Community interactions

Oppositional interactions

Predation

In predation, one organism kills and consumes another. Predation provides energy to prolong the life and promote the reproduction of the organism that does the killing. Predation influences organisms at two ecological levels. At the level of the individual, the prey organism has an abrupt decline in fitness, as measured by its lifetime reproductive success, because it will never reproduce again. At the level of the community, predation reduces the number of individuals in the prey population. The best-known examples of predation involve carnivorous

interactions, in which one animal consumes another

Think of wolves hunting moose, owls hunting mice, or shrews hunting worms and insects. Less obvious carnivorous interactions involve many small individuals consuming a larger one. Such group predation is common among social carnivores such as lions, hyenas, and wolves. Seeds are considered organisms. Under ideal circumstances, seeds grow to become plants. However, consumption of a seed kills the plant before it can grow, making seed consumption an

example of predation .

Not all predators are animals. Carnivorous plants, such as the Venus fly trap and the pitcher plant, consume insects. Pitcher plants catch their prey in a pool of water containing digestive enzymes, whereas the Venus fly trap captures an insect between the two lobes of a leaf and seals the insect inside with digestive enzymes. These plants absorb nutrients from the insects as they become available during digestion. On a microscopic scale, protozoa and bacteria also consume prey organisms. They play an important role in maintaining population sizes in microbial communities, which promotes the diversity of microorganisms and contributes to a stable community

structure .

Predation and Adaptation

Predation influences the fitness of both predators and prey. Individuals must both feed and avoid being eaten to survive and reproduce. Genetically-determined traits that improve an organism’s ability to survive and reproduce will be passed on to its offspring. Traits associated with improved predation for predators and escaping predation for prey tend to be positively

selected by natural selection .

Predators exhibit traits such as sharp teeth, claws, and venom that enhance their ability to catch food. They also possess extremely acute sensory organs that help them to find potential prey. Consider the ability of raptors to spot potential prey from over a kilometer away, the acute sense of smell of moles, the ability of owls to locate mice by sound, the ability of pit vipers to sense body heat when tracking prey, and the ability of bats and dolphins to echolocate. Predators catch their prey either by pursuing potential prey or by ambushing them. Organisms that give chase are capable of short bursts of speed. Those that lie in wait tend to be camouflaged to avoid detection.

Herbivory

Herbivory is the consumption of plant material by animals, and herbivores are animals adapted to eat plants. As in predator-prey interactions, this interaction drives adaptations

in both the herbivore and the plant species it eats .

For example, to reduce the damage done by herbivores, plants have evolved defenses, including thorns and chemicals. Scientists have identified thousands of plant chemical defense compounds, including familiar compounds such as nicotine and cocaine. To maximize nutrient intake, many herbivores have evolved adaptations that allow them to determine which plants contain

fewer defensive .

Some insects, such as butterflies, have chemical sensors on their feet that allow them to taste the plant before they consume any part of it. Mammalian herbivores often use their keen sense of smell to detect bitter compounds, and they preferentially eat younger

leaves that contain fewer chemicals .

Parasitism

In parasitism, an individual organism, the parasite, consumes nutrients from another organism, its host, resulting in a decrease in fitness to the host. In extreme cases, parasites can cause disease in the host organism; in these situations, we refer to them as pathogens. We divide parasites into two categories: endoparasites, which live inside the body of their hosts, and ectoparasites, which live and feed on the outside of the body of their host. Examples of endoparasites include flukes, tapeworms, fungi, bacteria, and protozoa. Ectoparasites include ticks and

lice, plants, protozoa, bacteria, and fungi .

In most situations, parasites do not kill their hosts. An exception, however, occurs with parasitoids, which blur the line between parasitism and predation. The best-known parasitoids include several species of wasp, which immobilize — but do not kill — a host by stinging it. The female then carries the host to a burrow, where she lays eggs within the host’s body. After the larvae hatch, they consume the living tissues of the host, eventually killing it

(Figure a)

Entomogenous fungi also act as parasitoids; they infect the bodies of insects, either through the mouth while foraging or by penetrating the outer cuticle of the insect’s body. Spores circulate inside the host, whose body provides the nutrients needed for fungal growth. Eventually, the fungal load becomes too great for the host, and the insect dies (Figure b). The major distinguishing difference between parasitoids and predators is that parasitoids feed on living tissue, whereas the predator kills its prey before, or in the process of consuming it.

