successfully integrate field and laboratory studies, the derivation of realistic models and appropriate experiments will require approaches different from those already used in terrestrial systems.

Nature, 2004

0.17, P N ¼ P P ¼ 0; light-harvesting machinery (chloroplasts), N I ¼ 0.113 and P I ¼ 0.0032 (ref... more 0.17, P N ¼ P P ¼ 0; light-harvesting machinery (chloroplasts), N I ¼ 0.113 and P I ¼ 0.0032 (ref. 6). As a neutral choice, we set the composition of the other biomass to the Redfield ratio, N o ¼ 0.0631, P o ¼ 0.00873; our results are relatively insensitive to these values. Assembly-machinery efficiency was set to m 0 ¼ 4.0 day 21 (g (g dry mass) 21 ) 21 (ref. 13). Maximum carbon-uptake efficiency was set to v 0 I ¼ w (16.8 g C day 21 g 21 ), using P b opt ¼ 20 mg C hr 21 (mg chl a) 21 (ref. 27), where superscript b indicates normalization to biomass, and the proportion of chlorophyll a in chloroplasts being 0.035 (ref. 28). Maximum N-and P-uptake efficiencies were set to v 0 N ¼ w (3.0 £ 10 3 g N day 21 (g N-uptake protein) 21 ) and v 0 P ¼ w (6.7 £ 10 3 g P day 21 (g P-uptake protein) 21 ), based on nutrient-transporter turnover times of 0.01 s (refs 29, 30). The minimum carbon quota was set to Q min,C ¼ 0.24w (ref. 12). The results are independent of cell weight because w cancels from the expression for optimal R a . The mortality rate was set to m ¼ 0.01 day 21 , chosen from the low end of the observed range to illustrate the lowest feasible value of allocation to assembly machinery. We fix the ratio of uptake machinery types to achieve colimitation during exponential growth, by equating the three terms in the minimum of equation . This results in R N ¼ 0.00317 R u , R P ¼ 0.000381 R u , and R I ¼ 0.996 R u , which agrees with the observation that nutrientuptake proteins are usually a much smaller component of biomass than chloroplasts.

Ecology Letters, 2004

The last 15 years has seen parallel surges of interest in two research areas that have rarely int... more The last 15 years has seen parallel surges of interest in two research areas that have rarely intersected: biodiversity and ecosystem functioning (BEF), and multispecies predator–prey interactions (PPI). Research addressing role of biodiversity in ecosystem functioning has focused primarily on single trophic-level systems, emphasizing additive effects of diversity that manifest through resource partitioning and the sampling effect. Conversely, research addressing predator–prey interactions has focused on two trophic-level systems, emphasizing indirect and non-additive interactions among species. Here, we use a suite of consumer-resource models to organize and synthesize the ways in which consumer species diversity affects the densities of both resources and consumer species. Specifically, we consider sampling effects, resource partitioning, indirect effects caused by intraguild interactions and non-additive effects. We show that the relationship between consumer diversity and the density of resources and consumer species are broadly similar for systems with one vs. two trophic levels, and that indirect and non-additive interactions generally do little more than modify the impacts of diversity established by the sampling effect and resource partitioning. The broad similarities between systems with one vs. two trophic levels argue for greater communication between researchers studying BEF, and researchers studying multispecies PPI.

