Plants most often live side-by-side with other plants. Neighbours compete locally with one another for light, water and soil nutrients. An individual plant may find itself alongside members of other species, “strangers” of its own species, or its own close kin (parents, offspring or siblings). There are two main schools of thought on how related neighbours should respond to one another: these are kin selection and niche partitioning. A team at McMaster University in Canada have reviewed empirical studies testing these hypotheses.
Kin selection predicts that relatives should cooperate with one another e.g. by competing less over limited resources. This is because genes encoding traits or behaviours that cause an individual to favour its close kin at some expense to its own fitness can still spread, because the genes are likely to be shared between those related individuals. The converse view, niche partitioning, is that neighbouring relatives will compete with one another even more fiercely compared with neighbouring strangers, because there is more overlap in how they access and use limiting resources. This is also descriptively referred to as the “elbow room” hypothesis. An example in plants is rooting depth, where non-related individuals would be more likely to tap resources at different levels in the soil.
Straight away, we can see that these two theories predict opposite outcomes when siblings grow side-by-side. Kin selection would predict that groups of altruistic siblings should grow better than groups of strangers, whereas niche partitioning says that groups of strangers, who exploit local resources in slightly different ways, should perform better. Intriguingly, different studies of kin competition have come up with varied results. Of the experiments reported on in the review, most found no difference in performance of related/unrelated stands of plants, nine found that groups of siblings outperform groups of strangers and 11 found the opposite situation.
The review authors point out that plant responses to kin depend on which resource(s) are limiting in the environment of a given species. Indeed, some sand dune plants growing in water- and nutrient-limited environments allocate more resources to their fine root systems when growing with strangers, compared with when they are alongside siblings. Another species (Impatiens pallida) growing in the forest understorey (where light is a limiting resource) boosts allocation of resources to shoots when growing among strangers, compared to when it is surrounded by siblings. There is also the important factor of whether plants can detect nearby kin. Several studies have proven root-to-root interactions between plants, including recognition of strangers/kin within a species. Even if kin are not directly detected, if it is likely that an individual plant will find itself in close proximity to siblings (e.g. if its seed dispersal mechanism ensures this, or if the environment is densely populated), the evolution of indiscriminate altruism to neighbours could be favoured.
Overall the authors dissuade experimenters from expecting the same response to kin proximity in all plant species, and even within a species under different environmental conditions. They also criticise the premise of many studies, which searched for either kin competition or kin cooperation, and argue that the reality of plant interactions is more complex. The full article is available at: http://rspb.royalsocietypublishing.org/content/279/1727/209.short
Image: A competitive environment: a dense stand of Himalayan Balsam (Impatiens glandulifera). Photograph by Cathy Shields
File A.L., Murphy G.P. & Dudley S.A. (2011). Fitness consequences of plants growing with siblings: reconciling kin selection, niche partitioning and competitive ability, Proceedings of the Royal Society B: Biological Sciences, 279 (1727) 209-218. DOI: 10.1098/rspb.2011.1995