Unravelling the role of mate density and sex ratio in competition for pollen

Mate density and sex ratio are commonly used concepts in pollination biology, but are not always clearly distinguished. Here we propose that mate density should only capture the number of male-phase flowers in a defined area and ignore female-phase flowers. Sex ratio is the ratio of male and female-phase flowers in a defined area and captures female-female competition. We use a spatially explicit simulation model to quantify the effect of variation in mate density and sex ratio for plant-pollinator systems characterized by combinations of high or low rates of pollen loss and deposition from pollinator to stigma and then assess the size of pollination neighbourhoods. In efficient systems with relatively little pollen loss, female-female competition is of overriding importance. In contrast, in wasteful systems with high pollen loss rates, mate density becomes the dominant factor and sex ratio is no longer consequential. These patterns were observed at both landscape and local scales. Systems with low deposition and low pollen loss rates are associated with large pollination neighbourhoods, which decline with increasing deposition and pollen loss rates. These results show that mate density and sex ratio should carefully be distinguished and highlight the complex way in which pollen loss interacts with deposition rate, which has not previously been appreciated.