Joint Effects of Habitat Heterogeneity and Species’ Life-History Traits on Population Dynamics in Spatially Structured Landscapes

Both habitat heterogeneity and species’ life-history traits play important roles in driving population dynamics, yet there is little scientific consensus around the combined effect of these two factors on populations in complex landscapes. Using a spatially explicit agent-based model, we explored how interactions between habitat spatial structure (defined here as the scale of spatial autocorrelation in habitat quality) and species life-history strategies (defined here by species environmental tolerance and movement capacity) affect population dynamics in spatially heterogeneous landscapes. We compared the responses of four hypothetical species with different life-history traits to four landscape scenarios differing in the scale of spatial autocorrelation in habitat quality. The results showed that the population size of all hypothetical species exhibited a substantial increase as the scale of spatial autocorrelation in habitat quality increased, yet the pattern of population increase was shaped by species’ movement capacity. The increasing scale of spatial autocorrelation in habitat quality promoted the resource share of individuals, but had little effect on the mean mortality rate of individuals. Species’ movement capacity also determined the proportion of individuals in high-quality cells as well as the proportion of individuals experiencing competition in response to increased spatial autocorrelation in habitat quality. Positive correlations between the resource share of individuals and the proportion of individuals experiencing competition indicate that large-scale spatial autocorrelation in habitat quality may mask the density-dependent effect on populations through increasing the resource share of individuals, especially for species with low mobility. These findings suggest that low-mobility species may be more sensitive to habitat spatial heterogeneity in spatially structured landscapes. In addition, localized movement in combination with spatial autocorrelation may increase the population size, despite increased density effects.     

Stage-structured ontogeny in resource populations generates non-additive stabilizing and de-stabilizing forces in populations and communities

Demographic heterogeneity influences how populations respond to density dependent intraspecific competition and trophic interactions. Distinct stages across an organism's development, or ontogeny, are an important example of demographic heterogeneity. In consumer populations, ontogenetic stage structure has been shown to produce categorical differences in population dynamics, community dynamics and even species coexistence compared to models lacking explicit ontogeny. The study of consumer–resource interactions must also consider the ontogenetic stage structure of the resource itself, particularly plants, given their fundamental role at the basis of terrestrial food webs. We incorporate distinct ontogenetic stages of plants into an adaptable multi-stage consumer–resource modeling framework that facilitates studying how stage specific consumers shape trophic dynamics at low trophic levels. We describe the role of density dependent demographic rates in mediating the dynamics of stage-structured plant populations. We then investigate how these demographic rates interact with consumer pressure to influence stability and coexistence in multiple stage-specific consumer–resource interactions. Results detail how density dependent effects across distinct ontogenetic stages in plant development produce non-additivity in the drivers of dynamic stability both in single populations and in consumer–resource settings, challenging the ubiquity of certain traditional ecological dynamic paradigms. We also find categorical differences in the population variability induced by herbivores consuming separate plant stages. Consumer–resource models, such as plant–herbivore interactions, often average out demographic heterogeneity in populations. Here, we show that explicitly including plant demographic heterogeneity through ontogeny yields distinct dynamic expectations for both plants and herbivores compared to traditional consumer–resource formulations. Our results indicate that efforts to understand the demographic effect of herbivores on plant populations may need to also consider the effects of plant demographics on herbivores and the reciprocal relationship between them.     

Linking spatial processes to life-history evolution of insect parasitoids

Understanding the evolutionary transition from solitary to group living in animals is a profound challenge to evolutionary ecologists. A special case is found in insect parasitoids, where a tolerant gregarious larval lifestyle evolved from an intolerant solitary ancestor. The conditions for this transition are generally considered to be very stringent. Recent studies have aimed to identify conditions that facilitate the spread of a gregarious mutant. However, until now, ecological factors have not been included. Host distributions and life-history trade-offs affect the distribution of parasitoids in space and thus should determine the evolution of gregariousness. We add to current theory by using deterministic models to analyze the role of these ecological factors in the evolution of gregariousness. Our results show that gregariousness is facilitated through inversely density-dependent patch exploitation. In contrast, host density dependence in parasitoid distribution and patch exploitation impedes gregariousness. Numerical solutions show that an aggressive gregarious form can more easily invade a solitary population than can a tolerant form. Solitary forms can more easily invade a gregarious, tolerant population than vice versa. We discuss our results in light of exploitation of multitrophic chemical cues by searching parasitoids and aggregative and defensive behavior in herbivorous hosts.     

