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.

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...

Interest in facilitative predator plant interactions has focused upon above-ground systems. Underground physical conditions are distinctive, however, and we provide evidence that bush lupine, Lupinus arboreus, facilitates the survival of the predatory nematode Heterorhabditis marelatus. Because H. marelatus is prone to desiccation and lupines maintain a zone of moist soil around their taproots even during dry periods, we hypothesized that dry-season nematode survival under lupines might be higher than in the surrounding grasslands. We performed field surveys and measured nematode survival in lupine and grassland rhizospheres under wet- and dry-season conditions. Nematodes survived the crucial summer period better under lupines than in grasslands; however, this advantage disappeared in wet, winter soils. Modeling the probability of nematode population extinction showed that, while even large nematode cohorts were likely to go extinct in grasslands, even small cohorts in lupine rhizospheres were likely to survive until the arrival of the next prey generation. Because this nematode predator has a strong top-down effect on lupine survival via its effect on root-boring larvae of the ghost moth Hepialus californicus, this facilitative interaction may enable a belowground trophic cascade. Similar cases of predator facilitation in seasonally stressful environments are probably common in nature.

Cycles in biological populations have been shown to arise from enemy-victim systems, delayed density dependence, and maternal effects. In an initial effort to model the year-to-year dynamics of natural populations of entomopathogenic nematodes and their insect hosts, we find that a simple, nonlinear, mechanistic model produces large amplitude, period two population cycles. The cycles are generated by seasonal dynamics within semi-isolated populations independently of inter-annual feedback in host population numbers, which differs from previously studied mechanisms. The microparasites compete for a fixed number of host insect larvae. Many nematodes at the beginning of the year quickly eliminate the pool of small hosts, and few nematodes are produced for the subsequent year. Conversely, initially small nematode populations do not over-exploit the host population, so the surviving hosts grow to be large and produce many nematodes that survive to the following year.

In spite of considerable effort to predict wildland fire behaviour, the effects of firebrand lift-off, the ignition of resulting spot fires and their effects on fire spread, remain poorly understood. We developed a cellular automata model integrating key mathematical models governing current fire spread models with a recently developed model that estimates firebrand landing patterns. Using our model we simulated a wildfire in an idealised Pinus ponderosa ecosystem. Varying values of wind speed, surface fuel loading, surface fuel moisture content and canopy base height, we investigated two scenarios: (i) the probability of a spot fire igniting beyond fuelbreaks of various widths and (ii) how spot fires directly affect the overall surface fire’s rate of spread. Results were averages across 2500 stochastic simulations. In both scenarios, canopy base height and surface fuel loading had a greater influence than wind speed and surface fuel moisture content. The expected rate of spread with spot fires occurring approached a constant value over time, which ranged between 6 and 931% higher than the predicted surface fire rate of spread. Incorporation of the role of spot fires in wildland fire spread should be an important thrust of future decision-support technologies.

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Duff fires (smouldering in fermentation and humus forest floor horizons) and their consequences have been documented in fire-excluded ecosystems but with little attention to their underlying drivers. Duff characteristics influence the ignition and spread of smouldering fires, and their spatial patterns on the forest floor may be an important link to the heterogeneity of consumption observed following fires. We evaluated fuel bed characteristics (depths, bulk densities, and moisture) of duff in a long-unburned longleaf pine (Pinus palustris Mill.) forest and corresponding spatial variation across 10 0 to 10 3 m scales. Fermentation and humus horizon depths both varied (ϳ100% coefficient of variation) but with moderate to strong spatial autocorrelation at fine scales. Fermentation bulk density varied less than humus bulk density, which varied considerably at fine scales. Fermentation horizons held more moisture (average 49%-172%) and were much more variable than humus following rainfall, which remained stable and relatively dry (average 28%-62%). Humus moisture was moderately autocorrelated at fine scales, but fermentation moisture was highly variable, showing no evidence of spatial autocorrelation under dry, intermediate, or wet conditions. Observations from this study highlight the underlying spatial variability in duff, informing future sampling and fire management efforts in these long-unburned coniferous forests.Résumé : Les feux d'humus (feux couvant dans l'horizon de fermentation et la couche d'humus de la couverture morte) et leurs conséquences ont été documentés dans des écosystèmes où le feu était exclu mais peu d'attention a été accordée à leurs causes sous-jacentes. Les caractéristiques de l'humus influencent l'allumage et la propagation des feux couvant et leur configuration spatiale dans la couverture morte pourrait avoir un lien important avec l'hétérogénéité de la combustion observée à la suite des feux. Nous avons évalué les caractéristiques des lits de combustibles (profondeur, densité apparente et teneur en humidité) dans l'humus d'une forêt de pin des marais (Pinus palustris Mill.) qui n'a pas brûlé depuis longtemps et les variations spatiales correspondantes à des échelles de 10 0 à 10 3 m. La profondeur de l'horizon de fermentation et celle de la couche d'humus variaient (coefficient de variation d'environ 100 %) mais avec une autocorrélation spatiale modérée à forte aux échelles fines. La densité apparente de l'horizon de fermentation était moins variable que celle de la couche d'humus qui variait considérablement aux échelles fines. L'horizon de fermentation avait une teneur en humidité plus élevée (49-172 % en moyenne) et était beaucoup plus variable que la couche d'humus après une pluie, laquelle demeurait stable et relativement sèche (28-62 % en moyenne). La teneur en humidité de l'humus était modérément autocorrélée aux échelles fines mais celle de l'horizon de fermentation était très variable et ne montrait aucun signe d'autocorrélation spatiale en conditions sèche, intermédiaire ou hum...

Burrowing crabs of the genus Uca inhabit tidal mudflats and beaches. They feed actively during low tide and remain in their burrows when the tide is high. The timing of this activity has been shown to persist in the absence of external light and tidal cues, indicating the presence of an internal timing mechanism. Researchers report the persistence of several variations in locomotor activity under laboratory conditions that cannot be explained by a single circatidal clock. Previous studies supported two alternative hypotheses: the presence of either two circalunidian clocks, or a circadian and circatidal clock to regulate these activity rhythms. In this paper, we formulate mathematical models to describe and test these hypotheses. The models suggested by the literature contain some important differences beyond the frequency of proposed clocks, and these are reflected in the mathematical formulations and simulation results. One hypothesis suggests independent phase oscillators, while the other hypothesis suggests that they are coupled in anti-phase. Neither model is able to recover all of the variations in locomotor acitivity observed under laboratory conditions. However, we propose a new model that incorporates aspects of both existing hypotheses and is able to reproduce all laboratory observations.