Demographic,mechanistic and density-dependent determinants of population growth rate: a case study in an avian predator

Identifying the determinants of population growth rate is a central topic in population ecology. Three approaches (demographic, mechanistic and density-dependent) used historically to describe the determinants of population growth rate are here compared and combined for an avian predator, the barn owl (Tyto alba). The owl population remained approximately stable (r approximately 0) throughout the period from 1979 to 1991. There was no evidence of density dependence as assessed by goodness of fit to logistic population growth. The finite (lambda) and instantaneous (r) population growth rates were significantly positively related to food (field vole) availability. The demographic rates, annual adult mortality, juvenile mortality and annual fecundity were reported to be correlated with vole abundance. The best fit (R(2) = 0.82) numerical response of the owl population described a positive effect of food (field voles) and a negative additive effect of owl abundance on r. The numerical response of the barn owl population to food availability was estimated from both census and demographic data, with very similar results. Our analysis shows how the demographic and mechanistic determinants of population growth rate are linked; food availability determines demographic rates, and demographic rates determine population growth rate. The effects of food availability on population growth rate are modified by predator abundance.     

The demographic basis of population regulation in columbian ground squirrels

Environmental factors influence the dynamics and regulation of biological populations through their influences on demographic variables, but demographic mechanisms of population regulation have received little attention. We investigated the demographic basis of regulation of Columbian ground squirrel (Spermophilus columbianus) populations under natural and experimentally food-supplemented conditions. Food supplementation caused substantial increases in population density, and population densities returned to pretreatment levels when the supplementation ended. Control (untreated) populations remained relatively stable throughout the study period (1981-1986). Because food resources regulated the size of the ground squirrel populations, we used life-table response experiment (LTRE) analyses to examine the demographic basis of changes in population growth rate and thus also demographic influences on population regulation. LTRE analyses of two food-manipulated populations revealed that changes in age at maturity and fertility rate of females generally made the largest contributions to observed changes in population growth rate. Thus, our results suggested that abundance of food resources regulated the size of our study populations through the effects of food resources on age at maturity and fertility rates. Our results also indicated that different demographic mechanisms can underlie population regulation under different environmental conditions, because lower juvenile survival substantially contributed to population decline, but in only one of the populations. Demographic analyses of experimental data, such as those presented here, offer a rigorous and unambiguous means to elucidate the demographic basis of population regulation and to help identify environmental factors that underlie dynamics and regulation of biological populations.     

Climate, food, density and wildlife population growth rate

1. The aim of this study was to derive and evaluate a priori models of the relationship between annual instantaneous population growth rate (r) and climate. These were derived from the numerical response of annual r and food, and the effect of climate on a parameter in the numerical response. The goodness of fit of a range of such deductive models to data on annual r of Soay sheep and red deer were evaluated using information-theoretic (AICc-based) analyses. 2. The analysis for sheep annual r showed negative effects of abundance and negative effects of the interaction of abundance and climate, measured as March rainfall (and winter NAO) in the best fitting models. The analysis for deer annual r showed a negative effect of deer abundance and a positive effect of climate measured as March rainfall (but a negative effect of winter NAO), but no interaction of abundance and climate in the best fitting models. 3. There was most support in the analysis of sheep dynamics for the ratio numerical response and the assumption that parameter J (equilibrium food per animal) was influenced by climate. In the analysis of deer dynamics there was most support for the numerical responses assuming effects of food and density (Ivlev and density, food and density, and additive responses) and slightly less support for the ratio numerical response. The evaluation of such models would be aided by the collection of and incorporation of food data into the analyses.     

Cyclicity and variability in prey dynamics strengthens predator numerical response: the effects of vole fluctuations on white stork productivity

Theory predicts that optimality of life-long investment in reproduction is, among other factors, driven by the variability and predictability of the resources. Similarly, during the breeding season, single resource pulses characterized by short periods and high amplitudes enable strong numerical responses in their consumers. However, it is less well established how spatio-temporal dynamics in resource supplies influence the spatio-temporal variation of consumer reproduction. We used the common vole (Microtus arvalis)—white stork (Ciconia ciconia) resource—consumer model system to test the effect of increased temporal variation and periodicity of vole population dynamics on the strength of the local numerical response of storks. We estimated variability, cycle amplitude, and periodicity (by means of direct and delayed density dependence) in 13 Czech and Polish vole populations. Cross-correlation between annual stork productivity and vole abundance, characterizing the strength of the local numerical response of storks, increased when the vole population fluctuated more and population cycles were shorter. We further show that the onset of incubation of storks was delayed during the years of higher vole abundance. We demonstrate that high reproductive flexibility of a generalist consumer in tracking the temporal dynamics of its resource is driven by the properties of the local resource dynamics and we discuss possible mechanisms behind these patterns.     

