Density‐dependent reproduction and pollen limitation in an invasive milkweed, Calotropis procera (Ait.) R. Br. (Apocynaceae)

Calotropis procera (Ait.) R.Br. (Apocynaceae), an invasive woody milkweed, has expanded its range in northern Australia affecting rangeland and pastoral productivity. While self-compatibility should enhance the species range expansion, spread of C.procera is limited by the availability of larger wasp and bee species that are able to vector its solid pollinia. Pollination efficiency is thus likely dependent on both pollinator abundance and plant density. Calotropis procera flowers year round in Australia but fruiting is limited to the warm months of the year when pollinators are most abundant, indicating that seasonal regulation of reproduction may be due to pollinator limitation. We examine the propositions that C.procera reproduction is regulated by the interaction between plant population density and pollinator pressure and that low pollinator pressure causes low per capita plant fecundity. All pollinators belonged to Order Hymenoptera and pollinator composition was similar at six of the seven sites. Fruit production per plant (fecundity) was lower above and below intermediate densities (350-550plantsha(-1)) of flowering plants with evidence of a weak Allee effect at lower plant density. Pollinator visitation rates per plant were low at high and low plant densities, and greatest at intermediate densities, while pollen supplementation experiments showed that C.procera is pollen limited (Pollen Limitation Index(fruit)=0.9) even at intermediate densities. Pollen limitation caused by low pollinator pressure at low plant densities and pollinator satiation at high plant densities may account for these fruit production trends. Management should be conducted in the colder months when pollinator pressure is low and plants are not reproducing. In addition, where stand eradication cannot be achieved in one attempt, management should reduce flowering plants to below intermediate densities where the fecundity per plant is low.