Ernest Dempsey—Recently findings of a noteworthy research made news, namely that unlike what was generally believed heretofore, organic productivity is not a key determinant of biodiversity in an area. The research was led by Peter Adler of Utah State University and involved dozens of researchers from over 40 different institutions. Responding to my call for an interview about the research, three researchers from this team answered my questions about biodiversity and the key factors influencing it. For participating in this conversation, I am thankful to the researchers: Dr. Stan Harpole (Iowa State University), Dr. Elizabeth Borer (University of Minnesota), and Dr. Eric Seabloom (University of Minnesota).
Ernest: Thank you all for taking time for this chat. Please tell our readers briefly about the importance of biodiversity for the life and environment around us.
Stan: Biodiversity is more than simply a list of all living organisms; it is also everything that species do, all the ecosystems services they provide in maintaining our “life support systems” (for free, in terms of economics); how they moderate climate, cycle carbon and nutrients, provide resources for higher consumers, pollinate crops, regulate pests, etc. Biodiversity also provides many other, less tangible services, like aesthetic beauty. Current rates of extinction—the permanent loss of species—are higher than any other time in the geological record.
Ernest: As I understand from the online information, till now, it has been generally accepted that biodiversity is dependent directly on primary productivity. Was it just an assumption or a scientific fact?
Stan: I would call it a confusion. Scientists have been interested in patterns of biodiversity since Aristotle and earlier. One of the earliest patterns people observed was that there tend to be more species in the tropics than temperate latitudes. Plant production also shows latitude patterns and so simple plots have been made over the years showing productivity on the X-axis and diversity on the Y-axis. But these are really just associational patterns or correlations. The question scientists are really interested in is what are the mechanisms or actual causal factors that determine how many species can coexist in a given habitat? I would argue that many ecologists have forgotten that question—what are the mechanisms—and become obsessed with the pattern and what the “true shape” of the pattern should look like, with many expecting to see a hump-shaped pattern. What we are arguing is that is important to move beyond pattern searching and try figure out what the causal factors are for biodiversity because, not only are many of these factors (e.g., actual limiting factors like water or nutrients or herbivores) also driving plant production, but are also being changed dramatically by human activity: things like nitrogen deposition, habitat loss, acid rain, warming, drought, etc.
Elizabeth: This means that if we retain a focus on factors that don’t control diversity, like productivity, we will not move toward solutions to our biodiversity crisis.
Ernest: How did you and your team come to test this understanding of relation between productivity and biodiversity?
Stan: Previous attempts that showed that there really is no “true shape” of the correlation between productivity and diversity used a statistical method called “meta-analysis”. Basically, one searches through all the published scientific literature and compiles all the relevant studies and their results into one big database and then analyzes it with the question: given all the studies available, what are the general trends in the data? When people have done this using hundreds of published studies, there has been no clear pattern that has emerged. But one challenge with this type of approach (meta-analysis) is that there are apples-and-oranges difficulties because each original research team might have used different methods or type of experiment or measured slightly different things in different ways; maybe all the variability in the results is partly due to the variability in methods.
Elizabeth: We realized that there was another way to answer this question—an actual sampling effort where everyone collected data using exactly the same methods. This is what we did.
Eric: We have extensive experience using meta-analysis to examine many questions relating to the drivers of productivity and diversity. This is a powerful and useful method, but it does have its limits as do all methods in science.
Ernest: So in what locations your team carried out research and what methodology you developed or followed?
Stan: What is unique about our approach is that we have established what is basically a “meta-experiment”. We (all the Nutrient Network collaborators, over 100 so far) are sampling grasslands all over the world in the same way: same plot size, same sampling methods, same protocols, same data formats, etc. What we have is the best, most consistent data yet to address this persistent prediction that there is some “true” pattern between productivity and diversity. Basically, we treat the “hump-shaped” pattern as a hypothesis and we test that against what, in statistics, we call a “null hypothesis”. Is there strong evidence for a relationship? Or how different is what we see from just a random set of points?
Elizabeth: For this analysis, we sampled grasslands from more than 40 locations on 5 continents. Our network is growing, and future analyses to answer new questions will include even more sites and continents.
Ernest: And you found that primary productivity is not a key determinant of biodiversity, right?
Stan: Exactly. On average, there was essentially no consistent relationship between productivity and diversity. Individual sites showed different patterns, but most were no different than what you would expect from a random cloud of points. This does not say that diversity or productivity are random, just that they do not correlate with each other when we compare apples to apples—other factors are driving diversity and productivity and that’s where the research needs to go.
