Empirical Evidence of Density Dependence in Populations of Large Herbivores (original) (raw)
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Ecology, 2007
Understanding the factors responsible for generating size variation in cohorts of organisms is important for predicting their population and evolutionary dynamics. We group these factors into two broad classes: those due to scaling relationships between growth and size (size-dependent factors), and those due to individual trait differences other than size (sizeindependent factors; e.g., morphology, behavior, etc.). We develop a framework predicting that the nonlethal presence of predators can have a strong effect on size variation, the magnitude and sign of which depend on the relative influence of both factors. We present experimental results showing that size-independent factors can strongly contribute to size variation in anuran larvae, and that the presence of a larval dragonfly predator reduced expression of these size-independent factors. Further, a review of a number of experiments shows that the effect of this predator on relative size variation of a cohort ranged from negative at low growth rates to positive at high growth rates. At high growth rates, effects of size-dependent factors predominate, and predator presence causes an increase in the scaling of growth rate with size (larger individuals respond less strongly to predator presence than small individuals). Thus predator presence led to an increase in size variation. In contrast, at low growth rates, size-independent factors were relatively more important, and predator presence reduced expression of these size-independent factors. Consequently, predator presence led to a decrease in size variation. Our results therefore indicate a further mechanism whereby nonlethal predator effects can be manifest on prey species performance. These results have strong implications for both ecological and evolutionary processes. Theoretical studies indicate that changes in cohort size variation can have profound effects on population dynamics and stability, and therefore the mere presence of a predator could have important ecological consequences. Further, changes in cohort size variation can have important evolutionary implications through changes in trait heritability.
Journal of Theoretical Biology, 1995
Several stage-structured population models are considered in which fecundity, mortality, and maturation to the next stage are allowed to depend on stage and within-stage age, and in which adults are assumed to be the dominant consumers of limiting resources. Local stability effects of changing the strength of regulation about a fixed equilibrium point are examined. The main result is that shifting the principal target of strongly density-dependent mortality from adults to progressively earlier phases of juvenile development reverses the stability effect of such mortality from locally stabilizing to destabilizing. This result holds for all the models studied and therefore appears to be fairly robust to the pattern of age-dependence in adult fecundity and the shape of the juvenile period's durational distribution. A simple intuitive interpretation is developed which can account for these and several related results in the literature. The necessity of distinguishing between life-historical delays and regulatory delays is also discussed.
Population density effects on longevity
Genetica, 1993
Population density, or the number of adults in an environment relative to the limiting resources, may have important long and short term consequences for the longevity of organisms. In this paper we summarize the way in which crowding may have an immediate impact on longevity, either through the phenomenon known as dietary restriction or through alterations in the quality of the environment brought on by the presence of large numbers of individuals. We also consider the possible long term consequences of population density on longevity by the process of natural selection. There has been much theoretical speculation about the possible impact of population density on the evolution of longevity but little experimental evidence has been gathered to test these ideas. We discuss some of the theory and empirical evidence that exists and show that population density is an important factor in determining both the immediate chances of survival and the course of natural selection.
Population dynamics of large herbivores: variable recruitment with constant adult survival
Trends in Ecology & Evolution, 1998
T he factors that explain changes in population size are a central theme in ecology, and long-term studies of population dynamics are of great interest for life history theory, population ecology, wildlife management and conservation biology 1,2 . Studies that can identify which vital rates are more variable (variability patterns) and which ones are more likely to influence overall changes in population size (elasticity patterns) are particularly useful 2 . Historically, large herbivores were sometimes considered unsuitable for the study of population ecology because their long generation time meant that interesting results could not be expected for several years. However, large herbivorous mammals are particularly suited to demographic studies because age classes are readily identifiable. In addition, many species are economically important or are useful indicator species for conservation. Recently, several researchers have published long-term studies of ungulates, mostly based on monitoring of marked individuals and often taking advantage of recent methodological developments of Capture-Mark-Recapture (CMR) modeling that account for differences in the probability of recapturing (or resighting) marked individuals, so that biological hypotheses can be tested reliably 3 .
Population dynamic consequences of delayed life-history effects
Trends in Ecology & …, 2002
Many aspects of population fluctuations can be captured with reasonable precision using simple, nonstructured models of the population renewal process . Such models include the familiar logistic equation, Ricker models and linear and nonlinear autoregressive models. Another frequently used class of models is age-or stage-structured models (e.g. matrix models ) that split the life history into relevant age or stage components. A common feature among these model types is that life-history traits, such as survival and fecundity, do not vary with time. The predicted dynamics of a model population are thus dependent only on the initial specification of the survival and fecundity rates.
Current caveats and further directions in the analysis of density-dependent population regulation
Oikos, 2008
An important issue in population ecology is to disentangle different density-dependent mechanisms that may limit or regulate animal populations. This goal is further complicated when studying long-lived species for which experimental approaches are not feasible, in whose cases density-dependence hypotheses are tested using long-term monitored populations. Here we respond to some criticisms and identify additional problems associated with these kinds of observational studies. Current caveats are related to the temporal and spatial scales covered by population monitoring data, which may question its suitability for density-dependence tests, and to statistical flaws such as the incorrect control for confounding variables, low statistical power, the distribution of demographic variables, the interpretation of spurious correlations, and the often used stepwise series of univariate analyses. Generalised linear mixed models are recommended over other more traditional approaches, since they help to solve the above statistical problems and, more importantly, allow to properly test several hypotheses simultaneously. Finally, several management actions aimed to recover endangered species, such as supplementary feeding, might be considered as field experiments for further testing densitydependence hypotheses in long-lived study models. We expect these opportunities, together with the most adequate statistical tools now available, will help to better our understanding of density-dependent effects in wild populations.
Journal of Animal Ecology, 2014
1. In seasonal populations, vital rates are not only determined by the direct effects of density at the beginning of each season, but also by density at the beginning of past seasons. Such delayed density dependence can arise via non-lethal effects on individuals that carry over to influence per capita rates. 2. In this study, we examine (i) whether parental breeding density influences offspring size, (ii) how this could carry over to affect offspring survival during the subsequent non-breeding period and (iii) the population consequences of this relationship. 3. Using Drosophila melanogaster, the common fruit fly, submitted to distinct breeding and non-breeding seasons, we first used a controlled laboratory experiment to show that high parental breeding density leads to small offspring size, which then affects offspring survival during the non-breeding period but only at high non-breeding densities. 4. We then show that a model with the interaction between parental breeding density and offspring density at the beginning of the non-breeding season best explained offspring survival over 36 replicated generations. 5. Finally, we developed a biseasonal model to show that the positive relationship between parental density and offspring survival can dampen fluctuations in population size between breeding and non-breeding seasons. 6. These results highlight how variation in parental density can lead to differences in offspring quality which result in important non-lethal effects that carry over to influence per capita rates the following season, and demonstrate how this phenomenon can have important implications for the long-term dynamics of seasonal populations.