Journal of Animal Ecology, 2006

One of the oldest questions in ecology is how species diversity in any given trophic level is rel... more One of the oldest questions in ecology is how species diversity in any given trophic level is related to the availability of essential resources that limit biomass (e.g. water, nutrients, light or prey). Researchers have tried to understand this relationship by focusing either on how diversity is influenced by the availability of resources, or alternatively, how resource abundance is influenced by species diversity. These contrasting perspectives have led to a seeming paradox '... is species diversity the cause or the consequence of resources that limit community biomass?' Here we present results of an experiment that show it is possible for species diversity and resource density to exhibit reciprocal causal relationships in the same ecological system. Using a guild of ladybeetle predators and their aphid prey, we manipulated the number of predator species in field enclosures to examine how predator diversity impacts prey population size. At the same time, we manipulated the abundance of aphid prey in discrete habitat patches within each enclosure to determine how smaller-scale spatial variation in resource abundance affects the number of co-occurring predator species. We found that the number of ladybeetle species added to enclosures had a significant impact on aphid population dynamics because interference competition among the predators reduced per capita rates of predation and, in turn, the overall efficiency of the predator guild. At the same time, spatial variation in aphid abundance among smaller habitat patches generated variation in the observed richness of ladybeetles because more species occurred in patches where predators aggregated in response to high aphid density. The results of our experiment demonstrate that it is possible for species diversity to simultaneously be a cause and eScholarship provides open access, scholarly publishing services to the University of California and delivers a dynamic research platform to scholars worldwide. a consequence of resource density in the same ecological system, and they shed light on how this might occur for groups of mobile consumers that exhibit rapid responses to spatial and temporal variation in their prey. Summary 1. One of the oldest questions in ecology is how species diversity in any given trophic level is related to the availability of essential resources that limit biomass (e.g. water, nutrients, light or prey). Researchers have tried to understand this relationship by focusing either on how diversity is influenced by the availability of resources, or alternatively, how resource abundance is influenced by species diversity. These contrasting perspectives have led to a seeming paradox '… is species diversity the cause or the consequence of resources that limit community biomass?' 2. Here we present results of an experiment that show it is possible for species diversity and resource density to exhibit reciprocal causal relationships in the same ecological system. Using a guild of ladybeetle predators and their aphid prey, we manipulated the number of predator species in field enclosures to examine how predator diversity impacts prey population size. At the same time, we manipulated the abundance of aphid prey in discrete habitat patches within each enclosure to determine how smaller-scale spatial variation in resource abundance affects the number of co-occurring predator species. 3. We found that the number of ladybeetle species added to enclosures had a significant impact on aphid population dynamics because interference competition among the predators reduced per capita rates of predation and, in turn, the overall efficiency of the predator guild. At the same time, spatial variation in aphid abundance among smaller habitat patches generated variation in the observed richness of ladybeetles because more species occurred in patches where predators aggregated in response to high aphid density. 4. The results of our experiment demonstrate that it is possible for species diversity to simultaneously be a cause and a consequence of resource density in the same ecological system, and they shed light on how this might occur for groups of mobile consumers that exhibit rapid responses to spatial and temporal variation in their prey.

Oikos, 2004

. Effects of species diversity on the primary productivity of ecosystems: extending our spatial a... more . Effects of species diversity on the primary productivity of ecosystems: extending our spatial and temporal scales of inference. Á/ Oikos 104: 437 Á/450. The number of studies examining how species diversity influences the productivity of ecosystems has increased dramatically in the past decade as concern about global loss of biodiversity has intensified. Research to date has greatly improved our understanding of how, when, and why species loss alters primary production in ecosystems. However, because experiments have been performed at rather small spatial and short temporal scales, it is unclear whether conclusions can be readily extrapolated to the broader scales at which natural communities are most likely to influence ecosystem functioning. Here we develop a simple patch-dynamics model to examine some of the scale-dependent and independent qualities of the diversity-productivity relationship. We first simulate a typical diversity-productivity experiment and show that the influence of species richness on productivity is temporally dynamic, growing stronger through successional time. This holds true irrespective of whether resource partitioning or a sampling effect is the underlying mechanism. We then increase the spatial scale of the simulation from individual patches to a region consisting of many patch types. Results suggest that the diversity-productivity relationship is not influenced by spatial scale per se, but that the mechanism producing the relationship can change from sampling effects within individual patches to resource partitioning across patch types composing the region. This change occurs even though model dynamics are the same at both scales, suggesting that sampling effects and resource partitioning can represent different descriptions of the same biological processes operating concurrently at differing scales of observation. Lastly, we incorporate regional processes of dispersal and disturbance into the model and show that these processes can amplify the effect of species richness on productivity, resulting in patterns not easily anticipated from experiments. We conclude that the relative control of community structure by local versus regional processes may be a primary determinant of the diversity-productivity relationship in natural ecosystems. Therefore, past experiments having focused only on local processes might not reflect patterns and processes underlying diversity-productivity relationships in communities where disturbance and dispersal regulate species biomasses.

