19.4 Community Ecology

There is no way to know if population growth will moderate to the point where the crisis will be avoided.

Population growth leads to the degradation of the natural environment.

Reducing the human impact on climate change has been attempted by many countries.
Many underdeveloped countries may be less likely to agree with provisions of a global climate change treaty if it means slowing their economic development.

In the United States, the role of human activity in causing climate change has become a politically charged issue.

We enter the future with a lot of uncertainty about our ability to control population growth and protect the environment.

The animation discusses the global impacts of human population growth.

Populations of one species never live in isolation from populations of other species.

An ecological community is formed by the interacting populations of a given habitat.
The number of species in the same area and relative abundance is known as the diversity of the community.
The diversity of tropical rainforests is so great that it cannot be accurately assessed, whereas areas with low species diversity, such as the glaciers of Antarctica, still contain a wide variety of living organisms.
Scientists study ecology at the community level to understand how species interact with each other.

The predator-prey relationship is a classical example of species interaction.

The definition of predator-prey interaction describes individuals of one population that kill and then consume another population.
Population sizes of predator and prey in a community are not always the same, and they may vary in cycles that seem to be related.
The most frequently cited example of predator-prey population dynamics is the cycling of the lynx and snowshoe hare, using 100 years of trapping data from North America.
The cycle of predator and prey population sizes takes about ten years, with the predator population lagging behind the prey population.
As the hare numbers increase, there is more food available for the lynx, which allows the population to increase as well.
When the lynx population grows to a threshold level, they kill a lot of hares, which leads to a decline in the population.
When the lynx population is low, the hare population size begins to increase due to the low pressure of prey.

The cycling of snowshoe hare and lynx populations in Northern Ontario is an example of predator-prey dynamics.

Predation and predator avoidance are very effective.

In later generations, any heritable character that allows an individual of a prey population to better evade its predator will be represented.
A trait that allows a predator to locate and capture its prey will lead to a greater number of offspring and an increase in the commonness of the trait within the population.
There are ecological relationships between populations that lead to adaptation.
The consumption of plants for food is one of the mechanisms that species have evolved to escape.
Defenses can be mechanical, chemical, physical, or behavioral.

The presence of armor in animals or thorns in plants discourages herbivory by discouraging physical contact.

Animals store chemical defenses from plants to prevent them from being eaten.
Secondary plant compounds that are toxic to animals and discourage consumption are produced by many plant species.

The honey locust tree uses thorns, a mechanical defense, against herbivores, while the foxglove uses a chemical defense, which can cause nausea, vomiting, convulsions, or death when consumed.

Many species use their body shape and color to avoid being detected.

The tropical walking stick is an insect with a twig-like body, which makes it hard to see when it is stationary against a background of real twigs.

Some species use coloration as a way of warning their prey that they are distasteful or poisonous.

The monarch butterfly caterpillar sequesters poisons from its food to make itself poisonous or distasteful to potential predators.
The caterpillar is bright yellow and black.
The caterpillar is able to pass the sequestered toxins on to the adult monarch, which is black and red, as a warning to potential predators.
The toxins produced by fire-bellied toads make them unattractive to their potential prey.
They have bright red or orange bellies which they use to advertise their poisonous nature and discourage an attack.
These are two examples of warning color.
The fire-bellied toad has bright colors on its belly to warn potential predator that it is toxic.

Even though they themselves may not be unpleasant to eat or contain toxic chemicals, other species have evolved mechanisms to mimic this coloration to avoid being eaten, even though they themselves may not be unpleasant to eat or contain toxic chemicals.

The harmless ones will be protected if they share the same predator.

Many insect species mimic the appearance of stinging, venomous insects, thereby discouraging prey.

Multiple species share the same warning coloration, but they all have their own defenses.

The commonness of the signal improves compliance.

You can find stunning examples of mimicry on this website.

Multiple species may compete to get resources that are limited within a habitat.

