Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Introduction • Ecology is the science that deals with all kinds of biological interactions. • Individuals of all species interact in various ways with individuals of their own and other species and with their physical environment. • The term “environment,” as used by ecologists, includes both abiotic (physical and chemical) factors, and biotic factors (all other organisms living in an area). • Behavioral ecology is the study of how animals make “decisions” (not necessarily conscious ones) that influence their survival and reproductive success. Responding to Environmental Variation • Throughout their lifespan, all organisms must make many decisions and life choices in a changing environment. • Some changes, such as the approach of danger, require an immediate response; others allow for a more gradual response. • Some plants detach their leaves in a wind storm and regrow leaves afterward. (See Figure 51.3.) • Lizards bask in the morning sun and then move to the shade when it gets too hot. • Organisms may evolve life cycles that anticipate cyclical environmental change. • Insectivorous birds leave high latitudes in autumn for more favorable wintering grounds; grazing animals follow the rains. (See Figure 53.2.) • Other animals hibernate until environmental conditions have improved. Animals choose where to live • The environment in which an organism normally lives is called its habitat. • The location in which an individual lives and the way it uses that environment strongly influence its survival and reproductive success. • For choosing a habitat, an animal seeks food, resting places, nest sites, and escape routes from predators. • Numerous factors influence how animals choose environments, but in general, habitat selection cues are good predictors of general conditions suitable for survival and reproduction. • An example of habitat selection cues is that used by red abalone. • The abalone begins its life as a motile larva and swims in the open ocean until its yolk sac is gone (about seven days). • Guided by chemical signals from coralline algae (the abalone’s food source), the abalone settles on the seafloor and metamorphoses into an adult. • The example of the flycatcher used in the textbook shows that animals can use the presence and success of already-settled individuals as an indication that the habitat may be good ground. (See Figure 53.4.) • When flycatchers arrive on their breeding grounds in the spring, they peer into the nests of other individuals. • In studies in which broods were artificially enlarged by researchers, birds settled preferentially in those areas. • An animal may leave an area temporarily or permanently if the population has grown too large to be supported there. • When a colony of the ant Lepidothorax albipennis has grown too large for its nest site, or the nest has been damaged, recruiter worker ants look for a new site. • The speed with which worker ants gather around a new site is related to recruiters’ judgments about how attractive it is. • Once a threshold number of workers has been recruited to a site, the recruiters begin carrying eggs and larvae from the old to the new site. • In this way the new colony has reached agreement on the best site, which may not have been the first one discovered. Defending a territory may improve fitness • The most common way for an animal to improve its survival and reproductive success is to establish an exclusive territory. • But to do so, the animal must use time and energy to chase away conspecifics as well as individuals of other species. • To understand the evolution of behavior, ecologists use a method called cost–benefit analysis. • A cost–benefit analysis is based on two assumptions: • That an animal has only a limited amount of time and energy to devote to any particular activity. • That animals generally do not perform behaviors whose total costs are greater than the sum of their benefits. • The cost of behavior has three components: • The energetic cost of behavior is the difference between the energy the animal would have expended had it rested and the energy expended in performing the behavior. • The risk cost of behavior is the increased chance of being injured or killed as a result of performing the behavior, compared to resting. • The opportunity cost of behavior is the sum of the benefits the animal forfeits by not being able to perform other behaviors during the same time interval. • An experiment conducted with Yarrow’s spiny lizards demonstrates all three costs. • Male lizards with implanted testosterone patrolled their territories more, performed more advertising displays, and expended more energy (energetic cost) than control males did. • They had less time to feed (opportunity cost), captured fewer insects, stored less energy, and died at a higher rate (risk cost). (See Figure 53.4 and Animated Tutorial 53.1.) • Because of these high costs of defending their territories, lizards do so vigorously in the months when females are receptive to mating, and they reduce these behaviors when the costs outweigh the benefits. Animals choose what foods to eat • Foraging theory tries to answer questions related to how an animal looks for and acquires food. • To construct a hypothesis about how a foraging animal should behave, a scientist first specifies the objective of the behavior and then attempts to determine the behavioral choices that would best achieve that objective. • This approach is known as