Fitness consequences of plants growing with siblings: reconciling kin selection, niche partitioning and competitive ability

Plant studies that have investigated the fitness consequences of growing with siblings have found conflicting evidence that can support different theoretical frameworks. Depending on whether siblings or strangers have higher fitness in competition, kin selection, niche partitioning and competitive ability have been invoked. Here, we bring together these processes in a conceptual synthesis and argue that they can be co-occurring. We propose that these processes can be reconciled and argue for a trait-based approach of measuring natural selection instead of the fitness-based approach to the study of sibling competition. This review will improve the understanding of how plants interact socially under competitive situations, and provide a framework for future studies.     

Local host competition in the evolution of virulence

The evolution of parasite life histories should usually have correlated effects on host survivorship and/or reproductive success. For example, parasites that reproduce more rapidly might be expected to cause greater reductions in host fitness. Important theoretical advances have recently been made on virulence evolution, but the results are not always consistent. Here I compare two models [ Q. Rev. Biol. 71 (1996) 37 ; Q. Rev. Biol. 75 (2000) 261 ] on the evolution of virulence that get qualitatively different results with respect to the effects of coinfection. I also construct a third model that attempts to connect these two formulations. The results suggest that parasite growth rates should increase as local host competition increases, unless relatedness is at equilibrium. In addition, the qualitative effect of adding coinfections on parasite growth rates depends critically on how the number of coinfections affects transmission success.     

Evidence for competition and cooperation among climbing plants

A plant's best strategy for acquiring resources may often depend on the identity of neighbours. Here, I ask whether plants adjust their strategy to local relatedness: individuals may cooperate (reduce competitiveness) with kin but compete relatively intensely with non-kin. In a greenhouse experiment with Ipomoea hederacea, neighbouring siblings from the same inbred line were relatively uniform in height; groups of mixed lines, however, were increasingly variable as their mean height increased. The reproductive yield of mixed and sibling groups was similar overall, but when adjusted to a common mean height and height inequality, the yield of mixed groups was significantly less. Where this difference in yield was most pronounced (among groups that varied most in height), mixed groups tended to allocate more mass to roots than comparable sibling groups, and overall, mixed groups produced significantly fewer seeds per unit mass of roots. These results suggest that, from the group perspective, non-kin may have wasted resources in below-ground competition at the expense of reproduction; kin groups, on the other hand, displayed the relative efficiency that is expected of reduced competitiveness.     

Intraspecific competition favors ant–plant protective mutualism

The dependency of the anti-herbivore defense on ant–plant protective mutualism often varies depending on abiotic and biotic conditions. Although intraspecific competition is a primary interaction between neighboring plants, its effects on ant–plant mutualisms have yet to be sufficiently elucidated. In order to determine the effects of intraspecific competition and competitor genotype on ant–plant mutualisms, I conducted competition and ant-removal experiments and examined their effects on damage to the leaves of Urena lobata var. tomentosa plants. I found that larger numbers of worker ants visited the plants growing with non-siblings than plants growing alone and that plants growing with non-siblings had a higher shoot to root ratio and secreted greater volumes of extrafloral nectar than plants growing alone and/or with siblings. Under the presence of both sibling and non-sibling competitors, I observed that when ants were removed from plants, those grown with conspecific neighbors were characterized by a higher percentage of damaged leaf area than plants harboring ants. The effect of ant exclusion on leaf damage was more pronounced in plants grown with non-siblings than those grown near siblings. However, when the plants were grown alone, I detected no significant difference in percentage leaf damage between the ant-excluded and ant-harboring plants. The results indicate that neighboring plants can exert strong effects on ant–plant protective mutualisms, thereby highlighting the need to take into consideration plant–plant interactions in studies on these mutualistic associations.