Community Ecology

The Role of Species within Communities

Communities are shaped by foundation species and keystone species, while invasive species disrupt the natural balance of an area.

Learning Objectives

Distinguish between foundation, keystone, and invasive species

Key Takeaways

Key Points

  • A community is defined by the structure of different species that occupy it and how those structures change over time.
  • Foundation species change the environment where other species live, modifying it to benefit the organisms that live there.
  • Keystone species maintain biodiversity; their removal can greatly alter the dynamics within the community.
  • Invasive species are non-native organisms introduced into an area that may be better competitors and reproduce faster than native species; they tend to upset the natural balance.

Key Terms

  • invasive species: any species that has been introduced to an environment where it is not native and has since become a nuisance through rapid spread and increase in numbers, often to the detriment of native species
  • community: a group of interdependent organisms inhabiting the same region and interacting with each other
  • keystone species: a species that exerts a large, stabilizing influence throughout an ecological community, despite its relatively small numerical abundance

Characteristics of Communities

Communities are complex entities that can be characterized by their structure (the types and numbers of species present) and dynamics (how communities change over time). Understanding community structure and dynamics enables community ecologists to manage ecosystems more effectively. There are three main types of species that serve as the basis for a community. These include the foundation species, keystone species, and invasive species. Each of these has a specific role in how communities are formed.

Foundation Species

Foundation species are considered the “base” or “bedrock” of a community, having the greatest influence on its overall structure. They are usually the primary producers: organisms that bring most of the energy into the community. Kelp, a brown algae, is a foundation species that forms the basis of the kelp forests off the coast of California.

Foundation species may physically modify the environment to produce and maintain habitats that benefit the other organisms that use them. An example is the photosynthetic corals of the coral reef. Corals themselves are not photosynthetic, but harbor symbionts within their body tissues (dinoflagellates called zooxanthellae) that perform photosynthesis; this is another example of a mutualism. The exoskeletons of living and dead coral make up most of the reef structure, which protects many other species from waves and ocean currents.

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Foundation species: Coral is the foundation species of coral reef ecosystems. The photosynthetic algae within the corals provides energy for them so that they can build the reefs.

Keystone Species

A keystone species is one whose presence is key to maintaining biodiversity within an ecosystem and to upholding an ecological community’s structure. The intertidal sea star, Pisaster ochraceus, of the northwestern United States is a keystone species. Studies have shown that when this organism is removed from communities, populations of their natural prey (mussels) increase, completely altering the species composition and reducing biodiversity. Another keystone species is the banded tetra, a fish in tropical streams, which supplies nearly all of the phosphorus, a necessary inorganic nutrient, to the rest of the community. If these fish were to become extinct, the community would be greatly affected.

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Keystone species maintain biodiveristy: The Pisaster ochraceus sea star is a keystone species. When this animal is removed from certain areas, its prey species greatly alters the dynamics of the ecosystem, reducing biodiversity.

Invasive Species

Invasive species are foreign species whose introduction can cause harm to the economy and the environment. These species have many ways of entering foreign environments, including through ship’s ballast water: when planes take off, organisms can sometimes become stuck in the cargo area. When the plane arrives in its destination, the organisms are now in a foreign environment. Travelers sometimes illegally smuggle items, such as fruits, plants, or even animals as pets, from one state or country to another..

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Invasive species threaten ecosystems: In the United States, invasive species such as (a) purple loosestrife (Lythrum salicaria) and the (b) zebra mussel (Dreissena polymorpha) threaten certain ecosystems. Some forests are threatened by the spread of (c) common buckthorn (Rhamnus cathartica), (d) garlic mustard (Alliaria petiolata), and (e) the emerald ash borer (Agrilus planipennis). The (f) European starling (Sturnus vulgaris) may compete with native bird species for nest holes.

Invasive species are often better competitors than native species, resulting in population explosions. These new species usually overtake the native populations, driving them to localized extinctions.

One of the many recent proliferations of an invasive species concerns the growth of Asian carp populations. Asian carp were introduced to the United States in the 1970s by fisheries and sewage treatment facilities that used the fish’s excellent filter feeding capabilities to clean their ponds of excess plankton. Some of the fish escaped, however, and by the 1980s, they had colonized many waterways of the Mississippi River basin, including the Illinois and Missouri Rivers.

