Hydrologic and behavioral constraints on oviposition of stream insects: implications for adult dispersal

The supply of recruits plays an important role in plant and animal populati on dynamics, and may be governed by environmental and behavioral constraint s on animals. Mated females of the mayfly genus Baetis alight on rocks prot ruding from streams, crawl under water and deposit a single egg mass under a rock. We surveyed oviposition and emergence of a bivoltine population of B. bicaudcatus in multiple stream reaches in one high-altitude watershed in western Colorado over 3 years to establish qualitative patterns at a regio nal scale (entire watershed), and quantitative patterns over six generation s at a local scale tone stream reach). We also measured characteristics of preferred oviposition substrates, performed experiments to test hypotheses about cues used by females to select oviposition sites, and measured mortal ity of egg masses in the field. Our goals were to determine whether: (1) hy drologic variation necessitated dispersal of females to find suitable ovipo sition sites; (2) the local supply of females could provide the supply of l ocal recruits; and (3) local recruitment determined the local production of adults. The onset of oviposition corresponded with the decline of spring r un-off, which differed dramatically among years and among sites within year s. However, eggs appeared before any adults had emerged in 8 of 22 site-yea rs, and adults emerged 2-3 weeks before any eggs were oviposited in 3 site- years. Furthermore, the size distribution of egg masses differed from that predicted by the size distribution of females that emerged from seven of ni ne stream reaches. Protruding rocks and eggs appeared earlier each summer i n smaller tributaries than in larger mainstream reaches, suggesting that hy drologic and behavioral constraints on oviposition may force females to dis perse away from their natal reach to oviposit, and possibly explain the pre dominantly upstream flight of Baetis females reported in other studies. Loc al oviposition rates in one third-order stream-reach increased rapidly as s oon as substrates protruded from the water surface, and females preferred l arge rocks that became available early in the flight season. However, femal es oviposited on <10% of all available rocks, and <65% of preferred rocks a s determined by an empirical model. These data indicated that the timing of appearance of suitable oviposition sites determined the phenology of local recruitment, but that preferred oviposition sites were not saturated. Thus , the magnitude of local recruitment was not limited by the absolute abunda nce of preferred oviposition sites. Only 22% of egg masses observed in the field suffered mortality during their embryonic development, and per capita Baetis egg mass mortality was significantly lower on rocks with higher den sities of egg masses. Thus, we suspect that specialized oviposition behavio r may reduce the probability of egg mortality, potentially compensating for the costs of dispersal necessary to locate suitable oviposition sites. Fin ally, the number of adults that emerged at one stream reach was independent of the number of egg masses oviposited over six generations of Baetis; and local recruitment was not a function of the number of adults of the previo us generation that emerged locally. The patterns of oviposition and emergen ce of Baetis found in this study are consistent with the following hypothes es. Recruitment of eggs in a stream reach is not limited by the local suppl y of adults, but is a function of the regional supply of dispersing adults, which are constrained by the spatial and temporal distribution of preferre d oviposition habitat. Furthermore, subsequent local production of adults is not a function of the supply of recruits, arguing for post-recruitment control of local populati ons by processes operating in the larval stage (e.g., predation, competitio n, dispersal, disturbance). Processes affecting larval and adult stages of Baetis act independently and at different scales, thereby decoupling local population dynamics of successive generations.