Life‐history strategy varies with the strength of competition in a food‐limited ungulate population
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Abstract Fluctuating population density in stochastic environments can contribute to maintain life‐history variation within populations via density‐dependent selection. We used individual‐based data from a population of Soay sheep to examine variation in life‐history strategies at high and low population density. We incorporated life‐history trade‐offs among survival, reproduction and body mass growth into structured population models and found support for the prediction that different life‐history strategies are optimal at low and high population densities. Shorter generation times and lower asymptotic body mass were selected for in high‐density environments even though heavier individuals had higher probabilities to survive and reproduce. In contrast, greater asymptotic body mass and longer generation times were optimal at low population density. If populations fluctuate between high density when resources are scarce, and low densities when they are abundant, the variation in density will generate fluctuating selection for different life‐history strategies, that could act to maintain life‐history variation.Keywords:
Density dependence
Ungulate
Population density
Life History Theory
Variation (astronomy)
Understanding population dynamics is critical for the management of animal populations. Comparatively little is known about the relative importance of endogenous (i.e. density‐dependent) and exogenous (i.e. density‐independent) factors on the population dynamics of amphibians with complex life cycles. We examined the potential effects of density‐dependent and ‐independent (i.e. climatic) factors on population dynamics by analyzing a 15‐yr time series data of the agile frog Rana dalmatina population from Târnava Mare Valley, Romania. We used two statistical models: 1) the partial rate correlation function to identify the feedback structure and the potential time lags in the time series data and 2) a Gompertz state‐space model to simultaneously investigate direct and delayed density dependence as well as climatic effects on population growth rate. We found evidence for direct negative density dependence, whereas delayed density dependence and climate did not show a strong influence on population growth rate. Here we demonstrated that direct density dependence rather than delayed density dependence or climate determined the dynamics of our study population. Our results confirm the findings of many experimental studies and suggest that density dependence may buffer amphibian populations against environmental stress. Consequently, it may not be easy to scale up from individual‐level effects to population‐level effects.
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In tropical areas, annual rainfall and predation have been reported to drive population dynamics of most species of large herbivores studied so far, with little direct empirical evidence for density-dependent responses of life-history traits. We here investigated in a game ranch in Zimbabwe density-dependent responses of body condition and recruitment in an impala ( Aepyceros melampus (Lichtenstein, 1812)) population that underwent an experimental drastic reduction of density within 2 years under similar rainfall and predation pressure. Body condition of all sex and age classes was lower during the high-density year than during the low-density year, suggesting increased competition for restricted resources at high density. In addition, we observed a significant increase in population recruitment (from 0.47 to 0.80 juveniles/female) as population density declined. Our study provides a rare example of a direct density-dependent response of body condition at the individual level in a tropical ungulate species, and indicates that food resource variation controls population dynamics of impala under constant and moderate predation pressure as is commonly reported in temperate populations of large herbivores.
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Population density regulation is a fundamental principle in ecology, but the specific process underlying functional expression of density dependence remains to be fully elucidated. One view contends that patterns of density dependence are largely fixed across a species irrespective of environmental conditions, whereas another is that the strength and expression of density dependence are fundamentally variable depending on the nature of exogenous or endogenous constraints acting on the population. We conducted a study investigating the expression of density dependence in Chlamydomonas spp. grown under a gradient from low to high nutrient density. We predicted that the relationship between per capita growth rate (pgr) and population density would vary from concave up to concave down as nutrient density became less limiting and populations experienced weaker density regulation. Contrary to prediction, we found that the relationship between pgr and density became increasingly concave-up as nutrient levels increased. We also found that variation in pgr increased, and pgr levels reached higher maxima in nutrient-limited environments. Most likely, these results are attributable to population growth suppression in environments with high intraspecific competition due to limited nutrient resources. Our results suggest that density regulation is strongly variable depending on exogenous and endogenous processes acting on the population, implying that expression of density dependence depends extensively on local conditions. Additional experimental work should reveal the mechanisms influencing how the expression of density dependence varies across populations through space and time.
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Bovidae
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The relative effect of top-down versus bottom-up forces in regulating and limiting wildlife populations is an important theme in ecology. Untangling these effects is critical for a basic understanding of trophic dynamics and effective management. We examined the drivers of moose (Alces alces) population growth by integrating two independent sources of observations within a hierarchical Bayesian population model. We used one of the largest existing spatiotemporal data sets on ungulate population dynamics globally. We documented a 20% population decline over the period examined. There was negative density-dependent population growth of moose. Although we could not determine the mechanisms producing density-dependent suppression of population growth, the relatively low densities at which we documented moose populations suggested it could be due to density-dependent predation. Predation primarily limited population growth, except at low density, where it was regulating. After we simulated several harvest scenarios, it appeared that harvest was largely additive and likely contributed to population declines. Our results highlight how population dynamics are context dependent and vary strongly across gradients in climate, forest type, and predator abundance. These results help clarify long-standing questions in population ecology and highlight the complex relationships between natural and human-caused mortality in driving ungulate population dynamics.