Ernest: So what does biodiversity primarily depend on?
Stan: One way to think about diversity is to use the concept of the niche: what are all the factors in an organism’s environment that it depends on for growth and reproduction (temperature, water, food, etc.), and in turn, how does an organism affect its environment by consuming resources (nutrient or other organisms) and producing waste? It’s that set of factors that limit organisms, often in different ways, that determines how many species can coexist. And for plants, those limiting factors also determine how much biomass they can produce. The question about soil microbes is really interesting because this is a really big question right now: how plants and microbes interact and how that affects diversity? We know that microbes like bacteria, fungi, and protozoa can interact with plants in a wide variety of ways, ranging from mutualistic (both sides benefit) to pathogenic (plant disease). And soil food webs are thought to be hugely more complex than other sorts of food webs we have studied. New genetic tools are just recently becoming available that we can start to look inside the “black-box” of soils.
Elizabeth: We know that diversity results from the net effect of nutrient availability and species interactions—but our great challenge is to sort apart and figure out which of these are most important in controlling diversity.
Ernest: Given the role of soil microbes in biodiversity, is the biodiversity likely to be affected negatively in places where pesticides are used periodically on a large scale?
Stan: Again, this is something we need to look at. We are changing our planet in many ways and usually faster than we can study the effects of those changes, let alone make accurate predictions about the consequences. How soil microbes will be affected and whole that in turn affects plant diversity and ecosystem function is poorly understood at this time.
Ernest: Last year, we had news of global warming casting a negative effect on biodiversity. How does this happen and whether on a global scale?
Stan: I prefer the term “global change”, because even though global temperatures are going up on average, there are many other human-driven changes that may be just as serious. It’s not just average global temperature that is changing, but also how extreme temperatures and rainfall events are becoming, and averages don’t capture how different regions are affected differently. Other global changes are also important: humans have roughly doubled the amount of nitrogen and quadrupled the amount of phosphorus available to plants through agriculture and burning fossil fuels, acidified our oceans, and are creating other challenges with land use and pollution. All of these changes are expected to have negative consequences for diversity.
Elizabeth: The biggest challenge for ecologists like us is sorting out when and where these changes will matter most for biodiversity.
Ernest: One thing I don’t want to miss asking is, do environmental scientists also research or debate better policymaking for environmental conservation, particularly biodiversity protection?
Stan: Yes, for example the Ecological Society of America publishes articles and reports for policy makers (Issues in Ecology; Frontiers in Ecology and the Environment), as do the American Association for the Advancement of Science and the National Academy of Sciences. Many ecologists are very serious about applying basic research to pressing environmental issues.
Elizabeth: Ecologists also fill prominent policy-making positions—for example, Jane Lubchenco is the under-secretary of commerce for oceans and atmosphere and has been the top administrator of NOAA since 2009. She has devoted much of her career to communicating science to non-science audiences and even started the Aldo Leopold Leadership Program that trains ecologists to communicate their research to the media and the policymakers.
Ernest: At the moment, what are some of the places in the world where biodiversity is most threatened?
Stan: The grassland systems we study with the Nutrient Network are strongly impacted because grasslands often coincide with agricultural and livestock activities. In fact, grasslands are the most critically threatened terrestrial habitat globally. For example, it’s estimated that less than one percent of prairie habitat is left in Iowa. Marine systems and tropical forests are systems that large in area and also relatively less studied in terms of biodiversity that are threatened, but the important thing is that humans are now a dominant force all over the globe.
Ernest: What can an ordinary person do in his daily life to contribute to protection of biodiversity?
Stan: I encourage my students to stay informed, think critically about sources of information, think about the interrelationships between different issues and factors. It’s important to remember that there are no simple solutions and focusing on just the effect of A on B while ignoring C through Z and possible feedbacks and interactions will likely lead to more problems down the road. But what is exciting is that there is enormous opportunity for creativity and solutions, and that there is still hope that many of the challenges we are facing can be turned around.
Ernest: Right. To wrap up, what is the next big question for you?
Stan: This is exactly my research: I want to understand how species coexist and what drives biodiversity, but also how those same factors that lead to coexistence are undergoing change globally and impacting biodiversity. Understanding the mechanisms behind biodiversity may give us better ability to predict and ameliorate the consequences of global change.
Ernest: Thank you so much Stan, Elizabeth and Eric for your precious time and sharing your knowledge!
Stan: Thank you for the opportunity to share our research!