tia of the water to drive her body with enough speed to overcome the fish, while holding her breath (oxygen being essential for metabolism to occur and birds, like us, get their oxygen from air) and maintaining all bodily functions. During the nesting season the amount of fish captured must meet her increased needs to produce eggs, and later to feed her young. Not all of the fish is converted into what the body needs, with the remainder ejected as excreta that contains concentrated nutrients often transferred from water to land.
Let us imagine a situation where cormorants really could eat all the fish. Visualize a swimming pool, stocked with 200 or so fish, each maybe 4 to 5 inches long, thus a perfect size for cormorants to swallow. Let us further pretend that these fish are sluggish in movement, thus easy to catch.
Now let us imagine two hungry cormorants placed in the pool. They must eat to survive, obviously, but what would happen if the only place they could eat was in the pool? At first the cormorants would do well, there being 200 fish distributed in the pool. It would be relatively easy to catch the first ones as none would be too far away if they were evenly distributed. If concentrated in one part of the pool they’d also be easy to catch. As the number of fish went down the cormorants would have to expend still more energy to catch each fish because each fish would have more space to flee.
Eventually one of two things would happen: In Scenario One the cormorants would eventually catch all the fish and then starve, because that was the only place where they could eat (and we are imagining fish who are conveniently sluggish in movement, so ultimately all could be caught). That would mean no fish and thus, soon after, no cormorants.
Scenario Two requires that we imagine the fish not to be so sluggish and the pool larger. The fish are fast-moving, and whoever owns the pool makes sure that they have lots to eat so that they maintain high energy levels.
In this state of affairs as the number of fish decreases, the amount of energy needed to catch the next one would increase, as in the first scenario. The difference is that in this imaginary situation there would eventually be so few fish that the amount of energy required to catch them would be more than they provided when caught and consumed, and the cormorants, having reached that point of diminishing returns, would die. There might be very few fish, but that would be a temporary condition. Fish produce lots of eggs, lots of young — some species producing hundred, even thousands — and since the cormorants would be gone, there would be a greatly increased survival of young fish so long as the pool’s owner keeps the fish food coming. Eventually, even with unlimited food, there would be so many fish that there wouldn’t be enough water to sustain them and they, too, might become extinct ... at least in the pool.
NEXT » Back in the Real World
Cormorants: Food Chains and Basic Ecological Principles
The Alewife, Alien Salmon and Trout, and the Double-Crested Cormorant
The Round Goby
The Difference Between Science and Management
The Agricultural Subsidy
The Missing Predator Argument
Semi-Science and Wildlife Management
So ... What Do Real Scientists Say?