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This study examines the extent to which plant population dynamics are affected by density—dependent processes, and at what stage(s) in the life cycle density—dependent effects occur. A natural population of Bouteloua rigidiseta, a perennial grass, was subjected to two changes of density, the addition of seeds and the removal of 30—40% of the adult plants. Censuses were conducted before and after these treatments, one year apart. Survival, growth, and reproduction were all highly size dependent. Seed additions increased proportionately the number of new recruits to the population, indicating that the size of this age class was limited by seed input; seed additions decreased the size of new recruits. Adult removal tended to increase the sizes of both new recruits and adults. Thus competition among adults, among recruits, and by adults against recruits, but not competition by recruits against adults, affected this population in a density—dependent fashion. No density—dependent treatment effects involving reproduction were found. There was a high level of environmental heterogeneity in the magnitude of treatment responses. The magnitudes of all responses to changes in density were much less than the magnitudes of the changes themselves, indicating that density—dependent effects were quite weak. The most probable cause was the decline of the B. rigidiseta population following a drought after the first census and before the treatments were effectively imposed. The highly variable climate of this region may make the relaxation or absence of density—dependent population regulation common. The size of new recruits to the population of Aristida longiseta, another perennial grass, was decreased by the increase in the number of B. rigidiseta new recruits, and the survival of A. longiseta recruits may have been increased. No other effects of the treatments on A. longiseta were found, and the treatments had no effect on the total densities of three other grasses. These results may also be a product of the decline in the B. rigidiseta population. The changes in abundances of the five species suggest that temporal partitioning of niche space is important in this community.
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Populations near their equilibrium are expected to show density‐dependence through a negative feedback on at least one demographic parameter, e.g. survival and/or productivity. Nevertheless, it is not always clear which vital rate is affected the most, and even less whether this dependence holds in open populations in which immigration and emigration are also important. We assessed the relative importance of population density in the variation of local survival, recruitment, proportion of transients (emigrants) and productivity through the analysis of detailed life‐histories of 4286 seabirds from a colony that reached an apparent demographic equilibrium after a period of exponential increase. We provide evidence that the role of population density and resource availability changes according to the demographic parameter considered. Estimates indicated that transients increased from 5% to 20% over the study period, suggesting an average turnover of about 1400 individuals per year. The parameters most influenced by population density alone were local survival and probability of transience. Recruitment was negatively associated with population density during the increasing phase but unexpected high values were also recorded at high population levels. These high values were explained by a combination of population size and food availability. Mean productivity varied with food availability, independently from population variations. The population density alone explained up to a third of the yearly variation of the vital rates considered, suggesting that open populations are equally influenced by stochastic and density‐independent events (such as environmental perturbations) than by intrinsic (i.e. density‐dependent) factors.
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Density dependence
Carrying Capacity
Life History Theory
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The relative effect of top-down versus bottom-up forces in regulating and limiting wildlife populations is an important theme in ecology. Untangling these effects is critical for basic understanding of trophic dynamics and effective management. We examined the drivers of moose abundance by integrating two sets of observations to create one of the largest existing spatiotemporal datasets on ungulate population dynamics globally. We documented a 20% population decline. At high density, moose were regulated by intraspecific competition. Predation primarily limited population growth, except at low density, where it was regulating. Harvest was largely additive and contributed to population decline. Our results provide strong evidence for density dependent predation, highlighting that population dynamics are context dependent and vary strongly across gradients in climate, forest type and predator abundance. These results clarify longstanding questions in population ecology and highlight the complex relationships between natural and human-caused mortality in driving ungulate population dynamics.
Ungulate
Density dependence
Population density
Population cycle
Trophic cascade
Population ecology
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This study examines the extent to which plant population dynamics are affected by density-dependent processes, and at what stage(s) in the life cycle density-dependent effects occur. A natural population of Bouteloua rigidiseta, a perennial grass, was subjected to two changes of density, the addition of seeds and the removal of 30-40% of the adult plants. Censuses were conducted before and after these treatments, one year apart. Survival, growth, and reproduction were all highly size dependent. Seed additions increased proportionately the number of new recruits to the population, indicating that the size of this age class was limited by seed input; seed additions decreased the size of new recruits. Adult removal tended to increase the sizes of both new recruits and adults. Thus competition among adults, among recruits, and by adults against recruits, but not competition by recruits against adults, affected this population in a density-dependent fashion. No density-dependent treatment effects involving reproduction were found. There was a high level of environmental het- erogeneity in the magnitude of treatment responses. The magnitudes of all responses to changes in density were much less than the magnitudes of the changes themselves, indicating that density-dependent effects were quite weak. The most probable cause was the decline of the B. rigidiseta population following a drought after the first census and before the treatments were effectively imposed. The highly variable climate of this region may make the relaxation or absence of density-dependent population regulation common. The size of new recruits to the population of Aristida longiseta, another perennial grass, was decreased by the increase in the number of B. rigidiseta new recruits, and the survival of A. longiseta recruits may have been increased. No other effects of the treatments on A. longiseta were found, and the treatments had no effect on the total densities of three other grasses. These results may also be a product of the decline in the B. rigidiseta population. The changes in abundances of the five species suggest that temporal partitioning of niche space is important in this community.
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Population density
Vital rates
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