Multiple stressors and ecological surprises

The expanding global human population, now about 7.5 billion, is increasing the pressure that we as a species put on the environment.  2016 was the warmest year ever recorded, and temperature records continue to be exceeded. Each year, more natural ecosystems are lost to dam construction, deforestation and urbanisation. Rates of species invasion are increasing, and pollution events continue to pressure native wildlife. Many ecosystems are now threatened simultaneously by these multiple human-caused stressors, yet we still know very little about their combined interactive impacts.

In our paper in Geo (Linking key environmental stressors with the delivery of provisioning ecosystem services in the freshwaters of southern Africa) we review the impacts of multiple stressors on ecosystem services in freshwater ecosystems in southern Africa (e.g. the Okavango Delta; see photo). We chose these systems because freshwaters contribute disproportionately to ecosystem services despite covering less than 1% of the earth’s surface. Freshwater systems are also especially vulnerable to environmental stressors and over exploitation, with water and fish protein growing in importance as commodities, and average species population declines since 1970 estimated at 81% (WWF Living Planet Report, 2016). Communities in southern Africa rely on freshwater ecosystems for critically important provisioning services, such as drinking water and food (e.g. inland fisheries)

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The Okavango Delta

We found evidence that water resources for drinking, agriculture, sanitation and power are declining because of both climate and land use change. In some areas, fish production increased because of dam construction or species invasions, but these stressors can have negative impacts elsewhere. Evidence also suggests that stressors can interact to alter one another’s impacts or promote the proliferation of further stressors.

Multiple stressors often cause impacts which are hard to predict because of both complex interactions between the stressors themselves, and interactions within communities (such as those between species in a food web). These unpredictable impacts have been termed ‘ecological surprises’ and global analyses indicate that they are very common (e.g. http://onlinelibrary.wiley.com/doi/10.1111/gcb.13028). This creates problems for decision makers when prioritising which stressors to manage or control, especially when it comes to the supply of the goods and services which we rely on from natural ecosystems.

We provide a framework to categorise multiple stressor effects on ecosystem services where they can either be additive (i.e. predictable and the sum of their independent effects) or four different types of non-additive ecological surprises.  For instance, nutrient enrichment in Lake Victoria (because of high nutrient inputs from the surrounding catchment) causes low oxygen levels, killing fish (Photo 2). At the same time the nutrients promote growth of invasive aquatic plants (water hyacinth) causing a successive and synergistic multi-stressor interaction whereby the increase in plant biomass triggers further fish kills in the lake. In addition, the introduction of non-native fish (Nile perch) caused a dramatic decline in native fish biodiversity but boosted the overall fishery catch in the lake, benefiting the surrounding populations (see figure below).

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With the growing population, it is becoming difficult to protect biodiversity and rely on our planet’s natural ecosystems for food and water security. Multiple stressors are causing a downward spiral, where our use of ecosystem services threatens the environment and therefore impairs the delivery of these services for future generations.  We need more research into multiple stressors and ecological surprises, and much more needs to be done to reduce the impact that humans have on the environment.

Michelle Jackson is a Researcher at Imperial College London. 

Drawing, remembering, knowing: natural history and the ecological imagination

By Meredith Root-Bernstein (Aarhus University)

Geo: Geography and Environment recently published my personal essay about how natural history practices have helped me to think about interdisciplinary research and collaborations.  I emphasize in the essay how developing and sharing habits of observing, interpreting, and considering the human contexts of nature can help form shared understandings as the basis for exchanges about social and natural sciences of the environment.  In that essay, I discuss seeing an espino (Acacia caven) with a liana growing on it in central Chile.  My research involves searching for the key problems and solutions for the conservation of a silvopastoral system (“espinal”) and the surrounding shrub and forest habitats in this mediterranean-climate zone.  The most common species in espinal is the espino (Acacia caven).  Yet, I had never seen an espino with a liana, and I became intrigued by trying to understand the potential ecological and social meanings of this unusual species assemblage.  Here, I expand on that essay with a discussion of a sketch of the liana and the espino.  While looking for something else I came across this drawing I made of the espino and its liana:

