Rediscovering Biology - Unit 12 Biodiversity: Expert Interview Transcripts
FPrime Recommended Article: Biodiversity and ecosystem stability: a synthesis of potential mechanisms underlying the relationship between diversity and stability. Nevertheless, incisive steps forward on this way are still lacking, and. Simply put, misjon.info is rich in content and convenient to use. . that biodiversity of an area has a large impact on the ecosystem stability of. There are two major philosophies that have been put forward to explain how that we came upon our discovery of the relation between diversity and stability.
For example, species that forage for pollen or nectar facilitate reproduction in the plants on which they forage, a function that is essential for the maintenance of plant populations, including in agricultural ecosystems McGregor ; Hoehn et al.
On coral reefs, grazing by fishes helps to maintain healthy, coral-dominated reefs Bellwood et al. One essential service provided by biodiversity is to stabilise the overall abundance of an assemblage of organisms that provides a particular ecosystem service or function, thereby making it less vulnerable to fluctuations in the abundances of individual populations.
Understanding diversity–stability relationships: towards a unified model of portfolio effects
This phenomenon, or components of it, has been characterised using a variety of terms e. This definition of the latter term is consistent with its use in other disciplines, such as finance Markowitz, and with its original use in the context of the DSR Tilman et al. In both model and experimental communities, stability is typically taken to be inversely related to the coefficient of variation of some measure of ecosystem function, such as total community abundance.
The DSR is the relationship between this measure of stability, and diversity here defined as the number of constituent populations. Usually, diversity is quantified as species richness, but communities can be stabilised by diversity at other levels of organisation as well, such as functional groups Bai et al.
However, stochastic competition models have consistently found portfolio effects e. Similarly, two decades of experimental research into DSRs indicates that portfolio effects are overwhelmingly present, but that their strength and magnitude varies considerably Campbell et al.
However, inverse portfolio effects, where stability decreases with diversity, also can occur in nature DeClerck et al. Several community properties have been identified as important determinants of the portfolio effect. Four that have received particular attention are asynchrony in population fluctuations, evenness of abundance, effects of diversity on total community abundance and the way in which temporal variability in abundance scales with its mean Cottingham et al.
Firstly, theoretical studies indicate that portfolio effects should strengthen as asynchrony in the fluctuations of a community's constituent populations increases Doak et al. Despite its importance in diversity—stability relationships, however, there is no consensus about how asynchrony should be measured, or about how it contributes to the DSR. A variety of metrics have been proposed, including coefficients of pairwise correlations of species' fluctuations in abundance Doak et al.
All of these metrics are still used in empirical studies e. Aboutwe started analyzing the data. Our first analysis suggested a really big effect on diversity. None of us believed it. We thought it was totally bogus. We thought it would be explained by something else. We actually put the data aside for a while, and then I had a visitor come to me on a sabbatical as a visiting professor, John Downing, from Iowa State University, and I would show John some of these results.
I'd say, well, here's a really funny result. It says, more diverse systems are more stable, but you know, it's really probably caused by some other variable.
Why don't we do that? We started saying, "Well, if diversity matters, why might it matter? How might it matter?
That led us to realize that the initial results in the diversity-stability experiment suggested that we had to do a real experiment where we had direct control on which species we were going to plot and how many. And so inwhich is a year before our first paper on this was even published, we started setting up this experiment.
So before we'd even gotten a paper accepted and published in a scientific journal, we'd had the insight that diversity would matter, but it was clear to us we had to control diversity experimentally to do it right. Anyone reading our first paper on this could say, well, sure. You happen to choose plots at different diversity for some reason when you started the experiment and you also treated them in different ways which caused the virtue to change, and the end result of that was that you saw what would look like a diversity stability effect, but maybe is really caused by something else that you didn't happen to measure.
