Who is Naïve, the invaders or the natives?

Why some species can invade natural ecosystems and many others cannot, is a question that doesn’t have an answer. Many ideas have been proposed to explain this, with relative success, but very low predictability. Most of the ideas have been focused on the factors that promote invasion (i.e. why successful invaders are successful). In a recent ideas paper Koen Verhoeven and collaborators propose a different approach. They ask the question, how ecological mismatches between natives and exotics can explain invasion? They propose a series of predictions based on plant defenses and plant enemies (herbivores, pathogens). They propose that the mismatches between exotic plants and their new enemies could explain their success or failure. For example, if a plant presents a new type of toxic chemical compound that the local herbivores have never encounter and cannot deal with, it would be a clear advantage for the plant (this is related to the novel weapons idea). On the other hand, if the plant has defenses that need to be trigger by a particular enemy (for example an induced defense triggered by a specific chewing insect) that could be a disadvantage for it. They propose that biotic resistance (when the native community resist the invasion) and enemy release (when an exotic invades due to experiencing less pressure by enemies than in its native range) are not oppose ideas but could be the different outcome of these mismatches.

This paper propose a very interesting approach to study some cases of successful and failed invasions, and I look forward for empirical tests of this idea.

Koen J. F. Verhoeven, Arjen Biere, Jeffrey A. Harvey, Wim H. van der Putten (2009). Plant invaders and their novel natural enemies: who is naïve? Ecology Letters, 12 (2), 107-117 DOI: 10.1111/j.1461-0248.2008.01248.x

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