Functional redundancy of scavenger guilds in sandy beach ecosystems
Ecosystem functioning can be compromised by loss of biodiversity, especially if overall functional diversity contained in the system is reduced. Losses of biodiversity are frequently linked to losses of habitat caused by human transformations, like urbanisation, of the landscape. Theoretically, species losses attributable to urbanisation are predicted not to alter overall ecosystem function, if functional redundancy (i.e. alternative species replacements) compensates for such losses. Here we test this expectation by measuring how coastal urbanisation affects a key ecological process: consumption of animal carcasses by scavengers in food webs. We measured this for the guild of vertebrate scavengers on sandy beaches, examining whether changes in guild composition resulted either in a loss of scavenging efficiency, or in a functional replacement of one guild by another while ecosystem functioning is maintained. Scavenging and scavengers were quantified, using baited camera traps, on a continental scale that encompassed tropical, subtropical and temperate shorelines situated over a broad gradient of urbanisation from wilderness beaches to coastal cities in Australia.
Urbanisation significantly altered ecological function with little functional redundancy in coastal food webs. Scavenger guilds varied considerably among beaches in different regions, and across the range of urbanisation intensity, which had significant effects on scavenging efficiency. When land-use changes were low, we found similar consumption rates, driven by raptors in the tropics and by foxes at higher latitude. On subtropical beaches where both raptors and mammals are functionally prominent, scavenging efficiency was significantly enhanced, suggesting additive functional effects of guild composition. In this region, increased urbanisation caused a substantial shift in the scavenger community structure. Iconic and functionally important raptors removed 70-100% of all fish carcasses from beaches with <10% urban land cover, but declined precipitously in abundance on urban beaches. Importantly, other vertebrates usually associated with urban settings (e.g. foxes, corvids) did not functionally replace raptors. Our study shows that 1) functional replacement is evident over large geographic distances at continental scales, provided that urbanisation remains low; 2) functional loss associated with coastal cities is not compensated by replacement of species; 3) effective conservation of ecological function in coastal domains requires us to identify thresholds of landscape transformations beyond which declines in function become irreversible.