Lesson Overview

1. Lesson Overview Population Dynamics (Part 2)

2. Limiting Factors • A limiting factor is a factor that controls the growth of a population. • Some—such as competition, predation, parasitism, and disease—depend on population density. • Others—including natural disasters and unusual weather—do not depend on population density.

3. Density-Dependent Limiting Factors • Density-dependent limiting factors operate strongly only when population density—the number of organisms per unit area—reaches a certain level. These factors do not affect small, scattered populations as much. • Density-dependent limiting factors include • 1. competition • 2. predation • 3. herbivory • 4. parasitism • 5. disease • 6. stress from overcrowding

4. 1. Competition • When populations become crowded, individuals compete for food, water, space, sunlight, and other essentials. • Competition can lower birthrates, increase death rates, or both. • Competition is a density-dependent limiting factor. The more individuals living in an area, the sooner they use up the available resources. • Often, space and food are related to one another. Many grazing animals compete for territories in which to breed and raise offspring. Individuals that do not succeed in establishing a territory find no mates and cannot breed. • Competition can also occur between members of different species that attempt to use similar or overlapping resources. • This type of competition is a major force behind evolutionary change

5. Predator-Prey Relationships • An interaction in which one animal (the predator) captures and feeds on another animal (the prey) is called predation. • Predators can affect the size of prey populations in a community and determine the places prey can live and feed. • This graph shows an idealized computer model of changes in predator and prey populations over time.

6. Predator-Prey Relationships • Sometimes, the moose population on Isle Royale grows large enough that moose become easy prey for wolves. When wolves have plenty to eat, their population grows. • As wolf populations grow, they begin to kill more moose than are born. This causes the moose death rate to rise higher than its birthrate, so the moose population falls • ` • As the moose population drops, wolves begin to starve. Starvation raises wolves’ death rate and lowers their birthrate, so the wolf population also falls • When only a few predators are left, the moose death rate drops, and the cycle repeats..

7. 2. Predation and Herbivory • The effects of predators on prey and the effects of herbivores on plants are two very important density-dependent population controls • Herbivory can also contribute to changes in population numbers. From a plant’s perspective, herbivores are predators • In some situations, human activity limits populations. • For example, fishing fleets, by catching more and more fish every year, have raised cod death rates so high that birthrates cannot keep up. As a result, cod populations have been dropping. • These populations can recover if we scale back fishing to lower the death rate sufficiently. 3. Herbivore Effects Humans as Predators

8. Keystone Species • Sometimes changes in the population of a single species, often called a keystone species, can cause dramatic changes in the structure of a community. • In the cold waters off the Pacific coast of North America, for example, sea otters devour large quantities of sea urchins. • Urchins are herbivores whose favorite food is kelp, giant algae that grow in undersea “forests.”

9. Keystone species • A century ago, sea otters were nearly eliminated by hunting. Unexpectedly, the kelp forest nearly vanished. • Without otters as predators, the sea urchin population skyrocketed, and armies of urchins devoured kelp down to bare rock. • Without kelp to provide habitat, many other animals, including seabirds, disappeared. • Otters were a keystone species in this community. • After otters were protected as an endangered species, their population began to recover. • As otters returned, the urchin populations dropped, and kelp forests began to thrive again.

10. 4. Parasitism and 5. Disease • Parasites and disease-causing organisms feed at the expense of their hosts, weakening them and often causing disease or death. . • Parasitism and disease are density-dependent effects, because the denser the host population, the more easily parasites can spread from one host to another • Some species fight amongst themselves if overcrowded. • In some species, stress from overcrowding can cause females to neglect, kill, or even eat their own offspring. • Stress from overcrowding can lower birthrates, raise death rates, or both, and can also increase rates of emigration. • . 6. Stress From Overcrowding

11. Symbioses • Any relationship in which two species live closely together is called symbiosis, which means “living together.” • The three main classes of symbiotic relationships in nature are mutualism, parasitism, and commensalism.

12. Mutualism • The sea anemone’s sting has two functions: to capture prey and to protect the anemone from predators. Even so, certain fish manage to snack on anemone tentacles. • The clownfish, however, is immune to anemone stings. When threatened by a predator, clownfish seek shelter by snuggling deep into an anemone’s tentacles. • If an anemone-eating species tries to attack the anemone, the clownfish dart out and chase away the predators. • This kind of relationship between species in which both benefit is known as mutualism.