You overcome all of those criticisms if you directly control diversity in a whole series of replicated plots. We have 40 plots that are planted to one species, some plots planted at two, 40 more planted to four, and then 40 more to eight, and 40 more to 16 species, and each plot contains a random draw of species from this whole group of prairie species that we use. And so you're able to look at the average response of all of these randomly chosen monocultures with the randomly chosen two species plots, and the fours, the eights, and the sixteens and you can find out from those analyses if there really is a diversity effect.
And their plots aren't just randomly chosen for what's in them. They're randomly spread across this whole 20 acres, so there's not some little corner over here where all the monocultures are planted, which would mean the difference might lie in the soil. They're spread all around this area all randomly.
So you use the rigorous methods of experimental design to eliminate all these possible confounding variables, such that you have an experiment that directly lets you test, "does diversity matter? The reason was that no one had an insight that diversity really would matter. There's an old idea that had been rejected, and it wasn't really a part of active ecology, so no one had tried it because it just didn't seem interesting.
How were you able to build on this model? We did start this experiment in before our results for diversity-stability had been published, but the only reason we were able to start the experiment is that we've been fortunate to be funded as a site for long-term ecological research. This is a grant from the National Science Foundation that gives us the flexibility to have a unique new idea, and go out and set up in the field and try it.
We had people who were working with us in the summer--our field assistants, our grad students and post docs--who all chipped in and got the experiment off the ground and running very, very quickly from when we first had the idea. And we knew when we did this, when we dedicated part of our long-term funding that we'd received to maintaining and sampling this experiment, we knew we could run it for a long enough period of time that we could see whatever the real results of diversity were.
And we were very fortunate that way, that we had that kind of long-term funding and that the National Science Foundation makes that kind of funding available for ecological research. It is critical that this kind of work be done at other places around the world. It requires that other governments or funding agencies be able to and willing to support other researchers to do this kind of work.
There was a wonderful experiment done in eight different European countries, supported by the European Union. It was just like our experiments-it started about four years later-but they did the work not just at one site like we did. It was across Europe, and by doing it across Europe you could draw much broader inferences, if you can show that there's a result. We showed a result that happened in one field in Minnesota. The work done in Europe showed that the same result, almost an identical result, just an amazingly identical result happened on average across all of these countries in Europe, which says it happened all across the European continent, that whole large mass, and it happened that they used different species of plants--whatever were common at each local site.
It happened with all different kinds of plants from the grasslands, from across all those whole eight countries of Europe.
One can draw a much broader inference, it was a wonderful experiment. They were funded for three years, and they were not given renewal for three more years because the science committee for the European Union said it was time to move on and ask some other question.
Unfortunately, at the end of three years, these experiments are barely established. I told you about our experiments. At the end of two years, we just finally had reasonable plants. We got rid of the weeds and our plants were finally big enough to start showing some responses. Our most highly significant results in terms of how clear the pattern is didn't happen until the 5th or 6th year, and the pattern's becoming clearer and clearer through time as these plants have longer and longer to interact.
We're in the 9th year now, and we have our clearest results ever from the 9th year. We see very distinct, clear effects of diversity on the function of these systems. That European experiment, which frankly was in my mind the best one like this ever set up, died before it ever had a chance for us to find out what its long-term implications were.
What are your hopes for your research and the field of ecology? One of the problems with ecology as a science is that the way it's been funded-it has always been funded very poorly. There is a minor amount of money coming into the science compared to the money that comes into some other disciplines like physics, which need large accelerators and even they're having trouble, or astronomy, which needs a very large radio telescopes or light gathering telescopes or the Hubble telescope and so on.
The sum spent on those things are huge compared to the money that's ever spent in ecology. And what this has meant is that, in ecology, if a result is found in one place in the world, once, we'll believe it probably applies to all places in the world forever, because we don't have the money to go out and do it again. And if you're just building up academic knowledge, and it's only of interest to academics and it has no practical application to humans, then maybe you can accept that.
