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Lecture 12

Lecture 12 - Predation and Herbivory

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Biology 2483A
Hugh Henry

LECTURE 12: PREDATORS AND HERBIVORY Introduction  Over half the species on Earth get energy by feeding on other organisms, in a variety of types of interactions  All are exploitation – a relationship in which one organism benefits by feeding on, and directly harming, another  Herbivore – eats tissues of living plants or algae  Predator – kills and eats other organisms, referred to as prey  Parasite – lives in or on another organism (its host), feeding on parts of it o Usually doesn’t kill the host o Some parasites (pathogens) causes disease  Not all organisms fit neatly into these categories. Some predators, such as wolves, also eat berries, nuts, and leaves  Parasitoids are insects that lay an egg on or in another insect host. After hatching, larvae remain in the host, which they eat and often kill Predators and Herbivores  Predators and herbivores both tend to feed on multiple individuals of prey or food plants in their lifetimes  Herbivores don’t usually kill the food organisms as predators do  Some predators forage throughout their habitat in search of food  Others are sit-and-wait predators, remaining in one place and attacking prey that move within striking distance. These include sessile animals, such as barnacles, and carnivorous plants  Predators tend to concentrate effort in areas that yield abundant prey o Example: wolf packs follow seasonal migrations of elk herds  Sit-and-wait predators such as spiders relocate from areas of scarce prey to areas where prey are abundant  Most predators eat a broad range of prey species, but specialist predators show a preference (e.g., lynx eat more hares than expected based on hare abundance)  Some predators concentrate on whatever prey is most abundant. For example, when researchers provided guppies with two kinds of prey, they ate disproportionate amounts of whichever prey was most abundant  These predators may form a search image of the most common prey type and orient toward that prey  Or, learning enables them to become increasingly efficient at capturing the most common prey  In some cases, prey switching is consistent with optimal foraging theory  Herbivores eat different parts of the plant  Large herbivores may eat all aboveground parts, but most specialize on particular plant parts  Leaves are most commonly eaten, because they are often the most nutritious part, except for seeds  Herbivores can reduce the growth, survival, or reproduction of plants  Herbivores that eat seeds can impact reproductive success  Some herbivores feed on plant fluids by sucking sap, etc.  Belowground herbivores can also have an impact  Most herbivores feed on a narrow range of plants. Most insect herbivores feed on only one or a few plant species  Other herbivores (e.g., grasshoppers) feed on a wide range of species.  Large browsers, such as deer, often switch from one tree or shrub species to another Adaptations to Exploitative Interactions  Organisms have evolved a wide range of adaptations that help them obtain food and avoid being eaten  Life changed radically with the appearance of macroscopic predators about 530 million years ago  Within a few million years, prey had evolved defenses, such as body armor and spines  Predators exert strong selection pressure on their prey: If prey are not well defended, they die  Herbivores exert similar selection pressure on plants  Adaptations to escape being eaten: o Physical defenses  Large size (elephants)  Rapid movement (gazelles)  Body armor (snails, sea urchins) o Warning or aposematic coloration – predators learn not to eat organisms that have toxins o Crypsis – the prey is camouflaged, or resembles its background o Mimicry – the prey resembles another organism that is toxic or very fierce o Behavior  Not foraging in open areas  Keeping lookouts  Defensive circles  There can be trade-offs between behavioral and physical defenses. Snails with thickest shells are the last to take refuge  Plants also have defenses  Some produce huge numbers of seeds in some years and hardly any in other years (masting)  The plants hide (in time) from seed-eating herbivores, then overwhelm them by sheer numbers  Some plants have growth responses that allow them to tolerate effects of herbivory  Compensation – removal of plant tissue stimulates new growth o Full compensation – no net loss of plant tissue  Removal of leaves can decrease self-shading resulting in increased plant growth. Removal of apical buds may allow lower buds to open and grow  In field gentians, early herbivory results in compensation, but later in the season it does not. If too much tissue is removed, or there aren’t enough resources for growth, compensation cannot occur  Plants have many structural defenses: o Tough leaves, spines and thorns, saw-like edges, pernicious (nearly invisible) hairs that can pierce skin o Some are induced defenses—produced in response to herbivore attack. Some cacti increase spine production after being grazed  Plants also have chemical defenses: o Secondary compounds—toxic chemicals to reduce herbivory o Other compounds attract predators or parasitoids that will attack the herbivores o Some are produced all the time; others are induced. Predator Adaptations  For any prey defense mechanism, there is usually a predator with a countervailing offense  Example: cryptic prey could be detected by smell or touch instead of sight  Physical features of prey capture: o Snakes can swallow prey that are larger than their heads. The skull bones aren’t rigidly attached to one another, which allows the snake to open its jaws massively o Some predators subdue prey with poisons (e.g., spiders) o Some use mimicry, blending into their environment so that prey are unaware of their presence o Some have inducible traits (e.g., a ciliate that adjusts its size to match the size of the available prey) o Some predators can detoxify or tolerate chemicals made by prey organisms  Garter snakes are able to eat the rough-skinned newt, which has large amounts of tetrodotoxin (TTX), a potent neurotoxin o Some garter snake populations are resistant to TTX, but there are costs  Resistant garter snakes can’t move as fast as nonresistant snakes  After swallowing a toxic newt, the sna
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