We review and synthesize recent developments in the study of the spread of invasive species, emphasizing both empirical and theoretical approaches. Recent theoretical work has shown that invasive species spread is a much more complex process than the classical models suggested, as long range dispersal events can have a large influence on the rate of range expansion through time. Empirical work goes even further, emphasizing the role of spatial heterogeneity, temporal variability, other species, and evolution. As in some of the classic work on spread, the study of range expansion of invasive species provides unique opportunities to use differences between theory and data to determine the important underlying processes that control spread rates.

Standardized sampling from many sites worldwide was used to address an important ecological problem.

The ecosystem engineering concept focuses on how organisms physically change the abiotic environment and how this feeds back to the biota. While the concept was formally introduced a little more than 10 years ago, the underpinning of the concept can be traced back to more than a century to the early work of Darwin. The formal application of the idea is yielding new insights into the role of species in ecosystems and many other areas of basic and applied ecology. Here we focus on how temporal, spatial and organizational scales usefully inform the roles played by ecosystem engineers and their incorporation into broader ecological contexts. Two particular, distinguishing features of ecosystem engineers are that they affect the physical space in which other species live and their direct effects can last longer than the lifetime of the organismengineering can in essence outlive the engineer. Together, these factors identify critical considerations that need to be included in models, experimental and observational work. The ecosystem engineering concept holds particular promise in the area of ecological applications, where influence over abiotic variables and their consequent effects on biotic communities may facilitate ecological restoration and counterbalance anthropogenic influences.

We review and synthesize recent developments in the study of the invasion of communities in heterogeneous environments, considering both the invasibility of the community and impacts to the community. We consider both empirical and theoretical studies. For each of three major kinds of environmental heterogeneity (temporal, spatial and invader-driven), we find evidence that heterogeneity is critical to the invasibility of the community, the rate of spread, and the impacts on the community following invasion. We propose an environmental heterogeneity hypothesis of invasions, whereby heterogeneity both increases invasion success and reduces the impact to native species in the community, because it promotes invasion and coexistence mechanisms that are not possible in homogeneous environments. This hypothesis could help to explain recent findings that diversity is often increased as a result of biological invasions. It could also explain the scale dependence of the diversity-invasibility relationship. Despite the undoubted importance of heterogeneity to the invasion of communities, it has been studied remarkably little and new research is needed that simultaneously considers invasion, environmental heterogeneity and community characteristics. As a young field, there is an unrivalled opportunity for theoreticians and experimenters to work together to build a tractable theory informed by data. KeywordsCommunity ecology, environmental heterogeneity hypothesis, impact, invader-driven heterogeneity, invasibility, spatial heterogeneity, spatial spread, temporal heterogeneity.Ecology Letters (2007) 10: [77][78][79][80][81][82][83][84][85][86][87][88][89][90][91][92][93][94] I N T R O D U C T I O NEarly theory for biological invasions treated the environment as if it were homogeneous in space and time (Skellam 1951). Similarly, few empirical studies of invasion directly address environmental heterogeneity, and experiments are designed to minimize its effects. In reality, invasions proceed in a highly heterogeneous world and in the context of existing communities of species. For example, important environmental drivers such as temperature, water, nutrients, sunlight and physical disturbances, are all variable at a range of spatial and temporal scales, as are the densities of species in the resident community. Recent developments in the theory of invasions suggest that environmental heterogeneity plays a defining role in whether the community can resist new invasions and the rate at which an invasion progresses. Heterogeneity is also likely to be an important factor in the outcome of invasions, changing the impacts on the community in the event of a successful invasion, including whether native species are driven to extinction and the extent to which species abundance patterns within the community are altered.In this review, we consider how environmental heterogeneity modifies the invasibility of the community and the impacts on the community following invasion. First, we define the key concepts and framework within w...

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