optimality modeling; the underlying assumption is that natural selection has molded the behavior of animals so that they solve problems by making the best choices available to them. • In the case of food selection, for example, optimality modeling would support the energy maximizing hypothesis: • If the most valuable prey type is abundant, a predator gains the most energy per unit of time spent foraging by taking only that prey type. • As the abundance of the most valuable prey type decreases, the predator adds increasingly less valuable prey to its diet. • An experiment studying bluegill fish prey choice supported the energy maximizing hypothesis. • Researchers ranked bluegill prey (water fleas) according to two features: energy quantity and energy investment needed by the predator to capture and consume the prey. • The most valuable prey was predicted to be the one that yields the most energy per unit of time invested. • In lab experiments, bluegills ate large or small water fleas if there were few total numbers of them; however, when there were many large water fleas present, the fish ignored the small prey and ate only large individuals. • The proportions of different-sized water fleas taken by the fish under different conditions were close to those predicted by the hypothesis. (See Figure 53. 5 and Animated Tutorial 53.2.) • Animals select certain foods for reasons other than the energy content offered; many species of mammals and birds, for instance, get minerals by eating mineral-rich soil. (See Figure 53.6.) • The evolutionary reason that humans add spices to their food is probably that spices have antimicrobial properties. (See Figure 53.7.) Choice of associates influences fitness • The most basic mating decision is the choice of a partner of the correct species. • Additional decisions are made based on the qualities of a potential mate, the resources it controls, and nest sites. • The reproductive behaviors of males and females are often very different, in part because of the costs of producing sperm and eggs. • Sperm are cheap to produce (energetically), and one male can sire a large number of offspring with the sperm he makes. • Therefore, males of most species can increase their reproductive success by mating with many females. • Eggs are typically more expensive to produce (energetically). • Consequently, a female is unlikely to increase her reproductive success by mating with many males. • The reproductive success of a female depends primarily on the quality of the genes she receives from her mate, the resources he controls, and the amount of assistance he provides in the care of her offspring. • Males use the resources they control and courtship behaviors to attract mates. • Male hanging flies offer females dead insects; the bigger the insect, the longer the female copulates and the more of her eggs he fertilizes. (See Figure 53.8.) • To improve their reproductive success, females need to assess the quality of potential mates according to “reliable” signals (such as the possession of a large dead insect by a male who is a good forager), at which males cannot cheat. • Experiments with bluethroat birds of Europe and Asia have shown that females respond to UV light reflected by the bright blue throat patch of males. They prefer normal males to those to whom sunscreen has been applied because intense plumage is an indicator of a male’s health. (See Figure 53.9.) The Evolution of Animal Societies • Social behavior evolves when individuals who cooperate with others of the same species have, on average, higher rates of survival and reproductive success than those achieved by solitary individuals. • Association for reproduction may consist of little more than joining of egg and sperm, but individuals of many species associate for longer times to care for offspring or reduce the risks of predation. • Today’s animal social systems are the result of long periods of evolution, but there are few traces in the fossil record. • Biologists infer possible routes of the evolution of social systems by studying current patterns of social organization. • Three concepts are important in understanding animal social systems: • They are best explained not according to how they benefit the species as a whole, but by how they benefit the individuals who join together. • They are dynamic and ever-changing. • The costs and benefits to specific individuals differ according to their age, sex, physiological condition, and status. Group living confers benefits but also imposes costs • Group living may improve hunting success or expand the range of prey that can be captured. • Hunting in groups, our ancestors were able to kill larger animals than they could have if they had hunted alone. • Small birds forage in flocks; flocking is shown to provide protection against predation. (See Figure 53.10.) • Social behavior has costs as well as benefits: Individuals in a group may compete for food, interfere with one another’s foraging, injure one another’s offspring, inhibit one another’s reproduction, or transmit diseases to their associates. In some species, parents care for their offspring • The most widespread social system is the family, an association of one or more adults and their dependent offspring. • If parental care or the breeding season lasts a long time, older offspring may be available to help parents care for younger siblings. • Florida scrub jays live on territories that contain a breeding pair and helper offspring who bring food to the