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Amphibians can be important consumers in both aquatic and terrestrial habitats and may represent an important energetic link between the two, particularly in the tropics, where amphibian species richness and abundance are high. In the past 20 years, amphibian populations have declined dramatically around the world; numbers have decreased catastrophically in protected upland sites throughout the neotropics, usually resulting in the disappearance of over 75% of amphibians at a given site, particularly those species that breed in streams. Most studies of amphibian declines have focused on identifying causes and documenting changes in adult abundance, rather than on their ecological consequences. Here, we review evidence for the potential ecological effects of catastrophic amphibian declines, focusing on neotropical highland streams, where impacts will likely be greatest. Evidence to date suggests that amphibian declines will have large-scale and lasting ecosystem-level effects, including changes in algal community structure and primary production, altered organic matter dynamics, changes in other consumers such as aquatic insects and riparian predators, and reduced energy transfers between streams and riparian habitats. Furthermore, because of habitat and functional differences between larvae and adults in most amphibians, the loss of a single species is akin to losing two species.

Amphibian populations around the world have been declining rapidly over the past two decades, particularly in upland regions of the neotropics, where a fungal pathogen that causes chytridiomycosis has decimated many regions. Despite increasing concern over these and other dramatic losses of biodiversity, little information is available on the overall ecological effects of amphibian declines. As part of the Tropical Amphibian Declines in Streams (TADS) project, I quantified major energy fluxes, secondary production, and macroinvertebrate community structure for two consecutive years in four stream reaches in the Panamanian uplands, two with healthy amphibian populations and two that had experienced amphibian declines in 1996-1997. Despite relatively high year-round inputs of allochthonous organic materials, storage of detritus in the stream channels was low compared to streams in temperate regions. Organic matter inputs and standing stocks were similar between pre- and post-decline streams, and did not differ appreciably with season. Seston export was a major energetic flux in these systems, and differences in the nutritional quality (C:N) of seston in pre- and post-decline streams suggested that the loss of tadpoles may decrease the quality of materials exported from these headwaters. At coarse scales (e.g., total abundance) macroinvertebrate assemblages were similar between pre- and post-decline sites, but there were noticeable differences in production and functional and taxonomic structure. Pre-decline reaches had higher shredder production and post-decline streams had higher scraper production. In addition, taxonomic differences between pre- and post-decline streams were also evident, with a shift from dominance of smaller scraper taxa in pre-decline sites (i.e. Psephenus) to larger-bodied scrapers such as Petrophila in post-decline reaches. Filterer production was dominated by hydropsychid caddisflies in pre-decline reaches, whereas black flies dominated filterer production in post-decline reaches. Overall, detritus and detritivores dominated energy flow in all study reaches. However, scrapers were well represented in these systems and appeared to be food-limited, particularly in pre-decline reaches where grazing tadpoles were still abundant. During the second year of my study, predicted amphibian declines began at the pre-decline site. The loss of amphibians through this year resulted in subtle shifts in macroinvertebrate functional and taxonomic structure, which correlated with changes in available food resources. Some grazing mayflies responded positively to declining tadpole populations and subsequent increased periphyton resources, suggesting a potential for some degree of functional redundancy in these systems. However, other grazers, such as the water penny beetle Psephenus, showed no response during the period of study. My results indicate that responses of remaining consumers to tadpole declines in streams may not be evident at some coarse scales (e.g., total abundance, biomass). However, differences in secondary production at the community and the functional level, along with assemblage structure changes were evident, with some individual taxa responding relatively quickly. Long-term studies in these same stream reaches will further illuminate the ultimate ecological consequences of these dramatic and sudden losses of consumer diversity.

