Toni Klauschies

Bimodal trait distributions with large variances question the reliability of trait-based aggregate models

Functionally diverse communities can adjust their species composition to altered environmental conditions, which may influence food web dynamics. Trait-based aggregate models cope with this complexity by ignoring details about species identities and focusing on their functional characteristics (traits). They describe the temporal changes of the aggregate properties of entire communities, including their total biomasses, mean trait values, and trait variances. The applicability of aggregate models depends on the validity of their underlying assumptions that trait distributions are normal and exhibit small variances. We investigated to what extent this can be expected to work by comparing an innovative model that accounts for the full trait distributions of predator and prey communities to a corresponding aggregate model. We used a food web structure with well-established trade-offs among traits promoting mutual adjustments between prey edibility and predator selectivity in response to selection. We altered the shape of the trade-offs to compare the outcome of the two models under different selection regimes, leading to trait distributions increasingly deviating from normality. Their biomass and trait dynamics agreed very well for stabilizing selection and reasonably well for directional selection, under which different trait values are favored at different times. However, for disruptive selection, the results of the aggregate model strongly deviated from the full trait distribution model that showed bimodal trait distributions with large variances. Hence, the outcome of aggregate models is reliable under ideal conditions but has to be questioned when confronted with more complex selection regimes and trait distributions, which are commonly observed in nature.

Trait adaptation promotes species coexistence in diverse predator and prey communities

Abstract Species can adjust their traits in response to selection which may strongly influence species coexistence. Nevertheless, current theory mainly assumes distinct and time‐invariant trait values. We examined the combined effects of the range and the speed of trait adaptation on species coexistence using an innovative multispecies predator–prey model. It allows for temporal trait changes of all predator and prey species and thus simultaneous coadaptation within and among trophic levels. We show that very small or slow trait adaptation did not facilitate coexistence because the stabilizing niche differences were not sufficient to offset the fitness differences. In contrast, sufficiently large and fast trait adaptation jointly promoted stable or neutrally stable species coexistence. Continuous trait adjustments in response to selection enabled a temporally variable convergence and divergence of species traits; that is, species became temporally more similar (neutral theory) or dissimilar (niche theory) depending on the selection pressure, resulting over time in a balance between niche differences stabilizing coexistence and fitness differences promoting competitive exclusion. Furthermore, coadaptation allowed prey and predator species to cluster into different functional groups. This equalized the fitness of similar species while maintaining sufficient niche differences among functionally different species delaying or preventing competitive exclusion. In contrast to previous studies, the emergent feedback between biomass and trait dynamics enabled supersaturated coexistence for a broad range of potential trait adaptation and parameters. We conclude that accounting for trait adaptation may explain stable and supersaturated species coexistence for a broad range of environmental conditions in natural systems when the absence of such adaptive changes would preclude it. Small trait changes, coincident with those that may occur within many natural populations, greatly enlarged the number of coexisting species.

Diversity, functional similarity, and top-down control drive synchronization and the reliability of ecosystem function.

The concept that diversity promotes reliability of ecosystem function depends on the pattern that community-level biomass shows lower temporal variability than species-level biomasses. However, this pattern is not universal, as it relies on compensatory or independent species dynamics. When in contrast within–trophic level synchronization occurs, variability of community biomass will approach population-level variability. Current knowledge fails to integrate how species richness, functional distance between species, and the relative importance of predation and competition combine to drive synchronization at different trophic levels. Here we clarify these mechanisms. Intense competition promotes compensatory dynamics in prey, but predators may at the same time increasingly synchronize, under increasing species richness and functional similarity. In contrast, predators and prey both show perfect synchronization under strong top-down control, which is promoted by a combination of low functional distance and high net growth potential of predators. Under such conditions, community-level biomass variability peaks, with major negative consequences for reliability of ecosystem function.

Functional diversity in a tritrophic system: Effects on biomass production, variability, and resilience of ecosystem functions

Diverse communities can adjust their trait composition to altered environmental conditions, which may strongly influence their dynamics. Previous studies of trait-based models mainly considered only one or two trophic levels, whereas most natural system are at least tritrophic. Therefore, we investigated how the addition of trait variation to each trophic level influences population and community dynamics in a tritrophic model. Examining the phase relationships between species of adjacent trophic levels informs about the degree of top-down or bottom-up control in non-steady-state situations. Phase relationships within a trophic level highlight compensatory dynamical patterns between functionally different species, which are responsible for dampening the community temporal variability. Furthermore, even without trait variation, our tritrophic model always exhibits regions with two alternative states with either weak or strong nutrient exploitation, and correspondingly low or high biomass production at the top level. However, adding trait variation increased the basin of attraction of the high-production state, and decreased the likelihood of a critical transition from the high-to the low-production state with no apparent early warning signals. Hence, our study shows that trait variation enhances resource use efficiency, production, variability, and resilience of entire food webs.

Integrating Food-Web and Trait-Based Ecology to Investigate Biomass-Trait Feedbacks

Biodiversity is rapidly declining while the frequency and strength of anthropogenically influenced changes in climate and land use is increasing (Chapin et al., 2000; Butchart et al., 2010). A diminished biodiversity leads to a reduced capability of ecological systems, such as individuals, populations, communities, and food webs, to buffer environmental changes and to maintain ecosystem functions and ecosystem services, leading, in turn, to a further decline in biodiversity. This profoundly impacts human well-being and our economy on a global scale (Naeem et al., 2009). Thus mechanistic understanding and models predicting future ecosystem responses to climate changes are an important basis for management decisions. We aim to understand how an understudied aspect of biodiversity, the biodiversityrelated flexibility of ecological systems, allows ecosystems to adjust their properties to altered abiotic and biotic conditions. Depending on the different facets of biodiversity (e.g., genetic, phenotypic, and species diversity) individuals, populations, and communities possess an inherent flexibility, which influences their dynamics and, consequently, those of the entire food web (Figure 8.1). For example, enhanced grazingmay lead to a higher proportion of less edible plants. This dampens the reduction of plant biomass, which likely will have a feedback on the biomass and community composition of the herbivores, e.g., the share of herbivorous species able to exploit less edible plants may increase. As a result, the advantage of being less edible is reduced and the edible plants may recover with positive consequences for their consumers. This promotes the coexistence of different plant types and hence biodiversity. Given such feedback loops, the responses of large, non-linear, and intricately interconnected networks, such as food webs, to altered conditions are very difficult to understand and to predict, but of outstanding importance for fundamental and applied ecology. Thus wemust broaden our very limited quantitative knowledge and predictive power on how biodiversity affects ecological dynamics and responses to environmental changes. Oneway to put this idea into practice is to combine approaches from food-web and traitbased ecology. For example, previous more descriptive considerations of biodiversity effects on overall temporal variability focusedmainly on the effects of the sheer number of