As one moves from the core to the periphery of a species' geographical range, populations occupy less favourable habitats and exhibit lower and more variable densities. Populations along the periphery of the range tend to be more fragmented and, as a result, are less likely to receive immigrants from other populations. A population's probability of extinction is directly correlated with its variability and inversely correlated with density and immigration rate. This has led to the prediction that, when a species becomes endangered, its geographical range should contract inwards, with the core populations persisting until the final stages of decline. Convinced by these logical but untested deductions, conservation biologists and wildlife managers have been instructed to avoid the range periphery when planning conservation strategies or allocating resources for endangered species. We have analysed range contraction in 245 species from a broad range of taxonomic groups and geographical regions. Here we report that observed patterns of range contraction do not support the above predictions and that most species examined persist in the periphery of their historical geographical ranges.

Aim We examined the range contraction of 309 declining species of animals and plants to determine if the contraction dynamics better matched predictions based on the demographic characteristics of historical populations (demographic hypothesis) or based on the contagion‐like spread of extinction forces (contagion hypothesis). Location Species included in the analysis came from all biogeographic regions. Methods We obtained range maps for 309 species from literature or through personal correspondence with authorities. Hypotheses were contrasted by examining the sequence of changes in the proportion (C) of the remnant range that fell within the central region of the historical range. Monte Carlo simulations and polynomial regressions were employed to examine changes in C during the process of range contraction. Results The results of the Monte Carlo simulations indicated that more species had observed range contractions consistent with the contagion hypothesis than expected by chance (z‐score = 2.922, P = 0.002). The Monte Carlo analysis also indicated that the number of species whose observed range contractions were consistent with the demographic hypothesis was no greater than expected by chance (z‐score = 0.337, P = 0.367). The results of the polynomial regression analysis for the two most common taxonomic groups (mammals and birds) and for all geographical regions (Australia, Africa, Eurasia, and North America) we examined also supported the contagion hypothesis. Main conclusions Most of the examined range contractions are consistent with the contagion hypothesis and that the most likely contagion is human related disturbance. These results have important implications for the conservation of endangered species.

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Old World Bluestems (OWB), introduced from Europe and Asia in the 1920s, recently have begun to raise concerns in the Great Plains. Despite suggestion in the late 1950s that OWB were weedy and negatively impacted biological diversity, they were widely introduced throughout the Great Plains for agricultural purposes. Anecdotal evidence suggests that OWB exhibit invasive characteristics that promote competitive exclusion of native species. The objective of our study was to quantify the competitive abilities of two OWB species (Caucasian bluestem; Bothriochloa bladhii (Retz.) S.T. Blake (= Bothriochloa caucasica (Trin.) C.E. Hubb.) and yellow bluestem; Bothriochloa ischaemum (L.) Keng) with three native grass species (big bluestem (Andropogon gerardii Vitman), little bluestem (Schizachyrium scoparium (Michx.) Nash), and sideoats grama (Bouteloua curtipendula (Michx.) Torr.)). A greenhouse target-neighbor study was conducted to assess both interspecific and intraspecific competition. A total of 480 pots (4.4 l) filled with native soil was used with all pair-wise combinations of species and four density treatments (six replications). Vegetative tiller height, above-and belowground biomass were measured at the end of 16 weeks. Both of the OWB significantly inhibited at least one growth parameter of the three native grass species, while most of the native species did not inhibit growth of either OWB species. Growth of B. ischaemum was enhanced when grown in association with S. scoparium. Based upon the results of our study of OWB competitive superiority and previous research, many of the characteristics possessed by OWB are found to be in common with known invasive species. Hence, we propose that two OWB are competitively superior to three common native prairie species providing them with the ability to invade and threaten the native grasslands of the Central and Southern Great Plains.