IMPACTS OF THE INVASIVE GRASS SALTWATER PASPALUM (PASPALUM VAGINATUM) ON AQUATIC COMMUNITIES OF COASTAL WETLANDS ON THE GALAPAGOS ISLANDS, ECUADOR.
Siemens, Tania Juniper
Invasive plants species pose a threat to ecosystem function, and island ecosystems are particularly vulnerable to their impact. The grass saltwater paspalum (Paspalum vaginatum) has successfully invaded Galapagos Islands, potentially threatening coastal lagoons and their globally significant avifaunal biodiversity. I conducted two studies to assess potential changes in invertebrate populations associated with increased P. vaginatum abundance. The first study evaluated the relationship between abundances of fiddler crab (Uca galapagensis) burrows and habitat conditions (including above ground P. vaginatum densities) measured along transects that spanned lagoon shore-line habitat. Results show that at intermediate P. vaginatum densities, fiddler crab burrow abundance increased proximal to the shoreline and at high water tables and demonstrates the importance of moisture in fiddler crab habitat selection. However, burrow densities were reduced at high and low P. vaginatum densities regardless of habitat. This result suggests that P. vaginatum invasion may benefit the crab at early stages of invasion, but once P. vaginatum reaches high densities fiddler crabs may be excluded from lagoon shoreline habitats. In the second study I assessed whether P. vaginatum is impacting aquatic invertebrates by comparing invertebrate communities across 4 lagoon habitats: P. vaginatum, the interface between P. vaginatum and open water (edge), open water, and emergent mangroves. The invertebrate community in P. vaginatum showed increased species richness and was characterized by more terrestrial species such as polychaetes, tabanid larvae, and syrphid larvae. Invertebrate communities in open water were dominated by highly abundant aquatic species such as corixids (Trichocorixa reticulata) and ostracods. Although invertebrates in P. vaginatum had greater mass/individual than those in water, edge and mangrove, average overall invertebrate biomass was similar in all habitats and across seasons. This suggests that the invasion of P. vaginatum does not affect annual average invertebrate production per se, but rather the relative contribution of each species to the overall biomass. To evaluate whether the shift in invertebrate communities may impact lagoon avifauna, I conducted bird observations, estimated maximum habitat potentially affected by P. vaginatum, and assimilated bird feeding ecology data into an impact assessment table. Results suggest that most waterbirds (flamingos, ducks, herons, migratory shorebirds) generally do not associate with P. vaginatum while food availability for terrestrial birds (yellow warblers, smooth billed ani, mocking birds) may increase. Furthermore, the P. vaginatum invasion degrades foraging habitat and food resources for waterbirds and with further increase may potentially occupy 90.6 % of the lagoon surface area. This potential habitat degradation poses a particular threat to the viability of the Galapagos flamingo (Phoenicopterus ruber glyphorhynchus) population, an endemic subspecies, since a further population reduction may increase extinction risk. My results show that the invasion of P. vaginatum is associated with a shift from an aquatic to a more terrestrial invertebrate community. The continued invasion of P. vaginatum degrades water bird and fiddler crab habitat. Control of P. vaginatum may be necessary to maintain the Galapagos lagoons status as an Internationally Important Wetland.
Galapagos Islands, lagoons, flamingo, Phoenicopterus ruber glyphorhynchus, Ramsar treaty, non-native, invasive plants, impacts, Paspalum vaginatum, saltwater paspalum, fiddler crabs, Uca galapagensis, coastal wetlands, invertebrate communities, water birds, Trichocorixa reticulata
dissertation or thesis