Phylogenetic restriction of plant invasion in drought-stressed environments: implications for insect-pollinated plant communities in water-limited ecosystems

Background: Plant-pollinator community diversity has been found
to decrease under conditions of drought stress, however research into the
temporal dimensions of this phenomenon remains limited. In this study, we
investigated the effect of seasonal drought on the temporal niche dynamics
of entomophilous flowering plants in a water-limited ecosystem. We
hypothesized that closely related native and exotic plants would tend to
share similar life history, and that peak flowering events would therefore
coincide with phylogenetic clustering in plant communities based on
expected phenological responses of plant functional types to limitations
in soil moisture availability. Location: Galiano Island, British Columbia,
Canada Methods: Combining methods from pollinator research and
phylogenetic community ecology, we tested the influence of environmental
filtering over plant community phenology across gradients of landscape
disturbance and soil moisture. Floral resource availability and community
structure were quantified by counts of flowering shoots. We constructed a
robust phylogeny to analyze spatial and temporal variation in phylogenetic
patterns across the landscape, testing the significance of the observed
patterns against a randomly generated community phylogeny. Phylogenetic
metrics were then regressed against factors of disturbance and soil
moisture availability. Results: Critical seasonal fluctuations in
floral resources coincided with significant phylogenetic clustering in
plant communities, with decreasing plant diversity observed under
conditions of increasing drought stress. Exotic plant species in the
Asteraceae became increasingly pervasive across the landscape, occupying a
late season temporal niche in drought-stressed environments. Main
conclusion: Results suggest that environmental filtering is the
dominant assembly process structuring the temporal niche of plant
communities in this water-limited ecosystem. Based on these results, and
trends seen elsewhere, the overall diversity of plant-pollinator
communities may be expected to decline with the increasing drought stress
predicted under future climate scenarios.