A population is comprised of organisms belonging to the same species, all living in the same area and interacting with each other
One measure of a population’s health is the dispersion of individuals within a population
Information about birth rate, death rate, and survivorship curves also show how populations grow and change
The population growth rate shows how the population size changes per population member per unit of time and depends on birth and death rates and migration
There are different types of migrations that affect population density
Under ideal conditions, populations show exponential growth; under more realistic conditions, limiting factors (density-dependent and density-independent factors) cause logistic growth
There are two major schools of thought about human population growth; the Neo-Malthusians and the cornucopians
Review Questions
Name two ways in which ecologists can get an idea of the health of a population.
For a secretive or highly mobile species, how might you determine population size?
What might cause a clumped or clustered dispersion?
In an altricial reproductive strategy used by robins and hummingbirds, the birds hatch helpless and naked. Parents invest little energy in just a few, small eggs. It is important these offspring survive, because there are so few. What strategies might parents use to ensure that their young survive?
How does a limiting factor such as food supply limit population size?
In human history, major advances in technology caused an increase in carrying capacity. What do you think these major advances were?
Name some environmental crises that support the idea that our human population has already grown beyond the carrying capacity resulting in environmental degradation.
Further Reading / Supplemental Links
http://www.brainpop.com/science/ourfragileenvironment/populationgrowth/preview.weml
http://eelink.net/pages/EE+Activities+-+Population
http://mathforum.org/t2t/faq/census.html
http://en.wikipedia.org/wiki/Population_ecology
Vocabulary
altricial
Newborn that are helpless at birth and require much parental care.
birth rate
Number of births per individual within the population per unit time.
carrying capacity
Maximum population size that can be supported in a particular area without degradation of the habitat.
death rate
Number of deaths within the population per unit time.
density-dependent factors
Promote competition between members of the same population for the same resource; food and space are examples.
density-independent factors
Act irregularly, regardless of how dense the population is; temperature and climate are examples.
dispersion
Spacing of individuals within a population.
emigration
Movement of individuals out of a population.
immigration
Movement of individuals into a population from other areas.
limiting factor
A living or nonliving property of a population’s environment, which regulates population growth.
population growth rate
How the population size changes per unit of time.
precocial
Newborn that are independent at birth or hatching and require little parental care.
Points to Consider
Now that you understand what makes up a population, what do you think makes up a community?
You have learned about some of the factors that limit populations. What do you think are some interactions that affect the community?
Lesson 23.3: Communities
Lesson Objectives
Explain what a community is.
Describe community interactions
Explain what competition is and how it affects the community.
Describe predation and how that affects prey density.
Explain what symbiosis is and give examples of different kinds of symbiosis.
Check Your Understanding
What is a population?
How do density-dependent factors promote competition between members of the same population?
Introduction
Now that we have examined the dynamics of a single species at the population level, we are now ready to move to the next higher level. This is the community level, where we look at how populations of different species that occupy the same area interact with each other. As we will see, there are a number of types of interactions, including competition, predation and symbiosis. These interactions in turn affect the species’ interactions with one another.
What is a Community?
A community is an assemblage within, the same area, of populations of different species interacting with each other. The term can be used in various ways with differences in meaning. For example, it may be limited to specific places, at specific times, or certain types of organisms. Thus, one may study the fish community in Lake Ontario or the fish in this lake during a specific period, such as the period before industrialization.
A community may also be defined according to the classification of and the geographic distribution of, species, as in an oak-hickory forest. On the other hand, a community might be defined according to function and behavior, as in a forest that is moderate in temperature (temperate) and sheds leaves annually (deciduous).
Community Interactions
Community interactions can be either intraspecific, that is between members of the same species, or interspecific, between members of different species. There are a number of different types of interactions, such as competition, predation, and symbiosis, which can be described as beneficial, detrimental or neutral. For example, competition could be looked at as having negative effects on the competing individuals or species, whereas mutualism, a type of symbiosis, could be seen as positive for the individuals involved.
As we examine different types of interactions in the next few sections, we will see more specifically why interactions are considered positive, negative, or neutral. We usually look at costs and benefits in terms of fitness, or survival and reproduction. These types of interactions may alter populations, communities, and even ecosystems, and the evolution of interacting species.
