TADS I: 2003 – 2006
This interdisciplinary collaborative proposal with investigators from 3 institutions, addressed central questions in ecosystem-level ecology that relate to the consequences of an extinction event. This phase of the study investigated changes in stream ecosystem structure and energy flow associated with the loss of stream-dwelling anurans. Data was collected at the reach scale for 2 years on 2 Panamanian highland streams with natural amphibian populations and 2 similar streams in a nearby region that has experienced recent extinctions.
The following questions were addressed: How are amphibian declines in tropical highlands altering stream food webs; what are the specific influences on primary producers, detrital resources, and other consumer groups in streams? What are the ecosystem-level consequences of dramatic declines in amphibian abundance and diversity in tropical highland stream and riparian habitats? How will the reduction of riparian anurans influence energy subsidies in the form of amphibian biomass to riparian habitats and predators?
TADS II: 2008 – present
Building on our prior efforts, we proposed a series of new experiments, focused on processes and function, which will allow us to more completely quantify the ecological consequences of the catastrophic loss of consumers.
Objectives: Quantify changes in stream nutrient cycling associated with consumer extirpations by performing 15N addition tracer experiments before and after an amphibian extirpation. Using methods developed through the NSF-funded Lotic Intersite Nitrogen eXperiment (LINX) we will quantitatively assess how extirpations alter nutrient cycling and food web flux rates, and perform more comprehensive natural abundance stable isotope analyses to better assess stream and riparian linkages that are likely altered. This study was conducted in El Valle region, located ~50-70 km east of El Copé, that are on, or just ahead of, the current disease front in 2006 and have since declined.We successfully performed a pre-decline 15N tracer addition in 2006 at El Valle where, as predicted, amphibians have since crashed. Analyses of these samples will allow us to build a quantitative model of N cycling in a stream with a full complement of amphibians. A post-decline 15N tracer addition was completed in 2008 in this same stream in order to assess changes in N cycling associated with amphibian extirpations.
Long-term Consequences of Amphibian Declines on Ecosystem Structure and Function in Panamanian Headwater Streams and the Roles of Amphibians in Nutrient Cycling
This project builds on prior investigations from TADS and examined the long-term (e.g., multiple years post-decline) consequences of amphibian declines on the structure and function of headwater streams in the Panamanian uplands. Ecological stoichiometry was used as a framework to assess the effects of amphibian declines on ecosystem processes (e.g., nutrient cycling). Tadpoles may regulate flows (e.g., rates and ratios) of nitrogen (N), phosphorus (P), and carbon (C) within the aquatic food web, thus influencing ecosystem processes by altering the supply of nutrients to other organisms through their feeding, excretion, and egestion. Nutrient pools and fluxes among tadpoles, food resources, and other consumers were compared at sites where amphibian declines have occurred and pre-decline sites.
Objectives: (1) Quantify long-term changes in macroinvertebrate community structure and seston quantity and quality in post-decline streams (El Cope and Fortuna); (2) Quantify tadpole and macroinvertebrate assemblage, density, and biomass at two of the last remaining pre-decline sites (Chucanti and Cana); (3) Measure N and P excretion and egestion rates, and C, N, and P composition body tissues of dominant tadpole and macroinvertebrate taxa and functional feeding groups (scrapers, grazers, filter feeders) in pre- and post- decline sites; (4) Measure C, N, and P content of basal food resources (e.g., detritus, algae) to examine the relationship between consumer and resource stoichiometry in pre- and post- decline sites. Results from this study will further our understanding of the role of consumers in neotropical headwater streams. These results will also provide critical information on relationships between consumer biodiversity and ecosystem processes, while examining the potential long-term ecological consequences of amphibian declines in these understudied systems.
Trophic Basis of Production in a Neotropical Stream Foodweb
Amphibians, especially in their larval stage, are considered key players in ecosystems by linking terrestrial and aquatic systems through the transfer of energy and nutrients. Amphibians in the Neotropics have been declining drastically, mainly due to the chytrid fungus infection. The lack of a conceptual model describing and predicting the structure and function of neotropical streams, limits ecologists to understand the effects of the amphibian decline on the ecosystem. Creating quantitative food webs, using trophic basis of production, will help answer whether allochthonous (i.e. external carbon sources) or autochthonous (i.e. in stream primary production) carbon sources are used throughout the food web, if and how much omnivory is present, and if seasonality and the loss of tadpoles affects the structure of tropical food webs. Trophic basis of production quantifies sources of energy and energy flow through consumers. This analysis uses the proportions of various food types (e.g. diatoms, animal material, detritus) ingested by consumers, estimates of assimilation efficiency (assimilation/ingestion) of each food type, net production efficiency (production/assimilation), and secondary production (elaboration of all heterotrophic biomass through time) to quantify energy flow pathways in a system. This research provides quantitative estimates of energy flow through a neotropical headwater stream food web during the dry and wet seasons, and will ultimately allow for an investigation of how this energy flow is affected by the loss of amphibians.