Relaxing tradeoffs helps metacommunity paradigms be more comparable and general
Four paradigms form current metacommunity theory: patch dynamics (PD), species sorting (SS), mass effects (ME), and neutral dynamics (ND). These paradigms differ in their assumptions and are based on competition. Consequently, it is difficult to test paradigms in empirical metacommunities. We address the underlying structure and scope of metacommunity theory by relaxing colonization/competition tradeoff assumptions and generalizing beyond competitive guilds. We simulated metacommunity assembly for: original metacommunity paradigms and slightly- or strongly-relaxed tradeoffs. Tradeoffs were generalized beyond competitive guilds by relating species establishment in a site to species richness (S) and the species' ranked occupancy (O) in the metacommunity; negative species interactions are implied but not specified. Simulations consisted of 100 species colonizing 100 sites for 500 time steps. Habitat heterogeneity is assumed for SS and ME and was implemented by disturbance. S was recorded at steady state conditions; O was sampled through time.
Original paradigms: As expected, patterns stabilized quickly for all paradigms and differed in mean S and ranked O. SS and ME maintained lower S, and only PD increased in local S through time, related to a gradual shift in O. Slightly-relaxed tradeoffs: local S increased modestly so that SS and ME paradigms were now intermediate in S. Results were otherwise consistent with original paradigms. Strongly-relaxed tradeoffs: General patterns among paradigms were retained for S and O, though successional trajectories were slower and rare species in the regional source pool were less likely to occupy metacommunities in SS or ME. Relaxed competition/ colonization tradeoffs helped improve metacommunity paradigms because: (a) paradigms can now more realistically include interspecific variation for colonization (SS, ME) and/or competition (ND) but retain essential differences; (b) maintain longer and more dynamic successional trajectories; (c) include some species in the regional pool that remain rare in a metacommunity; and (d) transcend a guild-based colonization/competition tradeoff to a more general regional/local tradeoff that may apply to more metacommunities.