University of Wyoming Laramie, Wyoming, United States
Background/Question/Methods
Dispersal is important for species coexistence in metacommunities with spatially variable environments. For example, the competition-colonization tradeoff is a well-known mechanism for coexistence, where species that are poor competitors can maintain viable populations if they have sufficiently high dispersal. The simplest dispersal strategy, commonly assumed in theoretical models, is passive, global dispersal; where individuals have a fixed propensity to emigrate from one patch and settle into any other patch with equal probability. However, in nature, species display a myriad of dispersal strategies, including variation in the development stage when individuals disperse, how far they move, and whether that movement is directed (i.e., active versus passive dispersal). The consequences of these dispersal strategies on variation-dependent mechanisms of coexistence remain less clear. We created a competitive metacommunity model with spatial variation and trade-offs between species in their preferred habitats, systematically altering the way that species moved, and keeping other parameters constant. We considered passive movement and a range of active dispersal scenarios including directed dispersal (species move towards patches with higher fitness) and density-dependent dispersal (species move towards or away from conspecifics). For each scenario, we quantified coexistence outcomes and the strength of spatial coexistence mechanisms using simulation-based partitioning approaches.
Results/Conclusions
For the default simulations with global dispersal, fitness-density covariance, a metric that quantifies the importance of intraspecific aggregation of individuals into habitats where individuals experience high growth rates, was the primary contributor to coexistence. Nevertheless, for the default simulations, increasing the per capita rate of dispersal decreased the importance of this mechanism. In contrast, for our simulations with habitat directed dispersal, fitness-density covariance increased by 66%, suggesting that species more effectively spatially aggregated into patches where they exhibited the highest fitness, and this mechanism remained the primary contributor to coexistence despite increasing dispersal rates. Interestingly, the spatial storage effect, which quantifies the potential benefits species receive from covariation in environmental conditions and competition across a patchy landscape, weakly contributed to overall coexistence (< 10 %), and did not change much between passive and active dispersal strategies. Overall, this work highlights how varying dispersal strategies can alter the strength of variation-dependent coexistence mechanisms, thereby hindering or promoting coexistence between species. This work also provides theoretical predictions that can be tested with experiments or observational data of organisms with varying dispersal regimes.
