Community-level variation in patterns of HP receipt
The average intensity and diversity of HP that plants receive not only vary among populations of the same species but can vary among entire plant communities. Recent studies on HP transfer networks have shown that community-wide patterns of HP transfer can vary spatially (e.g. Tur et al. 2016, Johnson & Ashman 2019). For instance, a study across nine sand dune plant communities in the Yucatan showed large variation in average HP proportion (2.3% - 20.8%) and HP richness (2.3 - 5.5 HP donor species) that plants receive across communities (Parra-Tabla et al. 2020). A recent global study also showed that patterns of HP receipt (likelihood and intensity) correlate with latitudinal and altitudinal biodiversity gradients, suggesting that plants growing in highly diverse regions of the world are more likely to engage in HP transfer interactions (Arceo-Gómez et al. 2019a). Although it is evident that patterns of HP receipt vary across large spatial scales most studies to date have been limited to evaluate patterns of HP receipt within single communities (e.g. McLernon et al. 1996, Montgomery & Rathcke 2012, Fang & Huang 2013, Arceo-Gómez et al. 2016b). Wide differences in pollinator species composition across regions around the globe may further contribute to global trends in HP receipt (Arceo-Gómez et al. 2019a). For example, large vertebrate pollinators common in tropical regions (e.g. bats and hummingbirds; Bawa 1990) can carry larger and more diverse HP loads (e.g. Borgella et al. 2001, Muchhala & Jarrin 2002, Muchhala & Thomson 2012) compared with invertebrate pollinators (e.g. beetles, bees, flies, butterflies) that are common in temperate areas (e.g. Barrett & Helenum 1987). These global differences in HP carriage and receipt may thus contribute to observed global patterns of floral diversification by imposing a wide-range of selective pressures via male (Moreira-Hernandez & Muchhala 2019) and female fitness (Morales & Traveset 2008, Ashman & Arceo-Gómez 2013). For instance, it has long been proposed that enhancing pollinator floral constancy is a strong force driving divergent evolution, but how much of this is due to HP avoidance versus competition for pollinator visitation is less known (e.g. Waelti et al. 2008; also see Moreira-Hernandez & Muchhala 2019). In fact, the pollination literature has largely overlooked the potential contribution of post-pollination process (via HP transfer) to outcomes of pollinator-mediated selection. HP receipt has been shown to influence the evolution of morphological traits (e.g. flower color and size; Armbruster et al. 1994, Muchhala & Thomson 2012, Hopkins & Rausher 2012), physiological processes (e.g. Kay & Schemske 2008), mating systems (e.g. Fishman & Wyatt 1999, Randle et al. 2018) and flowering phenology (Waser 1978). Thus, differences in HP receipt mediated by variation in plant and pollinator community composition among geographic regions across the globe may have the potential to generate evolutionary hostspots (Thompson 1999) and contribute to shape global patterns of plant biodiversity.