Magnus Huss

Predators with multiple ontogenetic niche shifts have limited potential for population growth and top-down control of their prey

Catastrophic collapses of top predators have revealed trophic cascades and community structuring by top-down control. When populations fail to recover after a collapse, this may indicate alternative stable states in the system. Overfishing has caused several of the most compelling cases of these dynamics, and in particular Atlantic cod stocks exemplify such lack of recovery. Often, competition between prey species and juvenile predators is hypothesized to explain the lack of recovery of predator populations. The predator is then considered to compete with its prey for one resource when small and to subsequently shift to piscivory. Yet predator life history is often more complex than that, including multiple ontogenetic diet shifts. Here we show that no alternative stable states occur when predators in an intermediate life stage feed on an additional resource (exclusive to the predator) before switching to piscivory, because predation and competition between prey and predator do not simultaneously structure community dynamics. We find top-down control by the predator only when there is no feedback from predator foraging on the additional resource. Otherwise, the predator population dynamics are governed by a bottleneck in individual growth occurring in the intermediate life stage. Therefore, additional resources for predators may be beneficial or detrimental for predator population growth and strongly influence the potential for top-down community control.

Ontogenetic specialism in predators with multiple niche shifts prevents predator population recovery and establishment

The effects of ontogenetic niche shifts on community structure and dynamics are underexplored, despite the occurrence of such shifts in the majority of animal species. We studied the form of niche shifts in a predator that exhibits multiple ontogenetic niche shifts, and analyzed how this life history complexity affects the size-structured predator-prey dynamics in the system. The predator represents either an ontogenetic generalist, exhibiting a partial shift to predation (in which case an alternative resource is also available) or an ontogenetic specialist, exhibiting a complete shift (in which case the predator depends entirely on predation). We showed two effects on community dynamics from accounting for a complete niche shift to predation: (1) occurrence of alternative stable community states (coexistence and a prey-only community state) and (2) occurrence of deterministic extinction following initially successful invasion (predators can invade an equilibrium with only prey, but are bound to go extinct after a few generations). Both phenomena are due to the match or mismatch in the timing of predators and suitably sized prey and the growth trajectory of the predator, which is plastic, due to the population feedback on available resources. In the case of persistence without invasion (alternative stable community states), slow growth during the pre-piscivorous life stage is necessary to stay in tune with the prey cycle; in the case of extinction following invasion, slow growth through the pre-piscivorous life stage causes the predator to reach the completely piscivorous stage when there is no prey available to feed upon. Somatic growth rates are directly coupled to food availability, which, in turn, is the result of density-dependent feedbacks in the system. Since they primarily determine these density-dependent feedbacks, the ontogenetic niche shifts in predator life history structure the community to a major extent.

Size- and food-dependent growth drives patterns of competitive dominance along productivity gradients

Patterns of coexistence among competing species exhibiting size- and food-dependent growth remain largely unexplored. Here we studied mechanisms behind coexistence and shifts in competitive dominance in a size-structured fish guild, representing sprat and herring stocks in the Baltic Sea, using a physiologically structured model of competing populations. The influence of degree of resource overlap and the possibility of undergoing ontogenetic diet shifts were studied as functions of zooplankton and zoobenthos productivity. By imposing different size-dependent mortalities, we could study the outcome of competition under contrasting environmental regimes representing poor and favorable growth conditions. We found that the identity of the dominant species shifted between low and high productivity. Adding a herring-exclusive benthos resource only provided a competitive advantage over sprat when size-dependent mortality was high enough to allow for rapid growth in the zooplankton niche. Hence, the importance of a bottom-up effect of varying productivity was dependent on a strong top-down effect. Although herring could depress shared resources to lower levels than could sprat and also could access an exclusive resource, the smaller size at maturation of sprat allowed it to coexist with herring and, in some cases, exclude it. Our model system, characterized by interactions among size cohorts, allowed for consumer coexistence even at full resource overlap at intermediate productivities when size-dependent mortality was low. Observed shifts in community patterns were crucially dependent on the explicit consideration of size- and food-dependent growth. Accordingly, we argue that accounting for food-dependent growth and size-dependent interactions is necessary to better predict changes in community structure and dynamics following changes in major ecosystem drivers such as resource productivity and mortality, which are fundamental for our ability to manage exploitation of living resources in, e.g., fisheries.

