Mucilage binding to ground protects seeds of many plants from harvester ants: A functional investigation

The seeds of many plant species produce mucilage on their surfaces that when wetted and dried, firmly adheres seeds to surfaces and substrates. Previous studies have demonstrated that seed anchorage to the ground can reduce seed predation, although only a few species have thus far been tested. Here we investigated whether binding to the ground reduces seed removal by harvester ants Pogonomyrmex subdentatus, an important granivore, for 53 species with mucilaginous seeds. We also explored functional traits that associate with seed removal risk to understand the ecological and evolutionary context of this granivory resistance trait. Using a field cafeteria choice experiment, we compared harvester ant seed removal of wetted ground‐bound seeds to dry unbound control seeds for these 53 species. We developed a simple assay to score dislodgement force. We examined whether this force, seed mass and seed mucilage production explained the interspecific variation in protection that we observed in field seed removal. We integrated these experiments with a broad scale test of correlates of seed attachment using a previously published dataset of attachment potential of mucilaginous seeds for 432 species, examining correlations of attachment potential with 13 plant traits and the climate characteristics of the species' range. Binding to the ground reduced seed removal in 42 of 53 species tested. The benefit increased with seed dislodgement force, which itself increased with mucilage production, but not with seed mass. In the larger dataset, shorter plant life span, higher temperature, more solar radiation, higher humidity, fewer wet days per year and higher seed density correlated positively with the odds of seed attachment. We also found that attachment potential showed a concave down quadratic relationship with latitude, peaking at roughly 30°. No strong evidence that any of the other six predictors correlated with attachment potential was found. We demonstrate that protection from granivores is a widespread convergent function of seed mucilage and is associated with mucilage production. We highlight the need for increased mechanistic investigations into this common but poorly studied trait, particularly in relation to functional drivers of the broad patterns we found. A free Plain Language Summary can be found within the Supporting Information of this article.