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.