Functional structure and diversity of invertebrate communities in a glacierised catchment of the tropical Andes

In many mountainous areas, glaciers feed streams characterised by harsh environmental conditions, such as low water temperature, high turbidity, low channel stability, and high temporal variability in flow. Additionally, in many glacierised catchments, the mixture of streams arising from different water sources (glacier melt, groundwater, rainfall) generates high levels of environmental heterogeneity, which enhance species turnover rates and increase regional diversity. Studies from mainly temperate regions have revealed some consistent patterns: a predominance of traits adaptive to harsh environmental conditions and reduced functional diversity with increased glaciality, both strongly related to environmental filtering. Here, we investigated variation in functional structure and diversity between macroinvertebrate communities from 15 stream sites, with different water sources (five glacier-fed, five groundwater-fed, and five mixed source) and level of glacier influence, in a glacierised catchment in the Ecuadorian Andes. Our results revealed functional differences between communities inhabiting the different stream types. As found in temperate regions, high levels of glaciality were associated with an increase of small-sized taxa that do not swim but are temporarily attached to or burrow in the substrate, have a flying-adult stage, and feed by collecting–gathering. Similarly, we found a general decrease in functional diversity at sites with higher glacier influence. A null modelling approach suggested that in some of our glacier-fed sites, environmental filtering may be the main driver of community assembly, whereas other mechanisms, mainly regional (such as dispersal), but also local (such as intraspecific competition), may gain importance as glacier influence decreases. Assemblage composition in streams in tropical glacierised catchments may be driven by both local and regional processes that generate a gradient of decreasing functional diversity with stronger glacier influence. However, lack of knowledge of relevant traits for taxa in tropical glacierised streams currently poses a substantial obstacle to predicting changes likely to arise from global warming and glacier melt in this region.