Chapter: Community Ecology

In ecology, a community is all the populations of all the species in a given area. Community ecology is the study between or among species within a given area.

 Species interactions

Species can interact in several different ways that a community ecologist would study. First, competition occurs when organisms use the same resource. And when organisms compete for resources, this lower the fitness for both. In other words, it is a negative relationship for both involved. When another organism consumes another, either by predation or parasitism, it increases the consumer’s fitness, while decreasing the victim’s fitness. In other words, it is a plus minus relationship. When two species benefit from their interaction, they are involved in a mutualistic relationship. In other words, it is a plus plus relationship. When one species benefits from a relationship benefits and the other is unaffected, this is said to be a commensalism. It is a plus zero relationship.

Competition

Competition is a minus minus relationship between species. When two organisms compete they lower both of their abilities to survive and reproduce. Competition between species of the same species is very common, and is known as intraspecific competition. And it intensifies as the population density increases. Whereas competition between species for the same resource is known as interspecific competition.

 Niche model

In community ecology, the niche model predicts that competition between species is limited because species that live in a specific area tend to use different resources. A great example of this is Darwin’s finches. The niche model predicts that different species in a given area utilize different resources. On a given island, there may be different species of finches. And in the case of Darwin’ s finches, they differ by their beak size. Species with smaller beaks can only eat smaller seeds; while species with larger beaks tend to eat larger seeds.

Fundamental vs. realized niche

The resources a species can tolerate are known as its fundamental niche. For example, a plant can live within a specific range of water availability. If the environment is too dry, the plant will shrivel and die. If the environment is too wet, The plants roots can not receive enough carbon dioxide to go through cellular respiration. However, when a species in in the same environment as one of its competitors, the range of its niche will change. This is known as its realized niche. In the example on this graph, we have two species that compete for the same resource. One is a strong competitor and the other is a weak competitor. When these species come into contact, the strong competitor squeezes out the weak competitor where their range of resources overlap. This is the weak species realized niche.

Competitive exclusion principle

In ecology, the competitive exclusion principle, sometimes referred to as Gause's law of competitive exclusion is a proposition which states that two species competing for the same resources cannot coexist if other ecological factors are constant. When one species has even the slightest advantage or edge over another, then the one with the advantage will dominate in the long term. One of the two competitors will always overcome the other, leading to either the extinction of this competitor or an evolutionary or behavioral shift towards a different ecological niche. The principle has been paraphrased into "complete competitors cannot coexist". Gause conducted an experiment with the protist, Paramecium. He grew two separately and they grew logistically as expected by population ecological models. However, when he grew the paramecium species together, one grew logistically as predicted by population ecology models. However, the other went locally extinct. In other words, Gause suggested that complete competitors can not exist.

Asymmetric vs. symmetric

The previous example was a case of asymmetric competition. In asymmetric competition, one species highly outcompetes the other. In this way one species suffers much greater fitness decline than the competitor. Asymmetrical competition is driven by competitive exclusion. Species that suffer equal decrease in fitness is known as symmetric competition. This is however a continuum. Most competition relationship lie somewhere between asymmetric and symmetric competition.

Consumption

Consumption is when one organism eats another. When animals eat plants, it is known as herbivory. When animals eat other animals, it is known as predation, but when only small amounts of tissue are consumed allowing the host to live, this is known as parasitism. 

Coevolutionary arms race

Predators have a tendency to evolve traits that make them stronger, faster, and more fierce. And as these more efficient predators evolved, their prey were selected for as well. Slower, weaker prey tended to be consumed in greater numbers, and natural selection selected prey were either unpalatable, more elusive or had defensive abilities.

Defenses

Consumption is so common that many organisms have evolved defenses against it. In fact, all animals are consumers. Some animals hide within their environment to avoid consumption. This is known as avoidance. Other animals have poison within them in order to inhibit predation. Many of these organisms are brightly colored warning potential predators of the dangers within. Fish and herds of mammals practice a schooling behavior as a defense mechanism. When a predator comes in contact with a school of fish, for example, the a shark can’t concentrate on a single fish and becomes confused. And species that are common prey also fight back. This is why elephants have tusks, moose are the most dangerous animals in North America, and porcupines have spines.

 Mimicry

Some animals take advantage of certain animals that have poison within them. They adopt the coloration of the dangerous animals with the advantage of avoiding predation. The three species in the above picture are all mimics of wasp, and none of them are toxic to predation. They evolved to resemble wasp and as a consequence they get consumed less.

Pseudocopulation

But animals don’t get to have all the fun. Orchids have also evolved to resemble wasps and bees. They also produce a pheromone. Pheromones are sex chemicals that animals use to attract each other. And orchids are so well adapted to resemble these insects that the males actually copulate with the orchids think they are females. This is nature at its most cunning.

What controls herbivores?