Threshold levels of generalist predation determine consumer response to resource pulses

Many ecological systems are characterized by brief periods of increased resource availability called resource pulses. Empirical studies suggest that some populations of primary consumers grow rapidly in response to resource pulses, but others instead remain at low abundance despite increases in resource availability. Previous theory suggests that the lack of increase in primary consumers might be due to predators, which can respond to increased prey density both numerically, by increasing their own population, and functionally, by killing prey at a faster rate. The complexity of potential population responses to resource pulses can be assessed with simulations, but analytical conditions determining when one observes qualitatively distinct dynamics have yet to be identified. Here we use a graphical method based on a bifurcation diagram to derive the conditions leading to qualitatively distinct steady state and transient prey population dynamics as levels of predation (abundance and diversity) vary. When predation thresholds are crossed, consumer populations respond numerically to increases in their resources and provide a secondary resource pulse to their predators and parasites. These community dynamics have broad implications for the impact of changing predator communities on insect and rodent population outbreaks, which are economically and epidemiologically important.     

Offspring size–number tradeoffs and food quality feedbacks impact population dynamics in a Daphnia–algae system

Population fluctuations can be affected by both extrinsic (e.g. weather patterns, food availability) and intrinsic (e.g. life‐history) factors. A key life‐history tradeoff is the production of offspring size versus number, ranging from many small offspring to few large offspring. Models show that this life‐history tradeoff in offspring size and number, through maturation time, can have significant impacts on population dynamics. However, few manipulative experiments have been conducted that can isolate life‐history effects from impacts of extrinsic factors in consumer–resource systems. We experimentally tested the effect of an offspring size–number tradeoff on population stability and food availability in a consumer–resource system. Using Daphnia pulex, we created a shift from many, small offspring being produced to fewer, larger offspring. Two sets of experiments were performed to examine the interaction of an extrinsic factor (light levels) and intrinsic population structure on dynamics, and we controlled for the ingestion pressure on algal prey at the time of the manipulation. We predicted that the tradeoff would impact internal consumer population characteristics, including biasing the stage structure towards adults, increasing adult size, and increasing average population‐level reproduction. This adult‐dominated stage structure was predicted to then lead to instability and a low quantity–high quality food state. Under all light levels, the manipulated populations became dominated by large adults. Contrary to predictions, the amplitudes of fluctuations in Daphnia biomass were lower in populations shifted to few–large offspring, representing higher stability in these populations. Furthermore, in high light conditions, a stable low Daphnia – high algae biomass (low food quality) state was observed in few–large offspring treatments but not in control (many–small offspring) treatments. Our results show a strong link between light levels as an extrinsic factor and the life‐history tradeoff of consumer offspring size versus number that impacts consumer–resource population dynamics through feedbacks with resource quality.     

Marten and fisher responses to fluctuations in prey populations and mast crops in the northern hardwood forest

Many forest tree species produce seed (mast) crops that are consumed by a variety of wildlife species and these pulsed resources may mediate interactions among predator and prey populations. In the northern hardwood forests of New York, we investigated interactions among mast production, prey abundance, and harvests of American martens (Martes americana) and fishers (Martes pennanti) during 1988–2009. Mast production for beech (Fagus grandifolia), sugar maple (Acer saccharum), and mountain ash (Sorbus americana) was synchronous and an alternate-year pattern in production was evident for most of the time series. We documented considerable temporal variation in summer small mammal relative abundance and our numerical response models received substantial support for 5 of the 8 species, indicating lagged responses to autumn mast crops. Trap response of martens to the autumn production of beech mast and mountain ash berries was immediate and numerical responses to the relative abundance of small mammal prey occurred during the preceding summer. The age structure of the marten harvest differed based on the dominant alternate-year pattern of summer prey relative abundance and autumn mast production (χ²₄ = 33.06, P < 0.001). The proportion of juvenile marten in the autumn harvest was 52% and 34% following summers when small mammal relative abundance was high and low, respectively and these differences resulted in a persistent cohort effect that was apparent until age 3.5. Trap response of fishers to the autumn production of beech mast was immediate and numerical responses to the relative abundance of Sciurid prey occurred during the preceding summer. Marten and fisher harvests fluctuated similarly among New York, Maine, and New Brunswick, which may indicate regional synchronization of mast crops and responses of martens and fishers to similar prey dynamics. A better understanding of how food availability influences demographic responses and trapping vulnerability of martens and fishers would aid our ability to manage harvests of these species on a sustained yield basis. © 2011 The Wildlife Society.     

