A test of the preference–performance hypothesis with stream insects: selective oviposition affects the hatching success of caddisfly eggs

Abstract

Summary Under the preference–performance hypothesis (PPH), oviparous females select oviposition sites that optimise the fitness of their offspring (eggs or larvae). The resulting distribution and fitness of offspring may have knock‐on effects for population distribution patterns and dynamics during larval and adult stages. We tested the PPH for Australian caddisflies from two genera (family: Hydrobiosidae) that oviposit in different flow conditions. Apsilochorema spp. oviposit in slow flowing water, whereas Ulmerochorema sp. favour fast flows. We expected hatching success to be higher in velocities favoured by ovipositing females. In a field experiment, newly laid egg masses of each species were exposed to experimental ‘fast’ and ‘slow’ flow treatments throughout development and monitored until they hatched or died. In a second field experiment, we placed egg masses in a range of velocities (0.0–1.5 ms−1) to determine the threshold beyond which eggs were damaged by shear forces. The results supported the PPH for one species. Apsilochorema egg masses were sheared from the substratum in fast flows, but hatched with 100% success in favoured slow flows. The threshold velocity for Apsilochorema was 0.6–0.7 ms−1, well beyond the natural oviposition range of up to 0.3 ms−1. Ulmerochorema eggs hatched in all flows, suggesting that flow‐related mortality of the egg stage is unimportant for this species. Oviposition in fast flows might enhance the fitness of Ulmerochorema larvae or ovipositing females instead. Vulnerability to shear forces appears to explain why Apsilochorema lay eggs exclusively in slow flows. Shear may be a common cause of mortality for lotic insect eggs, and unseasonal spates or regulated flows may significantly affect recruitment of larvae. Selective oviposition affects the spatial distribution and survival of eggs and thus affects larval supply, and these supply dynamics are under‐studied in stream ecology.