Long-term population studies of freshwater shrimps in headwear streams of the Luquillo Mountains of Puerto Rico since 1988 document the effects of two hurricanes (1989, 1998) and two droughts (1994, 2015) on distribution and abundance of detritivores and omnivores. In general, the high input of leaf litter from diverse tropical riparian forests provides a relatively continuous resource base for detritivores living in headwater streams. However, pulses of leaf litter occur during extreme disturbances such as hurricane-driven winds that remove entire leaf canopies and prolonged dry periods when riparian trees lose their leaves to lower leaf transpiration. These disturbances result in large pulses of leaf litter that increase some populations of detritivores more than others in the years following the pulsed inputs. Differences in movement along elevational gradients by detritivores and their omnivorous predators affect how the food webs are altered by this inter-annual variability in detrital food supplies.
The dominant shrimp detritivores (e.g. Atya lanipes, Xiphocaris elongata) form leaf-litter processing chains that increase trophic facilitation through inter-specific access to different sizes of suspended organic particulates. Some shrimp species (shredders) feed on the fungi and bacteria that microbially condition leaf litter and breakdown leaf litter into coarse and fine suspended particles. Other species of shrimp filter feed on the suspended leaf particulates and algae. As the number of shredders increases, the resource availability for filter-feeding consumers also increases. Facilitation among species results in increased availability of leaf-detrital particulates that stimulates population growth and increases densities of one or more species in mid-elevation pools. This facilitation is context-specific, positive-density dependent and results in changes in distribution and abundance of detritivores along an elevational gradient to 1000 masl.To avoid omnivorous shrimp species (e.g., Macrobrachium carcinus, M. crenulatum) when the detritivore populatons are high, some of these shrimp prey can move farther upstream where pools are shallow and not generally conducive to high densities of territorial omnivores. Only rarely (1994, 1995) did some of the omnivores move upstream apparently in search of higher densities of prey and in avoidance of territorial dominant Macrobrachium in lower-elevation pools during extreme drought.