The Introduced Species Problem - Laserfiche WebLink

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eliminate their vulnerability to enemies through adaptation, both behavioral and morphological (Fryer 1986). How do fishes reduce vulnerability to enemies, and how can species introductions can be so devastating? It has been known for centuries that: "... major aspects of species' lifestyles are moulded by the impact of natural enemies..." (Jefferies and Lawton 1984), and even the morphology of a species can be changed by this interaction. One of the most interesting fish examples is predator- induced morphological change in crucian carp ( Carassius carassius), whose populations increase their body depth in the presence of northern pike to avoid being eaten (Bronmark and Minor 1992). This morphological change is so dramatic that affected individuals have been mistakenly identified as a separate species. It takes time to develop the strategies that adapt one species to the activities of others, and once developed, these strategies can be made less effective with physical or biological alterations of habitat. Evolution usually occurs slowly (i.e., over geologic time scales), but abrupt changes such as caused by human activities can disrupt the relationships previously developed between species. For example, a reduction in turbidity in the Missouri River could be expected to favor predation by native sauger on chubs, sturgeon, or other species adapted to life in turbid water. However, the introduction of nonnative fishes that may be "preadapted" to life in the altered environments may place native predator and prey at a disadvantage. In this case, lack of exposure to the nonnative species also could result in a naivete' to predation (e.g., see Johnson et al. 1993), perhaps due to different foraging tactics, and etc. In this case, defense strategies that evolved in response to sauger predation may not be appropriate for introduced walleye, who are more adaptable, occupy different habitats, forage differently, select different prey, and as a consequence are even known to displace sauger in some habitats (Scott and Crossman 1973, Rawson and Scholl 1978). The addition of other predators such as pike, bass, sunfishes, rainbow smelt, etc, to a system that continues to be changed by physical habitat alteration makes it hard to imagine how the native prey could survive for long. But not all impacts caused by nonnative fishes are due to predation. Competition for food and space also can be a powerful force (e.g., Schoenherr 1981, Taylor et al. 1984), especially with an increased vulnerability to predation. For example, a decrease in turbidity can be expected to result in shifts in habitat use by predator -wary prey (Miner and Stein 1996). In this case, less food and fewer places to hide greatly exacerbate problems caused by increased predation and can result in species extirpations and possibly extinctions over time. A better understanding of vulnerability to other species can be aided by an appreciation of interactions that develop by coevolution (i.e., evolution of 2 interacting species that adapt to characteristics of the other and each enhances the fitness of the other). In this case, coevolution can be considered "reciprocal evolution" among 22