Diverse interactions and ecosystem engineering can stabilize community assembly

Abstract

The complexity of an ecological community can be distilled into a network, where diverse interactions connect species in a web of dependencies. Species interact directly with each other and indirectly through environmental effects, however to our knowledge the role of these ecosystem engineers has not been considered in ecological network models. Here we explore the dynamics of ecosystem assembly, where species colonization and extinction depends on the constraints imposed by trophic, service, and engineering dependencies. We show that our assembly model reproduces many key features of ecological systems, such as the role of generalists during assembly, realistic maximum trophic levels, and increased nestedness with mutualistic interactions. We find that ecosystem engineering has large and nonlinear effects on extinction rates. While small numbers of engineers reduce stability by increasing primary extinctions, larger numbers of engineers increase stability by reducing primary extinctions and extinction cascade magnitude. Our results suggest that ecological engineers may enhance community diversity while increasing persistence by facilitating colonization and limiting competitive exclusion.

Fig. 1. Model framework for ecological networks with multitype interactions and ecosystem engineering.

a Multitype interactions between species (colored nodes) and abiotic modifiers (black nodes). Trophic and mutualistic relationships define both species–species (S–S) and species–modifier (S–M) interactions; an engineering interaction is denoted by an engineer that makes a modifier, such that the modifier needs the engineer to persist. b An assembling food web with species (color denotes trophic level) and modifiers. The basal resource is the white node at the bottom of the network. c The corresponding adjacency matrix with colors denoting interactions between species and modifiers. d A species (*) can colonize a community when a single trophic and all service requirements are met. e Greater vulnerability increases the risk of primary extinction via competitive exclusion (competition denoted by dashed line) to species (†). The extinction of species (†) will cascade to affect those connected by trophic (††) and service (†††) dependencies.

Fig. 2. Food web structure over the course of assembly.

a Assembling communities over time from a pool of 200 non-engineering species. Steady state species richness is reached by t = 250. b The proportion of specialists as a function of assembly time (iterations). Diamonds denote expected values for functional (realized) trophic interactions at each point in time, and triangles denote expected values for potential trophic interactions (as if all trophic interactions with all species in the pool were realized), where the expectation is taken across replicates. Individual replicate results are shown for functional trophic interactions (small points). c The frequency distribution of trophic levels as a function of assembly time (iterations). Autotrophs occupy TL = 1. Measures were evaluated across 10⁴ replicates; see “Methods” for parameter values.

Fig. 3. Community structure and stability as a function of the frequency of service interactions.

a Structural nestedness of communities, measured as UNODF (Unipartite Nestedness based on Overlap and Decreasing Fill; measured using the R package UNODF v.1.2). The value reported is the mean value taken across the rows and columns of the adjacency matrix accounting for both trophic and service interactions. b Mean rate of primary extinction (where primary extinctions occur from competitive exclusion of consumers over shared resources) and c secondary extinction (which cascade from primary extinctions) as a function of service interaction frequency. d Species persistence as a function of service interaction frequency. Primary and secondary extinction rates were evaluated at the community level, whereas persistence was determined for each species and averaged across the community. Measures were evaluated for 10⁴ replicates; see “Methods” and Supplementary Note 2 for parameter values.

Fig. 4. Community stability as a function of the frequency of service interactions and modifiers per species.

a Mean rates of primary extinction, where primary extinctions occur from competitive exclusion of consumers over shared resources. b Mean rates of secondary extinction, which cascade from primary extinctions. c Mean species persistence. d The ratio $S^{*}/S_{\mathrm{u}}^{*}$, where $S_{\mathrm{u}}^{*}$ denotes steady states for systems where all engineered modifiers are unique to each engineer, and S^* denote steady states for systems with redundant engineering. Higher values of $S^{*}/S_{\mathrm{u}}^{*}$ mean that systems with redundant engineers have higher richness at the steady state than those without redundancies. Primary and secondary extinction rates were evaluated at the community level, whereas persistence was determined for each species and averaged across the community. Each measure reports the expectation taken across 50 replicates. See “Methods” and Supplementary Note 2 for parameter values.