CAUSES OF LOW LEAF-SHREDDING INSECT DIVERSITY IN NEOTROPICAL STREAMS

Pavel Ernesto García Soto

Abstract

Biological diversity peaks in tropical regions followed by a steady decline towards the poles. It is a pattern in γ-diversity that sometimes explains latitudinal patterns in local assemblages (α) and their species turnover (β), which suggests that similar ecological mechanisms might be operating at different latitudes. However, not all taxa have more species towards the equator. Aquatic shredder insect richness peaks in temperate streams relative to tropical streams, where local aquatic shredder insect assemblages have low α-diversity. Three mechanisms have been proposed as factors structuring aquatic shredder insect assemblages in tropical streams, leading to shredder assemblages with low α-diversity, 1) low dispersal capabilities, 2) environmental filtering, and 3) availability of low-quality leaf litter in tropical zones. Although several studies have described the pattern of low shredder α-diversity in tropical streams, few of them have tested the underlying mechanisms. Therefore, our understanding of the processes that generate local species-poor assemblages of shredders is incomplete, especially in Neotropical streams. In my dissertation, I tested how much environmental filtering and dispersal capabilities drove aquatic shredder assemblages along an elevation gradient in northern Central America. I tested how low-quality leaf litter influenced growth rates of a species in the genus Phylloicus, a pantropical aquatic shredder insect, with a field experiment of feeding manipulation to test. In Chapter 2, I show that distance decay relationships (DDR) between species assemblage similarity and spatial distance indicated lower dispersal capacity for shredder than non-shredder insects across the study river network. Shredder DDR decayed 1.6 times faster with Euclidean distance between pair of sites than with in-network distance between pair of sites, which suggests that overland dispersal may control variation among shredder assemblages. In Chapter 3, I show that phylogenetic diversity did not vary with increasing elevation for both, shredder and non-shredder insect assemblages. Phylogenetic divergence decreased with elevation for non-shredder insects while for shredder insects there was no change with increasing elevation, which suggest that the strength of environmental filtering was not changing with increasing elevation for shredder assemblages. Low-elevation shredder assemblages had larger trait clustering and phylogenetic dispersion than high-elevation shredder assemblages. Shredder assemblages had randomly dispersed phylogenetic and trait composition at all elevations. These findings suggest that ecological drift and competitive interactions may drive the observed phylogenetic patterns. In Chapter 4, I show that Phylloicus larvae had a potential N-P limitation when fed with leaf litter, both with high- and low-quality leaf litter. Instantaneous growth rate decreased with an increasing initial size of larvae, with a stronger decline in growth when larvae were fed with low-quality leaf litter in a warm stream. Given the high C:P and N:P among Neotropical plant species relative to invertebrates, it may be that the N-P limitation of shredders is common in Neotropical streams and support the hypothesis that leaf litter quality drives the low diversity of shredders in tropical streams. The results of my dissertation research supported the hypotheses of low dispersal capabilities and low-quality leaf litter as drivers of shredder insect assemblages but did not support the hypothesis of environmental filtering as a driver of the assemblage structure.