Environmental constraints on oviposition limit egg supply of a stream insect at multiple scales

Most studies of stream insects presume that egg supply is in excess and that juvenile mortality limits population density; fewer consider that density may be limited by egg supply. For species that oviposit on specific substrata, however, environmental constraints on oviposition sites may limit egg supply. Females of Baetis mayflies lay eggs masses on the underside of rocks that emerge above the water. We tested whether egg mass densities are constrained by emergent rock densities within and between streams, by counting egg masses. All emergent rocks were counted along 1 km lengths of four streams, revealing significant variations within streams and a more than three-fold difference across streams. In each stream, egg mass density increased with the density of emergent rocks in 30 m stretches. We used regression equations describing these small-scale relationships, coupled with the large scale spatial variation of emergent rocks, to estimate egg mass densities for 1 km stream lengths. These scaled estimates were positively associated with emergent rock density, suggesting environmental constraints on egg supply may limit population densities. Resource saturation (egg mass crowding) was inversely related to emergent rock density at the stream scale, but crowding did not compensate entirely for differences in emergent rocks.