Population Interactions and Community Ecology

Chapter Introduction

Three interacting populations. Ladybird beetles (Coccinella septempunctata) feed on aphids (order Hemiptera), which consume the sap of plants.

Why It Matters. . .

In some open woodlands in Central America, flocks of chestnut-headed oropendolas (Psarocolius wagleri), members of the blackbird family, build hanging nests in isolated trees (Figure 51.1). Female giant cowbirds (Molothrus oryzivorus) often bully their way into a colony, laying an egg or two in each oropendola nest. Cowbirds are brood parasites on oropendolas, tricking them into caring for cowbird young. The cowbird chicks grow faster than oropendola chicks, and they consume much of the food that the oropendolas bring to their own offspring. Because cowbird chicks take food away from their oropendola nest mates, we might expect adult oropendolas to eject cowbird eggs and chicks from their nests—but often they don't.

Figure 51.1. Potential victims of brood parasitism.

Chestnut-headed oropendolas (Psarocolius wagleri) rear their young in elaborate hanging nests. Some populations of oropendolas are subject to brood parasitism by giant cowbirds (Molothrus oryzivorus).

Why do some oropendolas care for offspring that are not their own? In an ingenious study conducted in the 1960s, Neal Smith of the Smithsonian Tropical Research Institute determined that cowbird chicks could actually increase the number of offspring that some oropendolas raise. Oropendola chicks are frequently parasitized by botfly larvae, which feed on their flesh. The aggressive cowbird chicks snap at adult botflies and pick fly larvae off their nest mates. Although cowbird chicks eat food meant for oropendola chicks, they also protect them from potentially lethal parasites; twice as many young oropendolas survive in nests with cowbird chicks as in nests without them.

In other areas of Central America, oropendolas build nests near the hives of bees or wasps. These oropendolas chase cowbirds from their colonies, and when a cowbird does manage to sneak an egg into one of their nests, the oropendolas frequently eject it. Why do oropendolas in these colonies reject cowbird eggs? Smith determined that the swarms of bees and wasps keep botflies away from the oropendola colonies. At these sites, twice as many oropendola chicks survive in nests without cowbirds as in those that include them. Thus the oropendolas derive no benefit from having cowbird chicks in their nests, and natural selection has favored discriminating behavior in oropendolas that nest near bees and wasps.

The story of the oropendolas, cowbirds, botflies, bees, and wasps provides an example of the population interactions that characterize life in an ecological community, an assemblage of species living in the same place. And as this story reveals, the presence or absence of certain species may alter the effects of such interactions in almost unimaginably complex ways. We begin this chapter with a description of some of the many ways that populations in a community interact. We then examine how population interactions and other factors, such as the kinds of species present and the relative numbers of each species, influence a community's characteristics.

Population Interactions and Community Ecology

51-1 Population Interactions

Population interactions usually provide benefits or cause harm to the organisms engaged in the interaction (Table 51.1). And because interactions with other species often affect the survival and reproduction of individuals, many of the relationships that we witness today are the products of long-term evolutionary modification. Before examining several general types of population interactions, we briefly consider how natural selection has shaped the relationships between interacting species.

Table 51.1. Population Interactions and Their Effects

Interaction	Effects on Interacting Populations
Predation	+/–	Predators gain nutrients and energy; prey are killed or injured.
Herbivory	+/–	Herbivores gain nutrients and energy; plants are killed or injured.
Competition	–/–	Both competing populations lose access to some resources.
Commensalism	+/0	One population benefits; the other population is unaffected.
Mutualism	+/+	Both populations benefit.
Parasitism	+/–	Parasites gain nutrients and energy; hosts are injured or killed.

Coevolution Produces Reciprocal Adaptations in Species That Interact Ecologically 51.1a

Population interactions change constantly. New adaptations that evolve in one species exert selection pressure on another, which then evolves adaptations that exert selection pressure on the first. The evolution of genetically based, reciprocal adaptations in two or more interacting species is described as coevolution.

Some coevolutionary relationships are straightforward. For example, ecologists describe the coevolutionary interactions between some predators and their prey as a race in which each species evolves adaptations that temporarily allow it to outpace the other. When antelope populations suffer predation by cheetahs, natural selection fosters the evolution of faster speed in the antelopes. Cheetahs then experience selection for increased speed so that they can overtake and capture antelopes. Other coevolved interactions provide benefits to both partners. For example, the flower structures of different monkey-flower species have evolved characteristics that allow them to be visited by either bees or hummingbirds (see Figure 21.7).

