Invasive species in marine ecosystems

Michael Neubert

Invasions by "introduced species" transported beyond their natural ranges are creating ecological and economic crises throughout the world. While estimating the economic costs of introduced species is a tricky and sometimes controversial enterprise, by any accounting these costs are staggering. US agricultural costs alone may be over $138 billion per year, and the Nature Conservancy estimates that just the 79 worst alien species have already cost the US economy $97 billion.

While agricultural pests garner the most media attention, invasions are also an increasing problem in marine systems, especially in coastal ecosystems. The US Fish and Wildlife Service reports more than 200 exotic species in the San Francisco Bay area alone, with one new species invading the Bay Area estuaries every 12 weeks. Examples of marine invaders are dishearteningly easy to come by:

Zebra Mussels Photo courtesy of Wisconsin Sea Grant

• Zebra and brown mussels—One of the more infamous freshwater invaders, the zebra mussel (Dreissena polymorpha), was recently joined by a marine cousin attempting its own invasion along the Texas Coast. The brown mussel (Perna perna), native to Brazil, was first spotted in 1990 near Corpus Christi and has since spread to cover over 900 miles of the Gulf coast. Population sizes remain small, but studies are already underway to find ways to control their spread because of the damage they can do; their weight can sink offshore buoys.
• European periwinkle—Introduced in Nova Scotia circa 1840, the European periwinkle (Littorina littorea) has since transformed much of the New England and Canadian Atlantic shoreline. By consuming marine plants that induce mud accumulation, they have dramatically increased the proportion of rocky shoreline compared to salt marshes and mud flats.
• Green crab—The green crab (Carcinus maenas), native to the European and North African Atlantic coasts, is a voracious consumer of many commercially valuable clams, oysters, and mussels. It is also an intermediate host of a parasitic worm that attacks shorebirds. Green crabs established a foothold in North America in the early 1800s, but first appeared on the West Coast in San Francisco in 1989. They are now spreading along the California, Oregon, and Washington coasts.

Tropical Green Algae Photo by A. Meinesz

• Tropical green algae—An aquarium culture of the tropical green algae Caulerpa taxifolia was accidentally released near Monaco in 1984. Today it covers thousands of hectares of the northwest Mediterranean, overgrowing the native seaweeds and sea grasses that serve as nurseries for valuable fish species. It has recently appeared in North America (near San Diego) and in Australia.

When an invader first appears, its geographic range is small. The rate of range increase, called the invasion speed, is an important aspect of the pattern of spread. Understanding the processes that control invasion speed helps managers decide where, how, and how quickly to act. Over the past several years, my colleagues (including Senior Scientist Hal Caswell of WHOI and Professor of Applied Mathematics Mark Kot from the University of Washington and Mark Lewis from the University of Utah) and I have been developing mathematical models of biological invasions. These models, formulated using "integrodifference" equations, permit great flexibility in incorporating dispersal distributions and demographic processes, and can be linked closely to empirical data.

Here is an example of how an integrodifference equation model works. Imagine a population of an invasive mussel along a stretch of coast. Let's say that in the year 2000 we measure the number of mussels per unit length of coastline at each spot on the coast. Now imagine that every mussel produces 10 offspring between 2000 and 2001, and then dies.

Each of these offspring has a probability of moving to a different location. To predict the number of mussels per unit length of coastline in 2001 at a given location x, we take the number of offspring produced at location y, multiply by the probability that they move to x, and add up the potential contributions from every location y along the coast. We repeat this process to determine the spatial distribution of mussels in year 2002 from the distribution in year 2001 and so on.

Of course, nature is not as simple as I have just described it. Individuals within a population differ in their rates of survival, growth, development, and reproduction; the environment varies from place to place and from year to year; and dispersal is determined by behavioral properties of the organism and physical transport processes. These factors combine to produce characteristic distributions of dispersal distance.

One focus of our research is to understand the effects of these real-world complications. While incorporating such complexities into a mathematical model is relatively easy, analyzing the resulting model is not. The art comes in judiciously eliminating the processes that are not important and incorporating the remaining processes in a way that still permits analysis.
Invasions by noxious pest species are not the only instances in which population growth and spread are important. The design of marine reserves and of biological control programs (introducing a predator to control the pest species) and the responses of animal and plant home ranges to climate change all depend on rates of population growth and dispersal.

We expect that our theoretical work using integrodifference equation models will help applied scientists deal with these important ecological issues. Financial support for our efforts has come from the National Science Foundation; The Charles Davis Hollister Fund for Assistant Scientists; a Texaco Research Award in Science, Technology and Public Policy; and the Lawrence J. Pratt and Melinda M. Hall Endowed Fund for Interdisciplinary Research.

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