Exponential growth in an invasive species

During a hurricane in 1962, five captive mute swans (Cygnus olor) escaped into the Chesapeake Bay, in Maryland. Since they were pinioned and therefore flightless, their chance of survival during the winter was considered negligible and no attempt was made to capture them. One pair, however, successfully nested. By 1975 the descendents of this original pair numbered approximately 200, and by 1986 totaled 264. By 1999 the estimated population of mute swans in the Chesapeake Bay was 3955 (Anonymous 2003, Sladen 2003, Craig 2003). In 2001 the Maryland Department of Natural Resources, in an effort to control the swan population, began shaking (addling) mute swan eggs or covering them with corn oil to terminate embryo development. Mute swans were also removed from Federal National Wildlife Refuges. The result was a decline to 3624 in 2002 (Anonymous 2003). As shown in Fig. 1.5, prior to these control efforts, the population was growing exponentially with an intrinsic rate of increase of 0.17 and a doubling time of four years! (As an exercise, try using Equation 1.10 to verify the doubling time.)

5000 r 4000

2 2000 CD

1000 0

1962 1972 1982 1992 2002

Census dates

Figure 1.5 Mute swan (Cygnus olor] population in the Chesapeake Bay since 1962.

1962 1972 1982 1992 2002

Census dates

Figure 1.5 Mute swan (Cygnus olor] population in the Chesapeake Bay since 1962.

So what's the problem? Swans are considered graceful, even "majestic," and are thought of as harmless by their admirers. However, mute swans, in addition to being a non-native species, have become permanent residents. That is, they do not migrate as do other swan species. Recent data show that an average adult swan eats 3.6 kg of submerged aquatic vegetation (SAV) a day (Craig 2003). This is occurring at a time when biologists are struggling to re-establish SAV in the Bay. Is it necessary to control the mute swan population? If so, how?

The Fund for Animals took the US Fish and Wildlife Service to court to stop its plan to kill 525 swans in 2003 (Craig 2003). The debate evidently will continue for the indefinite future.

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