Phenological turnover matters when making trait-based predictions of plant-pollinator interactions

Understanding the processes determining species’ interactions is key to
predicting and safeguarding ecological networks under rapid environmental
change. One approach to estimating interactions is to use morphologies of
taxa interacting across trophic levels to reveal suites of traits they are
more likely to interact with (i.e. a morphological trait
niche). Previous work studying these morphological trait niches
has typically used interactions between species that are pooled in space
and time. However, species assemblages, and the traits of individuals
within species, can change across even small landscapes over a season,
leading to morphological trait space being dynamically reshaped.
Therefore, it is unclear how morphological trait turnover affects our
inferences of trait niches, and our ability to answer this is in part
limited by a lack of individual-level trait data. Here, we
directly address this by studying a montane Arctic plant-pollinator
community over five growing seasons (>1,300 hours of fieldwork).
Specifically, we linked every recorded plant-bumblebee interaction with
the traits of the bee individual involved (n = 1,150), to investigate 1)
whether plant taxa (n = 10) exhibited bee trait niches by interacting with
specific regions of multidimensional trait space of visiting bumblebees,
and 2) how our inference of these trait niches was affected by considering
bumblebee trait turnover and plant taxon turnover over space and
time. When we did not consider turnover (i.e., interactions in
space and time were pooled), plant taxa demonstrated bee trait niches.
However, we next considered how bee trait space was reshaped over the
elevational and seasonal gradient (for example, with the emergence of
different castes), and how this reshaping co-occurred with different
spatiotemporal ranges of the plant taxa. From this, we found plant taxa no
longer interacted with a smaller area of bee community trait space than
expected by chance (i.e. no longer showed a bee trait niche), and that
seasonal reshaping of bee trait space was the primary driver of this
trend. Overall, in highly dynamic systems, like the Arctic, overlooking
community turnover could mask and even overestimate the ability of
morphology to explain interactions. Hence, determining how morphological
traits of individual interaction partners are in phenological synchrony at
localised scales will be fundamental to understanding the role morphology
plays in underpinning plant-pollinator interactions.