Interactions between protea plants and their animal mutualists and antagonists are structured more by energetic than morphological trait matching

Traits mediate mutualistic and antagonistic interactions between plants
and animals, and should thus be useful for predicting trophic species
interactions. Studies to date have examined the importance of
morphological trait matching for plant-animal interactions, but have
rarely explored the extent to which these interactions are shaped by
matching between energetic provisions of plants and energetic demands of
animals. We tested whether energetic and/or morphological trait matching
shapes interactions between Protea plant species and their interacting
animal mutualists and antagonists in the Cape Floristic Region, South
Africa. We recorded interactions between 22 Protea species, pollinating
insects, and vertebrates as well as seed predators (endophagous insect
larvae in protea cones) at 21 study sites. To relate species interactions
to matching trait pairs, we measured key morphological traits (shape and
size of flower heads and seed cones, and mouth part length as well as body
length) and quantified the animals’ energetic demands (metabolic rate)
together with the plants’ energetic provisions (nectar sugar amount,
seed-to-cone mass ratio). We calculated log ratios of both energetic and
morphological traits between animals and plants as predictor variables for
the number of observed interactions between Protea species and their
animal interaction partners. For both mutualistic and antagonistic
interactions, we found significant effects of morphological and energetic
trait ratios on the interactions between plants and animals. Trait ratios
accounted for 11% to 22% of the variation in species interactions.
Consistent with energetic trait matching, we found a hump-shaped
relationship between interaction frequency and log ratios of energetic
traits of animals and plants, indicating that interactions were most
frequent at intermediate log ratios between energetic demand and
provision. Effects of morphological trait ratios on interactions were
statistically supported in most cases but were variable in the magnitude
and shape of the predicted relationships. Across animal taxa and
interaction types, energetic traits had more consistent effects on
interactions between plants and animals than morphological traits. This
suggests that energy can function as an important interaction currency and
facilitate the understanding and prediction of trophic species
interactions.