Competition

Competition is a biological interaction among organisms of the same or different species associated with the need for a common resource that occurs in a limited supply relative to demand. In other words, competition occurs when the capability of the environment to supply resources is smaller than the potential biological requirement so that organisms interfere with each other. Plants, for example, often compete for access to a limited supply of nutrient , water, sunlight, and space.

Intraspecific competition occurs when individuals of the same species vie for access to essential resources, while interspecific competition occurs between different species. Stresses associated with competition are said to be symmetric if they involve organisms of similar size and/or abilities to utilize resources. Competition is asymmetric when there are substantial differences in these abilities, as occurs in the case of large trees interacting with plants of a forest understory.

Individuals of the same species have virtually identical resource requirements. Therefore, whenever populations of a species are crowded, intraspecific competition is intense. Intraspecific competition in dense populations results in a process known as self-thinning, which is characterized by mortality of less-capable individuals and relative success by more-competitive individuals. In such situations, intraspecific competition is an important regulator of population size. Moreover, because individual organisms vary in their reproductive success, intraspecific competition can be a selective factor in evolution.

Interspecific competition can also be intense if individuals of the various species are crowded and have similar requirements of resources. One ecological theory, known as the competitive exclusion principle, states that species with ecologically identical life styles and resource needs cannot coexist over the longer term; the competitively less-fit species will be displaced by the better fit species. Although it is debatable that different species could have identical ecological requirements, it is not difficult to comprehend that intense competition must occur among similar species living in the same, resource-limited habitat. In such situations, interspecific competition must be important in structuring

ecological communities and as an agent of natural selection.

Symbiosis

The word symbiosis literally means ’living together,' but when we use the word symbiosis in biology, what we're really talking about is a close, long-term interaction between two different species. There are many different types of symbiotic relationship that occur in nature. In many cases, both species benefit from the interaction. This type of symbiosis is called

mutualism .

Mutualism

An example of mutualism is the relationship between bullhorn acacia trees and certain species of ants. Each bullhorn acacia tree is home to a colony of stinging ants. True to its name, the tree has very large thorns that look like bull's horns. The ants hollow out the thorns and use them as shelter. In addition to providing shelter, the acacia tree also provides the ants with two food sources. One food source is a very sweet nectar that oozes from the tree at specialized structures called nectaries. The second food source is in the form of food nodules called beltian bodies that grow on the tips of the leaves. Between the nectar and the beltian bodies,

the ants have all of the food they need .

So, the ants get food and shelter, but what does the tree get? Quite a lot actually, you see the ants are very territorial and aggressive. They will attack anything and everything that touches the tree - from grasshoppers and caterpillars to deer and humans. They will even climb onto neighboring trees that touch their tree and kill the whole branch and clear all vegetation in a perimeter around their tree's trunk, as well. The ants protect the tree from herbivores and remove competing vegetation, so the acacia gains a big advantage from the relationship. In this case, the acacia is considered as a host because it is the larger organism in a symbiotic relationship upon or inside of which the smaller organism lives, and the ant is considered to be a symbiont, which is the term for the smaller organism in a symbiotic relationship that lives in or on the host.

A number of mutualistic relationships occur between multicellular organisms and microorganisms. Termites are only able to eat wood because they have mutualistic protozoans and bacteria in their gut that helps them digest cellulose. Inside our own bodies, there are hundreds of different types of bacteria that live just in our large intestine. Most of these are uncharacterized, but we do know a lot about Escherichia coli, which is one of the normal bacteria found in all human large intestines. Humans provide E. coli with food and a place to live. In return, the E. coli produce vitamin K and make it harder for pathogenic bacteria to establish themselves in our large intestine.

Commensalism

Commensalism is an association between two different species where one species enjoys a benefit, and the other is not significantly affected. Commensalism is sometimes hard to prove because in any symbiotic relationship, the likelihood that a very closely associated organism has no effect whatsoever on the other organism is pretty unlikely!!!!!. But, there are a few examples where commensalism does appear to exist. For example, the cattle egret follows cattle, water buffalo, and other large herbivores as they

graze .

The herbivores flush insects from the vegetation as they move, and the egrets catch and eat the insects when they leave the safety of the vegetation. In this relationship the egret benefits greatly, but there is no apparent effect on the herbivore.

Some biologists maintain that algae and barnacles growing on turtles and whales have a commesalistic

relationship with their hosts .