Voracious eaters and rapid reproducers, Asian carp may outcompete native species for food, potentially leading to native species extinctions. For example, black carp are voracious eaters of native mussels and snails, limiting this food source for native fish species. Silver carp eat plankton that native mussels and snails feed upon, reducing this food source by a different alteration of the food web. In some areas of the Mississippi River, Asian carp species have become predominant, effectively outcompeting native fish for habitat. In some parts of the Illinois River, Asian carp constitute 95 percent of the community’s biomass. Although edible, the fish is bony and not a desirable food in the United States. Moreover, their presence threatens the native fish and fisheries of the Great Lakes, which are important to local economies and recreational anglers. Asian carp have even injured humans. The fish, frightened by the sound of approaching motorboats, thrust themselves into the air, often landing in the boat or directly hitting the boaters.

One infested waterway of particular importance is the Chicago Sanitary and Ship Channel, the major supply waterway linking the Great Lakes to the Mississippi River. To prevent the Asian carp from leaving the canal, a series of electric barriers have been successfully used to discourage their migration; however, the threat is significant enough that several states and Canada have sued to have the Chicago channel permanently cut off from Lake Michigan. Local and national politicians have weighed in on how to solve the problem, but no one knows whether the Asian carp will ultimately be considered a nuisance, like other invasive species, such as the water hyacinth and zebra mussel, or whether it will be the destroyer of the largest freshwater fishery of the world.

Predation, Herbivory, and the Competitive Exclusion Principle

Predation and herbivory are two methods animals use to obtain energy; many species have developed defenses against them.

Learning Objectives

Distinguish between predation and herbivory and describe defense mechanisms against each

Key Takeaways

Key Points

  • Predation, the hunting and consuming of animals by other animals, often shows cyclical patterns of predator/prey population sizes; predators increase in numbers when prey species are plentiful.
  • Herbivory is the eating of plant material for energy and can assist the plants with seed distribution.
  • Plants have evolved spines and toxins to defend against being eaten by herbivores.
  • Animals use bright colors to advertise that they are toxic; mimicry to hide from predators; or have spines, shells, and scales to protect themselves.
  • Batesian mimicry is when a non-toxic species looks similar to a poisonous one, which deters predator attacks.

Key Terms

  • camouflage: resemblance of an organism to its surroundings for avoiding detection
  • herbivory: the consumption of living plant tissue by animals
  • Batesian mimicry: the resemblance of one or more non-poisonous species to a poisonous species, for example, the scarlet king snake and the coral snake

Predation and Herbivory

Most animals fall into one of two major categories when it comes to obtaining the energy they need to survive in the environment: predation or herbivory. An animal that hunts, kills, and eats other animals is called a predator. Examples of predators include tigers, snakes, and hawks. Herbivory, on the other hand, refers to animals that eat plant matter. Deer, mice, and most song birds are examples. To protect themselves against these feeding mechanisms, many organisms have developed methods that keep them from being eaten.

Predation is the hunting of prey by its predator. Populations of predators and prey in a community are not constant over time; in most cases, they vary in cycles that appear to be related. The most-often-cited example of predator-prey dynamics is seen in the cycling of the lynx (predator) and the snowshoe hare (prey), which is based on nearly 200-year-old trapping data from North American forests. This cycle of predator and prey lasts approximately 10 years, with the predator population lagging 1–2 years behind that of the prey population. As the hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. When the lynx population grows to a threshold level, they kill so many hares that the hare population begins to decline. This is followed by a decline in the lynx population because of scarcity of food. When the lynx population is low, the hare population size begins to increase due, at least in part, to low predation pressure, starting the cycle anew.

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Predator and prey population cycling: The cycling of lynx and snowshoe hare populations in Northern Ontario is an example of predator-prey dynamics. As hare populations increase,the lynx populations also increase due to increased food supplies. Many lynx eating many hares causes a decline in the hare population size. This results in a decrease in the lynx population. Then the cycle begins again.

Herbivory describes the consumption of plants by insects and other animals. Unlike animals, plants cannot outrun predators or use mimicry to hide from hungry animals. Some plants have developed mechanisms to defend against herbivory. Other species have developed mutualistic relationships; for example, herbivory provides a mechanism of seed distribution that aids in plant reproduction.

Defense Mechanisms against Predation and Herbivory

The study of communities must consider evolutionary forces that act on the members of the various populations contained within it. Species are not static, but slowly changing and adapting to their environment by natural selection and other evolutionary forces. Species have evolved numerous mechanisms to escape predation and herbivory. These defenses may be mechanical, chemical, physical, or behavioral.

Mechanical defenses, such as the presence of thorns on plants or the hard shell on turtles, discourage animal predation and herbivory by causing physical pain to the predator or by physically preventing the predator from being able to eat the prey. Chemical defenses are produced by many animals as well as plants, such as the foxglove which is extremely toxic when eaten.

