Friday, April 12, 2013
I usually start a post with the reason why a particular article caught my eye. Today, I’m not sure why my eyeballs glommed on to this paper, but they did. As it turns out, they have a pretty good taste in articles. Who knew?
An early view paper from the Journal of Ecology looks at how the large herbivores of the African savanna affect the diversity and community structure of plants. More and more studies these days are taking a phylogenetic approach to community ecology questions. That is, they are looking at the evolutionary development and history of a species or taxonomic group to explain the patterns that we currently see. This new study takes this approach in the subtropical woodland biome, or “bushveld,” of the Kruger National Park (KNP) in northeast South Africa. This area is home to 148 mammal species, of which 30 are large herbivores such as elephants, rhinos, and giraffes. The vegetation of this area consists of 1974 species, including 458 species of trees and shrubs. KNP varies from dense thicket, savanna woodlands to forests characterized by tall trees and a closed canopy. These plant communities are under occasional pressure from periodic fire and under constant pressure from large herbivores. This study looks at the impacts of these herbivores on the phylogenetic structure of woody plant assemblages and evaluates the impacts of their removal on plant community composition and structure.
The researchers used DNA sequencing to reconstruct the phylogeny of 448 species of trees and shrubs (using Amborella as an outgroup), representing 246 genera, 71 families and 31 orders. ... Let’s pause for a second to say “Wow!” ... Then they calculated divergence times and used speciation models on these data. Next was the community sampling along a north-south transect through KNP. This transect has enclosures situated along it where large herbivores are partly or fully excluded. These enclosures have been established in the park for between 8 and 43 years. Within 15 defined “ecozones,” the researchers surveyed 110 50x50 meter unrestricted herbivory plots and 15 50x50 meter plots in each of the five herbivore exclusion enclosures. Within these plots they recorded all species of trees and shrubs and the number of individuals per species (abundance). Then they evaluated various physical and mechanical plant defense traits (not including chemical defenses), using wood density of quantify plant resistance to physical damage and specific leaf area (SLA) as a proxy for leaf nutrient content.
All sorts of indices (MPD, MNTD, SR, Shannon, NRI, NTI) and statistics were used that I’m not going to go into because they will just confuse everyone. But when all the statistics were done, they found a latitudinal gradient in diversity with the highest diversity in the south and extreme north and low diversity in the center (which matches rainfall patterns). In parallel, they found shifts in community phylogenetic structure comparable to these changes in community diversity, indicating that the communities in the center are more highly phylogenetically clustered (composed of more closely related species). Plant diversity in the KNP is strongly spatially structured and this clustering is reflective of generalist browsing, the geomorphology of the area, and the patchy distribution of large herbivores. In plots where herbivory is unrestricted they found significant phylogenetic clustering of plant communities, likely the result of the heavy pressure from the herbivores. They also found that plant defense traits had a weak but significant phylogenetic signal, suggesting that they do not fit well into the simple model they used. When the researchers compared the enclosures to the unrestricted areas they found “that when megaherbivores are excluded, species diversity generally decreases, but changes in phylogenetic diversity [vary] by spatial location.” When large herbivores are excluded, plant community structure is contingent upon the initial community structure. If a community was initially overdispersed then the shift towards a more clustered community would occur by excluding large herbivores and vice versa. Ultimately, they conclude that the exclusion of large herbivores results in impoverished species communities.
I think that it is important to mention that these researchers were limited by the study design they had to work with. These enclosures had already been set up by other people. As such, the scientists encountered some issues that they would have been able to control for if they had set up the site themselves. Issues like enclosure age and location. For example, one enclosure was located on a river that periodically floods, altering local plant diversity. Such issues make it difficult for them to predict how specialist vs. generalist herbivores impact community structure and prevents them from drawing strong conclusions about the underlying causes for the patterns they saw.
On the upside, their results add valuable knowledge critical for predicting the impacts of overall herbivore decline on African ecosystems and local increases of these animals in protected reserves. This study “shows not only that large herbivores are key to maintaining woody plant diversity, but also that they may impose specific phylogenetic structure on plant communities.” Shifts in this structure have downstream consequences in that phylogenetic diversity can capture genetic and functional diversity, which has been linked to ecosystem productivity. And, ultimately, we want to keep our ecosystems happy and productive, right?
Kowiyou Yessoufou, T. Jonathan Davies, Olivier Maurin, Maria Kuzmina, Hanno Schaefer, Michelle van der Bank, & Vincent Savolainen (2013). Large herbivores favour species diversity but have mixed impacts on phylogenetic community structure in an African savanna ecosystem Journal of Ecology : 10.1111/1365-2745.12059
(image via Animals Time)