Use of small pond habitats by birds and its consequences for trematode infection in freshwater snails
Host abundance and diversity are integral to parasite transmission both among and between hosts. However for complex-life cycle parasites, measurement of mobile host diversity and abundance often occurs on a different spatial scale than sampling for parasites in less-mobile hosts making comparisons between host groups and parasite infections difficult. This is particularly important when infrequent transmission events have disproportionately large effects on parasite abundance, such as sporadic visits to small freshwater wetlands by bird hosts, which then transmit infections to other aquatic hosts. We aimed to (1) describe the seasonal pattern of bird use of small freshwater systems, and (2) determine the relationship between trematode infections in freshwater snails and bird abundance and diversity. To determine bird host use of wetlands, we established two trail cameras on three wetlands, capturing images hourly during daylight hours for 10 months (March until December 2011). To determine the relationship between trematode infections in snails and bird abundance and diversity, we subset the 10 month daily time series to match both the temporal resolution of trematode sampling and three additional sites monitored with trail cameras from June – August, 2010.
Using Loess smoothers of the 10 month daily bird abundance time series, we found that birds had strong temporal and seasonal trends, driven by a large winter peak in waterfowl abundance and smaller spring breeding peaks in passerine and waterfowl abundances, although these patterns varied between sites. We found that summer bird abundance and richness was driven mainly by surrounding habitat type (number of trees near shore, percent forest within 1 km, and wetland area), highlighting the importance of near shore habitat for wetland birds. In contrast, trematode richness and prevalence decreased in wetlands with larger surface areas but were positively related to bird abundance. We suggest that this pattern may be related to sites with more birds contributing more infectious material, but for this particular set of wetlands, larger sites may have had reduced size and quality of littoral zones, which may reduce transmission of trematodes to freshwater snails. Our results highlight how trail cameras can be useful in determining infrequent visitation by mobile hosts, and the relationship between these small scale visitations and complex life cycle parasites.