Ecological 'Tipping Points'? - How invasion of a potential canopy dominant affects landscape-scale ecosystem patterns

当气候条件发生较大幅度和较快的变化时，物种的主要反应是调整它们的地理分布，以便继续占据具有合适条件的地区。结果，物种入侵了以前没有的地区和景观。对于有可能成为生态系统优势物种的物种(例如，能够主宰森林树冠的树木)，这种入侵很可能导致入侵地区的生态系统发生显著变化。由于地貌的起伏或土壤发育的不同底物的存在，大多数景观支持一系列生态系统，这些生态系统占据着环境条件不同的地区。这种异质性有助于景观支持一系列物种的能力，在更异质的景观中，生物多样性更高。这种异质性还使更广泛的物种能够在适应气候变化的过程中在景观中活动。更为同质的地貌，或某些栖息地类型已经丧失的地貌，成为物种迁徙的障碍，从而限制了它们适应气候变化的能力。需要对潜在生态系统优势物种的入侵如何影响生态系统格局和景观异质性进行可靠的预测。这些预测将为制定景观管理计划和生物多样性保护战略提供信息，这些计划和战略将使物种最大限度地增加适应气候变化的机会。我们将收集数据，记录过去入侵对三个景观中生态系统模式的影响。我们将应用古生态学技术，主要是花粉分析和放射性碳测年，对自上一次冰川阶段结束以来在景观中小盆地积累的沉积物进行研究。通过研究每一景观中的一系列这样的小流域，以及每一种情况下的一个较大的流域，我们将获得个别植被组成变化的记录，以及区域植被组成变化的记录。总而言之，这些数据将使我们能够随着时间的推移重建景观上的生态系统模式。我们将重点关注大约8800到5800年前苏格兰松树入侵苏格兰西部高地的事件。在这次入侵之前，这一地区的森林一般都有橡树和榆树的混合树冠，还有一层淡褐色的下层。这种混合林地的一些林地仍然存在，但松林现在主导了剩余的大部分森林。我们将收集的数据将使我们能够确定松树景观的哪些部分受到了入侵，以及它的入侵对生态系统格局和整体景观异质性的影响。我们还将使用最先进的植被模型来模拟松树入侵。除其他外，该模型还使用入侵发生期间气候条件的模拟作为输入。我们将把与入侵相关的景观上的生态系统特征和格局的模拟变化与从古植被数据重建的格局进行比较。这将使我们能够可靠地评估该模型模拟此类入侵后果的能力。这一点很重要，因为这样的模型提供了为数不多的工具之一，可以用来预测物种对人为气候变化的反应所引起的入侵的潜在后果。为了说明它在这类应用中的潜力，我们将应用该模型来模拟将来一棵树冠优势树对这些景观的潜在入侵。我们的成果将与不同的利益相关者直接相关，为景观管理和生物多样性保护战略的制定提供信息。我们将确保将它们传达给这些利益攸关方，以及科学界和更广泛的公众。

When climatic conditions change by a relatively large amount and relatively rapidly, as they are projected to do this century, species' principal response is to adjust their geographical distributions so that they continue to occupy areas with suitable conditions. As a result, species invade regions and landscapes where they previously were absent. In the case of species with the potential to be ecosystem dominants (e.g. trees that can dominate forest canopies), such invasions are likely to lead to marked changes in the ecosystems of the invaded area. Most landscapes support a range of ecosystems occupying areas differing in environmental conditions because of the relief of the landscape or the presence of different substrates for soil development. Such heterogeneity contributes to the ability of a landscape to support a range of species, biodiversity being higher in more heterogeneous landscapes. This heterogeneity also enables a wider range of species to move through the landscape as they respond to climatic changes. More homogeneous landscapes, or those where some habitat types have been lost, act as barriers to the movement of species and thus limit their ability to adapt to climatic change. Reliable projections are needed of how invasions by potential ecosystem dominants may affect ecosystem patterns and landscape heterogeneity. Such projections will inform the development of landscape management plans and biodiversity conservation strategies that together will maximise species' opportunities to adapt to climatic changes. We will collect data documenting the impacts of a past invasion on ecosystem patterns in three landscapes. We will apply the techniques of palaeoecology, principally pollen analysis and radiocarbon dating, to sediments that have accumulated in small basins in the landscapes since the end of the last glacial stage. By examining a series of such small basins in each landscape, as well as one larger basin in each case, we will obtain records of the changing composition of individual stands of vegetation, as well as records of the shifting regional vegetation composition. Together, these data will allow us to reconstruct ecosystem patterns on the landscapes through time. We will focus our attention on the invasion of Scots Pine into the Western Highlands of Scotland between about 8800 and 5800 years ago. Prior to this invasion the forests of this region generally had mixed canopies of Oak and Elm, with a Hazel understorey. Some stands of such mixed woodland persist, but Pine now dominates most of the remaining forests. The data we will collect will allow us to determine which parts of the landscape Pine invaded and how its invasion impacted upon ecosystem patterns and overall landscape heterogeneity. We will also use a state-of-the-art vegetation model to simulate the Pine invasion. This model uses as inputs, amongst other things, simulations of climatic conditions during the period when the invasion took place. We will compare simulated changes in ecosystem character and patterns on the landscapes associated with the invasion with patterns reconstructed from the palaeovegetation data. This will enable us to evaluate the model's ability reliably to simulate the consequences of such an invasion. This is important because such models provide one of the few tools available to project the potential consequences of invasions arising from species' responses to anthropogenic climatic change. In order to illustrate its potential for such applications, we will apply the model to simulate one such potential future invasion of a canopy dominant tree into these landscapes. Our results will be of direct relevance to various stakeholders, informing the development of landscape management and biodiversity conservation strategies. We will ensure that they are communicated to such stakeholders, as well as to the scientific community and to the wider public.