What forces structure the diversity of ecological communities on local scales? One perspective is that local niche factors determine how many species can coexist locally. A very different perspective is that local diversity is mainly a function of immigration and regional diversity. Pioneering ecologist Robert MacArthur (1930–1972) made seminal contributions to theory embodying both perspectives. The dichotomy between the two perspectives has thus been termed “MacArthur’s paradox”.
In previous work, we built a unified mathematical model of island biodiversity and showed that increasing immigration with island area drives a transition from niche-structured to immigration-structured communities. The model predicts a biphasic island species–area curve that is commonly seen in nature. In a new paper just out in Theoretical Ecology, we explore this transition in more detail. We modify MacArthur & Wilson’s (1967) classical graphical paradigm of island biogeography to include niches and show that it leads to a biphasic species–area relationship (SAR), consistent with our previous mathematical model (see graph below). We show that three classic mathematical niche models predict a similar SAR when immigration is added. We also reconcile this biphasic island SAR with the classic triphasic SAR seen on mainlands, and predict a tetraphasic SAR in low-diversity mainland systems, with a sampling phase at very small spatial scales and an elusive niche-structured phase at intermediate spatial scales.
By continuing the unification of the niche and immigration perspectives, our work helps resolve MacArthur’s paradox. We propose experiments that manipulate immigration directly to explore the transition between the niche- and immigration-structured regimes. And we propose large-scale data analyses to find the elusive niche-structured phase of the SAR on mainlands.