Demographic and genetic Allee effects interact in depressing reproduction in a weedy grass (Lolium multiflorum)

Although a ‘rare weed’ or ‘rare invasive’ seems to be an oxymoron, the ability to survive short periods of small population sizes likely promotes colonization, range expansion, and invasion success. Allee effects that depress fitness in small populations are well known from declining species of conservation concern but rarely investigated in species with a predilection to naturally increase. We looked for just such an effect in the weedy annual ryegrass (Lolium multiflorum) by creating fifty small populations. The population sizes ranged from six to thirty plants at fixed density. Genetic diversity was manipulated factorially with population size by using putative non-siblings, half-siblings and reduced numbers of half-siblings. By cloning seedlings, the number of genotypes in a population was kept constant across population size treatments to avoid confounding demographic and genetic components. Regression and ANCOVA indicated that the percent seed set was reduced with reduction in demographic population size, consistent with pollen limitation. This effect was consistent across three of the four unrelated wild source populations. Decreased relatedness (postulated to increase genetic diversity) did not increase seed set rates, however the non-sibling treatment mitigated the effect of increased population size such that no demographic Allee effect was present in the absence of relatedness. This is consistent with either or both 1) self-incompatibility reducing the effective population size of pollen donors and thus limiting seed set; 2) inbreeding depression aborting ovules. Although widely shown in conservation biology, Allee effects, such as pollen limitation, have rarely been demonstrated in invasive species or grasses, much less the combination.