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).