It's sort of a low-budget method, and it's all you can do, so we live with it. But what is happening in ecology in the last 10 or 20 years is that what we find out is directly relevant for the quality of human life on earth, is directly relevant for public policy. The public policy debates about loss of biological diversity and the Endangered Species Act, the function of ecosystems, how should agricultural lands be managed, how should other lands be managed, wetland laws-all these things depend on scientific knowledge, and the scientific knowledge that we've been able to gather so far in ecology comes from a few very specific examples here and there.
I would assert that it's very important for a society to support experimental and observational long-term research in a variety of countries around the world, because we have to know general principles that we can use to really guide long-term national and international policy. And until we've really done it in many places, we don't know whether the results of Cedar Creek are unique to Cedar Creek. From the three-year experiment in Europe, they don't seem to be.
But I wouldn't want to have to make major international decisions based upon only one or two examples of some phenomenon. What would you most want teachers to understand? I guess the take-home lesson for me from this-there are two kinds.
One is the scientific take home lesson, and that is that we now have discovered the tradeoffs among species, the differences among species, which allow species to coexist; and we've discovered that those same differences that have allowed two or three different million species to evolve and coexist on earth, those same forces also mean that the number of species in an ecosystem influences how it functions, and that systems that are more diverse are more stable, more productive, they're more efficient users of resources, they're more efficient providers to society of services that humans need from natural ecosystems.
To me, that's sort of Lesson One. And Lesson Two, which I also think is very important, is that this kind of knowledge is very poorly understood by the general public, is very poorly understood by students and the public. We don't really realize, because of how we earn our livings, that so much of what humans need to live on this earth comes from other organisms, and it ultimately goes back to the natural and managed ecosystems that are on the earth and how we manage these.
Humans now, without trying to, we weren't deliberately trying to manage the whole earth, but we've in fact owned and managed all the lands of the earth and we almost manage all the oceans of the earth by how we harvest fish.
And we're doing these in ways right now that, as you learn more about them, are probably not very wise. We're not leaving the ability for future generations to live as well as we're currently living. And I guess that's Lesson Two that I see from this, that I think is a very important one, that we have to gain enough knowledge about how nature works and how we are interdependent with nature to help us find wiser ways to live on earth. In all of this, what is the role of climate change?
Global change and global climate change have become of increasing interest to society and to politicians and relevant for global international policy, and there are significant reasons to be concerned about climate change.
Everything that we know right now suggests that humans, by releasing carbon dioxide and other greenhouse gases, are changing climate at a rate more rapid than it's ever changed over thousands or tens of thousands of years. What is not often known is that climate is only one of a large number of ways that humans are now dominating global ecosystems, and there are other effects that humans are causing, which I and many ecologists assert are as strong as climate change, and have at least a serious implications for the future suitability and sustainability of earth.
Humans release as much nitrogen into all land ecosystems as all natural processes combined. That may not sound like it matters. Nitrogen is a good thing. It's a fertilizer, right? But the trouble is that organisms have evolved on Earth over the last three and a half billion years, an Earth where nitrogen is the main limiting factor.
Organisms have a whole array of specializations, which allow them to deal with low levels of nitrogen, and nitrogen varying in space and time, and so on, and this is one of the main ways that species are differentiated.
There has been great selection for efficiency in using this major limiting resource. But what happens when more and more of this just falls out of the sky, after it was used as fertilizer or created by combustion or fossil fuels, is that the organisms that were very efficient at using it get squeezed out by ones that aren't. And we've been doing some long-term nitrogen addition experiments trying to mimic the effects of nitro-deposition.
But we can detect this loss of diversity because we actually have permanent plots. We've been counting and measuring everything in them year after year after year for some years. Nobody else does this kind of thing.
People aren't out there counting all the species in other ruminant stands. So you might think, well, you know, humans will have an impact some place, but we'll save the nature here; we'll save a bit of nature there. Well, that's not true when there's climate change, because the climate affects the whole globe. It's not true for nitrogen deposition; it also affects the whole globe. Phosphorous, it doesn't rain out of the sky, but in agriculture we release a lot of phosphorous, which ends up going into rivers, streams, and so on, and into lakes, and the ocean.
