Intraspecific competition favors ant–plant protective mutualism

Abstract

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.

Intraspecific interaction of host plants leads to concentrated distribution of a specialist herbivore through metabolic alterations in the leaves

“…Local population density of host plants likely affects host plant quality, because it is linked to the interaction environment: host plants present at high density are exposed to direct intraspecific interaction, whereas host plants present at low density are exposed to direct interspecific interaction or no interaction. Many studies have reported that plant–plant direct interactions influence leaf traits (Barton & Bowers, 2006; Mraja et al., 2011; Muiruri et al., 2019) and herbivory (Hambäck & Beckerman, 2003; Muiruri et al., 2019; Yamawo, 2021). For example, plant competition for resources induces plastic changes in the plants' resource allocation; these changes can affect root or shoot growth.…”

Intraspecific interaction of host plant influences local distribution of specialist herbivores through metabolic alterations in leaves

“…Another possible hypothesis is that Aggregated plants invest more in defence than do Solitary plants through recognition of con-specific neighbours, because aggregated plants often consumed by specialist leaf beetles. Leaf-trait alteration based on neighbour recognition has also been reported in several plant species (Yamawo & Mukai 2020;Yamawo, 2015Yamawo, , 2021. Neighbour recognition can therefore be a cause of leaf-trait alteration in R. obtusifolius plants.…”

“…Therefore, variations in the interaction environment induce changes in expression of the chemical traits of host plant leaves (Barton & Bowers, 2006;Broz et al, 2010;Mraja et al, 2011;Muiruri et al, 2019), so they are likely to influence leaf herbivory and the distribution of herbivores. In fact, several studies have strongly suggested that changes in leaf traits in response to differences in the interaction environment of host plants influence herbivory or herbivore distribution (e.g., Broz et al 2010;Muiruri et al, 2019;Yamawo, 2021). However, to our knowledge, no experimental evidence has yet been provided for this effect.…”

“…In contrast, host plants present at low density are exposed to direct interspecific interaction or no interaction. Many studies have reported that plant-plant direct interactions influence herbivory (Hambäck & Beckerman, 2003;Muiruri et al, 2019;Yamawo, 2021). For example, plant competition for resources induces plastic changes in the plants' resource allocation; these changes can affect root or shoot growth.…”

“…For example, plant competition for resources induces plastic changes in the plants' resource allocation; these changes can affect root or shoot growth. The changes in resource allocation can also influence the expression of leaf thickness, leaf mass per area, and primary (essential nutrients) and secondary (potentially plant-protective compounds) metabolites in the leaves (Bartron & Bowers, 2006;Broz et al, 2010;Mraja et al, 2011;Takigahira & Yamawo, 2019;Yamawo, 2021). Therefore, variations in the interaction environment induce changes in expression of the chemical traits of host plant leaves (Barton & Bowers, 2006;Broz et al, 2010;Mraja et al, 2011;Muiruri et al, 2019), so they are likely to influence leaf herbivory and the distribution of herbivores.…”