Competition
Competition can be defined as an interaction between organisms of the same or different species, in which the “fitness” of one is lowered by the presence of another. Individuals compete for a limited supply of at least one resource, such as food, water, or territory. Fitness refers to the ability of a species to survive and reproduce.
Competition can be described in terms of the mechanisms by which it occurs, either directly or indirectly. For example, competition may occur directly between individuals via aggression or some other means, whereby individuals interfere with survival, foraging or reproduction, or by physically preventing them from occupying an area of the habitat. Indirect competition is when a common limiting resource acts as an intermediate. For example, use of a specific resource or resources decreases the amount available to others, thereby affecting the others’ fitness, or competition for space results in negatively affecting the fitness of one of the competing individuals.
Another type of indirect competition occurs when two species are preyed upon by the same predator. If the population size of one species increases, this would cause the predator population to increase, and might result in the other species’ population size decreasing.
Intraspecific competition occurs when members of the same species compete for the same resources, like food, space, or light. Two organisms competing for the same resource can adapt to such conditions. Thus, if two trees growing close together are competing for light, water, and nutrients, one may supercede the other by growing taller to get more available light or develop a more extensive root system to get m
ore water and nutrients. Such a situation results in survival for the organism that has better adapted to that environment.
Interspecific competition occurs when individuals of different species share a limiting resource in the same area, resulting in one of the species having lowered reproductive success, growth, or survival. For example, cheetahs and lions feed on similar prey. If prey is limited, then one species may catch more prey than the other and force the other species to leave the area. Lions sometimes steal prey killed by cheetahs. This could negatively affect the survival of the cheetahs.
According to the competitive exclusion principle, species less suited to compete for resources will either adapt, be excluded from the area, or die out. This is similar to what happens within a species. Evolutionary theory says that competition for resources within and between species plays an important role in natural selection (Table (below)).
In order for two species within the same area to coexist, they may develop different strategies. This is known as character displacement and an example of this is the different feeding adaptations, such as bill structure, that developed in Darwin’s Finches (Figure below).
Figure 23.11
An example of character displacement, showing different bill structures, reflecting different feeding strategies, in Darwins or Galapagos Finches. From Darwins Journal, 1845, during the voyage of the H.M.S. Beagle.
Main Features of Competition Type of Competition Description of Competition
Direct Competition Occurs directly between individuals via aggression or some other means.
Indirect Competition Occurs indirectly through a common limiting resource, which acts as an intermediate, and/or occurs between two species which are both preyed upon by the same predator.
Intraspecific Competition Occurs when members of the same species compete for the same resources, like food, nutrients, space, or light.
Interspecific Competition Occurs when individuals of different species share a limiting resource in the same area.
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Predation
Predation is an interaction where a predator organism feeds on another living organism or organisms, known as prey. Predators may or may not kill their prey prior to eating them. The key characteristic of predation is the direct effect of the predator on the prey population.
In all classifications of predation, the predator lowers the prey’s fitness, by reducing the prey’s survival, reproduction, or both. Other types of consumption, like detritivory, where dead organic material (detritus) is consumed, have no direct impact on the population of the food source.
Predation can be classified in a number of different ways. One way is to classify it functionally, according to the extent to which the predators feed on and interact with their prey. This type includes true predation, grazing, and parasitism. (Parasitism will be discussed later in this lesson.)
True predation is the type in which the predator kills and eats its prey. Some predators of this type, such as jaguars, kill large prey and dismember or chew it prior to eating it (Figure below). Others, such as a bottlenose dolphin or snake, may eat its prey whole. In some cases, the prey dies in the mouth or digestive system of the predator. Baleen whales, for example, eat millions of plankton at once, with the prey being digested afterward. Predators of this type may actively pursue prey, or wait for it to approach to within striking distance.
Figure 23.12
An example of a true predator, showing a lioness actively hunting warthogs in the western corridor of the Serengeti, in Africa.
In grazing, the predator eats part of the prey, but rarely kills it. Many of this type of prey species are able to regenerate or regrow the grazed parts, so there is no real effect on the population. For example, most plants can regrow after being grazed upon by livestock. Kelp regrows continuously at the base of the blade to cope with browsing pressure. Starfish, also, can regenerate lost arms when they are grazed on. Parasites feed in a similar way to grazers, but are noted for their close association with their host species, and will be discussed further in the next section on symbiosis.