Temperature-driven regime shifts in the dynamics of size-structured populations

Global warming impacts virtually all biota and ecosystems. Many of these impacts are mediated through direct effects of temperature on individual vital rates. Yet how this translates from the individual to the population level is still poorly understood, hampering the assessment of global warming impacts on population structure and dynamics. Here, we study the effects of temperature on intraspecific competition and cannibalism and the population dynamical consequences in a size-structured fish population. We use a physiologically structured consumer-resource model in which we explicitly model the temperature dependencies of the consumer vital rates and the resource population growth rate. Our model predicts that increased temperature decreases resource density despite higher resource growth rates, reflecting stronger intraspecific competition among consumers. At a critical temperature, the consumer population dynamics destabilize and shift from a stable equilibrium to competition-driven generation cycles that are dominated by recruits. As a consequence, maximum age decreases and the proportion of younger and smaller-sized fish increases. These model predictions support the hypothesis of decreasing mean body sizes due to increased temperatures. We conclude that in size-structured fish populations, global warming may increase competition, favor smaller size classes, and induce regime shifts that destabilize population and community dynamics.     

Stabilization of population fluctuations due to cannibalism promotes resource polymorphism in fish

Resource polymorphism is a well-known phenomenon in many taxa, assumed to be a consequence of strong competition for resources and to be facilitated by stable environments and the presence of several profitable resources on which to specialize. In fish, resource polymorphism, in the form of planktivore-benthivore pairs, is found in a number of species. We gathered literature data on life-history characteristics and population dynamics for 15 fish species and investigated factors related to the presence of such resource polymorphism. This investigation indicated that early cannibalism and low overall population variability are typically associated with the presence of resource polymorphism. These findings match previously reported patterns of population dynamics for size-structured fish populations, whereby early cannibalism has been shown to decrease temporal variation in population dynamics and to equalize the profitability of the zooplankton and macroinvertebrate resources. Our study suggests that competition alone is not a sufficient condition for the development of resource polymorphism because overly strong competition is typically associated with increased temporal variation (environmental instability). We conclude that although resource competition is an important factor regulating the development of resource polymorphism, cannibalism may also play a fundamental role by dampening population oscillations and possibly by equalizing the profitability of different resources.     

Capital and income breeding traits differentiate trophic match–mismatch dynamics in large herbivores

For some species, climate change has altered environmental conditions away from those in which life-history strategies evolved. In such cases, if adaptation does not keep pace with these changes, existing life-history strategies may become maladaptive and lead to population declines. We use life-history theory, with a specific emphasis on breeding strategies, in the context of the trophic match–mismatch framework to form generalizable hypotheses about population-level consumer responses to climate-driven perturbations in resource availability. We first characterize the income and breeding traits of sympatric caribou and muskoxen populations in western Greenland, and then test trait-based hypotheses about the expected reproductive performance of each population during a period of high resource variability at that site. The immediate reproductive performance of income breeding caribou decreased with trophic mismatch. In contrast, capital breeding muskoxen were relatively unaffected by current breeding season resource variability, but their reproductive performance was sensitive to resource conditions from previous years. These responses matched our expectations about how capital and income breeding strategies should influence population susceptibility to phenological mismatch. We argue for a taxon-independent assessment of trophic mismatch vulnerability based on a life-history strategy perspective in the context of prevailing environmental conditions.     

Stochastic population dynamics and life-history variation in marine fish species

Abstract We examined whether differences in life-history characteristics can explain interspecific variation in stochastic population dynamics in nine marine fish species living in the Barents Sea system. After observation errors in population estimates were accounted for, temporal variability in natural mortality rate, annual recruitment, and population growth rate was negatively related to generation time. Mean natural mortality rate, annual recruitment, and population growth rate were lower in long-lived species than in short-lived species. Thus, important species-specific characteristics of the population dynamics were related to the species position along the slow-fast continuum of life-history variation. These relationships were further associated with interspecific differences in ecology: species at the fast end were mainly pelagic, with short generation times and high natural mortality, annual recruitment, and population growth rates, and also showed high temporal variability in those demographic traits. In contrast, species at the slow end were long-lived, deepwater species with low rates and reduced temporal variability in the same demographic traits. These interspecific relationships show that the life-history characteristics of a species can predict basic features of interspecific variation in population dynamical characteristics of marine fish, which should have implications for the choice of harvest strategy to facilitate sustainable yields.     