Examination of the role of detritus food quality,phytoplankton intracellular storage capacity,and zooplankton stoichiometry on planktonic dynamics

Nutrient stoichiometric ratios are primary driving factors of planktonic food web dynamics. Ecological stoichiometry theory postulates the elemental ratios of consumer species to be homeostatic, while primary-producer stoichiometry may vary with ambient nutrient availability. The notion of phytoplankton intracellular storage is far from novel, but remains largely unexplored in modeling studies of population dynamics. We constructed a seasonally-unforced, zero-dimensional, nutrient–phytoplankton–zooplankton–detritus (NPZD) model that considers dynamic phytoplankton phosphorus reserves and quasi-dynamic zooplankton stoichiometry. A generic food quality term is used to express seston biochemical composition, ingestibility, and digestibility. We examined the sensitivity of the planktonic food web patterns to light and nutrient availability, zooplankton mortality, and detritus food quality as well as to phytoplankton intracellular storage and zooplankton stoichiometry. Our results reinforce earlier findings that high quality seston exerts a stabilizing effect on food web dynamics. However, we also found that the combination of low algal and high detritus food quality with high zooplankton mortality yielded limit cycles and multiple steady states, suggesting that the heterogeneity characterizing seston nutritional quality may have more complicated ecological ramifications. Our numerical experiments identify resource competition strategies related to nutrient transport rates and internal nutrient quotas that may be beneficial for phytoplankton to persevere in resource-limiting habitats. We also highlight the importance of the interplay between optimal stoichiometry and the factors controlling homeostatic rigidity in zooplankton. In particular, our predictions show that the predominance of phosphorus-rich and tightly-homeostatic herbivores in nutrient-enriched environments with low seston food quality can potentially result in high phytoplankton abundance, high phytoplankton-to-zooplankton ratios, and acceleration of oscillatory dynamics. Generally, our modeling study emphasizes the impact of both intracellular/somatic storage and food quality on prey–predator interactions, pinpointing an important aspect of food web dynamics usually neglected by the contemporary modeling studies.     

Grassland-herbivore interactions: how do grazers coexist?

We develop a new approach to modeling grazing systems that links foraging characteristics (intake and digestive constraints) with resource dynamics via the probability of encounter with different grass heights. Three complementary models are presented: the generation of a grass height structure through selective grazing; investigating the conditions for consumer coexistence; and, using a simplified resource structure, the consequences for consumer abundance. The main finding is that coexistence between grazers differing in body size is possible if a single-resource type becomes differentiated in its height structure. Large grazers can facilitate food availability for smaller species but with the latter being competitively dominant. The relative preference given to different resource partitions is important in determining the nature of population interactions. Large-body and small-body grazer populations can interact through competitive, parasitic, commensalist, or amensalist relationships, depending on the way they partition the resource as well as their relative populations and the dynamics of resource renewal. The models provide new concepts of multispecies carrying capacity (stock equilibrium) in grazed systems with implications for conservation and management. We conclude that consumer species are not substitutable; therefore, the use of rangeland management concepts such as "livestock units" may be inappropriate.     

Interactive effects of exogenous and endogenous factors on demographic rates of an African rodent

Exogenous and endogenous environmental factors can have simultaneous additive as well as interacting effects on life‐history traits. Ignoring such interactions can lead to a biased understanding of variability in demographic rates and consequently population dynamics. These interactions have been the focus of decades‐long debates on the mechanisms underlying small mammal population fluctuations. They have often been studied indirectly through seasonal effects, but studies considering them directly and more mechanistically are rare. We investigated the joint effects of exogenous (temperature, food availability) and endogenous (population density) factors on the demographic rates of a group‐living diurnal rodent, the African striped mouse Rhabdomys pumilio using nine‐year mark–recapture data from a population in the Succulent Karoo, South Africa. In general, higher temperatures and lower food availability were associated with higher survival, whereas high population densities were either beneficial or detrimental to survival depending on interacting food availability. High reproductive rates were related to lower temperatures, higher food availability and lower population density, and interactions among environmental factors mediated the strength of these relationships. Our study highlights the complex ways in which different environmental factors can interact to shape demographic rates and emphasizes the importance of explicitly including interactions among exogenous and endogenous factors into studies of population dynamics.     

