Impacts of an Invasive Snail (Tarebia granifera) on Nutrient Cycling in Tropical Streams: The Role of Riparian Deforestation in Trinidad, West Indies

Non-native species and habitat degradation are two major catalysts of environmental change and often occur simultaneously. In freshwater systems, degradation of adjacent terrestrial vegetation may facilitate introduced species by altering resource availability. Here we examine how the presence of intact riparian cover influences the impact of an invasive herbivorous snail, Tarebia granifera, on nitrogen (N) cycling in aquatic systems on the island of Trinidad. We quantified snail biomass, growth, and N excretion in locations where riparian vegetation was present or removed to determine how snail demographics and excretion were related to the condition of the riparian zone. In three Neotropical streams, we measured snail biomass and N excretion in open and closed canopy habitats to generate estimates of mass- and area-specific N excretion rates. Snail biomass was 2 to 8 times greater and areal N excretion rates ranged from 3 to 9 times greater in open canopy habitats. Snails foraging in open canopy habitat also had access to more abundant food resources and exhibited greater growth and mass-specific N excretion rates. Estimates of ecosystem N demand indicated that snail N excretion in fully closed, partially closed, and open canopy habitats supplied 2%, 11%, and 16% of integrated ecosystem N demand, respectively. We conclude that human-mediated riparian canopy loss can generate hotspots of snail biomass, growth, and N excretion along tropical stream networks, altering the impacts of an invasive snail on the biogeochemical cycling of N

Impacts of an Invasive Snail (\u3ci\u3eTarebia granifera\u3c/i\u3e) on Nutrient Cycling in Tropical Streams: The Role of Riparian Deforestation in Trinidad, West Indies

Non-native species and habitat degradation are two major catalysts of environmental change and often occur simultaneously. In freshwater systems, degradation of adjacent terrestrial vegetation may facilitate introduced species by altering resource availability. Here we examine how the presence of intact riparian cover influences the impact of an invasive herbivorous snail, Tarebia granifera, on nitrogen (N) cycling in aquatic systems on the island of Trinidad. We quantified snail biomass, growth, and N excretion in locations where riparian vegetation was present or removed to determine how snail demographics and excretion were related to the condition of the riparian zone. In three Neotropical streams, we measured snail biomass and N excretion in open and closed canopy habitats to generate estimates of mass and area-specific N excretion rates. Snail biomass was 2 to 8 times greater and areal N excretion rates ranged from 3 to 9 times greater in open canopy habitats. Snails foraging in open canopy habitat also had access to more abundant food resources and exhibited greater growth and mass-specific N excretion rates. Estimates of ecosystem N demand indicated that snail N excretion in fully closed, partially closed, and open canopy habitats supplied 2%, 11%, and 16% of integrated ecosystem N demand, respectively. We conclude that human-mediated riparian canopy loss can generate hotspots of snail biomass, growth, and N excretion along tropical stream networks, altering the impacts of an invasive snail on the biogeochemical cycling of N

Appendix B. Images of apparatus used for field experiments.

Images of apparatus used for field experiments

Appendix A. Detailed description of streams where surveys and experiments were conducted.

Detailed description of streams where surveys and experiments were conducted

Appendix E. Models 21–75 in the confidence set from which effects of the major threat classes on Chinook population trend are estimated for the small-scale analysis.

Models 21–75 in the confidence set from which effects of the major threat classes on Chinook population trend are estimated for the small-scale analysis

Appendix C. Model-averaged estimates of direct, indirect, and total effects of the four major threats on Chinook population trend, when populations spawning in the interior Columbia River basin are excluded from the large-scale analysis.

Model-averaged estimates of direct, indirect, and total effects of the four major threats on Chinook population trend, when populations spawning in the interior Columbia River basin are excluded from the large-scale analysis

Appendix B. Confidence set of models estimating the effects of major threats on Chinook population trend, when 79 populations spawning in the interior Columbia River basin are excluded from the large-scale analysis.

Confidence set of models estimating the effects of major threats on Chinook population trend, when 79 populations spawning in the interior Columbia River basin are excluded from the large-scale analysis

Appendix D. Path diagram depicting estimated effects of threat indicators on Chinook population trend, when populations spawning in the interior Columbia River basin are excluded from the large-scale analysis.

Path diagram depicting estimated effects of threat indicators on Chinook population trend, when populations spawning in the interior Columbia River basin are excluded from the large-scale analysis

Appendix A. Supplemental methods for estimating model-averaged effects and variance.

Supplemental methods for estimating model-averaged effects and variance

<i>Tarebia granifera.</i>

<p>The quilted melania snail has invaded freshwater habitats throughout much of the Neotropics. Photo credit: S. B. Snider.</p