Pathways to the density‐dependent expression of cannibalism, and consequences for regulated population dynamics
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Abstract Cannibalism, once viewed as a rare or aberrant behavior, is now recognized to be widespread and to contribute broadly to the self‐regulation of many populations. Cannibalism can produce endogenous negative feedback on population growth because it is expressed as a conditional behavior, responding to the deteriorating ecological conditions that flow, directly or indirectly, from increasing densities of conspecifics. Thus, cannibalism emerges as a strongly density‐dependent source of mortality. In this synthesis, we review recent research that has revealed a rich diversity of pathways through which rising density elicits increased cannibalism, including both factors that (a) elevate the rate of dangerous encounters between conspecifics and (b) enhance the likelihood that such encounters will lead to successful cannibalistic attacks. These pathways include both features of the autecology of cannibal populations and features of interactions with other species, including food resources and pathogens. Using mathematical models, we explore the consequences of including density‐dependent cannibal attack rates on population dynamics. The conditional expression of cannibalism generally enhances stability and population regulation in single‐species models but also may increase opportunities for alternative states and prey population escape from control by cannibalistic predators.Keywords:
Cannibalism
Density dependence
Population density
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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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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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Abstract The Eastern monarch butterfly population has significantly declined over the last two decades creating growing concerns around its conservation status. Here, we showed that the overwintering population exhibited a negative density-dependence (i.e. a negative effect on growth rate of the density in the previous year) and that, after accounting for the density effect, the population growth rate tended to decline over time. The negative time effect is probably linked to the host plant (i.e. milkweed) decline in North America. A negative density-dependence was also found in the time series of both egg density per host plant and adult density across North America suggesting the importance of a bottom-up, resource-driven regulation such as host plant limitation and/or of a top-down regulation through generalist natural enemies or diseases. The temporal stability of the density effect indicated that the negative density-dependence and the population decline are likely independent phenomena. One of the most common conclusions of previous research is that environmental stochasticity is the dominant key compounded driver of population dynamics. We showed that density dependence explained 37–50% of the total variation in growth rate in three independent datasets, indicating that several non-exclusive density-related mechanisms can be important in monarch 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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Cannibalism is a likely mechanism of population regulation in dragonflies that inhabit fish—free temporary ponds. I tested for density—dependent effects of cannibalism on survival and size structure of larvae of the dragonfly Tramea carolina in two short—term experiments in replicated artificial ponds. I was able to distinguish mortality due to cannabalism from other sources of mortality by preventing some populations from engaging in cannibalism. The first experiment included two density levels and the presence or absence of cannabilism in a factorial design. The no—cannibalism treatment was achieved by removing the labial palps from all individuals to prevent them from grasping large prey. T. carolina survived significantly better in the no—cannibalism populations at both densities, and this difference was greater at high density than low density, indicating that the proportion cannibalized was density dependent. A second experiment explored the form of the density—dependent cannibalism curve over a broad range of density. The results were adequately fitby a predator—prey model incorporating size structure, so that not all members of the population were vulnerable to cannibalism. In both experiments the survival of only the smaller instars was reduced, suggesting that cannibalism may reduce variation in the size distributions of dragonfly populations and contribute to emergence synchrony. The results demonstrated that cannibalism was strongly density dependent and may contribute to population regulation of dragonflies in temporary ponds.
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Sibling cannibalism among vundu Heterobranchus longifilis larvae started at the age of 4 days, with the prey caught tail-first then swallowed up to the head, which was eventually discarded (type I cannibalism). At 17 days old, this type of cannibalism vanished and was replaced by the ingestion of the whole prey (type II cannibalism), which could only be exerted by predators six times as heavy as their prey. Type II cannibalism consisted of a seemingly opportunistic ambush attack by a formerly passive predator towards a disorientated prey. It required no preliminary aggression or chase, or even contact with the prey, suggesting that the attack was not mediated by the tactile sense, and that cannibalism was independent of aggressive behaviour. When alternative food resources (formulated feed, live tilapia prey) were available, the intensity of cannibalism decreased but pellet-eaters or tilapia predators always achieved lower growth rates than those feeding on conspecifics, suggesting that cannibalism was the most advantageous foraging tactic. Losses to cannibalism among populations of 30-day old juvenile vundu with an initial ratio of 4% of cannibals were as high as 75·5–79·9% over 15 days. Predation peaked during the first days (up to 2·8 prey C−1 day−1), then vanished progressively as surviving prey grew faster than cannibals and escaped their predation. Cannibals preferred consuming the largest prey available with respect to the logistics of cannibalism (body weight ratio of 6·0). This preference for large prey was interpreted both as a foraging tactic aiming to maximize the energetic return, and as foraging strategy enabling the cannibals to exploit their prey as long as possible. Based on these data, comprehensive models of the impact of cannibalism on vundu populations were developed.
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Sibling cannibalism among vundu Heterobranchus longifilis larvae started at the age of 4 days, with the prey caught tail‐first then swallowed up to the head, which was eventually discarded (type I cannibalism). At 17 days old, this type of cannibalism vanished and was replaced by the ingestion of the whole prey (type II cannibalism), which could only be exerted by predators six times as heavy as their prey. Type II cannibalism consisted of a seemingly opportunistic ambush attack by a formerly passive predator towards a disorientated prey. It required no preliminary aggression or chase, or even contact with the prey, suggesting that the attack was not mediated by the tactile sense, and that cannibalism was independent of aggressive behaviour. When alternative food resources (formulated feed, live tilapia prey) were available, the intensity of cannibalism decreased but pellet‐eaters or tilapia predators always achieved lower growth rates than those feeding on conspecifics, suggesting that cannibalism was the most advantageous foraging tactic. Losses to cannibalism among populations of 30‐day old juvenile vundu with an initial ratio of 4% of cannibals were as high as 75·5–79·9% over 15 days. Predation peaked during the first days (up to 2·8 prey C −1 day −1 ), then vanished progressively as surviving prey grew faster than cannibals and escaped their predation. Cannibals preferred consuming the largest prey available with respect to the logistics of cannibalism (body weight ratio of 6·0). This preference for large prey was interpreted both as a foraging tactic aiming to maximize the energetic return, and as foraging strategy enabling the cannibals to exploit their prey as long as possible. Based on these data, comprehensive models of the impact of cannibalism on vundu populations were developed.
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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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