Consequences of an Allee Effect in the Invasion of a Pacific Estuary by Spartina Alterniflora

Abstract

Results from both a spatially explicit stochastic simulation model and a spatially implicit deterministic model show that a weak Allee effect can slow the spread of an invasive plant, Spartina alterniflora, in a Pacific Coast estuary. The mean rate of spread with the Allee effect is ϳ19%; removing the Allee effect results in a mean rate of spread of ϳ31%. Sensitivity analysis both with and without the Allee effect reveal that seedling establishment, inflorescence density, and outcrossed seed production are key factors determining invasion rate. When there is an Allee effect, the invasion rate is sensitive to variation in self-pollinated seed production. By structuring the population according to density classes instead of the more usual age or stage classes, the deterministic model demonstrates a novel way of representing density dependence and an Allee effect in population biology. This approach could be applied to model the population dynamics of any species that spreads from multiple foci and in which these foci later coalesce.

Pollen limitation causes an Allee effect in a wind-pollinated invasive grass ( Spartina alterniflora )

“…This tall, dense salt marsh grass has spread to cover Ϸ60 of the 230 square kilometers of intertidal lands since introduction from the Atlantic coast a century ago. This area occupied is far less than would be expected without the Allee effect (29).…”

“…A ''weak'' Allee effect, as can be the case with long-lived adults, causes a depressed per capita rate of growth at low population density, but it never becomes negative (27,28). A lack of mating opportunities among sparse or widely spaced individuals can result in an Allee effect and a slowing of the invasion (27)(28)(29).…”

“…1). Isolated colonists set Ͻ1͞10th the seed of plants that have grown vegetatively and coalesced with their neighbors (30), and this has caused a weak Allee effect (29). S. alterniflora spreads within Willapa Bay by seed that floats on the tide, with virtually no recruitment by rhizome fragments (30).…”

Spatial and temporal genetic structure in a hybrid cordgrass invasion

“…In open San Francisco Bay tidal flats, colonizing hybrid family groups growing in isolation have set selffertilized seeds, and have thus overcome the pollen limitation seen in invading, self-incompatible S. alterniflora in Willapa Bay, WA Taylor et al, 2004). There, pollen limitation of isolated plants caused an Allee effect that greatly slowed the tidal flat invasion Taylor et al, 2004).…”

“…In open San Francisco Bay tidal flats, colonizing hybrid family groups growing in isolation have set selffertilized seeds, and have thus overcome the pollen limitation seen in invading, self-incompatible S. alterniflora in Willapa Bay, WA Taylor et al, 2004). There, pollen limitation of isolated plants caused an Allee effect that greatly slowed the tidal flat invasion Taylor et al, 2004). In San Francisco Bay, these few self-compatible hybrids, having adapted to environmentally challenging conditions, maximize their reproductive fitness by producing large numbers of self-fertilized seeds in isolation (Daehler, 1998;Sloop et al, 2009).…”