Ecosystem Engineering

functional consequences for habitats

‘Ecosystem engineers’ are organisms that physically create or modify habitats, either through their activities or through the structures that they create. I have a long-standing interest in marine ecosystem engineers, especially in soft-bottom habitats.

Climate change-related range shifts have been widely documented for a great variety of terrestrial and marine organisms. When ecosystem engineers change their geographic range, cascading ecosystem-level effects may be expected. In Northwestern Europe, we showed that ecosystem engineering polychaetes in the Diopatra genus are expanding their ranges poleward in response to climate change. This range expansion is introducing large tube structures into sedimentary habitats that have historically lacked such structures. Diopatra tubes have well-known physical effects on habitats, altering local flow regimes, stablizing sediment at depth, enhancing surface scour, and enhancing the diversity and abundance of other organisms. Diopatra is expanding to habitats that are typically dominated by the burrowing lugworm Arenicola marina, an ecosystem engineer in its own right which has very different effects on habitats--an active burrower, A. marina mixes sediment, destabilizing the habitat and reducing the abundance and diversity of other organisms.  The resulting interaction between functionally different ecosystem engineers could well cause ecological changes in northern European coastal waters.

Latitudinal gradients in ecosystem engineering are an important component of large-scale variability in ecosystem function. I have recently shown strong geographic patterns in the ecosystem engineering activities of a tube-building worm, Diopatra cuprea, in the Eastern United States. D. cuprea facilitates algal communities by attaching algae to its tube. It also enhances the diversity and abundance of other animals by (i) directlly providng habitat, in the form of the tube and attached algae, and (ii) protecting smaller organisms from predation. The degree of protection depends on the abundance D. cuprea tubes as well as on the amount of algae attached, so D. cuprea’s ecosystem engineering effects have both behavioral and abundance-related components.

D. cuprea is abundant and copiously attaches algae throughout most of its range, from Cape Cod through Georgia. In this region, intertidal mudflats are commonly dominated by D. cuprea assemblages. This shfits abruptly in Florida, where D. cuprea abundance plummets and--most strikingly--it stops attaching algae to the tube almost entirely. The algal pattern represents a true behavioral shift: even in the laboratory, Florida worms eschew algal species that are readily attached by their northern counterparts. The mechanisms underlying these patterns are not immediately clear, but latitudinal gradients in predation and/or herbivory might play a role, as might changes in the physical properties of coastal sediments in Florida habitats. Fully understanding the mechanisms will likely require a deeper understanding of tube-worm-environment interactions.

Marine Ecosystem Engineers in a Changing World was an NSF-funded symposium that I conceived and co-organized for a recent Society for Integrative and Comaprative Biology meeting. An issue of Integrative and Comparative Biology was dedicated to papers from the symposium.