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

the importance of securing habitat connectivity along the altitudinal gradient to protect aquatic insect diversity effectively. Introduction Andean mountains provide an opportunity to examine the relationship between diversity pat- terns and community assemblages with environmental filters, such as elevation, topography, and temperature [1–3]. Previous studies of these patterns have found distinctive spatial organi- zation depending on the taxa under study. Generally, plant and animal species richness show a hump-shaped pattern, with maxima at intermediate elevations between 500–2,000 m a.s.l. [1, 4–6]. Additionally, a monotonic decrease has also been observed [6–8]. Conversely, mid-eleva- tion diversity peaks can occur when the elevation gradient is truncated on part of the eleva- tional gradient [9]. For birds, local species richness in a mountain forest in Bolivia peaked at around 1,000 m a.s.l., declined sharply near 1,750 m a.s.l., and then remained relatively con- stant [1]. In this study, elevation was the best single predictor, accounting for 78–85% of the variance in the data set. Herzog et al. [1] reported that a richness peak at 1,000 m a.s.l. resulted from an overlap of two distinct avifaunas (lowland and highland). Also, a recent study of fern diversity patterns across the tropics found that elevational richness patterns were symmetri- cally hump-shaped and overall richness was virtually equal along most of the mountains scat- tered along the latitudinal gradient [5]. The number of narrow-range species (i.e., endemics) along elevational gradients has also

been examined. However, this has been done to a much lesser extent. For example, Kessler [10] focused on the patterns of plant endemism between different taxa along an Andean eleva- tion gradient. In that study, he reported that most groups had hump-shaped patterns with maxima at different elevations and mainly at the same or higher elevations than species rich- ness maxima. Most taxa had the highest concentration of endemics in the narrowest and most fragmented elevational belts (> 2,000 m a.s.l.), presumably because of the fragmentation of species populations, the increase in niche partitioning, and harsher environmental conditions. Carvajal-Quintero et al. [11] found a similar pattern with a high concentration of endemic fish species at higher elevation due to the distance between headwater streams. Fjeldså et al. [12], in their analyses of bird endemism patterns across the Andes, documented that distribution patterns of endemic species along tropical elevational gradients are different from those of spe- cies richness and usually peak above 2,000 m a.s.l. The spatial patterns of endemism found in this study were inverse to species richness patterns and were presumably controlled by area, topography, ecoclimatic stability, and taxon-specific ecological traits. In a recent study of Andean aquatic insect clades, Polato et al. [13] found that insect species from lower elevations had markedly narrower thermal tolerances and lower dispersal capacity than high altitude spe- cies. These factors result in significantly greater population divergence (allopatric speciation),

higher cryptic diversity, higher speciation rates, and greater accumulation of species over time in lower elevations than in mid and high elevations. The patterns observed in birds, mammals, and plants have been studied scarcely in freshwa- ter organisms in this region [11, 14, 15]. Although longitudinal connectivity among river networks exists, elevation and other environmental gradients such as temperature and precipi- tation could act as barriers for freshwater species. Additionally, there is a lack of knowledge on the taxonomy and life history of freshwater insects, which is essential for understanding these patterns. For example, Holzenthal et al. [16] estimated that only about 30% of Trichoptera spe- cies in the tropical Andes are known to science. PLOS ONE Elevational biodiversity gradients of Neotropical caddisflies PLOS ONE | https://doi.org/10.1371/journal.pone.0272229 August 5, 2022 2 / 17 81028631). We also had the support of Universidad de Las Ame´ricas Research Grants: “Diversidad de Trichoptera en una Gradiente Altitudinal Neotropical” (AMB.BRT.18.01), "Mountain freshwater diversity, from taxonomy to functional genomics, and approximation from Trichoptera” (AMB.BRT.19.02" and Masphi Reserve. ERG was funded by a Peruvian government FONDECYT-CONCYTEC doctoral fellowship (grant contract number 277-2015- FONDECYT). ERG and RWH were also supported by University of Minnesota Agricultural Experiment Station projects MIN17-017 and 17-094. This support is gratefully acknowledged. The funders had no role in study design, data collection and

analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.