Contemporary landscape ecology is characterized by a flux of concepts and perspectives that reflect the differences in the origins of ideas and the ways of thinking, both of which are shaped by physical and cultural landscapes. The term 'landscape ecology' was coined in 1939 by the German geographer, Carl Troll, who was inspired by the spatial patterning of landscapes revealed in aerial photographs and the ecosystem concept developed in 1935 by the British ecologist, Arthur Tansley. Troll saw the need for combining the more structurally oriented geographical approach with the more functionally centered ecosystem approach, in order to allow for geography to acquire ecological knowledge of land units and for ecology to expand its analysis from local sites to larger regions. Thus, he defined landscape ecology as the study of the relationship between biological communities and their environment in a landscape mosaic on various spatial scales. At the same time, Troll also emphasized the
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Figure 2 A hierarchical and pluralistic view of landscape ecology. 'Hierarchical' refers to the multiplicity of organizational levels, spatiotemporal scales, and degrees of cross-disciplinarity in landscape ecological research. 'Pluralistic' indicates the necessity to recognize the values of different perspectives and methods in landscape ecology dictated by its diverse origins and goals. Reproduced from Landscape Ecology, 21, 2006,1-6, Cross-disciplinarity, landscape ecology, and sustainability science, Wu J, with kind permission of Springer Science and Business Media.
holistic totality of the landscape which was perceived as something of a gestalt (an integrated system organized in such a way that the whole cannot be described merely as the sum of its parts). This holistic and humanistic landscape perspective, focusing on landscape mapping, evaluation, conservation, planning, design, and management, was embraced and further developed primarily in Europe, and has become a hallmark of landscape ecology.
The concept of landscape ecology was introduced from Europe to North America in the early 1980s, and subsequently stimulated the rapid development of a stream of new ideas, theories, methods, and applications. As a result, the field of landscape ecology quickly flourished in North America, and became a widely recognized scientific discipline by the mid-1990s around the world. Some of the early publications in North America defined a landscape as a kilometers-wide land area with repeated patterns of local ecosystems. However, most landscape ecologists now consider such definition too narrow and rigid. Instead, the most widely accepted definition of landscape is simply a spatially heterogeneous area whose spatial extent varies according to research questions and processes of interest. This multiple-scale concept of landscape is more appropriate as it facilitates the theoretical and methodological developments of this interdisciplinary field by promoting micro-, meso-, and macroscale approaches. Despite their variations in details, the definitions of landscape ecology in North America all hinge on the idea of spatial heterogeneity. In particular, the North American landscape ecology focuses on the relationship between spatial pattern and ecological processes on multiple scales ranging from tens and hundreds of square meters to thousands of square kilometers in space and from a particular point to a period of several decades in time. Its primary goal is to understand the causes, mechanisms, and ecological consequences of spatial heterogeneity.
More specifically, North American landscape ecology has had a distinct emphasis on the effects ofspatial pattern on biodiversity, population dynamics, and ecosystem processes in a heterogeneous area. This research emphasis is practically motivated by the fact that previously contiguous landscapes have rapidly been replaced by a patchwork of diverse land uses (landscape fragmentation), and conceptually linked to the theory of island biogeo-graphy developed in the 1960s and the perspective of patch dynamics that began to take shape in the 1970s. Island biogeographic theory relates the equilibrium-state species diversity of islands to their size (area effect on species extinction rate) and distance to the mainland (distance effect on species immigration rate). The heuristic value of the theory is apparent for understanding the ecology of habitat patches submerged in a sea of human land uses. The patch dynamics perspective, on the other hand, treats ecological systems as mosaics of interacting patches of different size, shape, kinds, and history, emphasizing the transient dynamics and cross-scale linkages of such patchy systems. In this view, a forest is no more than a dynamic mosaic of tree gaps of various age, species composition, and biophysical properties; thus the dynamics of the forest can be adequately predicted by aggregating the behavior of individual tree gaps. The perspective of patch dynamics has been evident in the conceptual development of landscape ecology in the recent decades.
In summary, the European approach is more humanistic and holistic in that it emphasizes a society-centered view that promotes place-based and solution-driven research. In contrast, the North American approach is more biophysical and analytical in that it has been dominated by a biological ecology-centered view that is driven primarily by scientific questions. Here the author hastens to point out that this dichotomy most definitely oversimplifies the reality because such geographic division conceals the diverse and continuously evolving perspectives within each region. In fact, many ecologists in North America have recognized the importance of humans in shaping landscapes for several decades (especially since the Dust Bowl in the 1930s). Although humans and their activities have been treated only as one of many factors interacting with spatial heterogeneity, more integrative studies have been emerging rapidly in the past few decades with the surging interest in urban ecology and sustainability science in North America. On the other hand, the perspective of spatial heterogeneity has increasingly been recognized by landscape ecologists in Europe and the rest of the world. Thus, the current development of landscape ecology around the world seems to suggest a transition from a stage of diversification to one of consolidation (if not unification) of key ideas and approaches.
In fact, both the European and North American approaches can be traced back to the original definition of landscape ecology. Carl Troll's proposal to integrate the geographical and structural approach with the ecological and functional approach is best reflected in the pattern-process-scale perspective, which enhances the scientific rigor of landscape ecology. The holistic and humanistic perspective, on the other hand, epitomizes the idea of landscape as a nature-society coupled system embraced by Troll and others. This perspective is entailed by any attempt to tackle practical problems in real landscapes on broad scales. Both the European and North American perspectives are essential to the development of landscape ecology as a truly interdisciplinary science.
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