Tuesday, July 27, 2010

Enhanced biodiversity-ecosystem function relationships in polluted systems

*note: this text was adapted from an Editor's Choice I wrote for the Journal of Applied Ecology.

ResearchBlogging.orgIn this era of species loss and habitat degradation, understanding the link between biodiversity and functioning of species assemblages is a critically important area of research. Two decades of research has shown that communities with more species or functional types results in higher levels of ecosystem functioning, such as nutrient processing rates, carbon sequestration and productivity, among others. This research has typically used controlled experiments that standardize environmental influences and manipulate species diversity. However, a number of people have hypothesized that biodiversity may be even more important for the maintenance of ecosystem functioning during times of environmental stress or change rather than under stable, controlled conditions. It is during these times of environmental change that preserving ecological function is most important, as changes in function can have cascading effects on other trophic levels, compounding environmental stress. Therefore, explicitly testing how biodiversity affects function under environmental stress can help to inform management decisions.

Image from Wikimedia commons

In a recent paper in the Journal of Applied Ecology, Li and colleagues examine how algal biodiversity influences productivity in microcosms with differing cadmium concentrations. Cadmium (Cd) is a heavy metal used in a number of products and industrial processes, but it is toxic and Cd pollution is a concern for human populations and biological systems, especially aquatic communities. This is especially true in nations currently undergoing massive industrial expansion. In response to concerns about Cd pollution effects on aquatic productivity, Li et al. used algal assemblages from single species monocultures to eight species polycultures grown under a Cd-free control and two concentrations of Cd, and measured algal biomass.

Their results revealed that there was only a weak biodiversity-biomass relationship in the Cd-free teatment, which the authors ascribed to negative interactions offsetting positive niche partitioning. In particular, those species that were most productive in their monocultures were the most suppressed in polycultures. However, in microcosms with Cd present there were positive relationships between diversity and biomass. They attribute this to a reduction in the strength of competitive interactions and the opportunity for highly productive species to persist in the communities.

While a plethora of experiments generally find increased ecosystem function with greater diversity, Li et al.’s research indicates that the effect of biodiversity on function may be even more important in polluted systems. If this result can be duplicated in other systems, then this gives added pressure for management strategies to maintain maximal diversity as insurance against an uncertain future.

Li, J., Duan, H., Li, S., Kuang, J., Zeng, Y., & Shu, W. (2010). Cadmium pollution triggers a positive biodiversity-productivity relationship: evidence from a laboratory microcosm experiment Journal of Applied Ecology, 47 (4), 890-898 DOI: 10.1111/j.1365-2664.2010.01818.x

Thursday, July 22, 2010

Reinterpreting phylogenetic patterns in communities

Examining the phylogenetic structure of a community in order to understand patterns of community assembly has become an increasingly popular approach. A quick web search of “community”, “phylogenetics”, and “ecology” finds several hundred papers, most written in the last ten years.

Eco-phylogeneticists examine how patterns of evolutionary relatedness within communities may reflect the processes structuring those communities. In particular, a commonly tested hypothesis is the competition-relatedness hypothesis, which suggests that more closely-related species having more similar niches and therefore stronger competitive interactions, making coexistence between them less likely. As a result, if competition is important, communities may exhibit phylogenetic overdispersion, with species being less related on average than if drawn randomly from the regional species pool. The contrasting pattern, phylogenetic clustering, where species tend to be more closely related than expected, is often interpreted as being the result of strong environmental filtering, such that only a closely related group of species, best adapted to that environment, surviving in the community.

Evidence for the competition-relatedness hypothesis has been mixed, and since most tests of this hypothesis focus on patterns in observed data, conclusions about the underlying mechanism driving community phylogenetic patterns are rarely testable, and yet widely made.

In Mayfield and Levine (2010, Ecology Letters), the authors critique the current ecological justification for the competition-relatedness hypothesis, noting that it does not agree with a more current view of the processes driving species coexistence. As established by Chesson (2000, Annual Review of Ecology and Systematics), coexistence can involve both stabilizing forces (niche differences between species), and equalizing forces (fitness differences between species). In a simplistic example, plants using different soil types (niche differences) may coexist, while plants with similar high growth rates may exclude those species with lower growth rates (fitness differences). The final community should reflect the interplay of both these processes.

The implication of this view of species coexistence is that there is no preconceived phylogenetic pattern which should reflect competition: if species with the highest heights are competitively superior and exclude other species (coexistence driven by fitness differences), and height is a phylogenetically conserved trait, the community will appear to be phylogenetically clustered. Traditionally, a clustered pattern would not be considered to indicate the effects of competition. In fact, Mayfield and Levine show that the expected phylogenetic pattern depends entirely on whether niche and/or fitness differences are important and/or related to phylogenetic distance.

This suggest that conclusions in past studies may need to be reinterpreted. It also adds to the list of assumptions about evolutionary relatedness and ecological function which need to be tested: for example, how do niche and fitness differences tend to change through time? Do they tend to be conserved among closely related species? Does one or the other tend to dominate as a driver of coexistence in different systems? If nothing else, we need to be careful about making generalizations which don’t account for the differing evolutionary history, geographical location, and ecological setting that communities experience, when interpreting observed patterns in those communities.

Wednesday, July 7, 2010

Organic farming and natural enemy evenness

ResearchBlogging.orgThe basic reality of agricultural activity is that it reduces biological diversity, and these reductions in diversity potentially impact ecosystem services. But do some agricultural practices impact these services less than others? In a recent paper in Nature by David Crowder and colleagues, the question of how organic versus conventional farming affects predator and herbivore pathogen diversity and how this cascades to pest suppression. They show through a meta-analysis, that organic farms tend to support greater natural enemy evenness, and they hypothesize that greater evenness of enemies should better control pest populations, resulting in larger, more productive plants.

Picture from wikipedia

This result in itself is interesting, but they also carried out an elegant enclosure experiment where they manipulate the evenness of insect predators and pathogens and measure potato plant size. They found that even communities had the lowest herbivore densities and saw the greatest increases in plant biomass. Conversely, very uneven communities, typical of conventional farms, had the largest pest populations resulting in lower plant biomass accumulation.

While, multiple farming strategies are needed for adequate agricultural production, there are strong arguments for organic farms to be a important part of agricultural practice. These results show that organic farms have cascading effects on pest predators and pathogens and show that enemy evenness, as opposed to richness, has important ecosystem service consequences. To quote myself, evenness is a critical component of biodiversity, and much research has emphasized species richness, maybe at the detriment of studying evenness.

Crowder, D., Northfield, T., Strand, M., & Snyder, W. (2010). Organic agriculture promotes evenness and natural pest control Nature, 466 (7302), 109-112 DOI: 10.1038/nature09183