Results
In agreement with hypothesis 1, we observed an overall convergence of
the trait space occupied by subterranean communities in each island
(Figure 1), although the centroids of each island’s hypervolume were
grouped according to the island’s age. Interestingly, the largest trait
space was exhibited by Fuerteventura, despite being the island with the
lowest number of species (Table 1). When we compared the diversity
across islands, we found that beta diversity approached 1 between pairs
of islands (Figure 1), indicating full taxonomic replacement across
islands. Once again, this was in contrast with the patterns of
functional diversity, whose values ranged between 0 and 0.8, indicating
greater functional similarity among islands.
In agreement with hypothesis 2, the distribution of individual
contributions of species to the overall trait space of each island
largely varied across islands (Figure 2). In general, the largest
functional contribution for each individual species was found in the
senescent island Fuerteventura (average contribution per species =
67.196 ± 5.892), whereas the smallest was recorded in the mature island
Tenerife (average contribution per species = 2.348 ± 0.205), compared to
the remaining islands (Figure 2, Table 1). The distribution of the
densities for the functional contribution values were unimodal in all
the islands, except for Tenerife where they were markedly bimodal.
Regarding Hypothesis 3, partial mantel tests only yielded significant
correlations for the island of Tenerife (R = 0.207, p < 0.005)
between functional and geographical distances, with ecological distances
included as a confounding factor (Table S1). However, the absolute
values of the mantel r yielded the highest values for the mature
islands, decreasing towards the young and the senescent islands (Figure
2).
Null modelling (Hypothesis 4) yielded larger values for functional
richness than those observed in El Hierro (Observed, obs = 0.0376 ,
estimated, est = 0.0506 ± 0.0006), La Gomera (obs = 0.0238; est = 0.0463
± 0.0007), and Tenerife (obs = 0.0320, est = 0.0482 ± 0.0003); smaller
in La Palma (obs = 0.0520, est = 0.0496 ± 0.0004), Gran Canaria (obs =
0.0565, est = 0.0497 ± 0.0003), and Fuerteventura (obs = 0.0531, est =
0.0425 ± 0.0010). Functional evenness exhibited the opposite pattern:
predicted values exceeded observed values in La Palma (obs = 0.2498, est
= 0.2766 ± 0.0011), Gran Canaria (obs = 0.2421, est = 0.2728 ± 0.0010),
and El Hierro (obs = 0.2755, est = 0.2955 ± 0.0014), whereas in
Fuerteventura (obs = 0.3913, est = 0.3526 ± 0.0024), La Gomera (obs =
0.3536, est = 0.3118± 0.0017) and Tenerife (obs = 0.2817, est = 0.2627 ±
0.0008) the observed values were larger than those expected by chance
(Figure 3). Nevertheless, the differences between predicted and observed
values were only significant for the functional richness in Tenerife
(SES = –2.0134, p = 0.022; Table S2).