Here's a question:
Do artificial substrates favor non-indigenous fouling species over natives?

"Yes" suggests Wells Reserve postdoctoral research fellow Megan Tyrrell.

In aquatic environments, "fouling communities" are composed of numerous organisms, some native to our region and others "from away," that attach to underwater surfaces (substrates). Megan set up several experimental substrates in Wells Bay, left them in place for a few months, then brought them back to the lab to find out what was growing on them. Here's a photo of one small square of marble (a natural substrate) supporting a diverse community including tunicates, hydroids, gastropods, and bryozoans.

Megan found that on natural substrates, natives and exotics developed in equal abundance. After 3 months on artificial substrates, however, exotic species increased dramatically at the expense of native species. This suggests that adding artificial substrates to nearshore environments could facilitate the spread of exotic species at the expense of native organisms. Or, as Megan and co-author Jeb Byers concluded in their manuscript:*

Artificial surfaces may provide a novel context for competitive interactions, giving exotic species a more level playing field in an environment for which they otherwise might not be as well adapted compared to long-resident native species. Additions of artificial substrates to nearshore environments may disproportionately favor exotic species increasing local sources of exotic propagules to colonize all types of substrates.

Update: The manuscript was published in 2007.