Bernstein image

I had forgotten about the sketch, and I have also forgotten the exact circumstances of making it.  I am sure that I didn’t make it in situ, and a few days probably elapsed between seeing the tree and making the drawing. The structure of the trunk is hard to read.  First I thought it suggested that the tree is old and perhaps has ridges or hollows, but this doesn’t match the photograph.  I also couldn’t think of any example of thick, undulating or textured espino trunks.  Something was wrong, either with the drawing, my memory, or my knowledge of espinos.  Then, while walking past some trees here in Denmark with ivy on them, I realized by analogy that I had drawn the vines of the liana descending to the ground.

The liana seems to be partly imaginary.  I remember seeing red stems and green leaves, but I am fairly sure that there were no black drupes at the time and that I only saw images of them by looking up the species on the internet.  The drawing thus knits together memory and imagination to represent the way I was thinking about my observation.

The ambiguity of the sketch forced me to think about the visual and structural patterns that things make, and how those map onto our other kinds of knowledge and memory.   There are really two issues here: one is that the sketch was by nature approximate, hasty, and in this case not based on direct observation but rather memory and its own approximations.  All of these aspects confer an abstract nature on the sketch.  It excludes the inessential and retains only an impression, just enough to reconstruct what was seen.  The second issue though relates to my lack of experience thinking about and observing lianas.  This led to what might be a not-so-clear abstraction of a liana growing up a trunk, and certainly created ambiguity in interpretation.  But the ivy I saw that helped me to understand the sketched pattern of something I had only seen once before—a liana on an espino—taught me about lianas and vines in general.

An important part of natural history is personal memory, the accumulation of implicit and tacit knowledge.  How do we make these memories relevant to interpreting the future as well as the past?  Writing, sketching and showing others are all important means of communication, that emphasize different aspects of nature—the narratives and cycles, the structural patterns, the kinaesthetic and embodied knowledge of where, when and how.

It is well-known that natural history drawings have features that photographs do not: they can represent a general or ideal example of something, facilitating recognition, and they can bring attention to particular features or patterns through emphasis, selectivity and abstraction.

As I mention in the article, I think of natural history as seeking patterns, which can be used to interpret the past, but also potentially the future.  In my drawing, I imagined the visual effect of the liana on the espino in a season when it had fruit. In the Anthropocene, it might be interesting to think more about the natural history of the future.  How will places look, behave and feel under climate change? How will we read the landscape of abandoned infrastructures in the future?  What unexpected species pairing will we find somewhere next year, testimony to some casual event yesterday?  These visions don’t have to be apocalyptic—and they don’t have to be written.   Drawings can often be both more subtle and more complex than words.  They have their own logic of organization and representation.

I recently saw a short article in the ESA Bulletin about how ecologists can avoid midlife crises and burnout.  Going into the field from time to time was one suggestion.  I would also add to that that the practice of natural history, and the attempt to communicate it, if only to oneself later on, can be both enjoyable and meaningful.  It was a pleasant surprise to find this sketch that I had forgotten about, and it brought a new angle to what the liana and the espino taught me about the socioecological interactions of central Chile.

Who knows, practicing a little natural history on your days out might even inspire a new line of research, maybe an interdisciplinary one.  You don’t have to try to be serious and professional about natural history, which might take away the enjoyment of being in nature.  Play is an important way to explore the world, and its not just for children.  Many accomplished scientists, among others, take time to have fun with no clear purpose as a way to think better (see here  and here).  Later on, accumulated memories will certainly make something useful out of what you observe in nature for fun, whether it’s a publishable research project or some extra emotional attachment helping you to find satisfaction and motivation.  Indeed, my paper in GEO: Geography and Environment, and this blog post, were written just for fun and have helped me to recognise how important natural history is for my enjoyment of my job.

Meredith Root-Bernstein is a postdoctoral researcher in the Aarhus University Research on the Anthropocene (AURA) project, based in the Department of Bioscience, Aarhus University, Denmark