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.     

Quasi-Local Competition in Stage-Structured Metapopulations: A New Mechanism of Pattern Formation

A central question of ecology is what determines the presence and abundance of species at different locations. In cases of ecological pattern formation, population sizes are largely determined by spatially distributed interactions and may have very little to do with the habitat template. We find pattern formation in a single-species metapopulation model with quasi-local competition, but only if the populations have (at least) two age or stage classes. Quasi-local competition is modeled using an explicit resource competition model with fast resource dynamics, and assuming that adults, but not juveniles, spend a fraction of their foraging time in habitat patches adjacent to their home patch. Pattern formation occurs if one stage class depletes the common resource but the shortage of resource affects mostly the other stage. When the two stages are spatially separated due to quasi-local competition, this results in competitive exclusion between the populations. We find deep similarity between spatial pattern formation and population cycles due to competitive exclusion between cohorts of biennial species, and discuss the differences between the present mechanism and established ways of pattern formation such as diffusive instability and distributed competition with local Allee-effects.     

The effect of the Holling type II functional response on apparent competition

This article analyzes the classical 2-resource-1-consumer apparent competition community module with the Holling type II functional response. Two types of resource regulation (top-down vs. combined top-down and bottom-up) and two types of consumer behaviors (inflexible consumers with fixed preferences for resources vs. adaptive consumers) are considered. When resources grow exponentially and consumers are inflexible foragers, one resource is always outcompeted due to strong apparent competition. Density dependent resource growth relaxes apparent competition so that resources can coexist. As multiple attractors (either equilibria or limit cycles) coexist, population dynamics and community composition depend on initial population densities. Population dynamics change dramatically when consumers forage adaptively. In this case, the results both for top-down, and combined top-down and bottom-up regulation are similar and they show that species persistence occurs for a much larger set of parameter values when compared with inflexible consumers. Moreover, population dynamics will be chaotic when resource carrying capacities are high enough. This shows that adaptive consumer switching can destabilize population dynamics.     

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.     

The roles of history: age and prior exploitation in aquatic container habitats have immediate and carry‐over effects on mosquito life history

1. Per‐capita resource availability in aquatic habitats is influenced directly by consumer density via resource competition and indirectly via delayed resource competition when temporally non‐overlapping cohorts of larvae exploit the same resources. In detritus‐based systems, resources are likely to be influenced by the age of the aquatic habitat, as detritus changes in quality over time and may be replenished by new inputs. 2. For aquatic insects that exploit detritus‐based habitats, feeding conditions experienced during immature stages can influence fitness directly via effects on development and survivorship, but also indirectly by influencing adult traits such as fecundity and longevity. 3. Larval habitat age and prior resource exploitation were manipulated in a field experiment using the container mosquito Aedes triseriatus. 4. It was found that A. triseriatus from older habitats had greater larval survival, faster development and greater adult longevity. Exploitation of larval habitats by a prior cohort of larvae had a significant negative effect on subsequent cohorts of larvae by delaying development. 5. It is suggested that extended conditioning of detritus probably resulted in conversion of recalcitrant resources to more available forms which improved the quality of the habitat. 6. In a parallel study, evidence was found of carry‐over effects of habitat age and prior exploitation on adult longevity for A. triseriatus and Aedes japonicus collected from unmanipulated aquatic habitats. 7. These results indicate the importance of detritus dynamics and the discontinuous nature of resource competition in these mosquito‐dominated aquatic systems.     