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Defenses against predation and herbivory: The (a) honey locust tree (Gleditsia triacanthos) uses thorns, a mechanical defense, against herbivores, while the (b) Florida red-bellied turtle (Pseudemys nelsoni) uses its shell as a mechanical defense against predators. (c) Foxglove (Digitalis sp.) uses a chemical defense: toxins produced by the plant can cause nausea, vomiting, hallucinations, convulsions, or death when consumed. (d) The North American millipede (Narceus americanus) uses both mechanical and chemical defenses: when threatened, the millipede curls into a defensive ball, producing a noxious substance that irritates eyes and skin.

Many species use their body shape and coloration to avoid being detected by predators. The tropical walking stick is an insect with the coloration and body shape of a twig, which makes it very hard to see when stationary against a background of real twigs. In another example, the chameleon can change its color to match its surroundings. Both of these are examples of camouflage: avoiding detection by blending in with the background.

Some species use coloration as a way of warning predators they are not good to eat. For example, the cinnabar moth caterpillar, the fire-bellied toad, and many species of beetle have bright colors that warn of a foul taste, the presence of toxic chemical, and/or the ability to sting or bite, respectively. Predators that ignore this coloration and eat the organisms will experience their unpleasant taste or presence of toxic chemicals and learn not to eat them in the future. This type of defensive mechanism is called aposematic coloration, or warning coloration.

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Examples of aposematic coloration: (a) The strawberry poison dart frog (Oophaga pumilio) uses aposematic coloration to warn predators that it is toxic, while the (b) striped skunk (Mephitis mephitis) uses aposematic coloration to warn predators of the unpleasant odor it produces.

While some predators learn to avoid eating certain potential prey because of their coloration, 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. In Batesian mimicry, a harmless species imitates the warning coloration of a harmful one. Assuming they share the same predators, this coloration then protects the harmless ones, even though they do not have the same level of physical or chemical defenses against predation as the organism they mimic. Many insect species mimic the coloration of wasps or bees, which are stinging, venomous insects, thereby discouraging predation.

Symbiosis

Commensalism, mutualism, and parasitism are three symbiotic ways organisms interact with each other with differing degrees of benefit.

Learning Objectives

Differentiate among the types of symbiosis: commensalism, mutualism, and parasitism

Key Takeaways

Key Points

  • Commensalism is when two organisms share the same environment, where one benefits and the other is unharmed.
  • Trees and birds have a commensalistic relationship; the birds benefit from having a place to build their nests, while the trees are unharmed and not impacted by the bird’s presence.
  • Mutualism is when two species sharing the same environment both benefit from their interactions.
  • The protozoans living within the intestines of termites create a mutualistic relationship with them; the protozoans get a safe place to live while the termites get help digesting the cellulose in their diet.
  • Parasitism occurs when two organisms interact, but while one benefits, the other experiences harm.
  • Parasites harm their hosts, as with the tapeworm attaching itself to the intestine of a cow; the tapeworm absorbs the nutrients from the cow’s diet, preventing them from being absorbed by the cow.

Key Terms

  • mutualism: Any interaction between two species that benefits both.
  • commensalism: A sharing of the same environment by two organisms where one species benefits and the other is unaffected; e.g., barnacles on whales.
  • parasitism: Interaction between two organisms, in which one organism (the parasite) benefits and the other (the host) is harmed.

Symbiotic relationships, or symbioses (plural), are close interactions between individuals of different species over an extended period of time which impact the abundance and distribution of the associating populations. Most scientists accept this definition, but some restrict the term to only those species that are mutualistic, where both individuals benefit from the interaction.

Commensalism

A commensalistic relationship occurs when one species benefits from the close, prolonged interaction, while the other neither benefits nor is harmed. Birds nesting in trees provide an example of a commensal relationship. The tree is not harmed by the presence of the nest among its branches. The nests are light and produce little strain on the structural integrity of the branch. Most of the leaves, which the tree uses to obtain energy by photosynthesis, are above the nest, so they are unaffected. The bird, on the other hand, benefits greatly. If the bird had to nest in the open, its eggs and young would be vulnerable to predators.

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Commensalistic relationship: The southern masked-weaver bird is starting to make a nest in a tree in Zambezi Valley, Zambia. This is an example of a commensal relationship, in which one species (the bird) benefits, while the other (the tree) neither benefits nor is harmed.

Mutualism

A second type of symbiotic relationship, mutualism, is where two species both benefit from their interaction. Some scientists believe that these are the only true examples of symbiosis. For example, termites have a mutualistic relationship with protozoa that live in the insect’s gut. The termite benefits from the ability of bacterial symbionts within the protozoa to digest cellulose. The termite itself cannot do this; without the protozoa, it would not be able to obtain energy from its food (cellulose from the wood it chews and eats). The protozoa and the bacterial symbionts benefit by having a protective environment and a constant supply of food from the wood-chewing actions of the termite.