All species have an ecological niche.
A niche is the unique set of resources used by a species.
If there is an overlap in resource use and therefore competition between two species, then a trait that reduces reliance on the shared resource will be selected for leading to evolution that reduces the overlap.
The species that most efficiently exploits the resource will drive the other species to extinction if either species is unable to evolve to reduce competition.
They thrive in the laboratory when grown individually.

Symbiotic relationships are close and long-term.

Symbioses may be commensal, in which one species benefits while the other is neither harmed nor benefited; mutualistic, in which both species benefit; or parasites, in which the interaction harms one species and benefits the other.

A commensal relationship occurs when one species benefits from the other not being harmed.

Birds nest in trees as an example of a commensal relationship.
The tree isn't harmed by the nest in its branches.
The structure of the branch is light and the leaves are above the nest so they are unaffected.
The eggs and young of the bird would be at risk if it had to nest in the open.
It is difficult to prove that one partner does not benefit from the presence of the other in a commensal relationship.

The masked-weaver is making a nest in a tree.

This is an example of a commensal relationship in which one species benefits while the other is not harmed.

There is a mutualistic relationship between protists that live in the insect's gut.

The protists are able to digest cellulose.
The protists are able to digest cellulose because of the presence of symbioticbacteria in their cells.
Without the protozoa, the termite wouldn't be able to get the energy it needs from its food.
The protozoa benefit from having a protective environment and a constant supply of food.
The protists benefit from the enzymes provided by theirbacteria, while thebacteria benefit from a doubly protective environment and a constant source of food and water.
There is a mutualistic relationship between a fungus and a algae.
The physical structure of the lichen protects the algae from the elements and makes it more available to the algae in the atmosphere.
Many of the fungal partners are unable to live on their own, because the algae of lichens can live independently.

The host is usually weakened by the parasites as they steal resources from it.

Population and Community Ecology slows down this process to allow the parasites time to complete their reproductive cycle before they can spread to another host.

The reproductive cycles of parasites can be very complex.

The tapeworm can live inside the host's gut for several years, and it may grow to be over 50 feet long by adding segments.
The parasites move from one host to another in order to complete their life cycle.
Asexually reproducing in the human host and then sexually reproducing in the gut of blood-feeding mosquitoes, the organisms live inside the human host and red blood cells.
Malaria is spread from human to mosquito and back to human, one of many arthropod-borne infectious diseases of humans.

The life cycle of the tapeworm is shown in this diagram.

Communities can be characterized by their structure and dynamics, as well as how members and their interactions change over time.

Understanding community structure and dynamics allows us to manage our ecological communities.

One of the fundamental characteristics of a community is its biodiversity.

Ecologicalists use the number of different species in a particular area and their relative abundance as a measure of biodiversity.
The area could be a habitat, a biome, or the entire biosphere.
Ecologists are struggling to understand the causes of species richness.
The greatest species richness occurs near the equator and the lowest near the poles.

Foundation species have the highest relative abundance of other species.

The most abundant mammals in North America are found in the equator.

They are often primary producers.

The basis of the kelp forests off the coast of California is a species of brown algae.

Foundation species can physically modify the environment to create habitats that benefit other organisms.

There are examples of tree species found in a forest.
Most of the structure of the coral reef is made up of living and dead coral, which protects other species from waves and ocean currents.

Coral is the foundation of the coral reef.

Studies show that removing the organisms from communities will cause the population of mussels to increase, which in turn will affect the species composition.

The banded tetra, a fish in tropical streams, is one of the keystone species.
The banded tetra feeds mostly on insects from the land.
If these fish were to become extinct, the relationships between populations in the community would change a lot.

When non-native organisms are introduced to an area out of their native range, they change the community they invade.

If you enjoy a forest hike, take a summer boat trip, or walk down an urban street, you have likely encountered an Invasive species.

The Asian carp is an Invasive species in the United States.

Asian carp were introduced to the United States in the 1970s by sewage treatment facilities that used the fish's excellent filter feeding abilities to clean their ponds of excess plankton.

The fish colonized many waterways of the Mississippi River basin by the 1980s, including the Illinois and Missouri rivers.

Asian carp may out compete native species for food and lead to their extinction.

The nursery habitats for other fish are altered by removing aquatic plants.

