Tree decline and the future of Australian farmland biodiversity

Abstract

Farmland biodiversity is greatly enhanced by the presence of trees. However, farmland trees are declining worldwide, including in North America, Central America, and parts of southern Europe. We show that tree decline and its likely consequences are particularly severe in Australia's temperate agricultural zone, which is a threatened ecoregion. Using field data on trees, remotely sensed imagery, and a demographic model for trees, we predict that by 2100, the number of trees on an average farm will contract to two-thirds of its present level. Statistical habitat models suggest that this tree decline will negatively affect many currently common animal species, with predicted declines in birds and bats of up to 50% by 2100. Declines were predicted for 24 of 32 bird species modeled and for all of six bat species modeled. Widespread declines in trees, birds, and bats may lead to a reduction in economically important ecosystem services such as shade provision for livestock and pest control. Moreover, many other species for which we have no empirical data also depend on trees, suggesting that fundamental changes in ecosystem functioning are likely. We conclude that Australia's temperate agricultural zone has crossed a threshold and no longer functions as a selfsustaining woodland ecosystem. A regime shift is occurring, with a woodland system deteriorating into a treeless pasture system. Management options exist to reverse tree decline, but new policy settings are required to encourage their widespread adoption.countryside biogeography | grassy box woodlands | ranchland | regime shift | scattered trees T he future of farmland biodiversity is a major concern around the world (1-3). Farmland biodiversity is valuable in its own right, but also because it provides ecosystem services that are of direct benefit to agricultural production. For example, birds and bats control insect pests (4-6), and trees provide shade for livestock (7,8). Scattered trees occurring throughout the farmland matrix are prominent features of agricultural landscapes around the world, including in southern Europe (9, 10), North America (11, 12), Central America (13-15), and Australia (16). Farmland trees often represent relicts of largely cleared forest or woodland ecosystems (17,18) and are believed to play important roles in maintaining ecosystem function and farmland biodiversity (7,13,16). Southeastern Australia's temperate agricultural zone is part of a threatened ecoregion (19) where farmland trees are declining rapidly. In cropping landscapes, trees are cleared to make way for agricultural machinery (20,21). In livestock grazing landscapes, trees are declining because of a combination of natural or accelerated tree mortality coupled with widespread recruitment failure (22). The decline of scattered trees is increasingly recognized as a threat to biodiversity and associated ecosystem services, both in the academic literature (7,17,20,(22)(23)(24) and increasingly in conservation policy (25).Scattered trees are declining not only in Australia, b...

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