Stream invertebrate responses to a catastrophic decline in consumer diversity

Tadpoles are often abundant and diverse consumers in headwater streams in the Neotropics. However, their populations are declining catastrophically in many regions, in part because of a chytrid fungal pathogen. These declines are occurring along a moving disease front in Central America and offer the rare opportunity to quantify the consequences of a sudden, dramatic decline in consumer diversity in a natural system. As part of the Tropical Amphibian Declines in Streams (TADS) project, we examined stream macroinvertebrate assemblage structure and production for 2 y in 4 stream reaches at 2 sites in Panama. One site initially had healthy amphibians but declined during our study (El Copéé), and 1 site already had experienced a decline in 1996 (Fortuna). During the 1ˢᵗ y, total macroinvertebrate abundance, biomass, and production were generally similar among sites and showed no consistent patterns between pre- and post-decline streams. However, during the 2ⁿᵈ y, tadpole densities declined precipitously at El Copéé, and total macroinvertebrate production was significantly lower in the El Copéé streams than in Fortuna streams. Functional structure differed between sites. Abundance, biomass, and production of filterers generally were higher at Fortuna, and shredders generally were higher at El Copéé. However, shredder production declined significantly in both El Copéé reaches in the 2ⁿᵈ y as tadpoles declined. Nonmetric dimensional scaling (NMDS) based on abundance and production indicated that assemblages differed between sites, and patterns were linked to variations in relative availability of basal resources. Our results indicate that responses of remaining consumers to amphibian declines might not be evident in coarse metrics (e.g., total abundance and biomass), but functional and assemblage structure responses did occur. Ongoing, long-term studies at these sites might reveal further ecological consequences of the functional and taxonomic shifts we observed.