Redundancy in response to global change: Ecosystem processes buffered by functional diversity
A guest post from PLOS Ecology Reporting Fellow, Daniel Winkler on the 2015 PLOS One paper “Functional Resilience against Climate-Driven Extinctions – Comparing the Functional Diversity of European and North American Tree Floras” by Mario Liebergesell, Björn Reu, Ulrike Stahl, Martin Freiberg, Erik Welk, Jens Kattge, J. Hans C. Cornelissen , Josep Peñuelas , Christian Wirth featured in the PLOS Ecological Impacts of Climate Change Collection
A number of PLOS ONE and PLOS Biology’s papers illustrate that we can learn a lot about the future of global change dynamics by looking to the past. A recent study in PLOS ONE by Mario Liebergesell and colleagues did just that and is featured in the PLOS Ecological Impacts of Climate Change Collection 2015–2016. Liebergesell, from the German Centre for Integrative Biodiversity Research (iDiv) at the University of Leipzig, and his coauthors find evidence that climate-driven species loss at continental scales can be independent of changes in functional diversity; that species loss need not imply changes or declines in ecosystem processes. Their results have potentially dramatic implications for global change scenarios examining changes in ecosystem functioning.
Debate regarding the relative importance of an individual species versus the ecological function it provides to an ecosystem has been somewhat controversial over the past couple of decades (see Grime 1997 for a commentary on the beginning of the debate). Studies like Tilman et al. 1997 and Hooper and Vitousek 1997 were among the first to show that the number of species is not as important, or, in some cases, not important at all, when it comes to ecosystem functioning (for example plant productivity and soil processes). These studies and many that have followed have logically reframed discussions around conservation issues, global change impacts, and even ecosystem benefits to humans. That being said, we still seem to adore and want to protect and conserve our favorite species because they have the prettiest flowers, or are the most charismatic species we associate with conservation and environmental ethics, or simply because they are threatened and/or nearing extinction. We also value a singular definition of biodiversity when there are multiple types of biodiversity that may be more relevant than others to conservation goals and sustainable practices. Why do we care so much about individual species when the threat of global change strengthens its imminent control on all Earth systems? Perhaps this debate should be reserved for another occasion.
Glaciation events induced extensive extinctions in the Northern hemisphere 21,000 years ago and provide substantial explanation for Europe’s modern tree flora. These extinction events correlate with phylogenetic selection and regional extinctions due to cold tolerance. This is further evidenced by widespread taxa being more tolerant of cold growing season and winter temperatures than extinct or relict taxa. Liebergesell et al. are the first to test the influence of glacial extinction on the overall functional diversity of the two continents. They compare the tree floras of contemporary Europe and North America and examine how richness varies between the two continents as a result of past glacial climates, and whether functional diversity similarly compares.
They start with a whole-plant perspective and use a comprehensive trait matrix consisting of 26 traits that are largely important for tree responses to environmental drivers. Many of the traits they used are also directly relevant to ecosystem functioning (e.g., life form, leaf carbon to nitrogen ratio) and affect biogeochemical cycling. Overall, they selected contemporary, climatically-similar regions in the temperate zones of North America and Europe and compared trait space occupied by 66 European and 154 North American tree species.
The authors then compared the functional diversity of the continents using climatically similar sub-regions. Regions were selected based on statistical analysis of 19 bioclimatic variables from worldclim.org. This analysis was done separately for gymnosperms and angiosperms. Gymnosperms are plants like coniferous trees (pines, hemlocks, spruce, etc.) or others like gingko, the common trait being that all gymnosperms have “naked seeds” and do not flower. Angiosperms on the other hand do make flowers and fruits and are the most diverse group of land plants.
Liebergesell et al. were unable to detect differences in functional dispersion of gymnosperm species between Europe and North America. They note that North American gymnosperms appear to disperse more than European gymnosperms and that this variation can be attributed to life strategy differences that include how tolerate the species are of environmental stressors, how they acquire resources, and how they compete with other individuals.
Angiosperms, on the other hand, exhibited significant differences between the two continents, sub-regions, and smaller scales. European tree assemblages with higher species richness (the number of species in an area) levels consistently exhibit greater functional diversity than North American comparisons. Again, Liebergesell et al. attribute these observed differences to functional differences of specific taxa.
Liebergesell et al. conclude that a larger species pool does not imply higher functional diversity and that this can vary by organism group (i.e., gymnosperms vs. angiosperms). However, they go on to dissect their findings in terms of functional identity, limitations imposed by available distribution maps and map quality, the difficulty in identifying climatically equivalent regions, and the tremendous topographic variation between the two sampled regions (i.e., Europe has much larger topographic heterogeneity than the eastern United States). Nonetheless, their findings are exciting not only because of the observed trend in functional diversity but also because this trend (at least for angiosperms) is persistent across spatial scales!
Calls for the conservation and protection of biodiversity have been made many times in the past decade. These calls have largely been a response to human induced global change and its current and forecasted impacts on ecosystem functioning. In Half-Earth, Biologist E. O. Wilson recently joined in with perhaps the most audacious, but much needed, call of them all:
“In order to stave off the mass extinction of species, including our own, we must move swiftly to preserve the biodiversity of our plant.” – E. O. Wilson
Wilson calls for setting aside half the planet as permanently protected and that we include not only the charismatic species we overly study (my words, not his) but also the invisible-to-the-naked-eye-species including the macroinvertebrates and microorganisms directly responsible for a tremendous amount of ecosystem functioning. Liebergesell et al. maintain that they are not arguing against biodiversity preservation. In fact, they repeat that species rich communities most likely host functionally similar species and that these communities can buffer against changes in composition under climate change scenarios. Their study nicely demonstrates that biodiversity loss in Europe due to past glacial events has not led to a lower functional diversity.
Grime 1997 exuded a strong sense of reason, caution, and imperativeness when he wrote,
“It could be argued that the tide is turning against the notion of high biodiversity as a controller of ecosystem function and insurance against ecological collapse. However, such a stance would be as premature as that of the commentators who rapidly embraced early evidence of its supposed benefits.… The most immediate problem is to identify irreplaceable species and functional types and to discover whether there are situations in which ecosystem viability depends on unusually high biodiversity.”
– J. P. Grime
Liebergesell et al. exercise an equally strong sense of reason and caution, adding to the PLOS Ecological Impacts of Climate Change Collection 2015–2016.
Daniel Winkler is a PhD candidate at the University of California, Irvine and plant ecophysiologist interested in invasive species, evolutionary ecology, and climate change impacts on native communities in “extreme” environments. His field sites include much of the desert southwest, alpine regions of Colorado, the subalpine forests of Baja California, and the tundra of northern Japan. All of Daniel’s research focuses on climate change impacts on native systems, with an emphasis on parks and protected areas. You can follow him on Twitter @DanielEWinkler, his research on Facebook at www.facebook.com/GeoMustard/, or find more information on his website at www.winklerde.com.