Parasitism

A parasite is an organism that lives in or on another living organism, deriving nutrients from it. In this relationship the parasite benefits, but the organism being fed upon, the host, is harmed. The host is usually weakened by the parasite as it siphons resources the host would normally use to maintain itself. The parasite, however, is unlikely to kill the host. This is because the parasite needs the host to complete its reproductive cycle by spreading to another host.

The reproductive cycles of parasites are often very complex, sometimes requiring more than one host species. A tapeworm is a parasite that causes disease in humans when contaminated, undercooked meat such as pork, fish, or beef is consumed. The tapeworm can live inside the intestine of the host for several years, benefiting from the food the host is bringing into its gut by eating; it may grow to be over 50 ft long by adding segments. The parasite moves from species to species as it requires two hosts to complete its life cycle.

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Lifecycle of a parasitic tapeworm: This diagram shows the life cycle of a pork tapeworm (Taenia solium), a human worm parasite. The eggs of the tapeworm are ingested by the host. When they hatch, the worms travel through the wall of the intestine and begin to grow. Here, the parasite will absorb the nutrition from the host and continue to grow.

Ecological Succession

When disturbances occur, succession allows for communities to become re-established over periods of time.

Learning Objectives

Distinguish between primary and secondary succession following disturbances to communities

Key Takeaways

Key Points

  • After an environmental disturbance such as a volcanic eruption or forest fire, communities are able to replace lost species through the process of succession.
  • Primary succession occurs after a volcanic eruption or earthquake; it involves the breakdown of rocks by lichens to create new, nutrient -rich soils.
  • The first species to colonize an area after a major disturbance are called pioneer species; they help to form the new environment.
  • Secondary succession occurs after a disturbance such as a forest fire, where there is still some organic matter to allow new plants to grow.
  • Both types of succession take place over long periods of time and result in the communities reaching a state of equilibrium.

Key Terms

  • succession: an act of following in sequence
  • equilibrium: the condition of a system in which competing influences are balanced, resulting in no net change

Community dynamics

Community dynamics are the changes in community structure and composition over time. Sometimes these changes are induced by environmental disturbances such as volcanoes, earthquakes, storms, fires, and climate change. Communities with a stable structure are said to be at equilibrium. Following a disturbance, the community may or may not return to the equilibrium state.

Succession describes the sequential appearance and disappearance of species in a community over time. In primary succession, newly-exposed or newly-formed land is colonized by living things. In secondary succession, part of an ecosystem is disturbed, but remnants of the previous community remain.

Primary succession and pioneer species

Primary succession occurs when new land is formed or rock is exposed; for example, following the eruption of volcanoes, such as those on the Big Island of Hawaii. As lava flows into the ocean, new land is continually being formed. On the Big Island, approximately 32 acres of land are added each year. First, weathering and other natural forces break down the substrate enough for the establishment of certain hearty plants and lichens with few soil requirements, known as pioneer species. These species help to further break down the mineral-rich lava into soil where other, less-hardy species will grow, eventually replacing the pioneer species. In addition, as these early species grow and die, they add to an ever-growing layer of decomposing organic material, contributing to soil formation. Over time, the area will reach an equilibrium state with a set of organisms quite different from the pioneer species.

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Pioneer species are the first to colonize: During primary succession in lava on Maui, Hawaii, succulent plants are the pioneer species. After weathering breaks down the lava into soil, the plants are able to take root. These plants will help to further change the soil by adding nutrients to it.

Secondary succession

A classic example of secondary succession occurs in oak and hickory forests cleared by wildfire. Wildfires will burn most vegetation and kill those animals unable to flee the area. Their nutrients, however, are returned to the ground in the form of ash. Thus, even when areas are devoid of life due to severe fires, they will soon be ready for new life to take hold.

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Secondary succession in the forest: Secondary succession is shown in an oak and hickory forest after a forest fire. Those organisms that could not escape are killed, but their bodies decompose, adding nutrients to the soil. These nutrients provide the basis for new plants to grow.

Before a wildfire, vegetation is often dominated by tall trees with access to the major plant energy resource: sunlight. Their height gives them access to sunlight while also shading the ground and other low-lying species. After the fire, however, these trees are no longer dominant. Thus, the first plants to grow back are usually annual plants followed, within a few years, by quickly-growing and spreading grasses along with other pioneer species. Due to changes in the environment brought on by the growth of the grasses and other species, over many years, shrubs will emerge along with small pine, oak, and hickory trees. These organisms are called intermediate species. Eventually, over 150 years, the forest will reach its equilibrium point where species composition is no longer changing and resembles the community before the fire. This equilibrium state is referred to as the climax community, which will remain stable until the next disturbance.