Plant-insect interactions in a shifting coastal ecosystem: Avicennia germinans and its associated arthropods
Gruner, Daniel S
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The climate’s role in determining where species occur is increasingly well understood, but our ability to predict how biotic interactions both influence and respond to species’ range shifts remains poor. This is particularly important when considering climate-change-driven range shifts in habitat-forming species like mangroves, given their impact on ecosystem structure and function. In this dissertation, I consider the arthropods associated with the black mangrove, Avicennia germinans, to explore whether patterns of arthropod diversity affect the rate of a plant’s range expansion, and, in turn, how a range-expanding plant alters arthropod communities in habitats where it is invading. Among arthropods with the potential to influence plants’ range dynamics, pollinators can directly affect plant reproduction and ability to spread into new territory. Breeding system experiments reveal that A. germinans relies on pollinators for full fruit set, and surveys along the Florida coast show a substantial northward decline in the overall frequency of pollinator visits to A. germinans flowers. However, the decline in abundance of some common pollinator taxa is partly offset by an increase in the frequency of other highly effective taxa. Furthermore, range-edge A. germinans produce more flowers than southern individuals, contributing to high range-edge fecundity and enabling range expansion. As a woody plant with nectar-producing flowers, A. germinans is a novel resource for arthropods in the salt marshes where it is encroaching. To understand arthropod community assembly on these frontier mangroves, and how mangrove presence affects marsh arthropod community composition, I compare arthropod communities in these adjacent vegetation types. Arthropods form distinct communities on mangroves and marsh vegetation, with at least one A. germinans specialist already present in this range-edge population. However, neither mangrove proximity nor the abundance of mangrove flowers appears to influence salt marsh arthropod community structure, indicating that mangrove encroachment may lead to a net increase in arthropod diversity in coastal regions by increasing habitat heterogeneity. In sum, plants that rely on pollinators can avoid range-edge reproductive failure by attracting a diverse group of pollinating taxa, and range-expanding plants can rapidly alter invaded communities by shaping diversity at very local scales.