For many years, a major challenge in plant ecology has been the identification of plant traits that either enhance the benefits of mutualists or reduce the impact of antagonists such as herbivores and pathogens. Much work in this area has focussed on plant traits that reduce herbivory, such as the production of defence metabolites, and this interest has led to the emergence of plant defence theory. This theory predicts that levels of these defences are determined by a balance of costs and benefits: costs include direct production costs, increased vulnerability to enemies not defended against, and reduced attractiveness to mutualists, whereas benefits depend on the fitness value of the plant organ that would otherwise have been attacked and the change in the probability of attack.
A complication of defence theory is that many plant metabolites involved in herbivore defence are also involved in other biotic interactions, such as competition. Hence, theories of optimal defence investment may be incomplete if they do not consider other ecological roles of defensive traits.
With this in mind, Richard Lankau (left) and Daniel Kliebenstein of the University of California, Davis, USA, set up an elegant field experiment to examine how the production of sinigrin, the dominant glucosinolate defence compound in Brassica nigra (below), responded to the presence and absence of a herbivore (Deroceras reticulates, the grey garden slug), and competition, in plant genotypes artificially selected for high or low constitutive sinigrin levels. They then quantified the fitness costs and benefits of sinigrin production in these treatments.
Sinigrin induction response to herbivory was independent of a genotype’s constitutive sinigrin level. More surprising was the discovery that over two consecutive years, sinigrin concentrations increased in response to the presence of a competing plant, irrespective of the species of competitor. Moreover, the sinigrin induction response to the presence of a competitor differed between genotypes: those selected for low constitutive sinigrin levels were significantly more inducible in response to plant competitors than were genotypes with high constitutive levels.
The mechanism for the sinigrin induction response to competition is unclear and various suggestions are proposed to explain it. For example, it is suggested that it might be an adaptation to invest in costly allelochemicals when plants are stressed by competitors. Alternatively, the authors suggest that it might be an indirect response to competition, mediated by reduced light or nutrient availability in plants grown with competitors. Whatever the mechanism, these findings suggest that the fitness costs and benefits of sinigrin production agree with optimal defence theory in the absence, but not in the presence, of competition because of the differential induction effect caused by competitors. As argued by Lankau and Kliebenstein, this finding has broad implications because it suggests that predictions for optimal investment to defensive traits may be misleading if they do not incorporate all of the costs and benefits of traits involved in multiple interactions. As always, this issue of Journal of Ecology contains a host of other interesting and ground-breaking studies in aspects of plant ecology. We hope you enjoy reading it.
Editor, Journal of Ecology
Lankau, R.A. & Kliebenstein, D. J. (2009) Competition, herbivory, and genetics interact to determine the accumulation and fitness consequences of a defence metabolite. Journal of Ecology, 97, 78-88
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