The Human Release Hypothesis for invasive species

When my four year old son asked me what I found out during my PhD the answer did not sound exciting, nor novel, unfortunately it just sounded obvious. However, these obvious results could make a difference in the way we understand processes driving alien species abundance and their management. This is how I summarized my research to my son: “Shrubs grow bigger and are more abundant in Argentina (invasive range) than in Europe (native range) because they have more space there and no one cuts them down.” A few years of research in a nutshell.

Pictures (a) and (b) depict the situation for one particular alien shrub (Rosa rubiginosa) in its invasive range where populations expand over vast areas and where it is allowed to grow to its full size. The same species in its native range (c and d) is restricted to hedges, where it is regularly trimmed, and to highly managed, fragmented, rural landscapes.

Pictures (a) and (b) depict the situation for one particular alien shrub (Rosa rubiginosa) in its invasive range where populations expand over vast areas and where it is allowed to grow to its full size. The same species in its native range (c and d) is restricted to hedges, where it is regularly trimmed, and to highly managed, fragmented, rural landscapes.

Today I want to share with you an opinion article, where I, together with my co-authors, formulated the Human Release hypothesis around my seemingly obvious findings. We published our article in F1000 Research, whose guidelines led to lively discussion on this blog 2 years ago (immediate publication, open post-publication peer-review). We chose the term Human Release Hypothesis parallel to the established Enemy Release Hypothesis, which states that alien species benefit from escaping their natural enemies in the new range. In the case of our hypothesis the natural enemy is us, humans. Therefore, the difference in abundance of invasive species between regions is due to continuous land management and associated cutting of the invasive species that reduces population expansion and hampers growth. We illustrated this via a case study on an invasive rose species in its native range in Eurasia and in its invasive range in Australia, New Zealand, South, Africa, North and South America. This shrub builds up native population with 5 to 20 individuals whereas invasive populations consist of hundreds of individuals. Existing hypotheses of Invasion Ecology could not fully explain this pattern, but in line with the Human Release Hypothesis we only found trimming or removal of individuals in the native range. Moreover, according to the distribution of anthropogenic biomes native populations grow more often in villages and dense settlements than invasive populations.

According to the location of invasive and native rose populations the native range has a larger proportion of residential areas (data =anthropogenic biomes: Ellis and Ramankutty, 2008, Front Ecol Environ.)

According to the location of invasive and native rose populations the native range has a larger proportion of residential areas (land use categories =anthropogenic biomes: Ellis and Ramankutty, 2008, Front Ecol Environ.)

Mechanisms explaining the establishment and naturalization of alien species are well understood and human influences on these mechanisms recognized. The Human Release Hypothesis specifically addresses the effect of land use on the abundance of alien species that are already established in particular areas outside their native ranges. Invasive alien species impact their new environment especially through their high abundance, by building up invasive monocultures and replacing native communities. If release from humans would be the main driver of invasive alien species abundance, biological invasions could be prevented by preventing land abandonment, or by promoting restoration and monitoring of fallows. Particularly, in regions with low human population density land may be perceived as hyper abundant, providing ideal conditions for single disturbances followed by years without active management. Thus, the human factor in Biological Invasions demands interdisciplinary system knowledge, which can only be achieved by integrating the social and natural sciences.

This much about my obvious findings and if you like to know more about why we might yet need to pursue another hypothesis for Biological Invasions please check out our article:

Zimmermann H, Brandt P, Fischer J, Welk E, von Wehrden H (2014) The Human Release Hypothesis for biological invasions: human activity as a determinant of the abundance of invasive plant species. [v2; ref status: indexed, http://f1000r.es/4wp%5D F1000Research 3: 109. doi: 10.12688/f1000research.3740.2

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