When a small change in a condition or resource (e.g., amount of forest cover in a landscape) causes an abrupt change in a species’ abundance, a threshold response is said to occur. When viewed across a species’ geographic range, abundance is commonly thought to change in gradual manner: high abundances in the central portion of its range and low abundances at the range boundary. But humans now dominate many ecosystems through direct consumption (e.g., timber harvest) or by converting habitats to intensive land uses (e.g., agriculture), thus reducing the capacity of the system to support native biodiversity. Furthermore, because the human footprint is geographically uneven, it can lead to local extinctions within a species’ geographic range. The uneven pattern of human use suggests that abrupt abundance changes could occur anywhere within a species’ geographic range, not just near range boundaries. If such abrupt changes are more common than ecological conventional wisdom would suggest, then identifying where those abrupt changes (i.e, thresholds) occur on the landscape could serve as focal points for conservation action – in effect, using habitat planning and management to steer populations away from these threshold conditions.
For three large regions in the United States, we used the North American Breeding Bird Survey, U.S. National Land Cover Data, and a statistical modeling approach (multivariate adaptive regression splines) to assess whether land bird species’ abundances were associated with landscape composition and configuration in a threshold fashion.
Threshold relationships between abundance and landscape characteristics were exhibited by 42-60% of the species studied. The relationships were evident for five land types and five habitat guilds. We observed threshold relationships for more taxonomically diverse groups of bird species, a broader set of land types, and larger geographic extents than have been observed in similarly designed studies to date.
Initial finding from this study suggest that distribution and abundance theory can be refined to incorporate the influence of human development as an agent of abrupt spatial change in bird abundances. Our findings uncover 20 different kinds of bird–landscape threshold relationships that impose a much more complicated set of circumstances for multiple bird species management than has been widely recognized. Future research under this project will attempt to: (1) uncover why some species show threshold relationships and others do not; and (2) develop management and policy strategies that account for the complexities of multi-species management in conservation planning.
Landscape pattern, prevalence of avian persistence thresholds, and daunting conservation challenges. Presentations made to the North American Congress for Conservation Biology (13-16 July 2014); The Wildlife Society (25-30 Oct 2014)
Cross-pattern and cross-scale interaction effects of landscape characteristics on avian persistence thresholds: implications for broad-scale conservation. Presentations made to the International Association for Landscape Ecology (5-10 July 2015); The Wildlife Society (17-21 October 2015).
Gutzwiller, K.J., S.K. Riffell, and C.H. Flather. In review. Avian abundance thresholds, human-altered landscapes, and the challenge of assemblage-level conservation. Landscape Ecology 00:000-000.