Equilibrium and non‐equilibrium dynamics simultaneously operate in the Galápagos islands

Abstract

Island biotas emerge from the interplay between colonisation, speciation and extinction and are often the scene of spectacular adaptive radiations. A common assumption is that insular diversity is at a dynamic equilibrium, but for remote islands, such as Hawaii or Galápagos, this idea remains untested. Here, we reconstruct the temporal accumulation of terrestrial bird species of the Galápagos using a novel phylogenetic method that estimates rates of biota assembly for an entire community. We show that species richness on the archipelago is in an ascending phase and does not tend towards equilibrium. The majority of the avifauna diversifies at a slow rate, without detectable ecological limits. However, Darwin's finches form an exception: they rapidly reach a carrying capacity and subsequently follow a coalescent‐like diversification process. Together, these results suggest that avian diversity of remote islands is rising, and challenge the mutual exclusivity of the non‐equilibrium and equilibrium ecological paradigms.

Biodiversity Dynamics on Islands: Explicitly Accounting for Causality in Mechanistic Models

“…While spatially explicit and stochastic models were common in the earliest studies [22,27], more recently published models tend to be less often spatially explicit [28,29], which mirrors the popularity of the unified neutral theory of biodiversity (UNTB; [30]). Because the UNTB inspired a large portion of the later models, we considered it an additional category.…”

“…Two other studies used representations of real geography as their model arena [23,34] for the investigation of radiation or speciation histories. Only a single study actually fitted their model to data, using a maximum likelihood approach and dated phylogenies of Galápagos land birds to obtain rates of diversification and radiation [29].…”

“…com. SAR Cantrell et al [24] 1996 MI (h) ETIB, Allee effects disp., repr., comp., (st [29] 2015 MI (r) UNTB, ETIB disp., spec., ext., (st) 6 yes no yes yes 0 sp. com.…”

“…The models that simulate real-world systems [23,29,34] shed light on the mechanisms behind distribution patterns of real-world species; for instance, diversification of Philippine shrews shaped by competitive exclusion over dispersal ability [23], bluebirds on Bermuda as cryptic introductions [34] or presently increasing species richness in some bird lineages on the Galápagos, while other lineages already attained equilibrium dynamics [29]. The models of Birand and Howard [38] and Rosindell and Phillimore [39] were able to produce realistic species distribution and endemism patterns, when compared to empirical data of species numbers, while Valente et al [29] used Bayesian information criterion weights for selection of the best model that explains current diversification patterns on the basis of phylogenetic data.…”

Are there many different routes to becoming a global biodiversity hotspot?

“…More sophisticated methods for estimating rates of biotic assembly of entire communities have recently been developed (20), but these are computationally intensive and require phylogenies of major lineages with complete sampling of all species in a community and all their nearest relatives. Although this has been achieved for some groups on islands with modest species richness, such as Gálapagos birds (20), it has not yet been done for more species-rich clades that contribute substantial diversity to the major global biodiversity hotspots. Only through such studies will we be able to really compare trajectories of biodiversity hotspot assembly across the planet.…”