The Fenner School of Environment and Society, Australian National University, Canberra ACT 0200, Australia.
Proc Natl Acad Sci U S A. 2010 Nov 9;107(45):19597-602. doi: 10.1073/pnas.1008476107. Epub 2010 Oct 25.
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 self-sustaining 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.
农田生物多样性因树木的存在而大大提高。然而，包括北美、中美洲和欧洲南部部分地区在内的世界范围内，农田树木正在减少。我们表明，树木减少及其可能带来的后果在澳大利亚温带农业区尤为严重，该地区是一个受到威胁的生态区。我们利用有关树木的实地数据、遥感图像和树木的人口模型进行预测，到 2100 年，平均每个农场的树木数量将减少到目前水平的三分之二。统计栖息地模型表明，这种树木减少将对许多目前常见的动物物种产生负面影响，到 2100 年，鸟类和蝙蝠的数量预计将减少 50%。在所模拟的 32 种鸟类中有 24 种预计会减少，在所模拟的 6 种蝙蝠中全部会减少。树木、鸟类和蝙蝠的广泛减少可能导致牲畜遮荫和害虫控制等具有经济重要性的生态系统服务减少。此外，我们没有经验数据的许多其他物种也依赖于树木，这表明生态系统功能可能会发生根本变化。我们的结论是，澳大利亚温带农业区已经越过了一个门槛，不再作为一个自我维持的林地生态系统发挥作用。正在发生向无树草原系统恶化的系统转变。存在扭转树木减少的管理选择，但需要新的政策设置来鼓励广泛采用这些选择。
