Elsevier

Biological Conservation

Volume 127, Issue 3, January 2006, Pages 261-271
Biological Conservation

Forecasting faunal and floral homogenization associated with human population geography in North America

https://doi.org/10.1016/j.biocon.2005.04.027Get rights and content

Abstract

Human population and urbanization is unprecedented in its rate of growth and geographic scope. With the help of humans, exotic species have piggybacked their way to distant lands, which in combination with the loss of endemic native species, has led to the convergence of biological communities toward common and ubiquitous forms. However, the extent to which this “biotic homogenization” varies along gradients of human population size and urbanization remains mostly unexplored, especially at broad spatial scales. The present paper combines a recent conceptual model of homogenization with estimates of species invasions and extinctions to provide the first estimates of homogenization for five major taxonomic groups – land birds, freshwater fish, terrestrial mammals, plants, and freshwater reptiles and amphibians – at the continental-scale of North America (exclusive of Mexico). On average, the greatest levels of biotic homogenization were predicted for plants (22%) and fishes (14%), followed by reptiles/amphibians (12%), mammals (9%) and birds (8%). Substantial spatial variation in predictions of community similarity exists and emphasize that the outcome of species invasions and extinctions may not only increase community similarity, but may also decrease it (i.e., differentiation). Homogenization is predicted to be greatest for fish in southwestern and northeastern US, highest in eastern North America for plants, greatest for birds and mammals along the west coast of North America, and peak in southern US for reptiles and amphibians. We show that predicted change in community similarity for all taxonomic groups is positively related to human population size and urbanization, thus providing the first quantitative linkage between human population geography and homogenization for a number of major taxonomic groups at the continental-scale of North America. Our study helps identify regional hotspots of biotic homogenization across North America, thus setting the stage for future studies where more directed investigations of biotic homogenization along urban gradients can be conducted.

Introduction

The 20th century has witnessed the radical alteration of the world’s population geography. Human population size and urbanization continue to increase at unprecedented rates, and have exacted substantial ecological costs on the outlying rural environment (Wakermagel et al., 2002). As noted by the urban historian, Lewis Mumford (1961), the ecological impacts of urbanization are experienced far beyond the urban fringe, where “the inadequate and impaired carry capacity of the urbanized region is offset by the plundering of non-urban hinterlands”. Indeed, human activities are taxing the environment in a great number of ways, not in the least through the overexploitation of our natural resources (e.g., Fitzhugh and Richter, 2004).

Humanity’s migrations across, and subsequent urbanization of, the landscape have had innumerable effects on the many organisms in which we share this world (McKinney, 2002). Implications of human population geography (used here to refer to the size and distribution of human populations) include environmental degradation and the transport and introduction of foreign species, both of which are considered primary threats to native species persistence (Vitousek et al., 1997, Wilcove et al., 1998). In the face of increased human dominance of the biosphere, recent decades have seen significant changes in biodiversity. Global species diversity has decreased over time as a result of native species extinctions, but at regional and local scales species diversity has typically increased because the introduction of exotic species have outpaced the loss of native species (Sax and Gaines, 2003). However, increases in local or alpha-diversity is commonly at the expense of decreased beta-diversity or increased community similarity among regions. The process by which regionally distinct, native communities are gradually replaced by locally expanding, cosmopolitan, non-native communities is called biotic homogenization (McKinney and Lockwood, 1999). Biotic homogenization is considered among the greatest threats to biological life, and is now recognized as a distinct facet of the broader biodiversity crisis having significant ecological and evolutionary consequences (Olden et al., 2004). Evidence for homogenization is widespread and encompasses a number of faunal and floral groups (e.g., Rahel, 2000, Rejmánek, 2000, Blair, 2001, Duncan and Lockwood, 2001, Marchetti et al., 2001, Jokimäki and Kaisanlahti-Jokimäki, 2003, Crooks et al., 2004, McKinney, 2004a, Rooney et al., 2004).

