Scientists Find That Rain May Not Always Be a Welcome Thing to Waterbirds
By Johnny Gibbons
Scientists from the Smithsonian and colleagues have found that waterbird communities can be the "canary in the coal mine" when it comes to detecting the health of urban estuary ecosystems. Their research revealed that the types of waterbirds that inhabit urban estuaries are influenced not only by urban development, but also by a far more natural process—rain. The team's findings are published in the scientific journal PLoS ONE.
The scientists compared waterbird communities in estuaries of the Chesapeake Bay during 2002, a year of severe drought, to 2003, a year of high rainfall. During the drier year, the species of waterbirds present included both those that fed generally on many species of invertebrates and those that only fed on specific ones. However, the waterbird community was made up of most generalists the following year after heavy rain. The high rainfall increased nutrient runoff into the estuaries which reduced the estuaries' populations of small invertebrates. Because the dynamics of the invertebrate populations were affected, so in turn were the dynamics of the waterbird communities that fed on them.
"We're seeing more extreme rainstorms in this region, so our results give a snapshot of what bird communities in urban estuaries might look like in the future," said Colin Studds, lead author and postdoctoral fellow at the Smithsonian Conservation Biology Institute at the time of the research. "As urban development continues, we need new solutions for managing water quality or we stand to lose the Bay's iconic natural treasures."
Top members of estuary food chains such as waterbirds are not often considered when monitoring the stressors and indicators in the ecosystem. The team's research, however, shows that estuarine management could be improved by tracking the relationships between land development, water quality and waterbird communities. Understanding the relationships between the three could help improve strategies to protect these unique ecosystems as urban development increases.
This article summarizes the information in this publication:
Colin E. Studds, William V. DeLuca, Matthew E. Baker, Ryan S. King, Peter P. Marra. Land Cover and Rainfall Interact to Shape Waterbird Community Composition. PLoS ONE, 2012; 7 (4): e35969 DOI: 10.1371/journal.pone.0035969
Human land cover can degrade estuaries directly through habitat loss and fragmentation or indirectly through nutrient inputs that reduce water quality. Strong precipitation events are occurring more frequently, causing greater hydrological connectivity between watersheds and estuaries. Nutrient enrichment and dissolved oxygen depletion that occur following these events are known to limit populations of benthic macroinvertebrates and commercially harvested species, but the consequences for top consumers such as birds remain largely unknown. We used non-metric multidimensional scaling (MDS) and structural equation modeling (SEM) to understand how land cover and annual variation in rainfall interact to shape waterbird community composition in Chesapeake Bay, USA. The MDS ordination indicated that urban subestuaries shifted from a mixed generalist-specialist community in 2002, a year of severe drought, to generalist-dominated community in 2003, of year of high rainfall. The SEM revealed that this change was concurrent with a sixfold increase in nitrate-N concentration in subestuaries. In the drought year of 2002, waterbird community composition depended only on the direct effect of urban development in watersheds. In the wet year of 2003, community composition depended both on this direct effect and on indirect effects associated with high nitrate-N inputs to northern parts of the Bay, particularly in urban subestuaries. Our findings suggest that increased runoff during periods of high rainfall can depress water quality enough to alter the composition of estuarine waterbird communities, and that this effect is compounded in subestuaries dominated by urban development. Estuarine restoration programs often chart progress by monitoring stressors and indicators, but rarely assess multivariate relationships among them. Estuarine management planning could be improved by tracking the structure of relationships among land cover, water quality, and waterbirds. Unraveling these complex relationships may help managers identify and mitigate ecological thresholds that occur with increasing human land cover.
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