Temporal changes in fecundity and age at sexual maturity of southern elephant seals at Marion Island

Our objective was to examine the effect of variation in reproductive parameters on the demography of southern elephant seals at Marion Island. We used age-specific capture probabilities of breeding females in a Cormack-Jolly-Seber context to derive reproductive rates. We found that age at maturity declined and fecundity rates increased as the population declined, indicating a compensatory response. Fecundity rates ranged from 0.03 to 0.29 among 3-year-olds (mean=0.16), 0.18 to 0.50 in 4-year-olds (mean=0.40), and 0.28 to 0.50 in 5-year-olds (mean=0.45). We think that a relative increase in food availability, concomitant with the population decline, promoted earlier sexual maturity correlated with more rapid growth of juveniles when population abundance was lower. It is suggested that the relative importance of fecundity in population regulation in elephant seals has been underestimated. Moreover, it appears that the onset of sexual maturity may be the first demographic variable to change in response to a change in population density.     

Intra-guild interactions and projected impact of climate and land use changes on North American pochard ducks

The co-occurrence of functionally similar species is very common in nature, and is often put forward as a basis for ecosystem resilience to disturbance. At the same time, competition between similar species is also considered a strong driver of community composition. However, environmental stochasticity can alter this prediction, either because competitive abilities depend on time-varying factors or because covariance in species’ responses to environmental conditions masks the effect of competition. Interactions other than competition can also influence community dynamics but have received less attention. We used a simplified community of two sympatric duck species (redhead Aythya americana and canvasback A. valisineria) and a previously published analysis of 50 years of demographic data to parameterize a stochastic, density-dependent, stage-structured model. These ducks interact via nest parasitism (mostly of canvasback by redhead) in addition to competition for food resources, with consequences at the demographic level; these interactions are modulated by habitat availability (number of ponds in the study landscape). We found that if habitat availability decreased there was a high risk of quasi-extinction, and redheads, although initially able to maintain their numerical dominance, quickly became the least abundant species because they perform worse during droughts. If habitat availability increased, we found that the initially more rare canvasback would increase in relative abundance, albeit slowly. We interpret this as a shift from a community influenced by nest parasitism (which is detrimental to canvasback) to a community mostly driven by species-specific dynamics due to relaxation of resource limitation.     

Population cycles in voles and lemmings: state of the science and future directions

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Theoretical models of species' borders: single species approaches

The range of potential mechanisms limiting species' distributions in space is nearly as varied and complex as the diversity of life itself. Yet viewed abstractly, a species' border is a geographic manifestation of a species' demographic responses to a spatially and temporally varying world. Population dynamic models provide insight into the different routes by which range limits can arise owing to gradients in demographic rates. In a metapopulation context, for example, range limits may be caused by gradients in extinction rates, colonization rates or habitat availability. We have consider invasion models in uniform and heterogeneous environments as a framework for understanding non-equilibrium range limits, and explore conditions under which invasions may cease to spread leaving behind a stationary range limit. We conclude that non-equilibrial range dynamics need further theoretical and empirical attention.     

Predator population dynamics under a cyclic prey regime: numerical responses,demographic parameters and growth rates

The consequences of cyclic fluctuations in abundance of prey species on predator continue to improve our understanding of the mechanisms behind population regulation. Among predators, vole‐eating raptors usually respond to changes in prey abundance with no apparent time‐lag and therefore contradict predictions from the predator–prey theory. In such systems, the interplay between demographic traits and population growth rate in relation to prey abundance remains poorly studied, yet it is crucial to characterize the link between ecological processes and population changes. Using a mechanistic approach, we assessed the demographic rates associated to the direct and indirect numerical responses of a specialist raptor (Montagu's harrier) to its cyclic prey (common vole), using long term data from two adjacent study sites in France. First‐year survival rates were weakly affected by vole abundance, probably due to the fact that Montagu's harriers are trans‐Saharan migrants and thus escape the vole collapse occurring in autumn–winter. Recruitment of yearling as well as breeding propensity of experienced adult females were strongly affected by vole abundance and at least partially shaped the trajectory of the breeding population. We argued that the strong density dependent signal detected in predator time series was mostly the phenomenological consequence of the positive direct numerical response of harriers to vole abundance. Accounting for this, we proposed a method to assess density dependence in predator relying on a cyclic prey. Finally, the variation in Montagu's harrier population growth rates was best explained by overwinter growth rates of the prey population and to a lesser extent by previous residual predator density.     

Why don't predators have positive effects on prey populations?

Summary Three mechanisms by which increasing predation can increase prey population density are discussed: (1) Additional predation on species which have negative effects on the prey; (2) Predation on consumer species whose relationship with their own prey is characterized by a unimodal prey isocline; (3) Predation on species which adaptively balance predation risk and food intake while foraging. Possible reasons are discussed for the rarity of positive effects in previous predator-manipulation studies; these include the short-term nature of experiments, the large magnitudes of predator density manipulation, and various sources of bias in choice of system and interpretation of results.     

