Connecting macroscopic description of the island system with community processes using functional traits.
An emerging oceanic island represents an empty niche space susceptible to being colonised by individuals from different species, which will start competing for resources occupying empty niches and evolving there until, eventually, there will be no more niche space available for newcomers. In this process of colonisation and adaptation, species also establish networks of interactions that allow them to compress themselves through the niche space, optimising the flow of energy through the ecosystems. One might then predict that young islands, as they grow, will provide increasing levels of ecological opportunity facilitating the evolution of many species interacting loosely due to broad availability of resources. This is what we observe on the youngest islands of El Hierro and La Palma, where most of the available terrains are occupied by a network of lava tubes, fissures, and MSS-like habitats, with recent volcanic eruptions continually promoting the generation of new subterranean habitats. In those islands, we did not find a weaker correlation of ecological and geographical distances over the functional distances between pairs of species.
However, once the island matures towards its maximum level of geological complexity, ecological opportunities reduce as no newer habitat is generated. At that point, species have occupied all the available niche space. This leads to a higher ecological redundancy determined by the available resources, also because the more heterogeneous geological structure of the islands favours processes of parapatric speciation (Arnedo et al. 2007). At this stage, species interactions gain importance in modulating niche spaces and determining species distributions. This is what we expected to observe in the mature islands of La Gomera, Tenerife, and Gran Canaria. However, we only obtained a significant correlation between functional and geographical distances in Tenerife. This was probably influenced by confounding factors derived from the specific geological features in each island. For example, La Gomera is a relatively small island that concentrates most of its suitable subterranean habitats at an altitude of 830–1170 m, yet fragmented by the network of ravines and mountains of the Garajonay National Park, hindering dispersal events across them. Tenerife, instead, is much larger and the lava fields once flowing from El Teide have generated potential habitats along the entirely altitudinal gradient of the island.
Eventually, in a senescent stage of the islands, surviving species might still evolve trying to optimise their occupation of the niche space, while competition becomes less important as fewer species remain. Indeed, in Fuerteventura we found only a weak effect of functional and geographical/ecological distances between pairs of species. Interestingly, the island of Lanzarote does not harbour any subterranean terrestrial species, with the young, exposed and arid volcanic terrains of the Timanfaya National Park being colonised by surface species only (Oromí et al. 2004).