Numerous caddisfly species, including in the Family Hydrobiosidae, require emergent rocks (rocks that protrude from the water) as oviposition habitat. Each female lays a single gelatinous egg mass and attaches it to the underside of an emergent rock. Previous research identified species-specific preferences for rocks of different sizes or in different flow velocities, and for avoiding or congregating with egg masses of conspecifics and congenerics.
New egg mass census data at smaller scales of individual rocks and clusters reveals further species-specific differences. Ulmerochorema rubiconum and U. seona both lay in large clusters of up to hundreds of contiguous egg masses. Ulmerochorema seona egg masses tend to be concentrated into very large single-species clusters, often primarily in contact with conspecifics even in multispecies clusters, yielding highly-skewed distributions of egg mass numbers. Ulmerochorema rubiconum egg masses are more often laid in multispecies clusters, with fewer egg masses per cluster on average. Ethochorema turbidum and Taschorema evansi are less likely to lay in large clusters and are often found together. Apsilochorema gisbum, A. obliquum and Tasimia palpata tend to lay their egg masses as singles rather than in clusters of one or more species.
The relative abundance of species has shifted substantially over the past five years, with the previous numerically-dominant U. rubiconum now far less abundant. This may be a result of U. rubiconum being more commonly infected by the oomycete Saprolegnia spp. or recent cool summers and high water levels changing environmental conditions. In contrast, E. turbidum and T. evansi have become more abundant despite the higher prevalence of attack by the egg predator Orthotrichia armata on those two species. Thus, species-specific differences in oviposition habits and susceptibility to natural enemies may alter population fitness and have implications for species coexistence.