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 km2 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.