Submerged aquatic vegetation (SAV) plays an important role in aquatic systems, providing shelter, breeding habitat, and epiphytic forage for numerous fishes and aquatic macroinvertebrates. Since 1960, many lentic habitats in the southern U.S. have been invaded by Hydrilla verticillata, and in the last two decades this aggressive macrophyte has become the dominant species of SAV in the Atchafalaya River Basin in south central Louisiana. Because of its highly branched morphology and characteristically high densities, I found it difficult to quantitatively sample the macrofauna inhabiting hydrilla stands with traditional gears such as sweep nets, particularly under the canopy. As a consequence, I developed a suitcase trap that provided an efficient, quantitative method of sampling hydrilla-associated macroinvertebrates, and compared the abundance and taxonomic composition of samples collected with suitcase and sweep net samplers in dense hydrilla habitat. The suitcase trap is easy to deploy and retrieve, effective in all plant densities, permits estimation of macroinvertebrate densities by plant volume or dry weight, and is more effective than traditional sweep nets in describing the vertical distribution of macroinvertebrates inhabiting hydrilla-dominated littoral habitats.To provide a better understanding of the effects of exotic macrophyte invasions on the ecology of epiphytic invertebrates, and to identify possible management alternatives to mitigate detrimental impacts associated with these invasions on littoral habitat quality, I measured the density of vegetation-dwelling macroinvertebrates on exotic Hydrilla verticillata and native Ceratophyllum demersum in the Atchafalaya Basin. I collected a total of 34,996 macroinvertebrates from hydrilla and coontail habitat from May to August 2001 to determine how exotic hydrilla compares to native macrophytes in terms of macroinvertebrate habitat. Abundant macroinvertebrate taxa included Amphipoda, Decapoda, Diptera, Gastropoda, and Ephemeroptera. Overall, macroinvertebrate abundance between hydrilla and coontail was relatively similar. It is apparent from this study that, at least at the assemblage level, differences between abundance and distribution patterns of macroinvertebrates in different macrophyte species do exist. Principal component analysis displayed differences between macroinvertebrate assemblages, although the relative effects (and interactions) of declining or fluctuating water quality, macrophyte architecture, food resource quantity and quality, and predatory mortality on macroinvertebrate community composition remain to be identified.

Plant litter represents a major basal resource in streams, where its decomposition is partly regulated by litter traits. Litter-trait variation may determine the latitudinal gradient in decomposition in streams, which is mainly microbial in the tropics and detritivore-mediated at high latitudes. However, this hypothesis remains untested, as we lack information on large-scale trait variation for riparian litter. Variation cannot easily be inferred from existing leaf-trait databases, since nutrient resorption can cause traits of litter and green leaves to diverge. Here we present the first global-scale assessment of riparian litter quality by determining latitudinal variation (spanning 107°) in litter traits (nutrient concentrations; physical and chemical defences) of 151 species from 24 regions and their relationships with environmental factors and phylogeny. We hypothesized that litter quality would increase with latitude (despite variation within regions) and traits would be correlated to produce ‘syndromes’ resulting from phylogeny and environmental variation. We found lower litter quality and higher nitrogen:phosphorus ratios in the tropics. Traits were linked but showed no phylogenetic signal, suggesting that syndromes were environmentally determined. Poorer litter quality and greater phosphorus limitation towards the equator may restrict detritivore-mediated decomposition, contributing to the predominance of microbial decomposers in tropical streams.

The relationship between detritivore diversity and decomposition can provide information on how biogeochemical cycles are affected by ongoing rates of extinction, but such evidence has come mostly from local studies and microcosm experiments. We conducted a globally distributed experiment (38 streams across 23 countries in 6 continents) using standardised methods to test the hypothesis that detritivore diversity enhances litter decomposition in streams, to establish the role of other characteristics of detritivore assemblages (abundance, biomass and body size), and to determine how patterns vary across realms, biomes and climates. We observed a positive relationship between diversity and decomposition, strongest in tropical areas, and a key role of abundance and biomass at higher latitudes. Our results suggest that litter decomposition might be altered by detritivore extinctions, particularly in tropical areas, where detritivore diversity is already relatively low and some environmental stressors particularly prevalent. It is unclear whether stream detritivore diversity enhances decomposition across climates. Here the authors manipulate litter diversity and examine detritivore assemblages in a globally distributed stream litterbag experiment, finding a positive diversity-decomposition relationship stronger in tropical streams, where detritivore diversity is lower.