Unifying invader success and impact

Something that has continuously bothered me about our collective narrative concerning invasions has been the conflicting processes determining invader success and impact. Numerous studies (including some of my own) show that invaders are successful often because they are different from residents. That is, they are thought to occupy some unique niche. However, occupying a unique niche means that competition is minimized and these successful invaders should have relatively low impact on residents. Conversely, species that have large impacts are thought to be superior competitors, but why are they able to be so successful?

In a new paper in the Journal of Ecology, Andrew MacDougall, Benjamin Gilbert and Jonathan Levin use Peter Chesson's framework where ability for two species to coexistence (or conversely the strength of competitive exclusion) is a process relative to two factors -the magnitude of fitness differences and the degree of resource use overlap. Here competitive exclusion is rapid if species have a large fitness difference and high resource overlap, and slow if fitness differences are low. Species that are successful because of reduced resource overlap likely have little impact unless there are large fitness inequalities.

If we then view the invasions process on a continuum (see figure), then by determining basic fitness and resource use, we can predict success and impact. This is an exciting development and I hope it inspires a new generation of experiments.

MacDougall, A., Gilbert, B., & Levine, J. (2009). Plant invasions and the niche Journal of Ecology, 97 (4), 609-615 DOI: 10.1111/j.1365-2745.2009.01514.x

The sushi of tomorrow… Jellyfish rolls?

With the world’s fisheries teetering on the edge of collapse, familiar items at your local sushi bar might disappear in the near future. One candidate for replacing the Hamachi, Ikura, Maguru, Tai, and Toro on the menu is the jellyfish, which seems to be doing well – too well, actually – in today’s environment.

In recent years, jellyfish outbreaks have become more frequent and more severe. These outbreaks can have lasting ecological and economic consequences. They can wreak havoc on the tourist industry by closing beaches and harming swimmers, cause power outages by blocking cooling intakes at coastal power plants, reduce commercial fish abundance via competition and predation, spread fish parasites, burst fishing nets, and contaminate catches.

A review by Anthony Richardson and his collaborators suggests that human activities such as overfishing, eutrophication, climate change, translocation, and habitat modification have dramatically increased jellyfish numbers. Their research, which was published this week in Trends in Ecology and Evolution, highlights that the structure of pelagic ecosystems can abruptly transition from one that is dominated by fish to one that is dominated by jellyfish.

Richardson and his collaborators present a potential mechanism to explain how local jellyfish aggregations can spread, displace fish, and form an alternative stable state to fish-dominated ecosystems. Jellyfish are like the opportunistic weed of the sea, giving them an edge in environments stressed by climate change, eutrophication, and overfishing. In these disturbed environments, the abundance of jellyfish relative to filter-feeding fish increases until a tipping point is reached. Under normal conditions, filter-feeding fish keep jellyfish populations in check via competition for planktonic food and (perhaps) predation on an early life-stage of the jellyfish. At the tipping point, jellyfish numbers are such that they begin to overwhelm any control of their vulnerable life-cycle stages by fish predators. At the same time, jellyfish progressively eliminate competitors and predators via their predation on fish eggs and larvae. As jellyfish abundance increases, sexual reproduction becomes more efficient, allowing them to infest new habitats where fish might have formally controlled jellyfish numbers.

Richardson and his collaborators suggest that one way to hit the brakes on what they call the “the never-ending jellyfish joyride” is to harvest more jellyfish for human consumption. Jellyfish have been eaten for more than 1000 years in China, where they are often added to salads. In Japan they are served as sushi and in Thailand they are turned into a crunchy noodle concoction. Although the taste and texture of jellyfish might not be appealing to some westerners, I for one have yet to meet a sushi that I didn’t like. Of course, jellyfish harvesting is unlikely to return systems to their fish-dominated state if the stresses that caused the ecosystem shift remain.

Richardson, A. J., A. Bakun, G. C. Hays, and M. J. Gibbons. 2009. The jellyfish joyride: Causes, consequences and management responses to a more gelatinous future. Trends in Ecology and Evolution, 24 (6), 312-322 DOI: 10.1016/j.tree.2009.01.010

A mechanism on why communities of exotic species are less diverse than communities of native species

Plant communities dominated by exotics tend to be less diverse than plant communities dominated by natives. Apparently, few people have been curious enough to plan an experiment to try to further understand why this is the case. A recent paper in ecology letters Brian Wilsey and collaborators showed the results of an experiment designed to explore this. What they did is to create monocultures of a series of exotics and natives species, and mix cultures of exotics (a mix of 9 exotics, zero natives ) and mix cultures of natives (9 natives, zero exotics). They found that large exotics (plants with high aboveground biomass) tended to be even bigger when growing in mix cultures than in the monocultures, so big plants got bigger, which tend to reduce plant richness since it may displace other plants. On the other hand, for natives, small plants tended to get bigger, which is a mechanism for promoting biodiversity (communities may be more even). This research highlights the importance of understanding the mechanisms of plant coexistence and the fact that exotic species may behave very differently than native species.