The silver carp is a fish that competes with native fish.

The fish is not desired in the United States.
Their presence threatens the native fish and fisheries of the Great Lakes, which are important to local economies and recreational anglers.
Humans have been injured by Asian carp.
The fish are frightened by the sound of motorboats and jump into the air, landing in the boat or hitting boaters.

The Great Lakes and their prized salmon and lake trout are in danger.

Attempts are being made to prevent the carp from entering the Great Lakes through the Chicago ship and sanitary canal, which is the only connection between the Mississippi River and Great Lakes basins.
Several states and Canada have sued to have the Chicago channel permanently cut off from Lake Michigan because of the threat posed by the Asian carp.
Politicians have weighed in on how to solve the problem.
Governments have not been effective in slowing or preventing the introduction of invaders.

Issues of vital importance to the human food supply and economy are shown in the issues associated with Asian carp.

The study of members of a particular species occupying a habitat and the study of the interaction of all species within a habitat are some of the sciences used in socio- political issues.

A community with a relatively constant number of species is said to be at equilibrium.

The equilibrium is stable despite the changing relationships and species identities.
The community can either return to the equilibrium state or not.

Succession describes the appearance and disappearance of species in a community.

There is a sequential change in species until a more permanent community develops.

When new land is formed following an eruption of a volcano, it's called primary succession.

New land is being formed as lava flows into the ocean.
32 acres of land is added to the Big Island each year.

These species help to break down the mineral-rich lava into soil where other, less hardy but more competitive species, such as grasses, shrubs, and trees, will grow and eventually replace the pioneer species.

The area will reach an equilibrium state with a different set of organisms.

Succulent plants are the pioneer species in lava on Maui.

In oak and hickory forests that have been cleared of wildfire, the animals are killed unless they flee the area.

Ash is returned to the ground as a form of nutrition.
Although the community has been altered, there is a foundation for rapid recolonization in the soil.

Before the fire, the vegetation was dominated by tall trees with access to sunlight.

Their height gave them access to sunlight while also shading the ground.
The trees are no longer dominant after the fire.
The first plants to grow back are usually annual plants followed by grasses and other pioneer species within a few years.
shrubs emerge along with small pine, oak, and hickory trees due to changes in the environment brought on by the growth of grasses and forbs.
The organisms are called intermediate species.
The forest will reach its equilibrium point over the course of 150 years.
The climax community is characteristic of the climate and geology.

The equilibrium is a dynamic one with constant changes in abundance and sometimes species identities.

Secondary succession processes include the return of a natural environment after agricultural activities.

Secondary succession can be seen in the aftermath of a forest fire.

There is a sequence of people present at the same location.

Populations are individuals of a species.

Ecologists measure characteristics of populations.
Life tables can be used to calculate life expectancies.
The number of individuals surviving at each age interval is plotted.

Populations with unlimited resources grow quickly.

Population size will level off at the carrying capacity when resources become limited.

Populations are regulated by a variety of factors.

Life-history characteristics, such as age at first reproduction or the number of offspring, can evolve over time in a population.
There is a continuum between the two extremes.

The human population is growing fast.

Humans have increased their carrying capacity by using technology and fossil fuels.
Population growth can be predicted by the age structure of the population.
Unchecked human population growth could have dire long-term effects.

There are different species living in the same area.

The variety of these species is called biodiversity.
Many organisms have developed defenses against herbivory.
Two species are competing for the same resources in the same area.
commensalism, mutualism, or parasitism are some of the relationships that species may form.
The foundation and keystone species of the OpenStax book are described.
Communities respond to environmental changes by succession: the appearance of different plant species until a stable community structure is established.

The number of seal deaths would increase, but typically decreases with body size.

In stages 1 through 3, the carrying capacity of seals would decrease.

The seal population would not be affected by population change.

There are species that have many offspring at once.

A forest fire is an example.

In Africa a.

The proportion of the growth curve is determined by the type of country.

Which of the following isn't a way that humans have called it.

The population size of a species is related to the environment.

The first species to live on new land are called _____.