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.