And in rivers, streams and lakes, phosphorous has the same effect that nitrogen has on land. It's why phosphorous has to be removed from sewage, because the phosphorous in it was causing massive changes in lake quality, causing these scummy layers of blue algae to form in the lakes, and making them no longer of use for fishing or recreation.
Well, the same basic biological changes are happening to the earth ecosystems, terrestrial ecosystems, because of nitrogen. We are introducing exotic species. The world has major land masses that are separated from each other, and most organisms on their own couldn't get from North America to South America, much less from Europe to North America, and Australia, and people are moving organisms around willy-nilly. We are homogenizing the globe, and that has big effects on what's going on.
We are releasing pesticides which are moved around the world, and many people don't realize this but pesticides evaporate in the air like water does, and water, when it gets cold, comes out of the air as either rain or snow, and pesticides come out of the air when it's cold too-it takes negative 10, 20, 30 degrees, but it comes out as pesticide snow.
And you can go to pristine lakes 50 or a hundred miles away from the nearest pesticides, up at a high elevation on a mountain, but it's cold at high elevations of the mountains, and you can find deformities in the fish and pesticides in the fish. Pesticides that were used to try to stop malaria in some tropical habitat can end up in lakes in the Yukon Territory. There are these global cycles. There are many, many ways that humans are affecting the globe other than just climate change, and all of these are areas of I would assert significant concern, and that are at least as important as global climate change and they're probably going to interact with each other in ways that may not be very pleasant, and very few of these are very well understood at this point in time.
Has modern agriculture moved away from monocultures? If you look at all the lands of the earth and ask who manages those lands, the single largest group of land managers for the world is the farmers, the agriculturalists.
We estimate that by the Year about 50 years from now-that agriculturalists will be managing about half of the usable lands of the world. We're not talking about tundra, we're not talking about desert, we're not talking about mountains, but the land that actually in some ways you consider useable to humans--agriculturalists will be managing about half of that.
How that land is managed is incredibly important for the quality of life on the rest of the earth, because these lands produce lots of services and value to society. And one service, if you will, is production of food. That's a very important service. We need food, and agriculture has done wonderful things in the last 40 years at increasing global food production.
Understanding diversity–stability relationships: towards a unified model of portfolio effects
There was significant concern in the s that we might have massive starvation because food production around the world was not increasing as quickly as population. The green revolution had a dramatic and highly positive effect on that, and it has provided food that has prevented starvation, it has allowed people to be able to live and grow and develop their full physical mental capabilities and to contribute to society in meaningful ways.
It's greatly increased the quality of life. But there have been some side effects of that that were not appreciated, that are building up, and that we need to deal with in the long term if we're going to avoid some major problems that could be caused by agriculture.
To produce all of the food that we are producing right now, we now add as much nitrogen in fertilizers as was produced by the system in all various forms on its own naturally, so we sort of doubled the nitrogen cycle of the world. A lot of this nitrogen leaks out of farmlands and into groundwater, where it decreases its quality, and into the lakes, rivers, streams, and oceans and so on, and so it moves through the air and hits other land services.
We use pesticides, we use phosphorous, we use irrigation. And when we do that, we get a crop, a yield of food, but we no longer have these lands, which used to produce clean drinking water, for instance, pristine drinking water. We often farm soils in ways that we use up the nitrogen content and the carbon content of the soil, so we're sort of mining some soils as opposed to farming them sustainably.
And I think that as I look at the world and what the big issues are, one big issue is energy use. The other big issue I would assert is farming.
We must try to find ways-and we don't have quick easy answers, this is not an easy question-ways to farm that are going to maximize the net benefits that humans receive from that farming-the food and ecosystem services, the quality of the air, the quality of the water-that maximizes that total return to society.
Well, how can that be done? Can biodiversity contribute to that? And the answer is no and yes, depending upon what kind of agriculture you're looking at. In one sense, there are certain crops like corn, wheat, and rice that we know no other way to grow them efficiently except to grow them by themselves.
