- Position1 of 3›
- Invasive species harm native animals
- Argument‹3 of 3
Invasive species disrupt food chains
Invasive species eat native prey and kill native predators. Without prey, predators will starve. Without natural predators, the invasive species will reproduce, and their population will grow out of control.
The argument
This argument concerns the structural consequences of introducing a species into a habitat that did not evolve alongside it. A food chain is an arrangement of dependencies: each species is constrained by what it eats and by what eats it, and those reciprocal limits are what hold the populations in a habitat at workable levels. An invasive species enters that arrangement without the constraints that would have developed in it, and in doing so disrupts the habitat's food chain. The cane toad illustrates how the disruption can run in several directions at once. The toads were introduced deliberately, to get rid of beetles that were eating sugar cane — a narrow objective concerned with a single pest. What followed was not confined to that objective. The toads killed native predators, the animals that would ordinarily have exercised some control over a newly abundant species, and they also ate plants and small mammals. The consequence was that the introduced animal simultaneously removed the checks that might have contained it and consumed organisms that other native species depended upon. That combination is what makes the effect a disruption of the chain rather than the addition of one more link to it. When an invasive species begins consuming native species, the losses are not absorbed in isolation: each native population that is depleted was itself food or control for something else, so the effect propagates through the dependencies that the chain consists of. The habitat's established balance is thrown out and the species that relied on it are left without the prey, the cover or the predators that their own numbers were calibrated against. Because invasive species disrupt food chains in this way, they harm the native animals that those chains supported.
Premises
Counter-arguments
Invasive species can change the food chain for the better. The red fox was a helpful hand in thinning out wild rabbits in Australia. The rabbits were an invasive species that multiplied rapidly and became a problem. Therefore, the introduction of another invasive species worked well.
Rejecting the premises
[Rejecting P1] Invasive species can refine the food chain. [Rejecting P2] Native species can adapt to changes.