The last 15 years has seen parallel surges of interest in two research areas that have rarely int... more The last 15 years has seen parallel surges of interest in two research areas that have rarely intersected: biodiversity and ecosystem functioning (BEF), and multispecies predatorprey interactions (PPI). Research addressing role of biodiversity in ecosystem functioning has focused primarily on single trophic-level systems, emphasizing additive effects of diversity that manifest through resource partitioning and the sampling effect. Conversely, research addressing predator-prey interactions has focused on two trophiclevel systems, emphasizing indirect and non-additive interactions among species. Here, we use a suite of consumer-resource models to organize and synthesize the ways in which consumer species diversity affects the densities of both resources and consumer species. Specifically, we consider sampling effects, resource partitioning, indirect effects caused by intraguild interactions and non-additive effects. We show that the relationship between consumer diversity and the density of resources and consumer species are broadly similar for systems with one vs. two trophic levels, and that indirect and nonadditive interactions generally do little more than modify the impacts of diversity established by the sampling effect and resource partitioning. The broad similarities between systems with one vs. two trophic levels argue for greater communication between researchers studying BEF, and researchers studying multispecies PPI.

Ecology Letters, 2003

The suppression of agricultural pests has often been proposed as an important service of natural ... 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.

Bioscience, 2004

Empirical studies investigating the role of species diversity in sustaining ecosystem processes h... more Empirical studies investigating the role of species diversity in sustaining ecosystem processes have focused primarily on terrestrial plant and soil communities. Eighteen representative studies drawn from post-1999 literature specifically examined how changes in biodiversity affect benthic ecosystem processes. Results from these small-scale, low-diversity manipulative studies indicate that the effects of changes in biodiversity (mostly synonymous with local species richness) are highly variable over space and time and frequently depend on specific biological traits or functional roles of individual species. Future studies of freshwater and marine ecosystems will require the development of new experimental designs at larger spatial and temporal scales. Furthermore, to successfully integrate field and laboratory studies, the derivation of realistic models and appropriate experiments will require approaches different from those already used in terrestrial systems.

Nature, 2004

0.17, P N ¼ P P ¼ 0; light-harvesting machinery (chloroplasts), N I ¼ 0.113 and P I ¼ 0.0032 (ref... more 0.17, P N ¼ P P ¼ 0; light-harvesting machinery (chloroplasts), N I ¼ 0.113 and P I ¼ 0.0032 (ref. 6). As a neutral choice, we set the composition of the other biomass to the Redfield ratio, N o ¼ 0.0631, P o ¼ 0.00873; our results are relatively insensitive to these values. Assembly-machinery efficiency was set to m 0 ¼ 4.0 day 21 (g (g dry mass) 21 ) 21 (ref. 13). Maximum carbon-uptake efficiency was set to v 0 I ¼ w (16.8 g C day 21 g 21 ), using P b opt ¼ 20 mg C hr 21 (mg chl a) 21 (ref. 27), where superscript b indicates normalization to biomass, and the proportion of chlorophyll a in chloroplasts being 0.035 (ref. 28). Maximum N-and P-uptake efficiencies were set to v 0 N ¼ w (3.0 £ 10 3 g N day 21 (g N-uptake protein) 21 ) and v 0 P ¼ w (6.7 £ 10 3 g P day 21 (g P-uptake protein) 21 ), based on nutrient-transporter turnover times of 0.01 s (refs 29, 30). The minimum carbon quota was set to Q min,C ¼ 0.24w (ref. 12). The results are independent of cell weight because w cancels from the expression for optimal R a . The mortality rate was set to m ¼ 0.01 day 21 , chosen from the low end of the observed range to illustrate the lowest feasible value of allocation to assembly machinery. We fix the ratio of uptake machinery types to achieve colimitation during exponential growth, by equating the three terms in the minimum of equation . This results in R N ¼ 0.00317 R u , R P ¼ 0.000381 R u , and R I ¼ 0.996 R u , which agrees with the observation that nutrientuptake proteins are usually a much smaller component of biomass than chloroplasts.

