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Cormorants: Food Chains and Basic Ecological Principles

Published 01/25/11
By Barry Kent MacKay, Senior Program Associate

Double-breasted cormorant
(Painting by Barry Kent MacKay)

A cormorant lays only a few eggs. Depending on the species and the age of the individual, fish lay a great many eggs — dozens , hundreds, even many thousands. As a general guideline all those newly hatched fish are in competition with each other for resources (food) and in turn are close to the bottom of the food chain, consumed by various predators, in some cases including older, thus larger, individuals of their own species. Most of the fish of interest to anglers and commercial fishers are likely to die before they are big enough to be of interest to cormorants, but there are, at the outset, a great many of them.

We have seen that by the time cormorants reach a point of diminishing returns they move on, or die off, or fail to produce viable young. For the group, or “cohort,” of fish hatched that year, things are different. In the absence of those cormorants there is less competition for food, fewer predators, and so a greater percentage survives to breed, thus replenishing the species. This increased number of fish ultimately will benefit surviving cormorants and other predators, and they too will prosper. This effect is what is popularly known as the “balance of nature” whereby predator and prey influence the population sizes of each other, a process that has being going on for the 3 billion years since the beginning of life on Earth.

It is not quite as simple as what we have described, which is, in effect, a two-species food chain — the cormorant, and the species of fish it eats. In nature, and especially nature impacted in various ways by human activity, a vast number of other factors also influence the size of the population of both prey and predators. For example, heavy storms may increase the amount of silt (runoff) in the water, which might have the effect of reducing predation (because the fish are harder to see) or the prey (because it kills off fish, or the food the fish eat). Unusually cold weather might reduce or enhance the ability of fish to survive, thus reducing or increasing the amount available to cormorants and other predators. A new species of fish or invertebrate may enter the ecosystem, either on its own (for example, flooding might link two otherwise unconnected waterways, each with distinct species) or as a result of either accidental or deliberate human intervention, and this can have a variety of consequences that may enhance or decrease the ability of the environment to sustain either predator or prey, or both.

And apart from these and other factors that can create quick changes, there are so-called successional changes. These are changes that occur as a result of natural progressions. For example, as aquatic vegetation grows and dies and is replaced by new generation it can build up the land, diminishing the wetlands. Erosion can eliminate or create habitats for certain species. Open land can, through time, become heavily forested, which can have various impacts on adjoining wetlands.

Ecosystems are infinitely complex organizations of continued interactions among the organisms that inhabit them and the various inanimate parts of the ever-changing environmental whole, including weather. A volcanic eruption on the far side of the planet can have measureable effects on a pond in Iowa. A slight increase in average temperature can herald the advent of a new disease to the region. The ability of any given environment to sustain any given species — what is popularly known as the “carrying capacity” of the environment for the species in question – is not static, but rather is dynamic and ever-changing. Nature is a crucible in which genetic variables are continually tested as species either evolve to accommodate change, or die out. In the environment change is the only constant, although rate of change will vary enormously in response to many factors.

Many events that alter the carrying capacity of the environment to sustain a given species of plant or animal are subtle or indirect and may be subtle and hard to discern, but others are quite evident. The introduction of exotic species can have a profound effect on the environment. In the Great Lakes, zebra mussels, which are not native, have established themselves in vast numbers. These mussels are fairly small, but they grow in huge mats containing large numbers of individual organisms. They are what are called “filter feeders” because they ingest water and extract, or “filter out” the nutrients. Some of those nutrients are thereby removed from the environment to the detriment of the small fish who would otherwise eat them, and are, in turn, part of the food chain.

But the story is more complex. By removing detritus and micro-organisms from the water, zebra mussels can make the water clearer, thus it becomes easier for fish predators, including cormorants, to see their prey. The mussels are eaten by some wildlife species, including diving ducks called scaup. But this is not necessarily good for the predators because the mussels can accumulate toxins from the environment, and as toxins move up the food chain their effect multiplies in the bodies of their predators.

It is too soon to know how all this will work out, but it is sobering to realize that more than 136 alien species of animals and plants have entered the Great Lakes since the 1800s. Most have little or no measurable impacts on native wildlife, but some cause significant changes, and there is no doubt that there are more to come. And it is sobering to realize that in some river systems, over half the fish species present are alien, either purposely introduced to provide prey for anglers, or accidentally introduced as discarded aquarium fish or as escaped live bait.

Introduction: Why Cormorants Can't Kill All Fish

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Back in the Real World

NEXT » The Alewife, Alien Salmon and Trout, and the Double-Crested Cormorant

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So ... What Do Real Scientists Say?


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