Human-related processes drive the richness of exotic birds in Europe

Abstract

Both human-related and natural factors can affect the establishment and distribution of exotic species. Understanding the relative role of the different factors has important scientific and applied implications. Here, we examined the relative effect of human-related and natural factors in determining the richness of exotic bird species established across Europe. Using hierarchical partitioning, which controls for covariation among factors, we show that the most important factor is the human-related community-level propagule pressure (the number of exotic species introduced), which is often not included in invasion studies due to the lack of information for this early stage in the invasion process. Another, though less important, factor was the human footprint (an index that includes human population size, land use and infrastructure). Biotic and abiotic factors of the environment were of minor importance in shaping the number of established birds when tested at a European extent using 50!50 km 2 grid squares. We provide, to our knowledge, the first map of the distribution of exotic bird richness in Europe. The richest hotspot of established exotic birds is located in southeastern England, followed by areas in Belgium and The Netherlands. Community-level propagule pressure remains the major factor shaping the distribution of exotic birds also when tested for the UK separately. Thus, studies examining the patterns of establishment should aim at collecting the crucial and hard-to-find information on community-level propagule pressure or develop reliable surrogates for estimating this factor. Allowing future introductions of exotic birds into Europe should be reconsidered carefully, as the number of introduced species is basically the main factor that determines the number established.

Socioeconomic legacy yields an invasion debt

“…Recent studies have demonstrated that on broad spatial scales, the impact of human activities overwhelms the influence of climate and geography on species invasions (3,4,24). The results of the current analysis extend our understanding of the temporal dimension of this relationship.…”

“…sequent establishment in the wild (14,25). Such a delay probably results from the time necessary to exceed critical thresholds of available propagules (8) that will, in turn, depend on factors like the numbers of introduction events and of individuals introduced (7,8,24,26), the type of introduction pathway (9,27), the match between an alien-species' habitat requirements and the conditions in the new territory (9), the length of generation times (14,28,29), or the time necessary for genetic adaptations to the new environments (8). This multitude of potentially interacting factors renders any generalization about the magnitude of such a delay difficult and probably also explains why the relative strengths of historical and contemporary models are not consistent among taxonomic groups.…”

The more you introduce the more you get: the role of colonization pressure and propagule pressure in invasion ecology

“…Wonham & Pachepsky (2006) considered the influence of 'introduction rate' on temporal trends in the rate that invasive species accumulate at a site, where introduction rate is the number of species introduced in any given time period. Chiron et al (2009) concluded that 'community-level propagule pressure', defined as the number of exotic species introduced, was a major driver of the observed variation in the number of established species across Europe. In these contexts, propagule pressure, introduction rate and community-level propagule pressure are all synonyms of colonization pressure.…”

Disentangling the role of environmental and human pressures on biological invasions across Europe

“…Propagule pressure is therefore closely linked with human activities and, indeed, there is increasing evidence that macroeconomic and macroecological variables need to be integrated in the same model if we are to understand the drivers of biological invasions more precisely (1,21). To date, such studies are relatively few and most focus on one or a few taxonomic groups (24,25). A study aimed at revealing general determinants of biological invasions common to a wide range of taxonomic groups that would weigh the relative importance of ecological factors and human factors will be pivotal for policymakers and future management.…”