The spread of species beyond their native range can have severe impacts to the invaded ecosystems. No habitat is immune from
biological invasions; non-native species have been found virtually every place that has been thoroughly searched for them. Never-
theless, all habitats are not equally susceptible to invasions.
Non-natives often show up in human-disturbed habitats, while
some other habitats appear to resist invasions. We recently carried out an investigation on the incidence of non-native species found
in different marine habitats, at two spatial scales.
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| Elkhorn Slough National Estuarine Research Reserve researcher Susie Fork monitors mudflat communities at the slough. photo Kerstin Wasson |
First, we quantified number and abundance of species in samples of hard (rock and oyster shell) and soft (mudflat) substrates from the same sites in Elkhorn Slough, the major estuary in the Monterey Bay area. We found that 52 percent of the species on hard substrates were non-native, while only 21 percent of those on or within soft substrates were non-native. This difference was particularly pronounced for sessile (non-mobile) species: 77 percent versus 7 percent were non-native in hard versus soft substrates. Similar patterns were obtained for abundance: non-natives dominated hard, but not soft, substrates.
The simplest explanation for this difference is that boat traffic and oyster culturing, the main vectors introducing exotic species to Elkhorn Slough, disproportionately transported non-native species associated with hard substrate in the slough. Soft sediment invaders also face rigorous competition from the diverse native species inhabiting estuarine mudflats. In contrast, there are very few native hard substrate specialists in our estuaries, which historically had very little in the way of natural hard substrates. So non-native hard substrate specialists that encounter the extensive artificial hard substrates (gravel bars, jetties, docks, etc.) in estuaries today may face little competition from natives and thus readily establish there.
The second component of our study compared the incidence of non-natives in estuarine versus open coast habitats. We compiled a database for Elkhorn Slough, building on observations of scientists at Moss Landing Marine Laboratories and other regional institutions over many decades. Overall, we documented 527 invertebrate species for the slough. For the open coast, surveys by University of California Santa Cruz students in the 1970s and 1990s of rocky intertidal habitats of the Monterey Bay National Marine Sanctuary compiled a list of 588 invertebrate species.
The difference between these lists was striking. The absolute number of exotic species in the estuary was an order of magnitude higher than along the open coast (fifty-eight versus eight species), as was the percentage of the invertebrate fauna that was exotic (11 percent versus 1 percent). The eight non-natives found on the open coast made up a subset of the Elkhorn Slough invaders, and all were rare. In contrast, many of the fifty-eight non-natives in Elkhorn Slough were highly abundant. The finding that the more species-rich habitat -- the open coast -- is less invaded contrasts with many terrestrial examples, where native and exotic species richness appear to be positively correlated at a broad geographic scale.
Again, a variety of causes can be invoked to explain the bias of invasions in sheltered estuaries. Boat traffic and culture of non-native oysters in estuaries certainly enhanced introductions there. In addition, limited dispersal of larvae in the somewhat closed circulation of estuaries may enhance the establishment of breeding populations. Moreover, estuaries on this coast are geologically young and have less diverse invertebrate communities than their more ancient counterparts on the open coast. Consequently, invaders to estuaries may face less competition from natives than invaders on the open coast. For instance, the European green crab (Carcinus maenas) occurs in both habitat types along the geologically young North American Atlantic coast but is only found in estuaries along the Pacific coast, perhaps because the richer crab fauna of the rocky intertidal prevents it from becoming established there. Finally, open coast communities may be better able to resist invasions because their members evolved under conditions similar to the present ones, while those of estuaries have been subject to substantial human alterations in recent centuries. In California, estuaries have certainly been subject to more pollution and changes in habitat structure
than have rocky intertidal habitats of the open coast.
Our investigation revealed that marine invertebrate invasions
vary dramatically across habitats. In our region, the rocky intertidal of the open coast is barely invaded, while estuarine habitats are highly invaded. Estuaries thus require particular focus for prevention and early detection of invasive species. Within the estuary,
hard substrates are more heavily invaded than soft sediments. However, since hard substrates are mostly artificial, they may
merit less conservation focus than the rich and spatially limited mudflat communities that face many threats, including invasions.
Kerstin Wasson1, John Pearse2, and Katherine Fenn3
1Elkhorn Slough National Estuarine Research Reserve
2University of California Santa Cruz Long Marine Laboratory
3Miami University
For information on exotic species in sanctuary harbors, please see Ecosystem Observations 2003, p. 21.
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