Intraspecific phenotypic variation among alewife populations drives parallel phenotypic shifts in bluegill

Evolutionary diversification within consumer species may generate selection on local ecological communities, affecting prey community structure. However, the extent to which this niche construction can propagate across food webs and shape trait variation in competing species is unknown. Here, we tested whether niche construction by different life-history variants of the planktivorous fish alewife (Alosa pseudoharengus) can drive phenotypic divergence and resource use in the competing species bluegill (Lepomis macrochirus). Using a combination of common garden experiments and a comparative field study, we found that bluegill from landlocked alewife lakes grew relatively better when fed small than large zooplankton, had gill rakers better adapted for feeding on small-bodied prey and selected smaller zooplankton compared with bluegill from lakes with anadromous or no alewife. Observed shifts in bluegill foraging traits in lakes with landlocked alewife parallel those in alewife, suggesting interspecific competition leading to parallel phenotypic changes rather than to divergence (which is commonly predicted). Our findings suggest that species may be locally adapted to prey communities structured by different life-history variants of a competing dominant species.

Cohort Dynamics Give Rise to Alternative Stable Community States

Many ecological systems can exhibit alternative stable states (ASS), which implies that ecological communities may diverge depending on their initial state, despite identical environmental conditions. Here we present a new mechanism that can cause ASS in competition systems. Using a physiologically structured model of competing populations, representing Baltic Sea sprat and herring and their resources, we show how cohort-driven population cycles may result in priority effects leading to ASS. Similar mechanisms could, depending on mortality level, also result in a “resident strikes back” phenomenon. We argue that the prerequisites for the occurrence of ASS in our model system, that is, communities with competing populations exhibiting cohort cycles and variation in size at maturation, may be common in ecological systems.

Temperature‐dependent body size effects determine population responses to climate warming

Current understanding of animal population responses to rising temperatures is based on the assumption that biological rates such as metabolism, which governs fundamental ecological processes, scale independently with body size and temperature, despite empirical evidence for interactive effects. Here, we investigate the consequences of interactive temperature- and size scaling of vital rates for the dynamics of populations experiencing warming using a stage-structured consumer-resource model. We show that interactive scaling alters population and stage-specific responses to rising temperatures, such that warming can induce shifts in population regulation and stage-structure, influence community structure and govern population responses to mortality. Analysing experimental data for 20 fish species, we found size-temperature interactions in intraspecific scaling of metabolic rate to be common. Given the evidence for size-temperature interactions and the ubiquity of size structure in animal populations, we argue that accounting for size-specific temperature effects is pivotal for understanding how warming affects animal populations and communities.

Size-based ecological interactions drive food web responses to climate warming

Predicting the impacts of climate change on animal populations and communities requires understanding of feedbacks between direct physiological responses and indirect effects via ecological interactions. Food-dependent body growth and within-species size variation have major effects on dynamics of populations and communities through feedbacks between individual performance and population size structure. Moreover, evidence suggests a link between temperature and population size structure, but we lack an understanding of how this is mediated by species interactions when life history processes are food-dependent. Here, we use a dynamic stage-structured biomass model with food-, size- and temperature-dependent life history processes to assess how temperature affects coexistence, stability and size structure in a tri-trophic food chain. We show that predator biomass densities decline with warming either gradually or in the form of collapses, depending on which consumer life stage they predominantly feed on. Collapses occur when warming destabilizes the community and induces alternative stable states via Allee effects, which emerge when predators promote their own food source through predation. By contrast, warming at low temperatures stabilizes the community as limit cycles turn to fixed point dynamics, unless predators feed only on juveniles. Elevated costs of being large in warmer environments accelerate the decline in predator persistence and mean body size of the community. These results suggest that predator persistence in warmer climates may be lower than previously acknowledged when accounting for size- and food-dependence of life history processes, and that interactions within and between species can mediate the effects of warming on food web stability. Significance Climate warming is altering the dynamics and structure of aquatic ecosystems worldwide. Predicting food web reorganization under rising temperatures requires an understanding of physiological responses and ecological interactions of organisms, both of which depend on body size. We show that size variation within species, food-dependent growth and ecological interactions critically affect how food chains respond to warming. Specifically, warming can stabilize or destabilize food chains and expose predators to increased risk of sudden collapses, resulting in alternative stable food web states. Increasing temperatures can cause abrupt reductions in mean community body size, primarily due to loss of top predators. The potential loss of biodiversity and shifts in ecosystem stability are among the major challenges caused by a warming climate.