Stage-structured cycles generate strong fitness-equalizing mechanisms

For many organisms, rates of reproduction, growth and mortality depend on the amount of resources that an individual consumes. When resource abundances fluctuate through space and time, the realized life-history of an individual can change dramatically depending on the dynamics experienced. Previous studies have investigated the influence of resource-dependent rates on population dynamics, but none have considered how the feedback between non-equilibrium resource dynamics and resource-dependent life-histories influence natural selection and the maintenance of genetic diversity within populations. Here we demonstrate that different patterns of resource dynamics have a strong impact on natural selection in organisms with resource-dependent life-histories. Small-amplitude consumer-resource cycles, lead to lower rates of natural selection than do large-amplitude consumer-resource cycles. Parameterizing the model for a Daphnia-algal system, we demonstrate that resource-dependent life-history can explain the recently published observation that selection among Daphnia genotypes changed depending on the pattern of algal resource fluctuations. The characteristically asexual reproduction of Daphnia allows us to draw a much-needed link to the large body of competition theory that has emerged from community ecology. Our results reveal that the common ecological features of resource-dependent life-history and ontogenetic size-structure generate strong fitness equalizing mechanisms that likely contribute to the maintenance of diversity in natural systems. Electronic Supplementary Material Supplementary material is available in the online version of this article at accessible for authorized users.     

A newly discovered role of evolution in previously published consumer–resource dynamics

Consumer–resource interactions are fundamental components of ecological communities. Classic features of consumer–resource models are that temporal dynamics are often cyclic, with a ¼‐period lag between resource and consumer population peaks. However, there are few published empirical examples of this pattern. Here, we show that many published examples of consumer–resource cycling show instead patterns indicating eco‐evolutionary dynamics. When prey evolve along a trade‐off between defence and competitive ability, two‐species consumer–resource cycles become longer and antiphase (half‐period lag, so consumer maxima coincide with minima of the resource species). Using stringent criteria, we identified 21 two‐species consumer–resource time series, published between 1934 and 1997, suitable to investigate for eco‐evolutionary dynamics. We developed a statistical method to probe for a transition from classic to eco‐evolutionary cycles, and find evidence for eco‐evolutionary type cycles in about half of the studies. We show that rapid prey evolution is the most likely explanation for the observed patterns.     

Interspecific competition in small rodents: from populations to individuals

The role of interspecific competition in shaping animal and plant communities has formed one of the major issues in ecology for decades. Small mammals, mainly rodents, have been among the model systems used for research on interspecific competition. Most studies within small mammal systems in the past have examined effects of competition on population attributes such as on population size, habitat use, or population dynamics. Population-level responses are the cumulative effects of individual responses, however, the influence of competition on individual life-history traits has rarely been studied. Research on life-histories may bridge gaps between population biology and effects of competition on individual behaviour. In this paper, we review recent research approaches to interspecific competition in rodents based on census data and species assemblages, that use regression analysis, time series analysis, removal and exclusion experiments, and showcase our own experimental research on the effects of interspecific competition on individual life-history traits in boreal voles.     

Life-history variation in agricultural and wild populations of Erucastrum nasturtiifolium (Brassicaceae)

In the present study we relate the variability in life-history traits (such as flowering time and lifespan) of the herbaceous biennial–perennial Erucastrum nasturtiifolium (Brassicaceae) to habitat type. We studied plant populations from arable fields and from eroded mountain habitats, such as badlands and rocky slopes. Collection sites ranged from low basin to sub-alpine regions in the NE Iberian Peninsula. Plants were grown under common garden conditions to evaluate genetic variation among and within populations. Plants were also subjected to a resource gradient to detect genetic variation in phenotypic plasticity. The populations exhibited differentiation across a number of life-history traits (mainly flowering time and lifespan) and morphological traits related to growth (basal stem diameter, plant height and number of branches). This suggests that life-history differences among populations are genetically based. Moreover, our results show that variation in flowering time and lifespan are affected by habitat type independent of other abiotic factors such as altitude or continentality. Thus, populations from arable fields started flowering in their first year and displayed annual cycles, whereas those from wild habitats generally flowered in their second year and showed biennial or even perennial cycles. Intra-population differences in flowering time were observed in only one population, and were related to nutrient availability. We suggest that early-flowering and shorter lifespan populations of E. nasturtiifolium may have been selected from late-flowering and longer lifespan populations as part of a selective process ensuring survival and future offspring amidst unpredictable and frequently disturbed environments such as exist in many agricultural habitats.     