Ecology Letters, 2004

The last 15 years has seen parallel surges of interest in two research areas that have rarely int... more The last 15 years has seen parallel surges of interest in two research areas that have rarely intersected: biodiversity and ecosystem functioning (BEF), and multispecies predator–prey interactions (PPI). Research addressing role of biodiversity in ecosystem functioning has focused primarily on single trophic-level systems, emphasizing additive effects of diversity that manifest through resource partitioning and the sampling effect. Conversely, research addressing predator–prey interactions has focused on two trophic-level systems, emphasizing indirect and non-additive interactions among species. Here, we use a suite of consumer-resource models to organize and synthesize the ways in which consumer species diversity affects the densities of both resources and consumer species. Specifically, we consider sampling effects, resource partitioning, indirect effects caused by intraguild interactions and non-additive effects. We show that the relationship between consumer diversity and the density of resources and consumer species are broadly similar for systems with one vs. two trophic levels, and that indirect and non-additive interactions generally do little more than modify the impacts of diversity established by the sampling effect and resource partitioning. The broad similarities between systems with one vs. two trophic levels argue for greater communication between researchers studying BEF, and researchers studying multispecies PPI.

Journal of Animal Ecology, 2006

One of the oldest questions in ecology is how species diversity in any given trophic level is rel... more One of the oldest questions in ecology is how species diversity in any given trophic level is related to the availability of essential resources that limit biomass (e.g. water, nutrients, light or prey). Researchers have tried to understand this relationship by focusing either on how diversity is influenced by the availability of resources, or alternatively, how resource abundance is influenced by species diversity. These contrasting perspectives have led to a seeming paradox '... is species diversity the cause or the consequence of resources that limit community biomass?' Here we present results of an experiment that show it is possible for species diversity and resource density to exhibit reciprocal causal relationships in the same ecological system. Using a guild of ladybeetle predators and their aphid prey, we manipulated the number of predator species in field enclosures to examine how predator diversity impacts prey population size. At the same time, we manipulated the abundance of aphid prey in discrete habitat patches within each enclosure to determine how smaller-scale spatial variation in resource abundance affects the number of co-occurring predator species. We found that the number of ladybeetle species added to enclosures had a significant impact on aphid population dynamics because interference competition among the predators reduced per capita rates of predation and, in turn, the overall efficiency of the predator guild. At the same time, spatial variation in aphid abundance among smaller habitat patches generated variation in the observed richness of ladybeetles because more species occurred in patches where predators aggregated in response to high aphid density. The results of our experiment demonstrate that it is possible for species diversity to simultaneously be a cause and eScholarship provides open access, scholarly publishing services to the University of California and delivers a dynamic research platform to scholars worldwide. a consequence of resource density in the same ecological system, and they shed light on how this might occur for groups of mobile consumers that exhibit rapid responses to spatial and temporal variation in their prey. Summary 1. One of the oldest questions in ecology is how species diversity in any given trophic level is related to the availability of essential resources that limit biomass (e.g. water, nutrients, light or prey). Researchers have tried to understand this relationship by focusing either on how diversity is influenced by the availability of resources, or alternatively, how resource abundance is influenced by species diversity. These contrasting perspectives have led to a seeming paradox '… is species diversity the cause or the consequence of resources that limit community biomass?' 2. Here we present results of an experiment that show it is possible for species diversity and resource density to exhibit reciprocal causal relationships in the same ecological system. Using a guild of ladybeetle predators and their aphid prey, we manipulated the number of predator species in field enclosures to examine how predator diversity impacts prey population size. At the same time, we manipulated the abundance of aphid prey in discrete habitat patches within each enclosure to determine how smaller-scale spatial variation in resource abundance affects the number of co-occurring predator species. 3. We found that the number of ladybeetle species added to enclosures had a significant impact on aphid population dynamics because interference competition among the predators reduced per capita rates of predation and, in turn, the overall efficiency of the predator guild. At the same time, spatial variation in aphid abundance among smaller habitat patches generated variation in the observed richness of ladybeetles because more species occurred in patches where predators aggregated in response to high aphid density. 4. The results of our experiment demonstrate that it is possible for species diversity to simultaneously be a cause and a consequence of resource density in the same ecological system, and they shed light on how this might occur for groups of mobile consumers that exhibit rapid responses to spatial and temporal variation in their prey.