Although one can hypothesize a coevolutionary relationship between any two interacting species, documenting the evolution of reciprocal adaptations is difficult. As our introductory story about oropendolas and their parasites illustrated, coevolutionary interactions often involve more than two species. Indeed, most organisms experience complex interactions with numerous other species in their communities, and the simple portrayal of coevolution as taking place between two species rarely does justice to the complexity of these relationships.

Population Interactions and Community Ecology

51-1b Predation and Herbivory Define Many Relationships in Ecological Communities

Because animals acquire nutrients and energy by consuming other organisms, predation (the interaction between predatory animals and the animal prey they consume) and herbivory (the interaction between herbivorous animals and the plants they eat) are often the most conspicuous relationships in ecological communities.

Adaptations for Feeding

Both predators and herbivores have evolved remarkable characteristics that allow them to feed effectively. Carnivores use sensory systems to locate animal prey and specialized behaviors and anatomical structures to capture and consume it. For example, a rattlesnake (genus Crotalus) uses heat sensors on its head (see Figure 39.22) and chemical sensors in the roof of its mouth to find rats or other endothermic prey. Its hollow fangs inject toxins that kill the prey and begin to digest its tissues even before the snake consumes it. And elastic ligaments connecting the bones of its jaws and skull allow a snake to swallow prey that is larger than its head. Herbivores have comparable adaptations for locating and processing their food plants. Insects use chemical sensors on their legs and heads to identify edible plants and sharp mandibles or sucking mouthparts to consume plant tissues or sap. Herbivorous mammals have specialized teeth to harvest and grind tough vegetation (see Section 45-5).

All animals must select their diets from a variety of potential food items. Some species, described as specialists, feed on one or just a few types of food. Among birds, for example, the Everglades kite (Rostrhamus sociabilis) consumes just one prey species, the apple snail (Pomacea paludosa). Other species, described as generalists, have broader tastes. Crows (genus Corvus) consume food ranging from grains to insects to carrion.

How does an animal select what type of food to eat? Some mathematical models, collectively described as optimal foraging theory, predict that an animal's diet is a compromise between the costs and benefits associated with different types of food. Assuming that animals try to maximize their energy intake in a given feeding time, their diets should be determined by the time and energy it takes to pursue, capture, and consume a particular kind of food compared with the energy that food provides. For example, a cougar (Puma concolor) will invest more time and energy hunting a mountain goat (Oreamnos americanus) than a jackrabbit (Lepus townsendii), but the payoff for the cougar is a bigger meal.

Food abundance also affects food choice. When prey are scarce, animals often take what they can get, settling for food that has a low benefit-to-cost ratio. But when food is abundant, they may specialize, selecting types that provide the largest energetic return. Bluegill sunfish (Lepomis macrochirus), for example, feed on Daphnia and other small crustaceans. When crustacean density is high, the fish hunt mostly large Daphnia, which provide more energy for their effort; but when prey density is low, bluegills feed on Daphnia of all sizes (Figure 51.2).

Figure 51.2. An experiment demonstrating that prey density affects predator food choice.

Researchers tested the food size preferences of captive bluegill sunfish (Lepomis macrochirus) by offering them equal numbers of small, medium, and large-sized prey (Daphnia magna) at three different prey densities. Because large prey are the easiest to find, bluegills encountered them more frequently than small or medium-sized prey, especially at the highest prey density. The bluegills' selection of prey varied with prey density; they strongly preferred large prey when prey of all sizes were abundant.

Defenses against Herbivory and Predation

Because herbivory and predation have a negative impact on the organisms being consumed, plants and animals have evolved mechanisms to avoid being eaten. Some plants use spines, thorns, and irritating hairs to protect themselves from herbivores. Many plant tissues also contain poisonous chemicals that deter herbivores from feeding. For example, plants in the milkweed family (Asclepiadaceae) exude a milky, irritating sap that contains cardiac glycosides, even small amounts of which are toxic to vertebrate heart muscle. Other compounds mimic the structure of insect hormones, disrupting the development of insects that consume them. Most of these poisonous compounds are volatile, giving plants their typical aromas; some herbivores have coevolved the ability to recognize these odors and avoid the toxic plants. Recent research indicates that some plants increase their production of toxic compounds in response to herbivore feeding. For example, potato and tomato plants that have been damaged by herbivores produce higher levels of protease-inhibiting chemicals; these compounds prevent herbivores from digesting proteins they have just consumed, reducing the food value of these plant tissues.

