Densities of a perennial invasive tree, Acacia cyclops, decline in the 20 years since inception of biological control with two seed-reducing agents, a flower-galling midge and a seed-feeding weevil

Abstract

Two biological control agents, a flower-galling midge (Dasineura rubiformis, Cecidomyiidae), and a seed-feeding weevil (Melanterius maculatus, Curculionidae), are consistently and substantially reducing the seeding capacity of Acacia cyclops in South Africa. Simultaneously there has been a conspicuous decline in the density of the weed in some areas over the past 20 years. Besides the two biological control agents, indigenous granivores, especially striped field mice (Rhabdomys pumilio), destroy a considerable proportion of the seeds that are produced. Fire is another key factor driving the population dynamics of A. cyclops. Most wildfires kill all the existing trees in areas that burn and nearly all (ca. 90 %) of the seeds in the soil are scorched and destroyed during fires. Dormancy is broken in the surviving seeds, of which nearly all (88 %) germinate during the first winter-rainfall season, the rest doing so during the following winter so that two years after fire no seeds remain in the soil. Approximately 13 % of all the seedlings which germinate following fire events survive through their first hot, dry summer and become established to form the next cohort of plants. Historically, the surviving recruits were abundant enough to form a closed canopy within 3–4 years. This pattern was sustained because A. cyclops plants produced an abundance of seeds each year which accumulated in the soil, typically reaching numbers of 1000–4000 seeds m−2 within a few years of the preceding fire. Since the establishment of biological control, the numbers of seeds in the soil under existing trees has dropped with time, but a pool of dormant seeds persists which usually produces enough seedlings to allow the weed to still form a closed canopy after a first fire event. Since the inception of biological control, the cohorts of new plants that grow up after a first fire only yield a small fraction of their potential seed production, resulting in few if any seeds accumulating in the soil under these plants. When these areas under biological control burn a second time there are insufficient seedlings to replace the plants killed by the fire and densities in the new cohort of plants are regularly at least 95 % lower than the densities in the thickets before the two fires. While the process relies on spontaneous outbreaks of wildfires, the interaction of biological control, indigenous granivores and fire is bringing A. cyclops under excellent control.