Spatial and temporal variation of disturbance: effects on riparian and aquatic communities along two headwater streams in Puerto Rico

Abstract

The structure and composition of tropical ecosystems are often shaped by disturbance events, especially in areas that experience hurricanes and other severe storms. The structure of riparian forests along headwater streams in the Luquillo Long Term Ecological Research Site of Puerto Rico reflects the patchiness of periodic hurricane disturbance as well as the spatial pattern of past human land use. The result of these disturbance patterns is a clumped distribution of the dominant riparian tree species (the Sierra palm, Prestoea montana) within and between riparian forests along two headwater streams, Quebrada Prieta and Quebrada Toronja. Fruit-fall and frond input from the Sierra palm is an important source of food and refuge for stream consumers. Patchiness of palm fruit and frond availability may have important effects on the distribution patterns and relative abundance of consumers in headwater stream communities. I studied the effects of disturbance from Hurricane Georges on a dominant riparian tree species in the Luquillo Forest Dynamics Plot of the Luquillo Long Term Ecological Research Site in northeastern Puerto Rico. I established 300 m by 10 m riparian transects along the Prieta and Toronja, which were both dominated by the Sierra palm (Prestoea montana). Following Hurricane Georges, I periodically surveyed Sierra palm individuals for damage and subsequent recovery (measured by initial loss and subsequent regrowth of palm fronds), palm height, and reproductive status. I used Geographic Information System (GIS) data to determine variation in riparian topography (elevation, slope, and aspect) and tree densities within riparian transects. I analyzed logistic regression models to determine the influence of riparian topography, tree density, and palm height on spatial patterns of palm damage and recovery due to Hurricane Georges. Palm height was the factor most related to initial hurricane damage to palms, with tall canopy palms receiving the most damage. Post-hurricane palm recovery was also related to wind exposure, measured by higher elevation of riparian transects and lower surrounding tree densities. A larger proportion of palms were damaged along the Toronja (33%) than the Prieta (25%), which may be a result of historical human disturbance in the Toronja basin which led to a shift in forest species composition. Large canopy palms constitute a greater proportion of the riparian forest along the Toronja than the Prieta, and so had a greater proportion of palms that were exposed to stronger hurricane winds. These spatial patterns of hurricane damage to riparian forests often affect detrital input to stream pools, which can influence distribution patterns and relative abundance of stream consumers. Stream communities in the Luquillo Mountains of Puerto Rico support several species of omnivorous freshwater decapods. One of the most abundant predatory decapods is the freshwater crab, Epilobocera sinuatifrons. I studied habitat selection of freshwater crabs in stream pools within the same 300 m riparian transects along the Prieta and the Toronja. Nine study pools were established along each stream transect, and were periodically sampled for juvenile crabs, mayfly nymphs, and physical and substrate characteristics. Juvenile crabs and mayfly nymphs were abundant in all pools, but both showed higher numbers along the Prieta. Because the land use history in the Toronja drainage has resulted in a less stable channel and increased abundance of fine sediment in Toronja pools, the more stable pools along the Prieta may retain more leaf litter and provide better pool habitats for juvenile crabs. Temporal variation and within-pool habitat variables (substrate composition, pool depth, flow velocity, and heterogeneity of pool width) were best at explaining variations in juvenile crab distributions. To determine food web relationships of juvenile crabs within the stream community, I studied predation rates of juvenile crabs on mayfly nymphs, larval shrimp, and conditioned leaf disks collected from leaf packs within stream pools. Pools were apparently dominated by one mayfly species, Farrodes taino (Ephemeroptera: Leptophlebiidae). Feeding experiments demonstrated mayfly nymphs to be the preferred food source for juvenile crabs (8 mm to 18 mm carapace width), with feeding rates increasing with crab size. The observed predation rates were relatively high, ranging from 2 to 72 mayfly nymphs per crab per night. These feeding experiments suggest that aquatic insects may be an important source of protein for juvenile crab growth, and high aquatic insect predation rates by juvenile crabs may influence the low relative abundance of aquatic insects found in these two insular tropical streams. In summary, detrital inputs are an important source of food, cover, and substrate for aquatic communities in tropical headwater streams. Spatial patterns of riparian palm damage influence the spatial distribution of detrital inputs across a watershed, in tum affecting distributions and relative abundance of stream consumers. The magnitude and frequency of disturbance, such as hurricanes or high rainfall events, influence the standing stock of detritus in stream pools, the availability of refuge, and the stability of the stream environment.