Many animals have evolved an appearance that provides a passive defense against predation (Figure 51.3). Caterpillars that look like bird droppings, for example, may not attract much attention from a hungry predator. And as you learned in Chapter 1 (see Figure 1.9), cryptic coloration helps some prey (as well as some predators) to blend in with their surroundings.

Figure 51.3. Hiding in plain sight.

Some animals, such as (A) giant swallowtail butterfly larvae which resemble bird droppings and (B) some katydids that resemble insect-damaged leaves, do not attract the attention of predators.

Once discovered by a predator, many animals first try to run away. When cornered, they may try to startle or intimidate the predator with a display that increases their apparent size or ferocity (Figure 51.4). Such a display might confuse the predator just long enough to allow the potential victim to escape. Other species seek shelter in protected sites. For example, flexible-shelled African pancake tortoises (Malacochersus tornieri) retreat into rocky crevices and puff themselves up with air, becoming so tightly wedged between rocks that predators cannot extract them.

Figure 51.4. Startle defenses.

A short-eared owl (Asio flammeus) increases its apparent size when threatened by a predator.

Other animals defend themselves actively. North American porcupines (genus Erethizon) release hairs modified into sharp, barbed quills that stick in a predator's mouth, causing severe pain and swelling. Other species fight back by biting, charging, or kicking an attacking predator. Chemical defenses also provide effective protection. Skunks release a noxious spray when threatened, and some frogs and toads produce neurotoxic skin secretions that paralyze and kill mammals. Some insects even protect themselves with poisons acquired from plants. The caterpillars of monarch butterflies (Danaus plexippus) are immune to the cardiac glycosides in the milkweed leaves they eat. They store these chemicals at high concentration, even through metamorphosis, making adult monarchs poisonous to vertebrate predators.

Poisonous or repellant species often advertise their unpalatability with bright, contrasting patterns, called aposematic coloration (Figure 51.5). Although a predator might attack a black-and-white skunk, a yellow-banded wasp, or an orange monarch butterfly once, it quickly learns to associate the gaudy color pattern with pain, illness, or severe indigestion—and rarely attacks these easily recognized animals again.

Figure 51.5. Aposematic coloration.

Poisonous animals, like the harlequin toad (Atelopus varius) from Central America, often have bright warning coloration.

Mimicry, in which one species evolves an appearance resembling that of another (Figure 51.6), is also a form of defense. In Batesian mimicry, named for English naturalist Henry W. Bates, a palatable or harmless species, the mimic, resembles an unpalatable or poisonous one, the model. Any predator that eats the poisonous model will subsequently avoid other organisms that resemble it. In Müllerian mimicry, named for German zoologist Fritz Müller, two or more unpalatable species share a similar appearance, which reinforces the lesson learned by a predator that attacks any species in the mimicry complex.

Figure 51.6. Mimicry.

(A) Batesian mimics are harmless animals that mimic a dangerous one. The harmless drone fly (Eristalis tenax) is a Batesian mimic of the stinging European honeybee (Apis mellifera). (B) Müllerian mimics are poisonous species that share a similar appearance. Two distantly related species of butterfly, Heliconius erato and Heliconius melpomene, have nearly identical patterns on their wings.

Despite the effectiveness of many antipredator defenses, coevolution has often molded the responses of predators to overcome them. For example, when threatened by a predator, the pinacate beetle (Eleodes longicollis) raises its rear end and sprays a noxious chemical from a gland at the tip of its abdomen. Although this behavior deters many would-be predators, grasshopper mice (genus Onychomys) of the American southwest circumvent this defense: they grab the beetles and shove their abdomens into the ground, rendering the beetle's spray ineffective (Figure 51.7).

Figure 51.7. Coevolution of predators and prey.

(A) When disturbed by a predator, the pinacate beetle sprays a noxious chemical from its posterior end. (B) Grasshopper mice overcome this defense by shoving a beetle's rear end into the soil and dining on it headfirst.