Worksheet: Community Interactions (KEY)

Impacts, Issues: Fire Ants in the PantsA. Fire ants have invaded the United States.

1. Two species entered from Argentina in the 1930s.2. Both inflict very painful bites on humans and other animals.

B. Ecologists are using biological controls.1. Inside the fire ant, a parasitoid lays her eggs, which hatch to a larva that consumes the ant’s vital tissues.2. Another idea is to use microbes that will infect fire ants but not native species.

1.Which Factors Shape Community Structure?A. A habitat is a place where an organism lives; it is characterized by distinctive

physical features, vegetation, and the array of species living in it.1. A community is an association of interacting populations of different species living in a particular habitat.2. Five factors shape the structure of the community.

a. Interactions between climate and topography dictate rainfall, temperature, soil composition, and so on.b. Availability of food and resources affects inhabitants.c. Adaptive traits enable individuals to exploit specific resources.d. Interactions of various kinds occur among the inhabitants; these include competition, predation, and mutualism.e. The overall pattern of population sizes affects community structure.

3. Several community properties are the result of the factors above.a. Varying numbers of species are found in feeding levels from producers to consumers.b. Diversity tends to increase in tropical climates, creating species richness.

B. The Niche 1. The niche of each species is defined by the sum of activities and relationships in which it engages to secure and use the resources necessary for its survival and reproduction.2. The fundamental niche is the one that could prevail in the absence of competition; the realized niche results from shifts in large and small ways over time as individuals of the species respond to a mosaic of changes.

C. Categories of Species Interactions1. Interactions can occur between any two species in a community and between entire communities.2. There are several types of species interactions.

a. Indirect interactions: neither species directly affects the other (for example, eagles and grass).b. Commensalism: one species benefits while the other is not affected (for example, bird’s nest in tree).c. Mutualism: there is a symbiotic relationship where both species benefit.d. Interspecific competition: both species are harmed by the interaction.e. Predation and parasitism: one species (predator or parasite) benefits while the other (prey or host) is harmed.

2.Mutualism

A. The yucca moth feeds only on the yucca plant, which is completely dependent on the moth for pollination—a classic example of mutualism that is obligatory.

B. This example is a form of symbiosis, which implies an intimate and rather permanent interdependence of the two species on one another for survival and reproduction.

3.Competitive InteractionsA. There are two major categories of competition.

1. Competition within a population of the same species (intraspecific) is usually fierce and may result in depletion of a resource.2. Interspecific competition is less intense because requirements are less similar between the competitors.3. There are two types of competitive interactions regardless of whether they are inter- or intraspecific.

a. In interference competition, some individuals limit others’ access to the resource.b. In exploitation competition, all individuals have equal access to a resource but differ in their ability (speed or efficiency) to exploit that resource.

B. Competitive Exclusion1. Competitive exclusion suggests that complete competitors cannot coexist indefinitely.2. When competitors’ niches do not overlap quite as much, the coexistence is more probable.3. Differences in adaptive traits will give certain species the competitive edge.

C. Resource Partitioning1. Similar species share the same resource in different ways.2. Resource partitioning arises in two ways.

a. Ecological differences between established and competing populations may increase through natural selection.b. Only species that are dissimilar from established ones can succeed in joining an existing community.

4. Predator–Prey InteractionsA. Coevolution of Predators and Prey

1. Predators get their food from prey, but they do not take up residence on or in the prey.2. Many of the adaptations of predators and their victims arose through coevolution.

B. Models for Predator–Prey Interactions1. By the Type I model, each individual predator will consume a constant number of prey individuals over time, regardless of prey abundance.2. In the Type II model, the consumption of prey by each predator incereases, but not as fast as increases in prey density.3. By the Type III model, a predator response is lowest when prey density is at its lowest level and predation pressure lessens.

C. The Canadian Lynx and Snowshoe Hare1. Stable coexistence results when predators prevent prey from overshooting the carrying capacity.2. Fluctuations in population density tend to occur when predators do not reproduce as fast as their prey, when they can eat only so many prey, and when carrying capacity for prey is high.

5.An Evolutionary Arms Race

A. Adaptations of Prey1. Camouflaging is any adaptation in form, color, patterning, or behavior that allows a prey or predator to blend with its surroundings.2. Warning coloration in toxic prey offer bright colors or bold patterns that serve as a warning to predators.3. In mimicry, prey not equipped with defenses may escape predators by resembling toxic prey.4. Moment-of-truth defenses allow prey animals to defend themselves by startling or intimidating the predator with display behavior.

