Natural and Climate Change Mediated Invasions
Species distributions are constantly in flux. Biological and physical factors continually influence the rates of range expansions and contractions, altering the distribution of species in space and through time (MacArthur 1972; Brown 1995; Brown et al. 19
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Natural and Climate Change Mediated Invasions Steve I. Lonhart
3.1
Introduction
Species distributions are constantly in flux. Biological and physical factors continually influence the rates of range expansions and contractions, altering the distribution of species in space and through time (MacArthur 1972; Brown 1995; Brown et al. 1996). Ranges expand as individuals colonize new areas and contract as populations become locally extinct. Understanding how organisms respond to environmental changes and describing the underlying mechanisms are key research components in the fields of ecology and biogeography. Knowing where populations occur—and where they are absent—provides insights into the ecological and physical factors that regulate patterns of density and distribution (see also Chap. 2, Carlton). Historically, biological responses were due to natural processes and often occurred over long (geological) time scales. More recently, anthropogenic (i.e. human-mediated) processes have played an increasingly important role in driving patterns of density and distribution. In this chapter I will present biological invasions in the context of geographic range shifts, explore range shifts due to natural, anthropogenic, and artificial processes, and consider how climate change is already affecting species distributions.
3.2
The Geographic Range of a Species
The geographic range of a species is commonly defined as the known spatial extent of the species. Field guides often display range information as a map with polygons or shading to indicate species presence. Since species distributions are dynamic, a truly accurate assessment of the geographic range is nearly impossible. Instead, range maps represent estimates of distribution based on limited, often incomplete data and thus provide a general view of where a particular species occurs (Brown et al. 1996). Range limits, with the exception of G. Rilov, J.A. Crooks (eds.) Biological Invasions in Marine Ecosystems. Ecological Studies 204, © Springer-Verlag Berlin Heidelberg 2009
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a few species, are likely either under- or over-estimates of the actual geographic range. This combination of historic, recent, and anomalous range records is not likely representative of a species’ current range extent (Gaston 1994). For the majority of species, most of which receive little scientific attention, geographic ranges are likely underestimated due to inadequate sampling near range limits. A suite of biological and physical factors regulates the distribution and density of each species. Physiological tolerance limits set thresholds beyond which an individual cannot reproduce, grow, or survive, and ecological interactions (e.g., competition, predation, mutualism) further modify these limits. Physical barriers can also prevent species from dispersing to all suitable habitats. For marine species, such barriers are often land masses, such as continents. For example, since completion of the Suez Canal in 1869, over 200 marine species have invaded the Mediterranean Sea
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