Evolutionarily stable consumer home range size in relation to resource demography and consumer spatial organization

There is a large variation in home range size within species, yet few models relate that variation to demographic and life-history traits. We derive an approximate deterministic population dynamics model keeping track of spatial structure, via spatial moment equations, from an individual-based spatial consumer-resource model; where space-use of consumers resembles that of central place foragers. Using invasion analyses, we investigate how the evolutionarily stable home range size of the consumer depends on a number of ecological and behavioral traits of both the resource and the consumer. We show that any trait variation leading to a decreased overall resource production or an increased spatial segregation between consumer and resource acts to increase consumer home range size. In this way, we extend theoretical predictions on optimal territory size to a larger range of ecological scenarios where home ranges overlap and population dynamics feedbacks are possible. Consideration of spatial traits such as dispersal distances also generates new results: (1) consumer home range size decreases with increased resource dispersal distance, and (2) when consumer agonistic behavior is weak, more philopatric consumers have larger home ranges. Finally, our results emphasize the role of the spatial correlation between consumer and resource distributions in determining home range size, and suggest resource dispersion is less important.     

Simple life-history omnivory: responses to enrichment and harvesting in systems with intraguild predation

This article analyzes the nature of top-down and bottom-up effects and alternative states in systems characterized by life-history omnivory. The analysis is based on a three-species food web with intraguild predation (IGP). The top predator population has juvenile and adult stages, which consume the basal resource and the intermediate prey, respectively; the prey consumes only the resource. The per capita reproduction of the adult predators depends on their consumption rate of prey, while the maturation rate of the juvenile predators depends on their resource consumption rate. Enriching the resource can increase or decrease the abundances of one or both of the two consumer species; an increased density is more likely in the intermediate species than in the systems where IGP is not based on stage differences. Alternative states that have or lack the predator occur frequently, particularly when the prey population is capable of reducing the resource to very low densities. These results differ from those of several other recent models of life-history omnivory. They suggest that life-history omnivory may be one of the primary reasons why exploited populations undergo sudden collapses and why collapsed populations fail to recover in spite of large reductions in the exploitation rate.     

Physiologically structured models – from versatile technique to ecological theory

A ubiquitous feature of natural communities is the variation in size that can be observed between organisms, a variation that to a substantial degree is intraspecific. Size variation within species by necessity implies that ecological interactions vary both in intensity and type over the life cycle of an individual. Physiologically structured population models (PSPMs) constitute a modelling approach especially designed to analyse these size‐dependent interactions as they explicitly link individual level processes such as consumption and growth to population dynamics. We discuss two cases where PSPMs have been used to analyse the dynamics of size‐structured populations. In the first case, a model of a size‐structured consumer population feeding on a non‐structured prey was successful in predicting both qualitative (mechanisms) and quantitative (individual growth, survival, cycle amplitude) aspects of the population dynamics of a planktivorous fish population. We conclude that single generation cycles as a result of intercohort competition is a general outcome of size‐structured consumer–resource interactions. In the second case, involving both cohort competition and cannibalism, we show that PSPMs may predict double asymptotic growth trajectories with individuals ending up as giants. These growth trajectories, which have also been observed in field data, could not be predicted from individual level information, but are emergent properties of the population feedback on individual processes. In contrast to the size‐structured consumer–resource model, the dynamics in this case cannot be reduced to simpler lumped stage‐based models, but can only be analysed within the domain of PSPMs. Parameter values used in PSPMs adhere to the individual level and are derived independently from the system at focus, whereas model predictions involve both population level processes and individual level processes under conditions of population feedback. This leads to an increased ability to test model predictions but also to a larger set of variables that is predicted at both the individual and population level. The results turn out to be relatively robust to specific model assumptions and thus render a higher degree of generality than purely individual‐based models. At the same time, PSPMs offer a much higher degree of realism, precision and testing ability than lumped stage‐based or non‐structured models. The results of our analyses so far suggest that also in more complex species configurations only a limited set of mechanisms determines the dynamics of PSPMs. We therefore conclude that there is a high potential for developing an individual‐based, size‐dependent community theory using PSPMs.