B. Adaptive Responses of Predators1. Stealth and camouflage are used by predators.2. Some predators can just plain outrun their prey.

6.Parasite–Host InteractionsA. Parasites and Parasitoids

1. Natural selection tends to favor parasite and host adaptations that promote some level of mutual tolerance and less-than-lethal effects.2. Usually death results only when a parasite attacks a novel host or when the number of parasites overwhelm the host’s defenses.3. There are several kinds of parasites.

a. All viruses, some bacteria, protists, and fungi are parasites, even a few plants.b. Many tapeworms, flukes, roundworms, insects, and of course, those nasty ticks are also parasites.

4. Parasitoids are insects that develop inside other insects, which they devour and kill.5. Social parasites depend on the social behavior of another to complete the lifecycle; for example, cowbirds lay their eggs in the nest of other birds, which unknowingly incubate and hatch the cowbirds’ eggs

B. Uses as Biological Control Agents1. Parasites and parasitoids have five attributes that make them good control agents.

a. They are well adapted to the host species and their habitat.b. They are exceptionally good at searching for hosts.c. Their growth rate is high relative to that of the host species.d. They are mobile enough for adequate dispersal.e. The lag time between responses to changes in the numbers of the host population is minimal.

2. Care must be taken in releasing more than one kind of control agent in a given area due to the possibility of triggering competition among them and lessening their overall level of effectiveness.

7. Cowbird ChutzpuhA. Brown-headed cowbirds evolved in the North American Great Plains.

1. The birds followed the buffalo herds, which stirred up insects for the bird’s meals, as they roamed the land.2. The vagabond lifestyle did not lend itself to nesting in one place, so the cowbirds laid their eggs in the nests of other species.

a. The “host” species incubated the cowbird eggs as if they were their own.b. But the ungrateful cowbird hatchlings shoved the owner’s eggs out of the nest and demanded to be fed as rightful occupants.

B. As cattle replaced buffalo, the cowbirds adapted to the new herds.

1. Today, brown-headed cowbirds parasitize at least 15 species of native North American birds.2. One female bird can lay as many as thirty eggs in thirty nests in one breeding season.

8. Forces Contributing to Community StabilityA. A Successional Model

1. Ecological succession is the predictable developmental sequence of species in a community.

a. Pioneer species are the first to colonize an area, followed by more competitive species.b. A climax community is the most persistent array of species that results after some lapse of time.

2. Primary succession happens in an area that was devoid of life.a. Pioneer species help to improve soil fertility; they are usually small, low-growing plants with a short life cycle and an abundance of seeds.b. Gradually other, usually larger, species join or replace the pioneer species.

3. In secondary succession, a community reestablishes itself to a climax state after a disturbance that allows sunlight to penetrate.

B. The Climax Pattern Model1. It was once thought that the same general type of community would always develop in a given region because of constraints imposed by climate.2. According to the climax pattern model, a community is adapted to a total pattern of environmental factors—climate, soil, topography, wind, fires, etc.—to create a continuum of climax stages of succession.

C. Cyclic, Nondirectional Changes1. Community stability may require episodes of instability that permit cyclic replacement of equilibrium species, thus maintaining the climax community.2. A good example is the necessary fires in the forests of California that rid the areas of underbrush.

9. . Factors Contributing to Community InstabilityA. The Role of Keystone Species

1. A keystone species is a dominant species that can dictate community structure.2. For example, when sea stars (keystone predator on mussels) were removed from a habitat, mussels increased in number and in turn preyed on enough other species to reduce the community from 15 to 8.

B. Species Introductions Tip the Balance1. Geographic dispersal of species can occur in three ways.

a. A population might expand its home range by slowly moving into outlying regions that prove hospitable.b. During the course of a lifetime, individuals may be rapidly transported across great distances (jump dispersal), as in bilge water of large ships.c. A population may move out from its home range over geologic time, as by continental drift.

2. An exotic species is a resident of an established community that dispersed from its home range and became established elsewhere.

a. Some introduced species have proved beneficial: soybeans, rice, wheat, corn, and potatoes.b. Others have greatly altered the community structure—for the worse.