We have determined that predators control the populations of herbivores. This is known as the top-down control of herbivores. Herbivores can also be controlled by their food supply, plants. This is known as bottom-up control. In reality, herbivores are controlled by the combination of these factors.
Mutualism

Mutualisms are symbiotic relationships between species. And we have discussed several mutualistic relationships throughout this course. Mycorrhizal fungus exist in a mutualistic relationship with over 90% of all plant species ever studied. Fungi provide increased water absorbing ability while plants provide the fungi sugar. Flower plants have developed mutualistic relationships with their pollinator. The flowers get pollinated while the pollinators get food. Lichens are a mutualistic relationship between fungi and algae. Certain ants have a mutualistic relationship with acacia trees in the Amazon. The trees provide everything the ants need. They have thorns that are swollen into specialized chambers that serve as their homes. The trees also dispense food packets for the ants to consume. In return the ants attack anything that comes close to the tree. They attack animals that come too close. And they remove plants that would compete for light from the tree. Cleaner shrimp have a mutualistic relationship with certain fish species. They clean the inside of the fishes mouth, removing any parasites that may exist in the mouth of the fish. The shrimp gets food and the fish gets parasites removed.

Disturbance

In ecological terms a disturbance, is an event that removes biomass from a community. And this directly affect resource availability. For certain species (like plants) there are typically more available resources, like light and space. For other species, including most animals, resources are typically diminished. There are several factors that affect determine a disturbance regime, the type of disturbance, the frequency of the disturbance and how severe the disturbance is.

Disturbance regime

A disturbance regime is the characteristic type of disturbance common to an area. And there are several types, from fire to hurricanes, avalanches and drought. An interesting story exists regarding the great redwood forests of the west coast. The Smokey the Bear campaign sought to eliminate all fires in America. And it was so successful, that wildfires were all but a thing of the past. After several decades of a successful campaign, it was noted that there were no seedlings of redwoods. And in fact, there were no saplings of redwoods. In other words, there was no young generation of redwood trees. It was hypothesized that this was because the fire regime had been altered to such an extent that the seeds of the redwoods were no longer able to germinate. Fire was reintroduced into the great redwood forests, and low and behold, seels of the redwood trees began sprouting. The conclusion….redwoods require a fire regime.

Succession

Following a disturbance is succession. This is the recovery of communities following a severe disturbance. If the disturbance is so severe that the soil and all the organisms are removed, this is known as primary succession. Common examples of this include volcanism at a large scale and avalanches at a smaller scale. Secondary succession is a disturbance where some or all of the organisms are removed but the soil is more or less left intact. Examples of this include wildfires, hurricanes, and small floods or severe drought.

Successional communities

Following a very severe disturbance, succession of plant species typically follow a specific succession. The first plants to reach a new environment are known as ecologically as r-selected species, or pioneer species. They reason they are typically the first to reach a newly disturbed environment is that they have the ability to disperse their seeds long distances. These species are also typically small in size and short lived. Pioneer species typically share a specific suite of adaptations. The put most of their energy into reproduction, and are typically very poor competitors, but are able to tolerate severe abiotic conditions like poor soil quality. In other words, pioneer species live fast and die young. They are the rock stars of the plant world.  Late successional communities, in contrast are dominated by K-selected species. These are long-lived and typically large in size. They are also known as climax species. They live slow and long. K-selected species are typically very good competitors and have high energy storage in their seeds. This makes their seeds large and heavy, which greatly inhibits their ability to be disbursed. In this way climax species are slow to come to newly disturbed areas.

Climax communities

Frederick Clements was a powerhouse of early ecologists. He proposed that following a disturbance ecological communities follow a series of predictable stages from r-selected species towards K-selected species, eventually resulting in what he termed climax communities. He proposed that pioneering species consisted solely of r-selected species. Those weedy species are replaced by longer-lived more competitive herbaceous species known as an early successional community. Clements hypothesized that those herbaceous species were then replaced with shrubs and short-lived trees, known as a mid-successional community. And finally, the shrubs and trees are replaced by long-lived large tree species. And once this plant community was established, it tended to replace itself until another major disturbance attacked the community.

Theory of island biogeography

The theory of island biogeography proposes that the number of species found on an undisturbed island is determined by immigration and extinction. And further, that the isolated populations may follow different evolutionary routes, as shown by Darwin's observation of finches in the Galapagos Islands. Immigration and emigration are affected by the distance of an island from a source of colonists (distance effect). Usually this source is the mainland, but it can also be other islands. Islands that are more isolated are less likely to receive immigrants than islands that are less isolated. The rate of extinction once a species manages to colonize an island is affected by island size (area effect or the species-area curve). Larger islands contain larger habitat areas and opportunities for more different varieties of habitat. Larger habitat size reduces the probability of extinction due to chance events. Habitat heterogeneity increases the number of species that will be successful after immigration.