Conclusions
The drivers of variation discussed here may not be exhaustive, however, they illustrate the high potential for geographic variation in the intensity and effects of HP transfer interactions as well as in the evolutionary responses to HP receipt. So far, the study of pollinator-mediated plant-plant interactions has been almost entirely dominated by studies of pre-pollination interactions even though their outcomes are influenced by plant-plant interactions that take place on the stigma after pollen has been deposited. Therefore, it is paramount that we fully evaluate the causes, consequences and context-dependency of HP transfer interactions in order to gain a more complete understanding of the role that plant-pollinator interactions play in generating and organizing plant biodiversity. It is also important to acknowledge that the number of studies documenting patterns of HP receipt is still limited and strongly biased towards temperate systems (Arceo-Gómez et al. 2019a; Fig. 3). Studies on HP transfer dynamics in highly diverse regions such as in Africa and South America are largely underrepresented (Fig. 3; Arceo-Gómez et al 2019a). Biases in studies of HP receipt are not only geographical but also phylogenetic as large groups of plants have also been poorly represented in these studies (e.g. monocotyledons; Arceo-Gómez et al. 2019a). Thus, there is an urgent need to evaluate patterns of variation in HP receipt at larger spatial and phylogenetic scales. Knowledge of wide-scale patterns of HP receipt may help uncover its potential role in shaping patterns of plant diversity at a global scale.