Species diversity and biological trait function: Effectiveness of ant–plant mutualism decreases as ant species diversity increases

One of the major concerns of ecological and evolutionary research is the
prediction of community function in relation to the degree of
biodiversity. To clarify the relationship between biodiversity and the
expression of plant defence traits, we investigated ant–plant defensive
mutualism in the pioneer plant Mallotus japonicus at several sites across
Japan. The plant bears ant-attracting extrafloral nectaries (EFNs) and
food bodies (FBs) as indirect defence traits. To reveal variations in
ant–plant interactions in the field, we measured species richness of
nectar-feeding ants and interaction strength between each ant species and
the plant (visiting frequency of each ant species to the plant). We also
investigated the expression of EFNs and FBs in natural plant populations
at the study sites. To assess the defensive quality of each ant species,
we then experimentally estimated the aggressiveness of the dominant ant
species. To examine the link between ant species richness and the
defensive function of ants, we conducted an ant-exclusion experiment and a
common garden experiment to clarify whether the defence efficacy of ants
varied in relation to ant species richness. Ant species richness differed
among the study sites. We found negative relationships between ant species
richness and the mean interaction strength of ant species, the interaction
strength of aggressive ant species, and the number of EFNs per leaf. The
effectiveness of indirect defence by ants was poor at sites where ant
species richness was high. When cultivated in the same environment, plants
from sites with low ant species richness developed a larger number of EFNs
per leaf than those from sites with high ant species richness. Our results
suggest that facultative ant–plant defensive mutualism is weakened at
sites where ant species richness is high, resulting in a decrease in the
number of EFNs per leaf. Such a link between species richness and an
indirect trait function may help us to understand the evolutionary
patterns of various species traits in complex biological communities.