In recent times, unprecedented rates of biodiversity loss have resulted in a pressing need for greater understanding of key processes maintaining biodiversity. These processes are often studied in island communities because they exhibit low complexity relative to their mainland counterparts, which helps to simplify experiments, data analyses and mathematical models. Much previous work on islands has focused on local processes such as competition and predation, but this neglects processes operating at broader, regional scales. In particular, immigration can carry individuals of new species into a local area, and can therefore play a pivotal role in maintaining biodiversity.
The importance of immigration was recognized half a century ago by the seminal Theory of Island Biogeography, which postulated that the number of species on an island results from a dynamic balance between immigration and extinction. However, the theory did not explicitly consider the simultaneous effects of local and regional processes on the abundance dynamics of species populations in island communities, and hence did not predict which type of process was the dominant driver of island biodiversity.
In our work, we addressed the key knowledge gap identified by quantifying the relative influence of local and regional processes on species richness of islands, using a suite of dynamic models. Each model represents species populations competing for common resources on an island, with immigration from a mainland source; however, the details of interspecific competition differ among models. The models include the classic Lotka–Volterra model and Tilman’s model of resource competition.
We found that for all models examined, the equilibrium number of species was typically low when immigration intensity was low, regardless of the level of competition. In this “niche-structured” phase, species dynamics were governed by deterministic competition, with typically a few species coexisting per available niche. However, as the immigration intensity increased beyond a threshold, the number of species exhibited a rapid increase. In this “immigration-structured” phase, species dynamics were governed by stochastic immigration, which allowed many species to coexist per available niche. These trends in species richness were consistent with data from empirical datasets for 100 archipelagoes worldwide, covering five taxonomic groups and three archipelago types. Our results highlight the general importance of maintaining immigration to achieve high levels of island biodiversity, which is directly relevant to conservation and sustainable management.