L28. Predators, Prey, and Competition

Ecosystems and Biodiversity

R-report

L28. Predators, Prey, and Competition

What happens to a small pond when a new predator is added — and why do some species survive while others decline?

Big idea and a quick link to what you already know

In ecosystems, predators eat other organisms (prey). That direct action—one animal eating another—changes how many individuals of each species survive and reproduce. You have already studied population limits: populations rise or fall because of births, deaths, and resources. Predation is one of the causes of death that can strongly change a population’s size and how animals behave. This lesson focuses on three related ideas: how predators affect prey numbers, how prey change their behavior or traits to avoid being eaten, and how predators and prey can compete with other species for food and space. We will study a short, real-style investigation: two similar ponds, but in one a new fish predator is introduced. Watching the differences helps us see direct and indirect effects in a short time.

Direct effects: what predation does, and how prey respond

When a predator catches and eats an animal, the prey population usually drops. But the system doesn’t stop there. Prey animals often change where they feed, when they are active, or even their body traits over many generations. These changes affect survival and competition for remaining resources. Key factors that shape what happens (quick list):

• Predator hunting style — ambush versus pursuit affects which prey escape. Prey defenses — camouflage, speed, schooling, toxins, and hiding places reduce chances of capture. Resource levels — abundant food can help prey reproduce faster, offsetting losses from predation. Competition — when prey decline, other species that use the same food or shelter may rise or fall.

Indirect effects and competition: more than one winner or loser

Predators can trigger indirect changes that look like competition even if the species do not eat each other. For example, if a predator reduces a prey species that previously ate lots of algae, algae may grow more. That change benefits algae-eating animals, which now have more food and may increase in number. Sometimes two predators compete: a new predator may eat the same prey species as a native predator, reducing food for the native one. Competition can be direct (fighting over food) or indirect (one species reduces the food of another). These indirect links are important because they create chains of effects. In the short term you see numbers change; in the longer term you may see behavior, body size, or life cycle timing shift as species adapt to new pressures.

Short investigation: two island ponds, one new predator

This comparison shows direct predation (fish eating tadpoles and insects) and indirect effects (less grazing on algae, which changes plant cover and affects other animals). It also shows competition: the new fish competes with native insect predators for the same prey, reducing the insects’ food and their numbers. From this short study, managers could decide whether to remove the fish or allow the pond to reach a new balance, depending on goals for native species and human uses.

• Pond A (with new fish): tadpole and small insect numbers fall. Insects that hide in dense plants decline less than those that swim openly. Algae cover increases because fewer tadpoles are grazing it. Some small birds that ate the insects begin visiting less often. Pond B (no fish): tadpole and insect numbers stay higher and more stable. Algae stays at a lower cover because tadpoles keep eating it. Native predator insects and birds continue their usual patterns.

What this teaches us

Predators change ecosystems directly by removing individuals and indirectly by altering behavior, competition, and resource levels. A single new predator can lower some populations, allow others to rise, and shift who competes for food and space.

Paying attention to both direct and indirect effects helps predict outcomes and make better choices. In real management, scientists compare places with and without the predator (like the two ponds) to decide whether to remove the predator or adapt to a new balance.