45.6 Community Ecology

The human population continues to grow.

At some point the food supply may run out, but it's not easy to predict.
Future world population growth may vary from 6 billion to 16 billion people, according to the United Nations.

Population growth endangers the natural environment.

Many countries are trying to reduce their emissions of greenhouse gas carbon dioxide.
The treaties have not been approved by every country.
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 how we can curb population growth and protect the environment.

You can find a link to learn about the global impacts of human population growth on this website.

By the end of this section, you will be able to discuss the predator-prey cycle, give examples of defenses against herbivory, and give examples of symbiotic relationships between species.

Population growth and abundance are regulated by the interactions between these populations.

Populations living in the same area at the same time form a community.
The number of species occupying the same habitat and their relative abundance is known as species diversity.
The diversity of tropical rainforests is so great that it cannot be counted, whereas areas with low diversity, such as the glaciers of Antarctica, contain a wide variety of living things.
Understanding how species interact with each other and compete for the same resources is studied at the community level.

Predation is the consumption of prey by a predator.

Nature shows show the drama of living organisms killing each other.
In most cases, the population of predator and prey in a community varies in cycles that seem to be related.
The most frequently cited example of predator-prey dynamics is the cycling of the lynx and snowshoe hare using trapping data from North American forests.
The cycle of predator and prey lasts 10 years, with the predator population lagging behind the prey population.
There is more food available for the lynx as the hare numbers increase.
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 hares in Northern Ontario is an example of predator-prey dynamics.

The population cycling of the two species is controlled by models.

Studies have shown that density- dependent factors are important in the cycling.

There is a chance that the cycling is inherent in the hare population due to the effects of crowding on fecundity.

The hare cycling would cause the lynx to cycle because it is their main food source.
The more we study communities, the more complex they are, allowing us to derive more accurate and sophisticated models of population dynamics.

There is an interspecific relationship between the consumption of plants by insects and other animals.

Plants can't outrun or hide from hungry animals.
Some plants can defend themselves against herbivory.
The mechanism of seed distribution that aids in plant reproduction is provided by herbivory.

Evolutionary forces must be considered in the study of communities.

Natural selection and other evolutionary forces help species change and adapt to their environment.
Many species have evolved ways to escape.
The defenses may be mechanical, chemical, physical, or behavioral.

The presence of thorns on plants or the hard shell on turtles are mechanical defenses that discourage animal and herbivory by causing physical pain to the predator or by physically preventing the predator from being able to eat the prey.

The foxglove is extremely toxic when eaten and is produced by many animals and plants.

The honey locust tree (Gleditsia triacanthos) uses thorns, a mechanical defense against herbivores, while the Florida red-bellied turtle uses its shell as a mechanical defense.

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

The tropical walking stick is an insect with a twig shape that makes it hard to see when stationary against a background of real twigs.

Some species use color to warn their prey that they are not good to eat.

If a predator ignores this color and eats the organisms, they will experience the unpleasant taste of toxic chemicals and learn not to eat them in the future.

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.

If they share the same predator, this coloration will protect the harmless ones even though they don't have the same level of physical or chemical defenses.
Many insect species mimic the color of stinging, venomous insects, which discourages prey.

A bumblebee and a bee-like robber fly are examples of a harmless species mimicking a harmful one.

This type of mimicry is very rare and difficult to understand.
Eating a milk snake is unpleasant but not fatal for this type of mimicry.
The coral snake is protected by the predator because they learn not to eat snakes with this color.
The benefit for the less toxic species would disappear if the snake were killed by the predator.

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 that describes how they acquire the resources they need and how they interact with other species in the community.
It is not possible for different species to coexist in a community if they are competing for the same resources.
They thrive in the laboratory when grown individually.
When they are placed in the same test tube, P. aurelia outclasses P. caudatum for food, leading to the latter's eventual extinction.

When they compete for the same resources, Paramecium aurelia and Paramecium caudatum grow well individually.

If a population makes use of a different resource, a different area of the habitat, or feeds during a different time of day, the exclusion may be avoided.

Two organisms are said to occupy different niches.

Direct competition is minimized by the coexisting organisms.

Some scientists restrict the term to only those species that are mutualistic, where both individuals benefit from the interaction.

