Elevational biodiversity gradients in the Neotropics: Perspectives from freshwater caddisflies (Insecta: Trichoptera)

suggest that thermal conditions from higher locations seem to act as a physiological barrier, selecting for organisms with narrow physiological tolerances to temperature and limited distri- bution along tropical elevation gradients. Therefore, physiology and resource availability play a key role in shaping diversity gradients in Andean freshwater taxa. Our results suggest an increase in generic caddisfly β-diversity (taxa turnover among loca- tions) as the geographic distance increased along the elevation gradient (RELATE procedure results). We found no significant changes at the species level, albeit a similar trend, increased community composition dissimilarity by increased geographic distance, was observed (Fig 3). These findings partially confirm our second hypothesis, although our results are constrained by low sample effort and few locations. As expected, changes in community assemblages at the species level were higher than in generic composition. β-diversity patterns along elevational groups of localities were strongly supported by our null models, suggesting a strong, non-random, environmental filtering of Trichoptera communities (Table 5). This is in agreement with what we know of the Trichop- tera fauna of Ecuador [18], in which, of the 266 species with elevation records, only 9 species are present along the whole gradient, with 47 above, and 210 below 2000 m asl. The substantial community turnover along the elevational/thermal gradient has been previously reported for other Andean aquatic insect taxa [13, 53, 54]. Complementing this observation, a recent study

of aquatic insect communities in Brazil and Finland found that β-diversity patterns of tropical communities (i.e., closer to stochastic expectations) are explained by the low number of indi- viduals within these communities, which in turn are affected by ecological drift [55]. These authors suggested that ecological drift may drive variation in some small communities by changing the expected outcome of niche selection, increasing the chances of having communi- ties characterized by species with low abundances and narrow distributions, which was also observed in our study. Patterns of high endemism in the caddisfly faunas of high elevations in mountain ecosys- tems have been observed for several genera worldwide, such as Drusus in Europe [56] and Contulma in South America [49, 57]. Besides, species turnover among caddisfly communities in our study area was high (mean value of Whittaker’s indices: 0.85 ± 0.11), but similar to the values reported for the aquatic communities studied in Brazil (β-diversity = 0,87) [55]. Species turnover (β-diversity) for aquatic insects in tropical mountains can be even higher in glaciated streams suggesting that community composition dissimilarity in these insect communities increases with elevation [58, 59] and along environmental gradients. Community generic similarity was highly correlated with geographic proximity (Rho = 0.624; p = 0.016), whereas at the species level we found no correlation (Rho = 0.321; p = 0.2874), partially confirming our second hypothesis. This high species turnover pattern

suggests that for caddisfly species assemblages, environmental filters (i.e., minimum tempera- ture) along wide elevation gradients are more important than linear geographic proximity [60]. In tropical mountain ecosystems, changes in environmental conditions over short dis- tances become more relevant than localities closer to each other but with similar environments [21]. Thus, the bimodal pattern observed in the species β-diversity confirms the occurrence of sharp temperature shifts at 1,874 and 3,464 m a.s.l (Table 5). Lastly, shifts in community com- position were dominated by the loss of species, rather than their gain, at higher elevations. Conservation implications The high Trichoptera β-diversity found in this elevation gradient suggests that communities are highly ecologically specialized, with narrow range species and low abundances [18, 57]. Cli- mate change is forcing species to move upwards in mountain environments [61]. We expect PLOS ONE Elevational biodiversity gradients of Neotropical caddisflies PLOS ONE | https://doi.org/10.1371/journal.pone.0272229 August 5, 2022 12 / 17