Predation, Herbivory and Defence Mechanisms

Eat, or be eaten

A lynx chases a snowshoe hare across deep snow. A moose strips the bark of a young balsam fir. A monarch caterpillar munches milkweed leaves full of toxins. These are predation and herbivory, both +/− interactions: one species benefits and the other is harmed.

• Predation: the predator kills and eats its prey.
• Herbivory: an animal eats plants or algae. Usually the plant is not killed outright, but it loses energy and may grow or reproduce less.

In this lesson you will see how predators affect prey populations and the many defences that prey have evolved.

How predators affect prey populations

Predators respond to more prey in two ways:

• Functional response: each predator eats more when prey are abundant, up to a limit set by how long it takes to catch and digest each one (it gets full).
• Numerical response: the predator population grows when prey are abundant, through more births and survival, and through predators moving into the area.

When these responses make the percentage of prey eaten rise as prey become denser, predation is density-dependent and can help regulate the prey near a lower level. But because predators need time to breed, their numbers often lag behind the prey. The result can be linked cycles.

Remember from Grade 9: the Hudson's Bay Company fur records revealed a roughly 10-year cycle of snowshoe hares and Canada lynx, with lynx peaks following hare peaks. Long-term field studies, such as Charles Krebs and colleagues' experiments at Kluane Lake in the Yukon, showed that the hare cycle is driven by a combination of food supply and predation (by lynx, coyotes, owls and hawks), not predators alone. You will model linked cycles yourself in Investigation: Modelling Predator-Prey and Host-Parasite Populations.

Prey defences

Predators and prey coevolve (Grade 11 Evolution): better predators select for better-defended prey, and the reverse. Prey defences fall into several groups.

1. Physical (mechanical) defences: spines, shells, thick skin and hard seed coats. North American porcupines have about 30 000 quills that detach on contact. Turtles have shells; plants have thorns (hawthorn) and prickles (raspberry).

2. Chemical defences: toxins and bad tastes. Striped skunks spray a foul-smelling liquid. Milkweed makes toxic cardiac glycosides (cardenolides); monarch caterpillars store them, so monarch butterflies taste bad and can make birds vomit. Many plants make tannins, alkaloids and other defensive chemicals; some, like the bitter compounds in many wild leaves, are why we taste them as bitter.

3. Cryptic colouration (camouflage): blending in. Snowshoe hares moult from brown in summer to white in winter. Willow ptarmigan do the same. Many moths match tree bark.

4. Aposematic (warning) colouration: bold colours that say "I'm dangerous." Monarchs (orange and black), yellowjackets (yellow and black) and skunks (black and white) are easy to see and easy for predators to learn to avoid.

5. Mimicry: looking like something else (see below).

6. Behavioural defences: fleeing, hiding, alarm calls (red squirrels chatter), living in groups (more eyes to spot danger), and group defence: muskoxen form a defensive ring with calves in the centre. Some animals play dead: the Virginia opossum, now found in southern Ontario, can go limp and motionless when attacked.

Mimicry

• Batesian mimicry (named after Henry Walter Bates): a harmless species resembles a harmful one. Many hoverflies (flower flies) are striped like wasps but cannot sting. The mimic benefits because predators that have learned to avoid wasps also avoid it. Batesian mimicry works only if mimics are rarer than models; if mimics became too common, predators would learn that the pattern is often safe.
• Müllerian mimicry (after Fritz Müller): two or more harmful species share a similar warning pattern. Bumblebees, honeybees and yellowjackets share yellow-and-black patterns. Each species benefits, because predators learn the shared signal faster, so fewer individuals of each species are sacrificed while predators learn.

The monarch and the viceroy, an orange-and-black butterfly that looks very much like the monarch, were long used as the classic example of Batesian mimicry. Research since the 1990s has shown that viceroys are also distasteful to birds, so the pair is now usually considered Müllerian mimicry, a good reminder that scientific explanations are revised when new evidence appears.

Keystone predators: when one predator shapes a whole community

In the 1960s, ecologist Robert Paine removed a predatory sea star, Pisaster ochraceus, from rocky shore plots on the coast of Washington State. Without the sea star, mussels (its main prey) took over the space, and the number of species in the plots dropped sharply. Paine called the sea star a keystone species: like the keystone of an arch, removing it makes the structure collapse.

A Canadian example: sea otters eat sea urchins, and sea urchins graze kelp. After the fur trade wiped out sea otters along the British Columbia coast, urchins multiplied and kelp forests were reduced to bare "urchin barrens". Between 1969 and 1972, 89 sea otters from Alaska were released off the west coast of Vancouver Island. As the otters spread, urchins declined and kelp forests returned, providing habitat for fish and many other species. The effect also shows a trophic cascade: a predator's effect passes down through more than one feeding level.

For some coastal communities, the return of sea otters also brought difficult trade-offs, because otters also eat shellfish such as clams and crabs that people harvest. Some First Nations on the BC coast are working with scientists on how to manage these effects.