The important oxymoron of rare invasives: Decreased reproduction in smaller populations of the invasive grass Lolium multiflorum

Background/Question/Methods

Small populations can play important roles in biological invasions as founders of new invasions or survivors of control efforts. Theory and models have explored how population size and related factors (Allee effects) can affect invasion probability and dynamics. In contrast, there have been few empirical studies of the effects of small population size on reproduction within invasive populations in the wild, especially for plant species.

We identified 21 naturally-occurring small populations of the invasive annual grass Lolium multiflorum. To elucidate the effects of population size on population reproduction, we compared seed production of focal plants to three attributes of the population: population size, the focal plants’ floret neighborhood (which is a measure of the size and pollen production of nearby potential pollen donors), and focal plant size. The first two of those population attributes were weighted by inter-plant distance to account for variation in density and spatial arrangement. We incorporated the three population attributes, and their variances, into a principal component analysis, and used the resulting principal components as regressors against seed production response variables.

Results/Conclusions

Much of the variance in both proportional and absolute seed production is explained by the measured attributes of the population (adjusted R² = 0.45, 0.76 respectively). The proportion of florets producing a seed increased with larger floret neighborhoods and census population sizes, but not with maternal plant size or habitat. Aborted seed was also common, and largely explained by larger maternal plants and fewer, smaller (or more distant) pollen donors. This variation in seed set suggests an Allee effect, possibly due to pollen limitation, in this invasive plant species. The importance of such an effect is that variation in reproductive rates due to population size may limit our ability to predict invasion risks and rates early in the invasion process, particularly if these differences are not recognized. Although many studies measure species traits leading to invasion, these results remind us that invasiveness can vary among introductions within a species. More fundamentally, this study is one of only a few to specifically investigate reproduction in invasive plants across a range of population sizes.