Red wood ants (Formica rufa) help propagate invasive small balsam (Impatients parviflora) in accordance with the directed dispersal hypothesis

Abstract

Ants are a major source of soil disturbance and can influence vegetation dynamics and biodiversity by changing environmental conditions. Their seed collection activity supports dispersal and can favour the presence of species, including introduced, non-native and invasive species. Understanding of seed dispersal mechanisms is important for predicting plant distributions and invasive risk.Studies generally addressing myrmecochory have mostly focused on interactions between native species and few have considered the role of ants in spreading non-native plants. This study investigated effects of a native species Formica rufa (red wood ant) on the distribution of the invasive plant, Impatiens parviflora DC (small balsam). The study measured ant seed collection activity and plant and soil chemical characteristics to evaluate whether assumptions of the directed dispersal hypothesis obtained.Seed collection experiments showed that ants do not selectively collect small balsam seeds but collect them randomly at about the same rates as other organic and inorganic material. Non-targeted collection however suffices in dispersing small balsam seeds in or near the ant nest thus enhancing plant establishment, growth and reproduction.I. parviflora density in the forest floor was 3.06 plants per m2 while I. parviflora density reached 82.68 plants per m2 in and around ant nests I. parviflora individuals growing on nest mounds were significantly larger, showed more flowers and produced more seeds than plants established on the surrounding forest floor. Better growth of I. parviflora on and around ant nests corresponded with significantly higher nutrient content, especially of nitrates and available phosphorus in ant nests. Ant nests also showed higher pH values and available potassium, sodium and calcium.This study thus found that red wood ants contribute to the expansion of I. parviflora by soil transformation and non-targeted seed dispersal in an otherwise nutrient-poor environment where the plant would otherwise struggle to flourish.