DENSITY-DEPENDENT DISPERSAL OF BLACK FLY NEONATES IS MEDIATED BY FLOW

To understand and predict the dynamics of a population it is necessary to determine whether processes such as dispersal, growth and mortalit y are density-dependent and how these processes may influence patterns of abundance and distribution. Newly hatched individuals (neonates) a re a common dispersal stage in many terrestrial and marine invertebrat es, and may affect where, and in what abundance, older stages are subs equently found, hence underscoring the potential importance of supply- side processes in governing the ecology of those systems. In streams, benthic invertebrates might be expected to experience strong density-d ependent competition near oviposition sites due to the often clumped d istribution of eggs, yet the ecology of the early life-history stages has been poorly studied. In laboratory experiments, we examined whethe r newly hatched black ny larvae (Simulium vittatum) disperse from egg masses, via water currents, in a density-dependent fashion, and the li kelihood that the strength of density-dependence is modulated by curre nt speed. To understand better the mechanisms controlling neonatal dis persal, we also determined the amount of time an average larva spent f ighting, and the relationship between fights and dispersal events. The experimental results demonstrate that the dispersal rate of neonates from egg masses was strongly density-dependent. A second-order polynom ial regression model reflecting this density effect explained 91% and 75% of the variation in dispersal rates for the fast and slow current speed treatment, respectively. Dispersal was lower at fast than at slo w current speeds, indicating that these patterns of drift are not the result of passive dislodgment by water currents. Current speed also mo dified the effect of density on dispersal rate. The increase in disper sal with a unit change in density was lower at fast than at slow curre nt speeds. Increasing larval density and low current speed increased t he proportion of time a larva spent fighting, but most larvae did not disperse immediately after being attacked. The density effect suggests that dispersal by black fly neonates can be a voluntary response to r educed feeding rates stemming from competition with neighboring larvae . In general, it appears that the tendency of neonates to remain at th e oviposition site depends on the suitability of the microhabitat for feeding. The high dispersal rates we documented (up to 4.5% of individ uals min(-1)) occurred in response to levels of larval density, curren t speed, and food concentration that are probably typical of many fiel d settings. This implies that many neonates may also disperse in a den sity-dependent manner via water currents in the field. The distances t raveled by large numbers of dispersing neonates may decouple the numbe r of larvae in an area from the number of adults that oviposited there , which suggests that supply-side phenomena may be important in stream s. The development of a clearer understanding of the role of density-d ependent dispersal as a potential regulatory factor in black fly popul ations depends upon the assessment of the fate of drifting individuals , coupled with measurement of other sources of mortality in these popu lations.