A latitudinal gradient of beta diversity for exotic vascular plant species in North America

Determining relationships between the ranges of introduced species and geographical and environmental factors is an important step in understanding the mechanisms and processes of the spread of introduced species. In this study, I examined the beta diversity and latitude relationship for all naturalized exotic species of vascular plants in North America at a continental scale. Beta diversity was calculated as the absolute value of the slope of the relationship between the natural logarithm of the Simpson index of similarity (lnS) and spatial distance between pairs of state‐level exotic floras within four latitudinal zones examined. Relative contributions of spatial distance and environmental difference to species turnover between exotic floras were examined. I found that beta diversity decreased monotonically from low to high latitudes: beta diversity for the southernmost zone was shallower than that for the northernmost zone by a factor of 2.6. Regression models of lnS in relation to spatial distance and environmental (climatic and topographical) difference for each latitudinal zone demonstrated that the explanatory power of these variables diminishes monotonically with latitude: the explained variance in lnS is 70.4%, 62.1%, 53.9%, and 33.9%, respectively, for the four latitudinal zones from south to north. For the southernmost zone, 58.3% of the variance in lnS is explained by climate variables and topography, and spatial distance explains only 2.3% of the variance. In contrast, for the northernmost zone, more than half the amount (22.5%) of the explained variance in lnS is attributable to spatial distance, and the remaining (18.9%) of the explained variance is attributable to climate variables and topography.     

Are islands more susceptible to plant invasion than continents? A test using Oxalis pes‐caprae L. in the western Mediterranean

Aim We tested the relative vulnerability of islands to Oxalis pes‐caprae L. invasion compared to mainland regions. Oxalis pes‐caprae is a South African annual geophyte that reproduces via bulbils, and has spread in many Mediterranean and temperate regions of the world where introduced. Our study is one of the first detailed regional analyses of the occurrence and local abundance of a non‐native plant. Methods We conducted an extensive survey (2000 sampling points) to examine local and coarse‐scale patterns in both the occurrence and abundance of O. pes‐caprae on islands and in neighbouring mainland regions of Spain. Location We analysed occurrence (number of samples where present) and abundance (percentage cover) on two Balearic Islands (Menorca and Mallorca) and in two mainland administrative provinces of Spain (Murcia and València). Results Oxalis pes‐caprae occurrence was consistently higher on islands. Occurrence varied among habitats, being the highest in tree groves and the lowest in forests and shrublands. It was never found in these two habitats on the mainland. Mean O. pes‐caprae abundance was greatest in tree groves on the mainland, and in field margins and old fields on the islands. However, in general there were not significant differences in local abundances between island and mainland locations. Main conclusions These findings suggest that local processes (such as the biotic resistance of plant communities) are less important than coarse‐scale phenomena (such as environmental driving forces) in explaining differences in the invasion patterns observed between islands and adjacent mainland regions. We suggest that O. pes‐caprae has occupied a larger proportion of available habitats on islands due to: (1) its strong dependence on domestic animal and human‐mediated dispersal which are probably greater on the islands than in mainland areas, and (2) the smaller area encompassed by islands that, over a comparable period of time, enables a greater proportion of available habitats to be colonized (and hence higher occurrence) than equivalent larger mainland areas.     

Intercontinental biotic invasions: what can we learn from native populations and habitats?

The effectiveness of management strategies for invasive species is often hampered by a lack of clear understanding of the factors that limit species distributions. The distribution of exotic species, especially those that are invasive, are often so dynamic that limiting factors are difficult to identify. Comparisons of exotic species between their native ranges, where they are presumably close to equilibrium with controlling factors, and their ranges in areas of introduction can circumvent this difficulty. Such studies would help identify (1) limiting factors for distributions in native ranges, (2) factors associated with a high degree of invasiveness, (3) changes in genetics and morphology since introduction, which also might contribute to invasiveness, and (4) future directions and rates of invasion as a basis for developing detection/warning systems. Findings from such comparative studies would be highly valuable for understanding the dynamics of biological invasions and for improving the effectiveness of management to prevent or control invasives.     

Fish Communities of the Sacramento River Basin: Implications for Conservation of Native Fishes in the Central Valley, California

The associations of resident fish communities with environmental variables and stream condition were evaluated at representative sites within the Sacramento River Basin, California between 1996 and 1998 using multivariate ordination techniques and by calculating six fish community metrics. In addition, the results of the current study were compared with recent studies in the San Joaquin River drainage to provide a wider perspective of the condition of resident fish communities in the Central Valley of California as a whole. Within the Sacramento drainage, species distributions were correlated with elevational and substrate size gradients; however, the elevation of a sampling site was correlated with a suite of water-quality and habitat variables that are indicative of land use effects on physio-chemical stream parameters. Four fish community metrics – percentage of native fish, percentage of intolerant fish, number of tolerant species, and percentage of fish with external anomalies – were responsive to environmental quality. Comparisons between the current study and recent studies in the San Joaquin River drainage suggested that differences in water-management practices may have significant effects on native species fish community structure. Additionally, the results of the current study suggest that index of biotic integrity-type indices can be developed for the Sacramento River Basin and possibly the entire Central Valley, California. The protection of native fish communities in the Central Valley and other arid environments continues to be a conflict between human needs for water resources and the requirements of aquatic ecosystems; preservation of these ecosystems will require innovative management strategies.