Water hyacinth is among the most widespread invasive plants worldwide; however, its effects on waterbirds are largely undocumented. We monitored site use by waterbirds at Lake Chapala, the largest lake in Mexico and recently designated Ramsar site, to evaluate the potential influence of water hyacinth cover on species composition and aggregate measures of the waterbird community, including waterbird density, species richness, and Simpson's index of diversity. We examined the response of waterbirds to changes in percent water hyacinth cover at 22 independent sites around the lake during six study seasons from May 2006 to February 2008. We found little evidence to suggest that percent water hyacinth cover affected aggregate community measures; however, multivariate analysis of relative species composition suggested that water hyacinth cover corresponded with seasonal species composition (Canonical Correspondence r  =  0.66, P  =  0.007) when seasonal site cover averaged 17.7 ± 4.67% (winter 2007). Several migratory species were not observed during this season, which could suggest that some small-bodied migratory species avoided Lake Chapala during the winter of high water hyacinth cover. We suspect that observed changes in the waterbird community are in response to species-specific tolerances for water hyacinth and indirect abiotic and biotic effects of its presence (e.g., invertebrate and fish composition).

Nomenclature: Glyphosate, water hyacinth, Eichhornia crassipes (Mart.) Solms.

Management Implications: Water hyacinth is among the most widespread aquatic invasive plants in the world. Yet despite its global recognition, our understanding of how nonnative ecosystems and animal communities, specifically waterbirds, respond to the establishment of this species is relatively weak, which hinders our ability to choose the best potential management strategy. The waterbird community comprises herbivores as well as secondary and tertiary consumers; therefore, changes in community composition and abundance can greatly affect food web structure and energy flows within an affected ecosystem. In addition to their ecological role, waterbirds are the source of cultural and economic values in many regions. A shift in waterbird community composition resulting from water hyacinth establishment has the potential to enhance or reduce socioeconomic values associated with freshwater ecosystems. By understanding how waterbirds respond to water hyacinth, field practitioners (e.g., researchers, managers, and decision-makers) will be better informed when developing research and management priorities. Our results suggest that water hyacinth cover had little effect on aggregate measures of the waterbird community at Lake Chapala, including waterbird density, species richness, and diversity (Simpson's index of diversity), but that some small-bodied, migratory species might avoid sites and/or entire ecosystems with water hyacinth. Although the decision to control water hyacinth and other invasive plants is surely based on a wide assortment of socioeconomic objectives and desired benefits (i.e., protection of municipal water supply), we believe that the ecological response to nonnative plants and potential control methods should be evaluated. Our results suggest a benign relationship between water hyacinth and waterbirds, with the potential exception of some small-bodied, migratory species. Understanding the waterbird response to water hyacinth provides critical pieces in the ecological and socioeconomic puzzle of invasive plant science and management and can help managers evaluate the benefits and costs of control more holistically.