Increased variability in rainfall is resulting in changes in the ways that stream invertebrates are distributed along altitudinal gradients. The consequences of highly variable flows on decapod-dominated food webs in steep, headwater streams are especially influenced by prolonged drought. The long-term effects of dry periods on migratory species in non-seasonal tropical rainforests are unclear because low-flows are relatively rare events. Mechanisms regulating species distributions are uncertain but are likely density-dependent for some species of freshwater shrimp. Long-term observations (27 years) of the spatial distributions of migratory freshwater shrimp species in the Luquillo Mountains of Puerto Rico are based on use of baited traps. This record of changes in distributions led us to hypothesize that the movement of predatory species (Macrobrachium crenulatum, M. carcinus) is influenced by increased densities in downstream pools resulting from lower flows and contracting habitats following periods of upstream migration by juveniles. Historically dry periods during 1994 and 2015 provided an opportunity to determine how different periods of low flows might influence future dispersal of predatory species in the uppermost Rio Espiritu Santo drainage network (including the Rio Sonadora and the Quebrada Prieta).
We observed that predatory shrimp (Macrobrachium) dominated relatively simple food webs in upper-elevation and fed on relatively uniform densities of smaller shrimp species (Atya lanipes, Xiphocaris elongata). A combination of low rainfall and smaller pools resulted in unusually high densities of these predators in 1995 following the extreme drought in 1994. There was likely an increase in competition for cover (refuge from the largest individual Macrobrachium) that resulted in crowding and upstream movement of sub-dominant Macrobrachium to less- crowded headwater pools. After the dry period, pool sizes returned to pre-drought levels following high flows during and after Hurricane Georges in 1998. Macrobrachium no longer moved to the upper-most headwater habitats to avoid competition and remained at earlier densities in lower and mid-elevation pools. A different pattern of distribution occurred following the 2015 drought and apparently resulted in differences in timing of upstream migrations of the dominant species of Macrobrachium. In general, the peak upstream movement of Macrobrachium has occurred in June and July. Shifts in the timing and the amount of rainfall may alter the upstream movements of post-larvae and juveniles in ways that either increase or decrease crowding and social interactions among sub-dominant individuals.