The broader definition will be used in this discussion.

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.
The pilot fish and shark are examples of a commensal relationship.
The pilot fish are fed on the leftovers of the host's meals.

The bird is starting to make a nest in a tree.

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

Some scientists think that these are the true examples of symbiosis.

There is a mutualistic relationship between the organisms that live in the insect's gut.
The ability of the symbionts within the protozoa to digest is a benefit to the termite.
Without the protozoa, the termite wouldn't be able to get the energy it needs from its food.
The protozoa and the symbionts benefit from having a protective environment and a constant supply of food.
There is a mutualistic relationship between the two organisms.
The physical structure of the lichen protects the algae from the elements and makes certain nutrients in the atmosphere more available to the algae as they grow together.

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

The host is unlikely to be killed quickly because this would allow no time for the organisms to 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 may grow to be over 50 feet long by adding segments.

Two necessary hosts are needed to complete the life cycle of the parasites.

Malaria is a significant disease in many parts of the world.

Asexually reproducing in the gut of blood-feeding mosquitoes, the organisms live in human and red blood cells.
Malaria is spread from human to human by mosquitoes.

The life cycle of a pork tapeworm is shown in this diagram.

Communities can be characterized by their structure and how they change over time.
Community structure and dynamics can be understood by community ecologists.

Most of the energy in the community comes from organisms that are the primary producers.

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

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

The coral reef has photosynthetic corals.
Another example of a mutualism is the fact that corals have symbionts within their body tissues that perform photosynthesis.
Most of the reef structure is made up of dead and living coral, which protects other species from waves and ocean currents.

Coral is the foundation of the coral reef.

A community's biological complexity is measured by the number of different species in a particular area and their relative abundance.
The area could be a habitat, a biome, or the entire biosphere.
The greatest species richness can be found near the equator, which has warm temperatures, large amounts of rain, and low seasonality.
The lowest species richness can be found near the poles, which are less hospitable to life.
The predictability of climate is an important factor.
Foundation species have a higher relative abundance than other species.

The equatorial latitudes are associated with the greatest species richness for mammals in North and South America.

Pisaster ochraceus is a keystone species in the northwestern United States.

Studies show that removing this organisms from communities will cause populations of their natural prey to increase.

The community would be greatly affected by the extinction of these fish.

The Pisaster sea star is a keystone species.

When introduced to an area outside of their native range, invasive species can threaten the balance of the habitat.

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

Invasive species like the purple loosestrife and the zebra mussel are threatening some aquatic environments in the United States.

Common buckthorn, garlic mustard, and emerald ash borer are threats to some forests.
The European starling may compete with native bird species for nest holes.

The growth of Asian carp populations is one of the recent proliferations of an Invasive species.

The fish's ability to clean their ponds of excess plankton was one of the reasons why Asian carp were introduced to the United States.
Some of the fish escaped, however, and by the 1980s they had colonized many waterways of the Mississippi River basin.

Asian carp may out compete native species for food, which could lead to their extinction.

Black carp limit the food source for native fish species.
Silver carp eat plankton that native mussels and snails feed on, reducing this food source by a different change in the food web.
In some areas of the Mississippi River, Asian carp species are more popular than native fishes.
In the United States, the fish is not a desired food.
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 the boaters.

The Great Lakes and their prized salmon and lake trout are being threatened by these fish.

There are canals and rivers that lead into Lake Michigan.
The major supply waterway linking the Great Lakes to the Mississippi River is the Chicago sanitary and ship channel.
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.
Local and national politicians have weighed in on how to solve the problem, but no one knows if the Asian carp will be considered a nuisance or a threat to the world's largest freshwater fishery.

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

Population ecology and community ecology can be used to study socio- political issues like this.

Community dynamics change over time.

A stable structure is said to be at equilibrium.
The community can either return to the equilibrium state or not.

Succession is a description of the appearance and disappearance of species in a community.

Primary succession occurs when new land is formed or exposed after an eruption of a volcano.

New land is being formed as lava flows into the ocean.
32 acres of land is added to the Big Island each year.
As these early species grow and die, they add to an ever-growing layer of decomposing organic material and contribute to soil formation.
The area will reach an equilibrium state with a different set of organisms.