Janzen theorized that mountain passes pose a greater physiological barrier to dispersal in the tropics, and therefore, species turnover should be greater along elevation gradients in tropical than in temperate regions. However, differences in disturbance frequency and intensity caused by hydro-climatological changes along elevation gradients may disrupt this predicted pattern. We investigated temporal and spatial turnover in functional and taxonomic composition of insect communities along elevation gradients in tropical and temperate streams. In total, we studied 14 sites in Colorado’s Rocky Mountains (1400 m elevation range) and 10 sites in the Ecuadorian Andes (2200 m elevation range), with three to five years of stream insect community composition and physical environment data at each site. During our study, our sites experienced extreme disturbances (historically large precipitation events and subsequent flooding and catastrophic landslides). Our dataset includes time-points both prior to and following those large disturbance events in both Colorado in Ecuador. For functional trait analysis, in Colorado, we used previously published trait information for North American stream insects to assign mobility traits. In Ecuador, we performed colonization experiments and drift net surveys to characterize mobility traits, which have not been thoroughly studied for many Neotropical stream insect genera.
Results/Conclusions
Our results suggest that landscape-level factors may influence habitat stability and disturbance regimes, thereby determining community stability in both tropical and temperate systems. Across our temperate and tropical streams, disturbance intensity was significantly negatively related with elevation. Higher elevation streams had higher habitat stability and experienced less intense disturbances during our study period. Habitat stability and disturbance intensity was also significantly related with temporal taxonomic and functional turnover in stream insect communities both in Colorado and Ecuador. Subsequently, spatial patterns in species turnover and diversity along elevation gradients varied by year. For example, at a mid-elevation site that experienced intense disturbance by landslides in Ecuador, species richness ranged from 42 stream insect taxa prior to the landslides to 15 taxa the year following the landslide disturbance. In contrast, at a high elevation site in the same drainage, which did not experience landslide disturbance and had high habitat stability, species richness ranged from 22 species to 15 species during that same time period. Mobility traits were a significant factor driving species’ interannual persistence at both our tropical and temperate sites. Studies along elevation gradients should explicitly consider disturbance regime relationships with elevation, in addition to temperature gradients, especially in mountainous regions.