13. Parasitism • Tapeworms live in the intestines of mammals, where they absorb large amounts of their hosts’ food. • Fleas, ticks, lice, and the leech shown, live on the bodies of mammals and feed on their blood and skin. • These are examples of parasitism, relationships in which one organism lives inside or on another organism and harms it. • The parasite obtains all or part of its nutritional needs from the host organism. • Generally, parasites weaken but do not kill their host, which is usually larger than the parasite.

14. Commensalism • Barnacles often attach themselves to a whale’s skin.They perform no known service to the whale, nor do they harm it. Yet the barnacles benefit from the constant movement of water—that is full of food particles—past the swimming whale. • This is an example of commensalism, a relationship in which one organism benefits and the other is neither helped nor harmed.

15. Density-Independent Limiting Factors • Density-independent limiting factors affect all populations in similar ways, regardless of population size and density. • Unusual weather such as hurricanes, droughts, or floods, and natural disasters such as wildfires, can act as density-independent limiting factors. • A severe drought, for example, can kill off great numbers of fish in a river. • In response to such factors, a population may “crash.” After the crash, the population may build up again quickly, or it may stay low for some time

16. Lesson Overview Succession

17. Primary and Secondary Succession • Ecological succession is a series of more-or-less predictable changes that occur in a community over time. • Ecosystems change over time, especially after disturbances, as some species die out and new species move in. • Over the course of succession, the number of different species present typically increases.

18. Primary Succession • Volcanic explosions can create new land or sterilize existing areas. • Retreating glaciers can have the same effect, leaving only exposed bare rock behind them. • Succession that begins in an area with no remnants of an older community is called primary succession.

19. The first species to colonize barren areas are called pioneer species. • One ecological pioneer that grows on bare rock is lichen—a mutualistic symbiosis between a fungus and an alga.

20. Secondary Succession • Sometimes, existing communities are not completely destroyed by disturbances. In these situations, secondary succession occurs. • Secondary succession proceeds faster than primary succession, in part because soil survives the disturbance. As a result, new and surviving vegetation can regrow rapidly.

21. Secondary succession often follows a wildfire, hurricane, or other natural disturbance. • We think of these events as disasters, but many species are adapted to them. Although forest fires kill some trees, for example, other trees are spared, and fire can stimulate their seeds to germinate. • Secondary succession can also follow human activities like logging and farming. • Every organism changes the environment it lives in. • One model of succession suggests that as one species alters its environment, other species find it easier to compete for resources and survive. Why Succession Occurs

22. For example, as lichens add organic matter and form soil, mosses and other plants can colonize and grow. • As organic matter continues to accumulate, other species move in and change the environment further. • Over time, more and more species can find suitable niches and survive. • Ecologists used to think that succession in a given area always proceeds through the same stages to produce a specific and stable climax community. • Recent studies, however, have shown that succession doesn’t always follow the same path, and that climax communities are not always uniform and stable. Climax Communities

23. Succession After Natural Disturbances • Secondary succession in healthy ecosystems following natural disturbances often reproduces the original climax community. • Healthy coral reefs and tropical rain forests often recover from storms, and healthy temperate forests and grasslands recover from wildfires. • However, detailed studies show that some climax communities are not uniform. • Often, they have areas in varying stages of secondary succession following multiple disturbances that took place at different times. • Some climax communities are disturbed so often that they can’t really be called stable.

24. Succession After Human-Caused Disturbances • Ecosystems may or may not recover from extensive human-caused disturbances. • Clearing and farming of tropical rain forests, for example, can change the microclimate and soil enough to prevent regrowth of the original community. • Ecologists study succession by comparing different cases and looking for similarities and differences. • Researchers who swarmed over Mount Saint Helens after it erupted in 1980 might also have studied Krakatau, for example. Studying Patterns of Succession

25. Succession Continued • On both Mount Saint Helens and Krakatau, primary succession proceeded through predictable stages. • The first plants and animals that arrived had seeds, spores, or adult stages that traveled over long distances. • Hardy pioneer species helped stabilize loose volcanic debris, enabling later species to take hold. • Historical studies in Krakatau and ongoing studies on Mount Saint Helens confirm that early stages of primary succession are slow, and that chance can play a large role in determining which species colonize at different times.