Oikos, 2004

. Effects of species diversity on the primary productivity of ecosystems: extending our spatial a... more . Effects of species diversity on the primary productivity of ecosystems: extending our spatial and temporal scales of inference. Á/ Oikos 104: 437 Á/450. The number of studies examining how species diversity influences the productivity of ecosystems has increased dramatically in the past decade as concern about global loss of biodiversity has intensified. Research to date has greatly improved our understanding of how, when, and why species loss alters primary production in ecosystems. However, because experiments have been performed at rather small spatial and short temporal scales, it is unclear whether conclusions can be readily extrapolated to the broader scales at which natural communities are most likely to influence ecosystem functioning. Here we develop a simple patch-dynamics model to examine some of the scale-dependent and independent qualities of the diversity-productivity relationship. We first simulate a typical diversity-productivity experiment and show that the influence of species richness on productivity is temporally dynamic, growing stronger through successional time. This holds true irrespective of whether resource partitioning or a sampling effect is the underlying mechanism. We then increase the spatial scale of the simulation from individual patches to a region consisting of many patch types. Results suggest that the diversity-productivity relationship is not influenced by spatial scale per se, but that the mechanism producing the relationship can change from sampling effects within individual patches to resource partitioning across patch types composing the region. This change occurs even though model dynamics are the same at both scales, suggesting that sampling effects and resource partitioning can represent different descriptions of the same biological processes operating concurrently at differing scales of observation. Lastly, we incorporate regional processes of dispersal and disturbance into the model and show that these processes can amplify the effect of species richness on productivity, resulting in patterns not easily anticipated from experiments. We conclude that the relative control of community structure by local versus regional processes may be a primary determinant of the diversity-productivity relationship in natural ecosystems. Therefore, past experiments having focused only on local processes might not reflect patterns and processes underlying diversity-productivity relationships in communities where disturbance and dispersal regulate species biomasses.

The last 15 years has seen parallel surges of interest in two research areas that have rarely int... more The last 15 years has seen parallel surges of interest in two research areas that have rarely intersected: biodiversity and ecosystem functioning (BEF), and multispecies predatorprey interactions (PPI). Research addressing role of biodiversity in ecosystem functioning has focused primarily on single trophic-level systems, emphasizing additive effects of diversity that manifest through resource partitioning and the sampling effect. Conversely, research addressing predator-prey interactions has focused on two trophiclevel systems, emphasizing indirect and non-additive interactions among species. Here, we use a suite of consumer-resource models to organize and synthesize the ways in which consumer species diversity affects the densities of both resources and consumer species. Specifically, we consider sampling effects, resource partitioning, indirect effects caused by intraguild interactions and non-additive effects. We show that the relationship between consumer diversity and the density of resources and consumer species are broadly similar for systems with one vs. two trophic levels, and that indirect and nonadditive interactions generally do little more than modify the impacts of diversity established by the sampling effect and resource partitioning. The broad similarities between systems with one vs. two trophic levels argue for greater communication between researchers studying BEF, and researchers studying multispecies PPI.

Ecology Letters, 2003

The suppression of agricultural pests has often been proposed as an important service of natural ... 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.