Now in those crops, we use genetic diversity within those crops to help us fight pests that attack the plants, whether it's a disease or insects. We use genetic diversity within the crops to help us find a variety of wheat or corn or rice that gives us the best yield on this kind of soil and in this kind of climate, and people in agriculture are working very hard to make sure we don't lose the natural genetic diversity of these crops, and are making sure they can find the diversity and use it to maximize the yields that we do get.
So we need to use the diversity of those crops but it's not as if we can plant corn, wheat, and rice and five other grains in the same field and get a reasonable yield. They don't necessarily grow well together. They're hard to harvest, and so that idea of using biological diversity for high production agriculture is unlikely to make it as far as planting many things together.
But if I talk about the land or the earth, of all the land dedicated to agriculture, about one-quarter is dedicated to growing crops intensively, a single corn, wheat or rice crop in an area. The other three-quarters of the land is dedicated to a more pastoral approach, where you have pastures and you're grazing animals on those pastures, and it is in those conditions where biological diversity can be a very important tool to use.
Parameter a is an examined in population dynamics by Anderson et al. The scaling power z of the variance lies between 1 and 2. It is also worthwhile stressing that Eq. Metric for the Diversity—Stability the one originally used by Doak et al.
Taking into account Eq. In that case, taking into community relative stability defined by Eq. Figure 1 shows the tionship of the variance. This criterion appears to be variation of b as a function of N for different values of z more appropriate to evaluate the performance of a and a given value of a: For z52; b is greater than 1 and multi-specific community than the average value of all tends to increase the community relative stability when the single-species communities Vandermeer and passing from RS to RS0: According to most empirical Schultz, ; Garnier et al.
Consequently, this result illus- previously proposed by Doak et al. This new generally outweighs that of monocultures when the equation is more general than the equation proposed by Doak et al. Variation of the coefficient b relating RSc;N community creases as diversity increases. In the following section we relative stability with respect to the most productive species to RS0c;N will only examine numerical results inferred from the community relative stability with respect to the species having a mean equation giving community relative stability Eq.
Diversity—Stability Relationships comparison is based on the mean of all possible community relative stability RSc;N: Consequently, when monocultures rather than on the most productive or species biomass follows any random distribution for most stable one Garnier et al. One can note that these results are in Eqs. This implies that species z52 the most productive species is also the most stable.
The b coefficient productivity with respect to the previous analysis made making no decisive impact on the variation of relative by TilmanFig. The differ- introduction of a strong heterogeneity in productivity. This stability RSc;N is equal to unity whatever species simple ranking often allows one to predict dominance in diversity N: However, as shown in Fig. The Wardle et al. As could be anticipated, a negative value of r results of several experiments where primary production tends to considerably enhance community relative exhibits a positive relationship with plant species and stability, but at the same time it generates a mathema- functional-group diversity, in contradiction with pat- tical limitation in the number of species simply because terns often observed in nature Loreau et al.
Our all the correlation coefficients are assumed to be theoretical examination, however, suggests that their identical. This limitation does not appear when different global effect, as indexed by a; systematically offsets the values of the coefficient of correlation between indivi- portfolio effect. This seems a paradoxical result when dual species are allowed e.
This apparent contradiction stems from our overall stability. Conversely, positive values of r lower the community relative stability, and this effect can be 3. Effect of Interaction and Correlation strong enough to reverse the expected direction of between Species variation of stability with diversity. This effect is illustrated in Fig. Nevertheless, its experimental equal. This can be useful since Lehman and Tilman demonstration remains questionable and fiercely de-using different models of multi-species competi- bated e.
However, Negative covariance can be generated by interspecific making r a function of N would lead us beyond the competition whereas positive covariance generally re- scope of the present analysis. Diversity—Stability Relationships 4.
Biodiversity and ecosystem stability: a synthesis of underlying mechanisms.
The new variation of productivity among species equation proposed Eq. This extended equation partially r correlation coefficient between species contradicts the results previously obtained by Doak et al. RSc;N community relative stability and Tilman.