Interplay between the paradox of enrichment and nutrient cycling in food webs

Nutrient cycling is fundamental to ecosystem functioning. Despite recent major advances in the understanding of complex food web dynamics, food web models have so far generally ignored nutrient cycling. However, nutrient cycling is expected to strongly impact food web stability and functioning. To make up for this gap, we built an allometric and size structured food web model including nutrient cycling. By releasing mineral nutrients, recycling increases the availability of limiting resources for primary producers and links each trophic level to the bottom of food webs. We found that nutrient cycling can provide a significant part of the total nutrient supply of the food web, leading to a strong enrichment effect that promotes species persistence in nutrient poor ecosystems but leads to a paradox of enrichment at high nutrient inputs. The presence of recycling loops linking each trophic level to the basal resources weakly affects species biomass temporal variability in the food web. Recycling loops tend to slightly dampen the destabilising effect of nutrient enrichment on consumer temporal variability while they have opposite effects for primary producers. By considering nutrient cycling, this new model improves our understanding of the response of food webs to nutrient availability and opens perspectives to better link studies on food web dynamics and ecosystem functioning.     

How to understand species’ niches and range dynamics: a demographic research agenda for biogeography

Range dynamics causes mismatches between a species’ geographical distribution and the set of suitable environments in which population growth is positive (the Hutchinsonian niche). This is because source–sink population dynamics cause species to occupy unsuitable environments, and because environmental change creates non‐equilibrium situations in which species may be absent from suitable environments (due to migration limitation) or present in unsuitable environments that were previously suitable (due to time‐delayed extinction). Because correlative species distribution models do not account for these processes, they are likely to produce biased niche estimates and biased forecasts of future range dynamics. Recently developed dynamic range models (DRMs) overcome this problem: they statistically estimate both range dynamics and the underlying environmental response of demographic rates from species distribution data. This process‐based statistical approach qualitatively advances biogeographical analyses. Yet, the application of DRMs to a broad range of species and study systems requires substantial research efforts in statistical modelling, empirical data collection and ecological theory. Here we review current and potential contributions of these fields to a demographic understanding of niches and range dynamics. Our review serves to formulate a demographic research agenda that entails: (1) advances in incorporating process‐based models of demographic responses and range dynamics into a statistical framework, (2) systematic collection of data on temporal changes in distribution and abundance and on the response of demographic rates to environmental variation, and (3) improved theoretical understanding of the scaling of demographic rates and the dynamics of spatially coupled populations. This demographic research agenda is challenging but necessary for improved comprehension and quantification of niches and range dynamics. It also forms the basis for understanding how niches and range dynamics are shaped by evolutionary dynamics and biotic interactions. Ultimately, the demographic research agenda should lead to deeper integration of biogeography with empirical and theoretical ecology.     

Accommodating environmental variation in population models: metaphysiological biomass loss accounting

1. There is a pressing need for population models that can reliably predict responses to changing environmental conditions and diagnose the causes of variation in abundance in space as well as through time. In this 'how to' article, it is outlined how standard population models can be modified to accommodate environmental variation in a heuristically conducive way. This approach is based on metaphysiological modelling concepts linking populations within food web contexts and underlying behaviour governing resource selection. Using population biomass as the currency, population changes can be considered at fine temporal scales taking into account seasonal variation. Density feedbacks are generated through the seasonal depression of resources even in the absence of interference competition. 2. Examples described include (i) metaphysiological modifications of Lotka-Volterra equations for coupled consumer-resource dynamics, accommodating seasonal variation in resource quality as well as availability, resource-dependent mortality and additive predation, (ii) spatial variation in habitat suitability evident from the population abundance attained, taking into account resource heterogeneity and consumer choice using empirical data, (iii) accommodating population structure through the variable sensitivity of life-history stages to resource deficiencies, affecting susceptibility to oscillatory dynamics and (iv) expansion of density-dependent equations to accommodate various biomass losses reducing population growth rate below its potential, including reductions in reproductive outputs. Supporting computational code and parameter values are provided. 3. The essential features of metaphysiological population models include (i) the biomass currency enabling within-year dynamics to be represented appropriately, (ii) distinguishing various processes reducing population growth below its potential, (iii) structural consistency in the representation of interacting populations and (iv) capacity to accommodate environmental variation in space as well as through time. Biomass dynamics provide a common currency linking behavioural, population and food web ecology. 4. Metaphysiological biomass loss accounting provides a conceptual framework more conducive for projecting and interpreting the population consequences of climatic shifts and human transformations of habitats than standard modelling approaches.