The role of human population geography in homogenizing faunas and floras of North American is the subject of this paper. Anthropogenic change to natural environments associated with humans is considered one of the primary threats to biodiversity (Sala et al., 2000), but what remains unclear is the manner such impacts influence rates and specific outcomes of species invasions and extirpations and therefore shape patterns of biotic homogenization. Current ecological knowledge support the potential linkage between human population geography and homogenization. First, a wealth of empirical evidence supports strong associations between human population size and the number of threatened/extinct and introduced species (e.g., Kerr and Currie, 1995, Kirkland and Ostfeld, 1999, McKinney, 2001a, McKee et al., 2003). Second, evidence also points strongly to the more obvious importance of species invasions and extinctions in driving the homogenization of biotas (McKinney and Lockwood, 1999, Rahel, 2002, Olden et al., 2004).

By virtue of these empirical relationships, one would expect that biotic homogenization should vary directly as a function of human population size and urbanization. Indeed, quantitative evidence for this association exists at small spatial scales for birds (Blair, 2001, Blair, 2004, Jokimäki and Kaisanlahti-Jokimäki, 2003, Crooks et al., 2004) and fishes (Walters et al., 2003), although much progress is still needed on this topic (see subsequent papers in this special issue). Because only a limited number of studies have formally quantified homogenization, there is a need to establish statistical associations between characteristics of human populations and homogenization so that we can forecast spatial and temporal patterns of biotic homogenization. This is particularly important given that studies of homogenization at broad spatial scales have focused almost exclusively on freshwater fishes (e.g., Rahel, 2000, Taylor, 2004), and we therefore know very little about levels of homogenization for most other taxonomic groups.

The objective of this study is to explore interrelationships between gradients of human population geography and patterns of biotic homogenization across North America. We use estimates of species invasions and extinctions for political divisions of North America and the predictive model of Olden and Poff (2003) to predict patterns biotic homogenization/differentiation for 5 major taxonomic groups – land birds, freshwater fish, terrestrial mammals, plants, and reptiles and amphibians. Estimates of faunal and floral homogenization are then associated with human population size and urbanization across these regions with the goal of better understanding the correlative nature of this relationship at broad spatial scales for multiple taxonomic groups.

Section snippets

Methods

Our analysis was fourfold. First, we calculated measures of human population geography for the 63 political divisions (10 provinces and 3 territories of Canada and 50 states of the United States) of North America (excluding Mexico). Second, using electronic databases and published sources we quantified the number of threatened/endangered/extinct (hereafter called TEE species) and non-native species for each division for each of 5 major taxonomic groups – land birds, freshwater fish, terrestrial

Predictions of faunal and floral homogenization

Predictions of homogenization/differentiation for North America varied greatly among the taxonomic groups and within taxonomic groups depending on the invasion–extinction scenario (Table 1). Here, we limit our interpretations to those invasion–extinction scenarios that are supported by the literature to be operating at a broad spatial scale, but for completeness present the results for all scenarios. Model predictions according to scenario I1 – describing invasions by cosmopolitan species

Discussion

Biotic homogenization is considered one the most important forms of biotic impoverishment worldwide (McKinney and Lockwood, 1999) and is recognized as an important component of the modern biodiversity crisis (Olden et al., 2004). The phrase biotic homogenization is increasingly used in the literature, although limited empirical quantification continues to impede our ability to make robust statements regarding its magnitude and spatial extent for different taxonomic groups. In the present study

Conclusion

The future is certain to bring considerable ecological shuffling as people influence ecosystems in various ways, not the least through both purposeful and accidental introduction of species. Humans are the primary agents of homogenization, yet we have many interests in slowing its progress. Urban areas provide perhaps the best example of biotic homogenization and at the same time may also provide the best opportunity for mitigation. The majority of the American public lives in or near urban

Acknowledgements

J.D.O. would like to thank co-author Michael McKinney and Julie Lockwood for the opportunity to participate in the special symposium on urbanization and biotic homogenization. We are especially indebted to Frank Rahel and Chris Taylor for kindly providing the fish homogenization data for the US and Canada. We thank Sam Otterstrom for advice regarding the quantification of urbanization, Gina Schalk for assistance in summarizing the data from the COSEWIC report, and the numerous individuals

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