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变化时，物种的主要反应是调整它们的地理分布，以便它们继续占据条件适宜的地区。结果，物种入侵了它们以前不存在的地区和景观。对于有可能成为生态系统支配者的物种（例如可以支配森林树冠的树木），这种入侵可能导致被入侵地区的生态系统发生显著变化。大多数地貌支持一系列生态系统，这些生态系统占据的区域因地貌起伏或土壤发育的基质不同而环境条件不同。这种异质性有助于景观支持一系列物种的能力，生物多样性在异质性更高的景观中更高。这种异质性也使更广泛的物种在应对气候变化时能够在景观中移动。较为同质的景观，或某些生境类型已经丧失的景观，成为物种移动的障碍，从而限制了它们适应气候变化的能力。需要可靠地预测潜在生态系统优势种的入侵可能如何影响生态系统格局和景观异质性。这些预测将为景观管理计划和生物多样性保护战略的制定提供信息，这些计划和战略将最大限度地增加物种适应气候变化的机会。我们将收集记录过去入侵对三种景观生态系统模式影响的数据。我们将应用古生态学的技术，主要是花粉分析和放射性碳测年，在景观中的小盆地，因为末次冰期结束积累的沉积物。通过研究每种景观中的一系列这样的小盆地，以及每种情况下的一个较大的盆地，我们将获得单个植被组成变化的记录，以及区域植被组成变化的记录。总之，这些数据将使我们能够重建随着时间的推移景观的生态系统模式。我们将把注意力集中在大约8800年至5800年前苏格兰松树入侵苏格兰西部高地的问题上。在这次入侵之前，这一地区的森林通常有橡树和榆树的混合树冠，还有榛树的下层。一些这样的混合林地仍然存在，但松树现在占据了大部分剩余的森林。我们将收集的数据将使我们能够确定松树入侵了景观的哪些部分，以及它的入侵如何影响生态系统模式和整体景观异质性。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，这一模型使用入侵发生期间气候条件的模拟作为输入。我们将比较模拟的生态系统特征和模式的变化与入侵的格局重建从古植被数据。这将使我们能够评估模型的能力，可靠地模拟这种入侵的后果。这一点很重要，因为这类模型提供了为数不多的工具之一，可用于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。为了说明它的潜力，这样的应用程序，我们将应用该模型来模拟一个这样的潜在的未来入侵的冠层优势树到这些景观。我们的研究结果将与各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。我们将确保向这些利益攸关方以及科学界和广大公众传达这些建议。

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.