Ecosystem engineers are species capable of modifying their environment in a manner that changes the availability of physical or environmental resources for that species or other species living in the same habitat. For some invasive ecosystem engineers, their ability to modify their environment can have a direct effect on their rate of spread. Spartina alterniflora, a salt marsh grass that has invaded several regions along the Pacific coast, is an invasive ecosystem engineer that is capable of increasing the height of sediment above average tidal height. If Spartina increases sediment levels towards the optimal sediment height for reproduction, its ability to act as an ecosystem engineer increases its reproductive rate. Additionally, climate change may affect this system via sea-level rise. As the average tidal height increases, relative sediment levels will decrease, which will affect the distribution of habitat of varying sediment heights. This has the potential to alter the spread rates of Spartina. We develop and numerically analyze an integrodifference equation model of the spread of an invader capable of ecosystem engineering, in an environment heterogeneous in sediment level, subject to a constant amount of sea-level rise each generation.
Invasion proceeds most quickly for intermediate rates of habitat modification relative to rates of sediment erosion, because habitat modification increases sediment heights to optimal values for growth during the initial stages of invasion. However, high rates of modification can increase sediment heights beyond optimal values, slowing the invasion. If the habitat is initially suboptimal for growth of the invader, with many areas of very low sediment height, spread rate will be slow initially, but accelerate as habitat modification changes suboptimal habitat into optimal habitat. Conversely, if the initial habitat distribution is optimal, spread rate will eventually decline as the habitat is modified past the optimal sediment level. When most habitat is suboptimal, sea-level rise causes the spread rate of the invader to slow, because it counteracts the effects of habitat modification. However, when the habitat has been modified beyond optimal levels, this same counteraction between sea-level rise and the effects of habitat modification accelerates the spread of the invasion.