Biodiversity Good, Extinction Bad, Climate Change Worse

This phylogenetic tree of life was created by David Hillis, Derreck Zwickil and Robin Gutell. It depicts the evolutionary relationships of about 3,000 species throughout the Tree of Life. Less than 1 percent of all the known species. Download the pdf from the Hillis Lab.

I hope we can all agree that: biodiversity = good, extinction = bad. This incredibly simplistic statement could be taken a number of ways, but, as we are doing with so many things lately, let's look at it through the lens of global climate change. How important is maintaining biodiversity? How bad is extinction? And how do these factors affect the function of ecosystems?

It has been established that the current rate of species extinction has far outpaced those rates we see in the fossil record. By "far outpaced," we're talking about a sixth mass extinction within 240 years (that's the projection as of now at least). There have been hundreds of experiments that have tackled this question of biodiversity and ecosystem processes, particularly in plant systems. Take a big statistical spoon and mix all the experiments together and you find that the loss of plant biodiversity affects biomass production and decomposition. Experiments to manipulate biodiversity in controlled environments have actually found that biodiversity can act as an independent variable that directly controls such ecosystem functions as nutrient cycling and biomass production. Studies have shown that greater biodiversity also increases these effects over time, likely by either a saturating response curve (large increases in ecosystem functioning as species are added to communities, leveling off after a while, with any additional species only increasing ecosystem functioning by small amounts) or a linear response curve (think: straight[er] line). These temporal aspects of diversity-productivity relationships are still somewhat obscure, particularly the mechanisms of these changes over time. Now add global climate change. It is uncertain the sizes these effects will be and how the direct effects of other types of environmental change (like atmospheric composition, nutrient pollution, etc.) will affect ecosystem functioning.

So far the month of May has yielded some big, interesting papers on the impacts of biodiversity loss. A paper by Peter Reich et al. in Science takes a look at the time component of biodiversity loss and how it affects the growth curves I mentioned above. In this paper, the authors present data from two long-running (≥13 years) grassland biodiversity experiments at the National Science Foundation's (NSF) Cedar Creek Long-Term Ecological Research (LTER) site in Minnesota, USA: the “Cedar Creek Biodiversity Experiment” (BioDIV), planted in 1994–1995, and the “Biodiversity, CO2, and N Experiment” (BioCON), planted in 1997. At these sites a number of plots are planted with different numbers of species of plants including various C3 and C4 plants and nitrogen (N)-fixing and non-fixing dicotyledonous herbs. The authors looked at the effects of diversity on biomass productivity and found that productivity (aboveground and belowground) increased and became less saturating over time; the diversity-productivity relationship became more linear and less strongly decelerating over time. Their evidence suggests that this may be due to the accumulating effects of complementary resource acquisitions and use and such ecosystem feedback effects as soil N cycling. Basically, the plants are complementing each other, increasing the functional diversity of the system. The greater the diversity of plants the more natural components (carbon, water, etc.) of the system can be capitalized on over time, a result that short-term experiments may underestimate.

The Cedar Creek LTER  site (Credit: David Tilman, UMN)

These ideas were discussed in a perspective paper by Bradley Cardinale, published in the same issue of Science. Here he points out that if Reich et al.'s conclusions prove to be general then they will have quantified how ecological impacts of extinction scale through time. Certainly not an insignificant conclusion. The Reich et al. paper doesn't spend a whole lot of time delving into niche theory (relational position of a species or population in an ecosystem, the where and how an organism makes its living), to the point that the word "niche" isn't even in their paper. But it is essentially what they are talking about, or at least hinting at.When species are accessing different resources then they are filling different niches, and the more diverse the species the more niches they can exploit. Cardinale knows quite a bit on this topic as he himself published a very nice study last year where he used a model system of stream biofilms, experimentally adding extra niche opportunities, to test the effects of algal biodiversity on water quality, showing that the more species in a stream the more ecosystem functions increased. Cardinale's examination of the Reich et al. paper points out some interesting points about the consequences these curves may have for conservation, specifically making the point that if the conclusions of the study hold true then biodiversity loss has probably already begun to degrade essential ecosystem processes.

Figure from Cardinale (2012)

Another paper published this month in Nature also takes a look at biodiversity loss as a driver of ecosystem change. In their study, David Hooper et al. use a series of meta-analyses of published data to look at the magnitude of the effects of species loss on productivity and decomposition. They focused on these two processes because they are major biological processes influencing carbon storage and other ecosystem services. Their analysis statistically summarized existing data, compared the environmental effect sizes to the estimated effects of species loss derived from a database of 192 peer-reviewed publications, summarized the results of 16 experiments that simultaneously manipulated plant species richness and some other environmental change variables (elevated CO₂, nutrient pollution, etc.), and assessed a large range of projections of species loss. This analysis showed that the biodiversity loss in the 21st century could rank as one of the major drivers of ecosystem change. In areas where local species loss is low (1-20%) there will be negligible effects. In areas of intermediate loss (21-40%), species loss is expected to decrease biomass production by 5-10 percent. In areas of high species loss (41-60%), the effects would rank alongside other major drivers such as warming, ozone, and acidification. They estimate that a 50% species loss will reduce biomass production by 13%. These reductions in biomass and decomposition don't sound like a lot, but, at least for decomposition, they are equal or greater than the effects of CO₂ or nitrogen. Hooper et al. also found that species loss would need to exceed that of prior mass extinctions (≥75% loss) to rival those environmental changes that have the greatest effect on primary production. Not a senerio that we will probably see globally, but locally or within certain taxa it could be realized if current extinction rates continue. Additionally, the types of species that are lost also have a huge effect. A good example of this was shown nicely in the Reich et al. paper above. When you add in other environmental changes to the meta-analysis it reinforces these conclusions.

I hope I've made my point that biodiversity = good, extinction = bad, and climate change = worse. I also hope I (and these authors) made the case that the loss of biodiversity isn't just a consequence but rather a major driver in key processes that affect our planet. Think about it.

You can read more in the articles:

Reich, P., Tilman, D., Isbell, F., Mueller, K., Hobbie, S., Flynn, D., & Eisenhauer, N. (2012). Impacts of Biodiversity Loss Escalate Through Time as Redundancy Fades Science, 336 (6081), 589-592 DOI: 10.1126/science.1217909

Cardinale, B. (2012). Impacts of Biodiversity Loss Science, 336 (6081), 552-553 DOI: 10.1126/science.1222102

Hooper, D., Adair, E., Cardinale, B., Byrnes, J., Hungate, B., Matulich, K., Gonzalez, A., Duffy, J., Gamfeldt, L., & O’Connor, M. (2012). A global synthesis reveals biodiversity loss as a major driver of ecosystem change Nature DOI: 10.1038/nature11118

Cardinale, B. (2011). Biodiversity improves water quality through niche partitioning Nature, 472 (7341), 86-89 DOI: 10.1038/nature09904

And here are some additional write-ups:
NSF story "Ecosystem Effects of Biodiversity Loss Rival Climate Change and Pollution"
NSF story "Plant Diversity Is Key to Maintaining Productive Vegetation" also at Science Daily

And some related websites you may want to visit:
Cedar Creak LTER site
LTER Network