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).