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

Eating Aliens: a genius scheme by the hunting lobby?

To all of you who are leading an omnivorous, carnivorous, herbivorous (…) life. Have you heard about the invasivores? People who believe that they can reduce the impact of harmful invasive species by eating them.

One of these invasivores has just written a book about his hunting and cuisine adventures: Jackson Landers, Eating aliens. One man`s adventures hunting invasive animal species.

Don`t feed the alien, eat the alien! This delicious little fellow is a nutria, native to South America and it has been introduced to North America, Europe, Asia and Africa.

Don`t feed the alien, eat the alien! This delicious little fellow is a nutria, native to South America and it has been introduced to North America, Europe, Asia and Africa.

I do not have the book at my disposal, but had one quick look at some preview pages provided by amazon. There I stumbled upon this paragraph: “It`s easy to shrug and say the problem is so big, the numbers of starlings and carp so great, that humans couldn´t possibly get rid of them all. But consider that human beings have historically succeeded in eliminating animals in such numbers. Recklessly, we´ve driven many formerly plentiful species to extinction or to extirpation from a section of their range. The difference is motivation, not capability.” Like for other global man-made problems the management of invasive species is lack of motivation, not capability. However, the management of invasive species stands out because humans have proven in the past that they are able to drive species to extinction through hunting, whereas to my knowledge we have never proven that we are able to redistribute food and resources across the globe in a equitable manner.

If you are interested in Landers book, Sara E. Kuebbing and colleagues have published a comprehensive review (Kuebbing et al. 2013, Biological Invasions) and I will just summarize their pros and cons.

Pros:

-the book educates the reader about each species´ introduction history and the ecological impact of each invasive animal.

-it will serve as important education for readers unfamiliar with the issue of invasive species.

Cons:

-the actual cooking and eating receive much less attention than the chase, and given the ostensible goals of the book, the imbalance may help motivate some hunters to chase invasives but will not help bring connoisseurs and chefs into the fold.

-some invasive species were in fact introduced to serve as a food source, and because of the negligible appetite for these species they were left to spread.

-ubiquitous doesn´t mean easy to hunt. It takes more than a few weekend-hunting tourist to decimate invasive populations.

– once an invasivore movement creates viable job opportunities, will these same hunters be willing to walk away from their livelihood as populations decline?

-most hunters do not want to eradicate entire populations, because what would be left to hunt?

The war of the roses

Recently, I switched on my computer, ready to analyse my data, when I learned that my work was obsolete. I work on invasive species, that is species introduced in areas outside their native range, and invasion science seems to be a rather elusive discipline. Invasion science is now about 30 years old (the Scientific Committee on Problems of the Environment put biological invasions on top of its research agenda in 1983) but already, its obituaries are being written.

Publications like “Another call for the end of invasion biology” by Loïc Valéry et al. or “Don’t judge species on their origins” by Mark Davis et al. and the latest “The Anthropocene could raise biological diversity” by  Chris D. Thomas, are calling for an end of the native vs. non-native dichotomy. While I agree that it is important to challenge and philosophically discuss a young growing discipline, I find that current discussions are excessive and keep us from getting the job done: That is, to understand the mechanisms behind biological invasions and translate these into useable, useful management schemes. My fist reflex when hearing the call to put an end to the native vs. non-native dichotomy was: “This is the easy way out!”. We, as global citizens, have promoted the spread of non-native species across the globe. Surely we can’t now sit back and rest the case on catchphrases like “ecosystem services” and “biological diversity in the Anthropocene”.

Luckily, I could turn to a really helpful field guide from David M. Richardson and Anthony Ricciardi, which underpins my reflex with real facts. It is a field guide for invasion science, just published in Diversity and Distributions. There, they summarize and invalidate the criticisms of invasion science in one table. Yes, non-native species drive species extinction. Yes, non-native invaders are ecologically more disruptive than native invaders (species that vigorously spread). No, increase of species introduction does not raise biodiversity. Focusing on species counts is a misleading approach. It neglects genetic swamping (hybridizations), reproductive interference and that species extinction due to invasive species takes time.

I agree with the critics that it is sometimes difficult to determine if a species is in fact non-native, or that we should prioritize management efforts on species that are most harmful, and that we should keep in mind climate change when we discuss range expanding species. However, these range expanding species can only expand their range with our help. We assist them to overcome natural borders, which they could have never crossed with their own dispersal mechanisms. In this light I also think we need to focus more on culture and not only on ecology. About five years ago, I listened to a talk from Prof. John Maron, University of Montana. He emphasized that by biotic exchange we lose uniqueness. We lose uniqueness of landscapes across the globe. I don’t want to travel to Africa, South-America, North-America, and Europe just to see the same flora and the same fauna. I want to see unique biomes, with all that is attached to it.

Below you see a post-card from one of my study areas. I studied the sweetbriar rose during my PhD in its native range in Europe and in parts of its invasive range in Argentina. This post-card is from Argentina. You can see six species on this post card with the title “forest flowers of Patagonia, Argentina”. Who can spot the non-native plants? I tell you this much: half of them are not native to South America. Two plants on this card are from Europe (and I already gave away one of them, the sweetbriar rose) and one is from North-America. People from Patagonia do believe that these plants are native to their area and that they are typical Patagonian plants. Losing uniqueness.

postal card from Patagonia, Argentina  (who can spot the non-native species?)

postal card from Patagonia, Argentina
(who can spot the non-native species?)

Who we are: Heike Zimmermann

Hi, I am Heike and currently a PostDoc in Joern’s lab. I should have written this introduction in March 2013, because this is when I started working for Joern … and now I have only three months left! My small research grant from Leuphana University is running out in February. “Working for Joern ” is actually kind of bending the truth. I am fortunate to work on my little side project, invasive species, quite independently – and when I am in need for some solid frameworking I can turn to Joern.

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Invasive species have been the focus of my research since my PhD project, which aimed to identify the mechanisms responsible for the sweetbriar rose’s successful invasion in Argentina.
My predecessors wrote something about their motivation and experiences that made them choose the job they now enjoy doing. I am sorry but I probably have to disappoint you. I am just a very pragmatic person, and for me it was just always clear that I wanted to do something with nature, in nature and of ecological relevance. Always being the time that I can remember thinking about stuff like “having a job”. I guess I can offer one important moment in my life that showed me the value of nature. That was the first I left Central Europe. When I was 15, I stayed one year in the U.S. and one of the greatest things I did there was a trip to the Great Lakes District at the Canadian border. That was the first time of my life when I was in a place where we did not run into another person for two weeks, that was the first time of my life when I saw something like real wilderness. That is when it dawned on me that the forests of my childhood were just plantations, well pruned assemblies of trees.
Later, I was able to travel to South America and Central Asia and see a lot more wilderness, but also its destruction and fragility. So there you have my motivation, after all!
If you want to learn more about my work, please visit my website at: https://sites.google.com/site/heikezimm/

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