Number of conspecifics and reproduction in the invasive plant Eschscholzia californica (Papaveraceae): is there a pollinator‐mediated Allee effect?

Abstract

The component Allee effect has been defined as ‘a positive relationship between any measure of individual fitness and the number or density of conspecifics’. Larger plant populations or large patches have shown a higher pollinator visitation rate, which may give rise to an Allee effect in reproduction of the plants. We experimentally tested the effect of number of conspecifics on reproduction and pollinator visitation in Eschscholzia californica Cham., an invasive plant in Chile. We then built patches with two, eight and 16 flowering individuals of E. californica (11 replicates per treatment) in an area characterised by dominance of the study species. We found that E. californica exhibits a component Allee effect, as the number of individuals of this species has a positive effect on individual seed set. However, individual fruit production was not affected by the number of plants examined. Pollinator visitation rate was also independent of the number of plants, so this factor would not explain the Allee effect. This rate was positively correlated with the total number of flowers in the patches. We also found that the number of plants did not affect the seed mass or proportion of germinated seeds in the patches. Higher pollen availability in patches with 16 plants and pollination by wind could explain the Allee effect. The component Allee effect identified could lead to a weak demographic Allee effect that might reduce the rate of spread of E. californica. Knowledge of this would be useful for management of this invasive plant in Chile.

A lack of genetically compatible mates constrains the spread of an invasive weed

“…Studies that used natural variation in population sizes found that small and isolated populations at the invasion front suffered low seed set as a result of limited pollen availability ( Spartina alternifora , Davis et al , 2004; Solanum rostratum , Zhang & Lou, 2015), suggesting that Allee effects could slow down invasion at its distribution edge. Other studies have tested for Allee effects in invasive species by experimentally manipulating population sizes, finding reduced seed set in small populations in some cases (Elam et al , 2007; Firestone & Jasieniuk, 2013; Anic et al , 2015; Lachmuth et al , 2018) but not in others (Cappuccino, 2004). In experiments manipulating both population size and relatedness (as a proxy for S‐allele diversity), seed set decreased with both factors in an additive manner, with effects of population size stronger than effects of relatedness (Elam et al , 2007; Firestone & Jasieniuk, 2013).…”

Pollination in the Chilean Mediterranean‐type ecosystem: a review of current advances and pending tasks

“…At the population level, Anic et al . () studied the effect of the number of individuals on plant reproductive success (component Allee effect) using the invasive Eschscholzia californica (Papaveraceae) as model system. Pollination and reproductive variables were recorded in replicated patches of varying density.…”

Effects of small-scale clustering of flowers on pollinator foraging behaviour and flower visitation rate

“…Moreover, the link between visitation rate and seed 34 set is not straightforward. Percentage seed set may increase with the number of flowers when 35 visitation rate also increases [16,28], but it may be reduced in self-incompatible species due to 36 geitonogamous pollination which occurs when a single pollinator visits multiple flowers on the same 37 plant [15,29,30]. 38 At the local scale, plants often grow in groups of multiple individuals, which we refer to as 39 clusters.…”

“…Pollinators also tend to visit a decreasing proportion of flowers in larger 48 reproduction can be very nuanced. Percentage seed set was reported to be independent of cluster 49 size [35], or increasing in response to higher visitation rate per flower in clusters with more 50 flowers [34,36]. However, seed set may also depend on the density of plants within the cluster [37], 51 on their genetic compatibility [38], and on species-specific consequences of geitonogamous 52 pollination whose frequency may vary with cluster size [30].…”