The timing of species assembly shapes the diversity and functioning of simulated food webs
Ecologists have developed a theory of food web disassembly to predict the consequences of species extinctions on persistence and ecosystem functioning. Much less is known about the reversed process of food web assembly. Such a theory could help us understand ecosystems restoration after major perturbations. Currently, many food web models used to study the emerging properties of complex ecological networks assume that the species of a regional pool assemble simultaneously, neglecting the impact of colonization history. However, under sequential assembly, differences in abundances between resident species and new colonizers will have important consequences on competitive outcomes and thereby on the diversity and functioning of food webs, potentially leading to alternative stable states.
In this study, we compared simulated food webs built from the same species pool but with either simultaneous or sequential assembly. We also varied the time laps between successive introductions of species. We asked to what extent the diversity and the functioning of food webs depends on the timing of species assembly.
We found that the diversity of simulated food webs depends strongly on the timing of species assembly. Simultaneous assembly leads to more diverse ecosystems. Strict sequential assembly (when equilibrium is reached before each new colonization event) induces stronger selection on new colonizers because of competition with residents. In that case, the top-down regulation, which limits the exploitative competition, appears only progressively, and thus has a weaker effect on basal species. Sequential assembly results in less diverse communities dominated by more competitive species. When the species assembly is faster, new colonizations prevent competitive exclusion and increase the diversity.
We suggest that food web theory should integrate more realistic scenarios of food web assembly. Sequential assembly allows capturing processes like the intensity of ecological filters that may be crucial during ecosystem recovery after perturbations. We expect the diversity of the food webs to decrease if perturbations slow down the colonization rate. Moreover, the strong ecological filtering we observed under sequential assembly may impact significantly the food web structure and functioning after recovery.