10.Exotic Invaders

A. The Plants That Ate Georgia1. Kudzu was introduced into the United States from Japan in 1876.

a. Freed from its native biocontrol agents, it grew, and still grows, uncontrollably.b. Herbicides can help, but they taint the water supplies; goats will eat it, but even they have their limits.

2. Asians use a starch made from kudzu, so we harvest, process, and ship it to them.

B. The Alga Triumphant1. A hybrid, sterile strain of the alga Caulerpa escaped from some marine aquarium into the wild.

a. It reproduces by runners and does well in a variety of habitats, including polluted waters.b. Its toxin repels herbivores that might keep it in check.

2. It has invaded the coastal waters of the United States, but has been eradicated off the coast of California.

C. The Rabbits That Ate Australia1. In 1859, a landowner in Northern Australia released two dozen European rabbits.

a. There were no natural predators.b. Hundreds of millions of rabbits now destroy grasses, undermine the soil, and cause erosion.

2. In 1951, the government introduced a myxoma virus, which killed off rabbits initially, but slowed as resistance developed.

11. Biogeographic PatternsA. Mainland and Marine Patterns

1. The number of species increases from the Arctic regions to the temperate zone to the tropics.2. Biodiversity is favored in the tropics for three reasons.

a. More rainfall and sunlight provides more food reserves.b. Species diversity is self-reinforcing from herbivores to predators and parasites.c. Traditionally, the rate of speciation has exceeded the rate of extinction.

B. Island Patterns1. Islands distant from source areas receive fewer colonizing species (distance effect).2. Larger islands tend to support more species (area effect).3. Species numbers increase on new islands and reach a stable number that is a balance between immigration rate for species new to the island and the extinction rate for established species.

12. Threats to BiodiversityA. Even though extinctions have occurred, biodiversity is still very great.

1. Prokaryotes dominated until the Cambrian period when abundant oxygen favored emergence of eukaryotes.2. Extinctions have occurred rapidly, but recovery is measured in millions of years.

B. The Newly Endangered Species1. Another major extinction event is underway.

a. About 300 mammals are on the endangered species list.b. These species are endemic to only one geographic region and thus are very vulnerable to human encroachment activities.

2. As humans, we are threatening other species with habitat losses, species introductions, and illegal wildlife trading.

C. Habitat Losses and Fragmentation1. Habitat loss may be a physical reduction of suitable places to live as well as a loss of habitat due to chemical pollution.2. Habitats may also be chopped into isolated patches (habitat fragmentation), which has three effects.

a. It increases the habitats boundaries, making species more vulnerable to predators, environmental factors, and disease.b. The patches may not be large enough to support the population numbers needed for breeding.c. There may not be enough food to sustain the population.

3. The equilibrium model of island biogeography says that a 50 percent loss of habitat will drive about 10 percent of its endemic species to extinction.

a. This could happen to land-based habitat islands (parks, reserves) that are surrounded by destructive human activities (logging, urbanization).b. Indicator species, such as birds, provide warning of changes in habitat and impending loss of diversity.

4. Exotic species that move into a new habitat are responsible for almost 70 percent of the cases where endemic species are driven to extinction.5. Overharvesting also reduces biodiversity.

D. Conservation Biology1. It is a systematic survey of the full range of biological diversity.2. It attempts to decipher the evolutionary and ecological origins of diversity.3. It attempts to identify methods that might maintain and use biodiversity for the good of the human population.

13. Sustaining BiodiversityA. Identifying Areas at Risk

1. Because it is impossible to make a global survey of all species, scientists have identified hot spots where habitats with the greatest number of species found nowhere else are in danger of extinction.2. Various hot spot inventories can be combined to define an ecoregion.

B. Economic Factors and Sustainable Development1. The goal is to convince individuals, and governments, that sustaining biodiversity has more value than destroying it.2. Biodiversity will be best protected when its species can be used over the long term for the good of local economies.

a. Strip logging is a proposal that would provide for a profitable, yet sustainable, way to harvest trees for wood; it would be used in sloped terrain with a number of streams, providing for regrowth of new saplings.b. Ranching can be done in riparian zones.

1) Riparian zones are valuable strips of vegetation along a stream or river.2) Cattle destroy riparian zones, but restricting cattle from these areas is costly.3) Rotation of cattle into and out of the zones combined with different breeds is a way of managing the riparian zones.