Large-scale manipulation of oviposition substrata affects egg supply to populations of some stream-dwelling caddisflies

Oviposition substrata are a crucial resource for many stream-dwelling insects and may have a strong effect on population densities. From February to March 2010, we conducted a large-scale experiment manipulating the density of oviposition substrata available to two taxa, Cheumatopsyche spp. and Ecnomus spp. in Hughes Creek (south-east Australia). These caddisflies oviposit on hard surfaces underwater. Hughes Creek has a sandy bed, and females rely predominantly on bark and wood as oviposition substrata. Bark density was manipulated in 25-m-long sites, creating a range of bark surface areas (SAs) spanning an order of magnitude, with appropriate controls. Estimates of the number of egg masses and bark SAs were obtained 22 and 55days after the experiment commenced in each site. To characterise taxon-specific oviposition site preferences and test whether preferences were dependent on overall bark availability, the conditions (velocity and emergence) of individual substrata were manipulated within sites. Egg masses were enumerated at approximately weekly intervals for 7weeks. We found a strong relationship between the SA of bark at sites and the number of masses of Ecnomus spp., demonstrating that oviposition is limited by substrata availability. In contrast, the number of Cheumatopsyche spp. egg masses was not related to the total amount of bark available. Ecnomus spp. showed a weak preference for emergent compared to submerged bark, but did not respond to water velocity. Cheumatopsyche laid eggs predominantly in fast flows, regardless of bark emergence, which may explain the lack of a site-level response because only a small proportion of bark occurred in fast flows even when bark was plentiful. Our results suggest that the strength of oviposition preferences may depend on overall background densities of substrata. Oviposition preferences and substrata availability interact to set the initial distribution (and density) of the next generation. Our study has wide implications for the management of streams with soft sediment beds, where bark and wood inputs from riparian vegetation provide not only food and habitat for larvae but also oviposition substrata that are critical to successful recruitment for some taxa.