Within-species variation in HP receipt and effects
First, future studies should more widely evaluate the variation in the intensity of HP receipt for one or multiple species across their geographic distributions. So far, very few studies have documented changes in the dynamics of HP receipt for a single species across spatial scales (e.g. compared to changes in pollen transfer network structure), and the spatial scales studied tend to be small (two or very few communities; e.g. Arceo-Gómez & Ashman 2014a, Arceo-Gómez et al. 2018; but see Waites & Agren 2004). So far, we have very little empirical evidence of the extent of within-species variation in HP receipt across natural communities. Such studies would constitute an important first in advancing our understanding of the relevance of geographic mosaics of HP transfer interactions in nature, as well as of its potential ecological and evolutionary consequences. Second, studies that evaluate the mediators of HP transfer dynamics (conspecific flower density, plant and pollinator community composition) and the directionality of their effects (increase or decrease HP receipt) are key if we aim to understand the underlying drivers generating geographic variation in HP interactions. This can be achieved via experimental studies, where flower density and plant and pollinator community composition are manipulated in the lab (e.g. Thomson et al. 2019) or field conditions (e.g. de Waal et al. 2015, Bruckman & Campbell 2016, Brosi et al. 2017), or by taking advantage of existing natural variation in the field (e.g. Arceo-Gómez & Ashman 2014a, Albor et al. 2019). It is also important to note that while some sources of variation are expected to vary inconsistently across the landscape (e.g. conspecific density and spatial arrangement) others may vary in a more predictable manner (e.g. species diversity, resource availability). This latter more predictable type of variation (i.e. geographic gradients) could then be used to formulate and test specific predictions regarding the role of these drivers in mediating variation in HP transfer interactions. For instance, we could expect an increase in HP receipt with increasing co-flowering diversity and a decrease in HP effects with increasing resource availability. The importance of these mediators in influencing HP transfer dynamics should also be evaluated singly and in combination (e.g. de Waal et al. 2015, Thomson et al. 2019). This is becoming more feasible with the development of powerful analytical techniques that allow for simultaneous evaluation of multiple independent variables (e.g. structural equation modeling; Albor et al. 2019).
Third, when evaluating HP effects, it is important that we move beyond effects in species-pairs and start incorporating the complexity of HP interactions within natural communities by acknowledging the intricacies of HP loads (e.g. Arceo-Gómez et al. 2011) and the diversity of co-flowering species involved (e.g. Fang & Huang 2013, Arceo-Gómez et al 2019b). Along these same lines, it is also important to design studies that help understand how effects revealed by greenhouse experiments may reflect expected outcomes in the field (e.g. Briggs et al. 2016), and how these effects vary across a wide range of environmental (Celaya et al. 2015) and biotic conditions (e.g. Arceo-Gómez et al. 2016a). Such tests are necessary in order to gain a more complete understanding of the causes and consequences of HP receipt in natural communities and how these may contribute to generate geographic mosaics of selection.