nest. • Parental care is altruistic—it involves tremendous costs for parents and helpers. How has it been possible, therefore, for altruism to evolve? Altruism can evolve by means of natural selection • Altruistic behaviors are most easily understood in terms of close genetic relatedness between performers and recipients. • An individual contributes to its own individual fitness by producing offspring and may also help relatives in ways that increase their fitness • By helping its relatives, an individual can increase the representation of some of its own genes in the population. This process is known as kin selection. • Together, individual fitness and kin selection determine the inclusive fitness of an individual. • Occasional altruistic acts may eventually evolve into altruistic behavior patterns if the benefits of increasing reproductive success of relatives outweigh the costs in terms of the individual’s own reproductive success. • Example of the white-fronted bee-eater: • Breeding pairs of white-fronted bee-eaters are assisted by nonbreeding adults who help them incubate their eggs and feed nestlings. Helpers choose nests with young that are most closely related to them. (See Figure 53.11.) • This behavior likely evolved through kin selection. Individual birds do not gain anything other than inclusive fitness, and nests with helpers produce more fledglings than nests without helpers. Eusociality is extreme social behavior • Species (e.g., ants, bees, wasps) whose social groups include sterile individuals are said to be eusocial. • In these species, worker females defend the group against predators or bring food to the colony, but they do not reproduce; only a few females, known as queens, reproduce. • Some ant species have soldiers with large defensive weapons. (See Figure 53.12.) • Genetic factors may facilitate the evolution of eusociality. • Among the Hymenoptera, a diploid egg hatches into a female and a haploid egg hatches into a male. • Therefore, if a female mates with only one male, her daughters share all of their father’s genes but only, on average, half of their mother’s. • Because workers are more genetically similar to their sisters than they would be to their own offspring, they increase their own fitness by caring for their sisters rather than by reproducing themselves. • Eusociality may be favored if establishment of a new colony is difficult and dangerous. Nearly all eusocial animals construct elaborate nests or burrow systems within which their offspring are reared. • High predation rates may account for the eusociality of naked mole-rats, who live underground in colonies of 70–80 individuals and restrict breeding to a single queen and several kings. • Inbreeding—the mating of individuals who are genetically related—could help explain the evolution of eusociality among many hymenopteran species in which queens mate with many males, and among termites and naked mole-rats, in which both sexes are diploid. • (See Videos 53.1, 53.2, and 53.3.) Behavioral Ecology, Population Dynamics, and Community Structure • The ways in which organisms make decisions about habitats, food, and associates may have important implications for the structure and function of ecological systems. • First, animals with complex social organizations often achieve high abundances. • Second, the decisions animals make about the above three matters may influence the range of habitats and foods used by a species. Social animals may achieve great abundances • For ants and termites, living in colonies allows them to harvest vital resources from other organisms. • Most of the individuals and biomass of arthropods in the canopies (tree-tops) of tropical rainforests are social ants. • Termites, which live in dense colonies, are the primary consumers of plant tissues in the savannas of Africa, extracting nutrients from soil at great depths. (See Figure 53.13.) • Both actively cultivate fungi that break down difficult-to-digest plant tissues, including wood. • Some ants tend phloem-sucking aphids and other insects that eject sugar-rich anal drops and provide the ants with the carbohydrates they need. • Social living also enables organisms to use temporally and spatially patchy foods. • The wildebeest, which travels in large herds, is the most abundant wild mammal in Africa. (See Figure 53.2.) • Social organization allows humans to live in high densities and to specialize in different activities. (See Figure 53.14.) Interspecific interactions influence animal distributions • Despite the “rule of thumb” accuracy of the optimality modeling approach, interspecific interactions may prevent animals from living in those environments in which they would do best. • Individuals of a behaviorally dominant species may be able to exclude individuals of a subordinate species from its preferred foraging areas. • Experiment with hummingbirds: • Hummingbirds extract nectar from flowers and defend patches from other hummingbirds. • In an experiment in Arizona, investigators set up feeders with artificial nectar, some rich in sucrose (and with blue bee guards), others containing a more dilute solution (and with yellow bee guards). • Hummingbirds quickly learned which were the high-quality feeders. • Larger male blue-throated hummingbirds dominated smaller male black-chinned hummingbirds, keeping them away from the rich feeders. • Nevertheless, the smaller hummingbirds achieved about the same rate of energy from the dilute feeders because they were able to feed longer at them.