Seed production is reduced by small population size in natural populations of the invasive grass Lolium multiflorum

Abstract

As founders of new invasions or survivors of control efforts, small populations can play important roles in biological invasions. Theory and models have explored how population size can affect invasion probability and dynamics. In contrast, there have been few studies of small population size and invasions in the wild, especially for plants. Here, we identified 21 naturally-occurring small populations of the invasive annual grass Lolium multiflorum to elucidate the effects of population size on plant reproduction and potential invasiveness. We compared seed production of focal plants to three attributes of the population: population size, a focal plant’s floret neighborhood (a measure of the size and pollen production of nearby potential pollen donors), and focal plant size. Population size and floret neighborhood were weighted by inter-plant distance to account for variation in density and spatial arrangement. Variation among populations in the proportion of florets producing a seed was explained by population size and floret neighborhood. In contrast, the focal plant’s size did not significantly affect the seed production proportion. A decrease in reproduction due to the size and density of the population (while accounting for maternal size) is an Allee effect, which may limit the ability to predict invasion risks and rates early in the invasion process, particularly if the effect is not recognized.