Applications of island biogeography theory beyond islands
Islands provide a powerful tool to address ecological questions because they represent discrete environments offering multiple replicates of comparable eco-evolutionary processes. Furthermore, there is a well-established theory that postulates a connection between the macroecological patterns observed in a set of islands and the identity and interactions between the species that integrate their species’ assemblages (Wilson 1969). In its initial formulations, however, island biogeographical models were essentially neutral, insofar as they only relied on species identity and endemism, treating all species as functionally equivalent. This is analogous to say that the position of each species on the niche space is random and does not affect the properties of the overall island biodiversity. We now know this is a strong assumption, which can be relaxed by incorporating a functional description of the species occurring in each island (Hubbell 2011; Rosindell et al. 2011). This way, the position of each species is defined in the n-dimensional overall trait space of each island, allowing us to make further assumptions on how the presence of different, non-random combinations of traits affect the macroecological status of the islands: a crucial step to connect community scale patterns with island-level observations.
Indeed, the results of null-modelling confirms that the combination of traits in each island is not random and can be linked to the predictions of the island biogeography model. In other words, our analysis suggests a causal relationship among the island age, the changes in the niche availability and the overall island’s biodiversity. Our results indicate how in young islands functional richness and evenness approach the values expected by change for the same species richness, illustrating a multidirectional expansion of the trait space as a consequence of the species radiation and ecological opportunity. However, as an island matures, the traits space becomes smaller and more ordered when compared to the null-model, reflecting how the existence of a more connected network of species interactions optimizes the use of the available niche space. Subsequently, as an island is eroded and its niche space reduced, species will start to be extirpated. An initial stage of this phase can be seen in Gran Canaria, where the null-modelling reveals that the trait space functional richness is larger than expected by chance, whereas evenness approaches the randomly predicted values. Indeed Gran Canaria, despite being younger than Fuerteventura, has already undergone a long erosive period, characterized by a lack of volcanism and massive landslides dramatically affecting its geomorphology. The trait space of Fuerteventura resembles the values obtained by null-modelling, illustrating the collapse of the island’s ecological network during its final ontogenetic stage, just before it will disappear under the ocean.