Regional variation in responses of wetland-associated bird communities to conversion of boreal forest to agriculture

Globally and in Canada's boreal forest, extensive deforestation has occurred because of agricultural conversion. However, consequences of forest loss for bird assemblages associated with wetlands and their associated riparian areas and shoreline forests are poorly understood. Using the multivariate approach, Threshold Indicator Taxa Analysis (TITAN), we assessed the response of bird communities to an agricultural conversion gradient at two spatial scales: (1) locally within 500 m of a wetland, and (2) throughout a 5 x 5 km landscape. We compared results from a study area in Manitoba surrounded by agriculture (DMMB) to those from a landscape where agriculture is encroaching from the southern edge in east-central Alberta (ECAB). Both species-level and community-level changes tended to occur at lower levels of agricultural conversion in DMMB than in ECAB, particularly at the landscape scale. Community-level changes were more gradual and reached a single maximum at the wetland scale, whereas there were two to three distinct community-level change-points at the landscape scale. Species responding positively (15 in ECAB and 18 in DMMB) to agricultural conversion were typical of open-country ecoregions, while species that responded negatively (13 in each of ECAB and DMMB) tended to be those for which loss of forest cover represented direct loss of habitat. For species common to both regions, direction of response (+ or -) was typically consistent, but specific change-points differed. Where conversion of forest to agriculture is unavoidable in boreal forests, limiting the total amount of forest and wetland vegetation loss around wetlands and within the landscape matrix to <= 30%, along with wetland preservation, will have the greatest benefit to conserving bird communities typical of boreal wetlands and their adjacent riparian areas and forests.