Overview & Lesson Sequence

Focus on Standards & Assessment

Materials Needed, Preparation & Planning, Management Strategies

Background Info

Intro Activity
Why Study Predator - Prey interactions?

Activity 1
Observation of a predator-prey interaction

Activity 2
What makes a predator successful?

Research Project
Designing a predator - prey experiment

Optional Activity
Biological Control vs. Pesticides

Case Study
Rabbits in Australia

References

Glossary

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Background Info

Predators
Predation is a way of life in which the primary source of food is obtained by the catching, killing and eating of other animals. Important predators of arthropods include vertebrate animals such as birds, bats, frogs, rodents and fish, and invertebrate animals such as insects and spiders. Predatory arthropods can be as large or larger than their prey and are often fast moving and possess good searching capabilities.  They use many different strategies for catching and subduing their prey such as a web, raptorial legs, or paralytic saliva.  Some predatory arthropods are predators in only one life stage, such as larval hover flies (Serphidae) or throughout all active stages, such as ladybird beetles.

Evolution of Mouthparts
Predators like the ladybird beetle and the praying mantid bite and chew their prey while others (green lacewing larvae and big-eyed bugs) use a sharp beak-like structure to suck fluid from their prey. The feeding structures of these and all insects evolved from the chewing mouthparts typical of insects that existed 400 million years ago. Insect mouthparts changed over time to form feeding apparatus capable of ingesting solid food, filtering particulate matter from water, and siphoning and sponging liquid foods. This evolution of insect mouthparts was discovered through the study of fossils dating to the late Devonian Period and extending to the early Cretaceous. Fossil records indicate that adaptation occurred through fusion of the mouthpart regions common to all insects (the labrum, hypopharynx, mandibles, maxillae, and labium). The evidence of feeding adaptations in ancient insects corresponds with the emergence of different food sources. For example, fossil records of insects with mouthparts suitable for collecting nectar and pollen coincide with the emergence of flowering plants. Fossil records of plant damage and insect gut contents corroborate this evidence.

Prey
In order to survive and reproduce, many prey have evolved cryptic coloration or hard body armor to escape predation.  Others sequester poisonous or distasteful plant toxins to avoid being eaten and still other prey exhibit threatening or hiding behaviors that increase their chances of surviving and reproducing. Some common prey, such as aphids and rabbits, have amazing reproductive capacities whereby they produce large numbers of young in a short amount of time in the hopes that at least a few will reach adulthood and reproduce.  Those prey that develop structures or behavior that thwart predators will survive, but predators are also developing structures and behaviors that allow them to catch and eat prey.  In many ways the predator-prey relationship is like an arms race that has reached détente.

Insect Note
People eat insects, too.  Human preparation and consumption of insects is called entomophagy.

Predators in Agriculture
Although no one has taken a census of all species of predatory arthropods important in agriculture, the number would likely reach into the thousands.  For example, there are about 400 North American species in the ladybird beetle (Coccinellidae) family alone. Many species have repeatedly been shown to be important in crop systems.

Most predators are considered generalists in that they will capture and eat almost anything that they can find within the range of habitats that they live. Some groups tend to specialize, such as the ladybird beetle in the genus Stethorus, which feeds only on spider mites. The adults of many important predators require alternate foods, or can subsist on supplemental foods, especially flower nectar and pollen.

Prior to the insecticide revolution in the 1940's, an estimated 7% of the world’s crops were destroyed by insect pests. Today some authorities estimate losses at 13%. This increase in crop destruction over the last half of the century is due to a number of modern agriculture practices including mass planting a single commodity over several hundred acres, called monoculture. Although monoculture is practical and cost effective, it eliminates natural plant and insect diversity creating an unstable environment in which opportunistic pests can thrive. Increased crop losses can also be attributed to the destruction of the natural enemies of insect pests by pesticides and to the absence of refugia (untreated land) to which beneficial insects can escape. These problems, coupled with increased environmental awareness, have forced agronomists to seek more environmentally safe and cost-effective pest control strategies that treat the target pest without destroying its natural enemies. The use of Bacillus thuringiensis (Bt) is a good example. Bt, a naturally occurring bacterium, has long been applied to both organic and conventionally grown crops to control insect pests. When the bacterium is ingested, it produces a toxin that attacks the gut of its host. There are many strains of Bt, each with an ability to attack a specific group of insects.  For example Bacillus thuringiensis  subspecies kurstaki only harms the digestive tract of lepidopterous insects (butterflies and moths), while Bt subspecies israelensis  attacks only mosquitoes and blackflies in the order diptera.  Bacillus thuringiensis is not harmful to humans and other vertebrates.

Genetic material from Bt has been successfully transferred into living plants to form built-in resistance i.e. transgenic crops.  Surveys of a transgenic Bt potato field in Oregon, planted to control the Colorado Potato Beetle, showed that transgenic plants do not harm predatory insects or reduce their effectiveness because the predator does not feed on plant tissue. In fact, the big-eyed bugs (a predator present in the fields) more effectively controlled the beetle population in transgenic fields over non-transgenic fields.  However, a study of aphids on transgenic potatoes in Scotland showed that predatory insects could be affected indirectly.  Aphids were apparently capable of sequestering the Bt toxin from the transgenic crop and transferring it to the ladybird beetle predators, reducing reproduction and longevity of the beneficial beetles. This suggests that the use of transgenic Bt crops may in fact affect populations of predators.

Host/Parasite Relationship
Parasites are a special form of predator.  They usually do not devour the host organism, at least until the parasite can reproduce and be transmitted to another host.  Over time, some hosts and parasites have co-evolved to the extent that parasites live permanently with their host organism. In many cases, complex life cycles have evolved in which the parasite moves from one prey species to the next over its life cycle, for example, the parasite that causes River Blindness must first complete part of its life cycle in a blackfly before being transmitted to a human to complete it's life cycle. The host may be harmed or simply used as a vector to transmit the parasite into another organism. Insects are often vectors or carriers of parasites that cause human disease for example, ticks can carry the Lyme disease bacterium and mosquitoes can carry malaria parasites, but neither insect appears to be harmed by the presence of the parasite.

Because of these very elaborate adaptations, most parasites are specific to one or two hosts. The parasite must find satisfactory conditions in a host organism in order to establish itself, grow, reproduce and possibly transmit its offspring to another host. For example, cat and dog hookworms cannot be transmitted to humans because humans do not provide the appropriate environment for them to develop and reproduce. Other organisms can affect more than one host, for example, trichina worms can cause trichinosis in humans when humans eat infected pork that is not thoroughly cooked.  One positive aspect of species-specific parasites is that they can be used to control pest organisms without infecting other, possibly beneficial, organisms such as with Bt described above.