Wilsey, B., Teaschner, T., Daneshgar, P., Isbell, F., & Polley, H. (2009). Biodiversity maintenance mechanisms differ between native and novel exotic-dominated communities Ecology Letters, 12 (5), 432-442 DOI: 10.1111/j.1461-0248.2009.01298.x

Researcher spotlight: Tadashi Fukami

Increasingly, ecological explanations for extant community patterns are relying on dynamics operating across multiple spatial and temporal scales, linking small and large scales, and the here and now with evolutionary history. The traditional boundaries of sub-disciplines are blurring. I think that few other young scientists straddle these boundaries as successfully as Tad Fukami, and new assistant professor in the Department of Biology at Stanford University. Tad uses a broad array of theoretical and experimental approaches to understand how ecological communities are put together. From laboratory microcosms to rat-infested islands, and from the computer to remote locations, he is able to pull together disparate pieces of information into a central narrative about the assembly of communities.

I asked him why the question of community assembly interested him so much, and he gives much credit to his advisors, Jim Drake (also my PhD advisor) and Dan Simberloff both in the Department of Ecology and Evolutionary Biology at the University of Tennessee. But more than this, he says that:

“you need to look into the historical background of species interactions to understand the apparently inexplicable variation in the way species interact and the way communities are structured by the interactions.”

and certain aspects of this research obviously excite him. He goes on to say:

“One particularly intriguing thing is the great effect that small chance events that cause variation in early immigration history can have on long-term community development.”

Most ecologists gain their expertise by coming to understand and appreciate the details and intricacy of particular organisms or ecosystems. But Tad is especially noted for his use of an amazingly broad assemblage of systems and methods. I asked him why he used so many different systems, and how he chose those to test his ideas. He said that his work has benefitted from many exciting collaborations and that he has:

“been very lucky to meet many great people who have expertise on specific organisms and systems that a person with diffuse interests like me doesn't have.”

But I think that there may be something deeper and more reassuring. That is, the fact that one could study a multitude of systems, testing the basic dynamics of community assembly, means that there are regularities in how communities are assembled. That you can study stochastic historical events in bacterial microcosms and inform your understanding of plant succession means that while we individually take on these, at times, daunting research projects, our collective understanding of ecological processes are threaded together in a great fabric. And no one is a microcosm of this more than Tad Fukami.

Key recent papers

Fukami, T., Beaumont, H. J. E., Zhang, X.-X. & Rainey, P. B. (2007) Immigration history controls diversification in experimental adaptive radiation. Nature 446: 436-439.

Fukami, T., Wardle, D. A., Bellingham, P. J., Mulder, C. P. H., Towns, D. R., Yeates, G. W., Bonner, K. I., Durrett, M. S., Grant-Hoffman, M. N. & Williamson, W. M. (2006) Above- and below-ground impacts of introduced predators in seabird-dominated island ecosystems. Ecology Letters 9: 1299-1307.

Fukami, T., Bezemer, T. M., Mortimer, S. R. & Van der Putten, W. H. (2005) Species divergence and trait convergence in experimental plant community assembly. Ecology Letters 8: 1283-1290.

Grazers chew, cereal gets sick

Managing plant disease is a major part modern agricultural practice, so it is important to understand the basic ecological dynamics of plant diseases. Some theoretical studies have found that the prevalence of plant diseases can be affected by the amount of herbivory in a system. Given that human land-use and the removal of top predators from many ecosystems has fundamentally changed the abundance and distribution of many herbivores, the repercussions of herbivory can have important cascading consequences throughout foodwebs –including disease dynamics. In the first experimental study of the interaction between herbivory and plant disease, the forthcoming paper in PNAS by Elizabeth Borer and colleagues, shows that increased exposure to large herbivores (e.g., mule deer) resulted in higher disease prevalence in the plant community. The disease they studied, barley and cereal yellow dwarf virus (shown in the photo), is transmitted by aphids, so herbivory does not cause increased transfer. Rather, herbivores changed community composition resulting in higher abundances of very susceptible species, creating a feedback where higher abundances resulted in higher infection rates due to a larger pool of potential hosts near by. These results are important for two reasons. First, this particular virus is an important agricultural disease. Secondly, we need to take a whole-community approach to understanding disease dynamics because these dynamics are not only a property of host-vector-pathogen interactions, but are subject to direct and indirect effects from interactions with other community members.

E. T. Borer, C. E. Mitchell, A. G. Power, E. W. Seabloom (2009). Consumers indirectly increase infection risk in grassland food webs Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0808778106

Whence diversity?

It is a truism to say that ecological communities are diverse. They often contain dozens or hundreds or thousands of species that represent many of the deep origins in the tree of life. A recent paper by Prinzing and colleagues published in Ecology Letters tested the hypothesis that communities of plants that include more of the ancient divergences from the evolutionary tree of plants should also contain a greater diversity of physical traits. They examined over 9000 plant communities and found that those that contain fewer evolutionary lineages actually had greater trait diversity than those randomly assembled from more lineages. This result reveals that when communities are assembled from a few lineages (likely due to strong environmental selection -e.g., drought tolerance) those members tended to have evolved large differences. That is, while species may be constrained to certain habitat types due to their evolutionary heritage, successful coexistence depends on maximizing differences.
Andreas Prinzing, Reineke Reiffers, Wim G. Braakhekke, Stephan M. Hennekens, Oliver Tackenberg, Wim A. Ozinga, Joop H. J. Schamine, Jan M. van Groenendael (2008). Less lineages more trait variation: phylogenetically clustered plant communities are functionally more diverse Ecology Letters, 11 (8), 809-819 DOI: 10.1111/j.1461-0248.2008.01189.x