COS 74-2 - Expanding the Constraint-based model of Dynamic Island Biogeography via trait-space plots and simulations for fragmented landscapes under global change

Abstract: Directional and cyclical environmental changes frequently result in archipelagos of islands or fragmented habitat patches, affecting the composition of communities. Predicting such turnover remains limited by difficulties integrating the roles of species traits and past environments. The recently proposed Constraint-based model of Dynamic Island Biogeography (C-DIB) invokes hysteresis (dependency on both the present and the past) to fill this gap, considering alternating roles of traits related to extinction vs. those affecting colonization. Here, we aim to expand the C-DIB to facilitate empirical tests of its predictions. To do so, we introduce trait-space plots of expected patterns of presence/absence of each species based on: a) traits related to colonization and extinction; and b) scenarios of environmental cycling. These plots portray on the x-axis each species’ connectivity needs (mediated by traits germane to colonization) and on the y-axis its areal requirements (based on traits relevant to sustaining a population, hence avoiding extinction). Current environmental conditions lead to clear deterministic predictions for species falling in much of the trait-space (presence for current connectivity; absence for current isolation and insufficient area). In contrast, expectations for species corresponding to current isolation yet sufficient area should be contingent on the history of the system. We then simulated patterns of occupancy for a system where connectivity and area covary linearly over environmental cycles. Videos of the results illustrated: loss of species as patches became isolated and smaller; but gain only with increases in connectivity. After many cycles, the simulations yielded expected gradual patterns of average occupancy for parts of trait-space dominated by determinism. In contrast, a stark gradient occurred in the rest of the space (especially subject to contingency): high occupancy for species with medium connectivity requirements and low areal needs, to low occupancy for those with high connectivity requirements and medium areal needs. Uncertainty (indicating hysteresis) particularly increased towards species with both the highest connectivity requirements and smallest areal needs. These plots and confirmation of their expectations via simulations now allow empirical tests of community-level predictions regarding trait values for species present vs. absent on given patches. In ongoing related work, we and collaborators are doing so for small mammals in montane mesic forests (“sky islands”) in Venezuela and Mexico. Other future directions include adding effects of time lags, biotic interactions, evolution, and stochasticity. In conclusion, the C-DIB has broad potential for forecasting the effects of environmental change in systems of real and habitat islands.