Most studies of stream invertebrates assume that larval mortality limits population densities in stream channels. However, the success of females in laying eggs sets the upper limit on prospective larval densities. For species that oviposit on specific substrata, densities of suitable oviposition habitat can limit egg supply, which then constrains larval densities.
Female, hydrobiosid caddisflies in the genera Ulmerochorema and Apsilochorema lay egg masses on the undersides of stream rocks that emerge above the water’s surface. We tested the hypothesis that egg mass densities are constrained by emergent rock densities over small spatial scales. We conducted a manipulative experiment in which fixed areas of the stream bed (“arenas”, each a few square metres in area) contained different numbers of suitable, emergent rocks. Replicate arenas were spread across riffles in five sites in the Little R. Victoria during summer of 2018. All oviposited eggs were counted and identified over several weeks.
Two species of Apsilochorema responded to emergent rock density by laying more egg masses in arenas with high and intermediate numbers of emergent rocks compared to arenas with low numbers of emergent rocks. In contrast, two species of Ulmerochorema did not show this pattern. Instead, U. rubiconum laid higher numbers of egg masses on individual rocks when the latter were located in low density arenas, while U. seona showed strong differences between sites.
Hydrobiosid caddisflies may make decisions about egg-laying at different spatial scales. Apsilochorema females seem able to assess suitable oviposition habitat across whole riffles whereas Ulmerochorema females appear unable or unwilling to seek out high density patches. Thus maternal behaviours interact with the spatial patchiness of oviposition habitat, potentially changing egg supply to different locations.