Smaller species but larger stages: Warming effects on inter- and intraspecific community size structure

Abstract

Global warming can alter size distributions of animal communities, but the contribution of size shifts within versus between species to such changes remains unknown. In particular, it is unclear if expected body size shrinkage in response to warming, observed at the interspecific level, can be used to infer similar size shifts within species. In this study, we compare warming effects on interspecific (relative species abundance) versus intraspecific (relative stage abundance) size structure of competing consumers by analyzing stage-structured bioenergetic food web models consisting of one or two consumer species and two resources, parameterized for pelagic plankton. Varying composition and temperature and body size dependencies in these models, we predicted interspecific versus intraspecific size structure across temperature. We found that warming shifted community size structure toward dominance of smaller species, in line with empirical evidence summarized in our review of 136 literature studies. However, this result emerged only given a size-temperature interaction favoring small over large individuals in warm environments. In contrast, the same mechanism caused an intraspecific shift toward dominance of larger (adult) stages, reconciling disparate observations of size responses within and across zooplankton species in the literature. As the empirical evidence for warming-driven stage shifts is scarce and equivocal, we call for more experimental studies on intraspecific size changes with warming. Understanding the global warming impacts on animal communities requires that we consider and quantify the relative importance of mechanisms concurrently shaping size distributions within and among species.

Keywords: adult; body size; competition; diet preference; global warming; juvenile; optimum; stage; temperature; zooplankton.

FIGURE 1

Three modeled communities. I: Two unstructured consumer species feeding on two resources. II: One stage‐structured consumer species feeding on two resources. III: Two stage‐structured consumer species feeding on two resources. Squares and circles represent different biomass compartments of food webs, and are labeled: $A$ and $A_i$: adult; $C_i$: consumer; $J$ and $J_i$: juvenile; $R_L$: large resource; $R_S$: small resource. Different sizes of circles symbolize different body masses of consumers. Solid arrows between squares and circles represent feeding links pointing in the direction of biomass flows, with the parameter $p$ indicating the diet preference (=feeding niche dissimilarity between competing consumers; 0.5–1 in I and II, 0–1 in III). Dashed arrows between circles represent biomass flows between consumer stages related to maturation and reproduction

FIGURE 2

Consumer persistence (solid black lines), biomass dominance (dashed black lines), and alternative stable states (solid red lines) boundaries in temperature–diet preference space, with a size–temperature interaction present both in the maximum resource density $R_{\mathit{max}}$ and in the temperature optimum of the maximum consumer ingestion rate $I_{\mathit{max}}$. (a) Two unstructured consumer species feeding on two resources (Community I). (b) One stage‐structured consumer species feeding on two resources (Community II). In (a), left and right solid lines represent the persistence boundary of, respectively, consumers $C_S$ and $C_L$, that is persistence is possible above the lines (=coexistence, marked $C_L+C_S$). On the left‐hand side of the dashed line, community biomass is dominated by the large consumer $C_L$, and on the right‐hand side by the small consumer $C_S$. Consumer $C_S$ becomes extinct >41.2°C. In (b), the consumer extinction boundary is marked with a solid black line. All equilibria are stable

FIGURE 3

Stable‐state (equilibrium) consumer biomass densities along the temperature gradient. Left panels (a, b) show model results with no size–temperature interaction. Right panels (c, d) show model results with a size–temperature interaction present both in the maximum resource density $R_{\mathit{max}}$ and in the temperature optimum of the maximum consumer ingestion rate $I_{\mathit{max}}$. Upper panels (a, c) represent Community I, and lower panels (b, d) represent Community II. Red lines in panel (d) show an alternative stable state for biomass equilibrium densities. In all panels, the diet preference $p=0.85$

FIGURE 4

Consumer persistence regions (colored areas) and biomass dominance boundaries (dashed lines of respective colors) in temperature–diet preference space for Community III (two stage‐structured consumer species feeding on two resources). A size–temperature interaction was present both in the maximum resource density $R_{\mathit{max}}$ and in the temperature optimum of the maximum consumer ingestion rate $I_{\mathit{max}}$. On the left‐hand side of the dashed lines, community biomass is dominated by juveniles $J_i$ and on the right‐hand side by adults $A_i$. Note that the biomass dominance curve for the small consumer ($J_S,A_S$) overlaps with the persistence boundary of the large consumer ($J_L,A_L$). All equilibria are stable

FIGURE 5

Summary of the literature review of published studies (136 articles) on warming‐induced changes in intraspecific and interspecific size structures of competitive zooplankton communities. The numbers of studies that reported the given observation (164 in total) are divided into three categories: shift to smaller species/stages (red bars), shift to larger species/stages (blue bars), or no observed effect (gray numbers between the bars). This is further divided into two levels: experimental (filled bars) and observational (empty bars). A full description of the review methods, analysis, as well as reference list and their short description, are found in Appendix S3