Figure 23.13
An example of Batesian mimicry, where the Viceroy butterfly () (right) mimics the unpalatable Monarch butterfly () (left). Both species are avoided by predators to a greater degree than either one would be otherwise.
Another way of classifying predators is by degree of specialization. Many predators, such as pandas and the snail kite, specialize in hunting only one species of prey, or certain classes of prey. Others, such as humans, leopards, and dogs, will kill and eat a wide variety of species. Specialists are usually well adapted in capturing their prey, but prey may be equally well adapted to escaping the predator. This helps to keep both populations in equilibrium. Almost all specialists will usually successfully switch to other prey, or may resort to scavenging, or even a vegetarian diet, if the preferred prey is extremely scarce.
Predators play an ecological role, in that they may increase the biodiversity of communities by preventing a single species from becoming dominant, as in the grazers of a grassland. Introduction or removal of these dominant keystone species, or changes in its population density, can have drastic effects on the equilibrium of many other populations in the ecosystem.
The act of predation can be broken down into four stages: detection of prey, attack, capture, and consumption. At each stage, predator and prey have adaptations for obtaining food and avoiding predation (Table (below), respectively. One mechanism to avoid detection is camouflage (Figure below), where species have an appearance (color, shape or pattern) which helps them blend into the background. Mimicry is a related phenomenon where a species uses appearance to copy another species, and which is used by both predators and prey (Figure above).
Main Features of Predation Type of Predation Description of Predation
True Predation Predator kills and eats its prey
Grazing Predator eats part of the prey, but rarely kills it
By degree of Specialization Predator specializes in hunting only one species of prey, or certain classes of prey, or predator kills and eats a wide variety of prey species
Figure 23.14
Camouflage by the dead leaf mantis, , makes it less visible to both its predators and prey. If alarmed, it lies motionless on the rainforest floor of Madagascar, Africa, camouflaged among the actual dead leaves. It eats other animals up to the size of small lizards.
Other anti-predator adaptations involve mobbing behavior, where a prey species cooperatively attacks or harasses a predator, as in crows and smaller birds working together to drive away a hawk. Prey may also suggest is is unprofitable to chase, as in the case of a Thomson’s gazelle stotting (jumping into the air with the legs kept straight and stiff, and with a visible white rear) to let predators know not to give chase. This is known as advertisement of unprofitability.
Anti-predatory Adaptations
Advertisement of Unprofitability
Prey species advertises in order to let predator know not to give chase
Mobbing Behavior
A prey species cooperatively attacks or harasses a predator
Mimicry
Species uses appearance to copy another species, and is used by both predators and prey
Camouflage
Species have an appearance which helps them blend into background
Symbiosis
The term symbiosis commonly describes close and often long-term interactions between different species, in which at least one species benefits. The symbiotic relationship may be characterized as being mutualistic, commensalistic, or parasitic. In mutualism, both species benefit; in commensalism, one species benefits while the other is not affected; and in parasitism, the parasitic species benefits, while the host species is harmed.
Symbiotic Relationships
Parasitism
Parasitic species benefits, while host species is harmed.
Commensalism
One species benefits, while the other is not af
fected.
Mutualism
Both species benefit.
Mutualistic relationships include the large percentage of herbivores that have gut fauna that help them digest plant matter, coral reefs that have various types of algae living inside, and the relationship between the Ocellaris clownfish and the Ritteri sea anemones. In the latter example, the clownfish protects the anemone from anemone-eating fish, and in turn, the stinging tentacles of the anemone protect the clownfish from its predators (Figure below).
Commensal relationships may involve an organism using another for transportation or housing, such as spiders building their webs on trees, or may involve an organism using something another created, after the death of the first.
Parasites include those that either live within the host’s body, such as hookworms, or those that live on its surface, such as lice. In addition, parasites may either kill the host they live on, or rely on the host's surviving. Parasites are found not only in animals but also in plants and fungi.
Figure 23.15
A mutualistic relationship between the Ocellaris clownfish and the Ritteri sea anemone. Myako Island, Japan. The fish protects the anemone from anemone-eating fish and the anemone protects the clownfish from its predators, with its stinging tentacles. The clownfish has a special mucus which protects it from the tentacles.
Lesson Summary
A community is an assemblage within the same area, of populations of different species interacting with one another.
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