The Influence of Multiple Scales of Environmental Context on the Distribution and Interaction of an Invasive Seagrass and its Native Congener
Hannam, Michael Patrick
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Predicting invasive species impacts is complicated by the variable context in which they occur. Furthermore, species invasion is an inherently multi-scale process, war- ranting examination at multiple spatial scales. In its introduced range, the invasive seagrass <italic>Zostera japonica</italic> often grows at higher elevations than its native congener, <italic>Zostera marina</italic>, but at some sites the congeners overlap. At overlapping sites, the two seagrasses often grow in a patch mosaic associated with intertidal microtopography. I examine the role of multiple scales of environmental context in the vertical zonation of <italic>Z. japonica</italic> and <italic>Z. marina</italic>, and the relationship of meter to decameter scale microtopography to the local distribution and biotic interactions of<italic> Z. marina</italic> and <italic>Z. japonica</italic>. Nearshore intertidal topography, hydrodynamic exposure, and tidal range were examined as abiotic predictors of the deep extents of <italic>Z. japonica</italic> and <italic>Z. marina</italic>, the shallow extent of<italic> Z. marina</italic> and the elevation overlap of the two species, both at within site and among site spatial scales. Bottom profile complexity was the most consistently important predictor studied, confirming the importance of the geomor- phic template on the zonation of these species. <italic>Z. japonica's</italic> deep extent was well- predicted by <italic>Z. marina's</italic> shallow extent, but <italic>Z. marina's</italic> shallow extent was best predicted by physical factors. Using a terrestrial laser scanner, I mapped and monitored a one hectare study site for three years. The location and shape of microtopographic features were generally stable from year to year, but the magnitude of local relief was variable. <italic>Z. marina</italic> was more likely to occur in water-filled depressions than on well-drained mounds, but the presence of<italic> Z. japonica</italic>, was better predicted by low shoot densities of <italic>Z. marina</italic>. Transplant experiments revealed that <italic>Z. marina</italic> suppressed <italic>Z. japonica</italic> shoot densities, more so in pools than on mounds. Topographic context remained the most influential predictor of <italic>Z. marina</italic> responses, even when I transplanted<italic> Z. marina</italic> into higher densities of <italic>Z. japonica</italic>. I show that decameter to kilometer-scale environmental context influences the vertical zonation of <italic>Z. marina</italic> and <italic>Z. japonica</italic>, and that meter to decimeter-scale microtopographic relief influences their local spatial distribution. Furthermore, centimeter-scale topographic relief can modify competitive outcomes and promote decameter-scale coexistence of these two species.
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