Dominance of non-native species increases over time in a historically invaded strandline community

Aim: We lack a robust understanding of whether exotic species, in addition to causing changes immediately after establishing, might continue to increase in dominance long after invasion events occur. To address this, we resurveyed strandline plant communities, which are likely to have been invaded for over two centuries. Location: Northeastern USA. Methods: We resurveyed the richness and cover of native and exotic plants in 2008 and 2009 at 18 sites, which had originally been surveyed in 1998. We examined whether native and exotic dominance had changed, whether native-rich sites were less likely to be impacted by exotics over time, whether changes in dominance were driven by large changes in a small number of outlier species or by small, incremental changes among many species and whether disturbance mediated any of these relationships. Results: Exotic dominance increased across sites. Initial native diversity was unrelated to patterns of exotic dominance during resurveys. The identity of species that were outliers with respect to changes in distribution or cover varied between resurvey years. Significant changes in exotic-to-native richness ratios at sites were detectible with or without the inclusion of outlier species, but changes in abundance ratios were only significant when outlier species were included. Disturbance across sites was not correlated with species richness, cover, or changes in dominance. Main conclusions: In this historically invaded community, exotics have increased in dominance over the last decade. This change is not due solely to the success of a few hyper-dominant species, but also to the cumulative effect of small changes in distribution among many species. It remains unclear whether patterns observed are due to invasion processes that are playing out very slowly through time or to some other explanation. Our findings highlight the need for a more robust understanding of the long-term dynamics of species invasions

Body size evolution of palaeo-insular mammals: Temporal variations and interspecific interactions

Aim: We investigated the hypothesis that body size evolution of mammals is strongly influenced by ecological interactions, resulting in evolutionary divergence in body size in species-rich (e.g. mainland) biotas, and convergence on the size of intermediate but absent species in species-poor (e.g. insular) biotas. Location: Mediterranean palaeo-islands. Methods: We assembled data on temporal variation in body size of palaeo-insular mammals and associated variation in ecological characteristics (colonization or extirpation of mammalian competitors and predators) for 19 species of fossil, non-volant mammals across four large (> 3640 km2) islands ranging between the late Miocene and Holocene. These are the only fossil species for which fine-detailed time series are available at present. Results: Our results are consistent with predictions based on an ecological interactions hypothesis of body size evolution. Following colonization (or first appearance in the insular fossil record) small mammals (such as mice, shrews and pikas) tended to increase in body size. These trends, however, ceased or were reversed following colonization of the focal islands by mammalian predators or competitors. Main conclusions: While body size evolution is likely to be influenced by a variety of characteristics of the focal islands (e.g. climate, area, isolation and habitat diversity) and species (e.g. diet, resource requirements and dispersal abilities), temporal trends for palaeo-insular mammals indicated that the observed trends for any particular species, island and climatic regime may be strongly influenced by interactions among species. Ultimately, invasion of a competitor often leads to the extinction of the native, insular species. © 2013 Blackwell Publishing Ltd