Chapter Six POPulation

section one

The Levels of complexity found in the natural world are individuals, populations, communities, ecosystems, and biosphere.


  • individuals - a simple organism. Scientists who study individuals know that they are the unit of natural selection, and they have to reproduce and be able to survive on its own.
  • population - the individuals that belong to the same species and live in a given area at a particular time. When scientist study population they study the dynamics of it. The scientists have to understand that the populations are always changing.
  • community - all of the populations of organisms within a given area. Communities exist within and ecosystem. Scientists who study communities know that it is always changing.
  • ecosystem - Consists of all biotic and abiotic components in a specific location. Scientist who study an ecosystem study the flow of energy and matter.
  • biosphere - Areas on the Earth that are occupied by living organisms. Largest and most complex system. Scientist who study the biosphere are interested in the movement of water, air, and heat that is around the globe.

Section two

(contrast the ways in which density-dependent and density-independent factors affect population size) DEFINITIONS:

  • Population Size (N) - total number of individuals within a defined area at a given time.
  • Population Density - total number of individuals per unit area at a given time.
  • Population Distribution - description of how individuals are distributed with respect to one another

Density-dependent and density-independent factors affect the population size in many ways. Density-dependent regulation is affected by factors that affect the death rates and birth rates such as competition and predation. Density-independent can be affected by factors that are similar to density-dependents factors, such as abiotic factors and environmental situations. (For example: weather conditions that are very severe.)


  • Population sex ratio - The ratio of males to females in a population. (Example : The ratio of male raccoons to female raccoons.)
  • Population age structure - A description of how many individuals fit into a particular age categories in a population.
  • Density-Dependent factor - Influences an individual's probability of survival and reproduction that depends on the size of a population. (Example: food and shelter)
  • Density-Independent factor - A factor that has the same effect on an individual's probability of survival and the amount of reproduction at any population. (Example: Deforestation )
  • Limiting Resource - A resource that a population is not able to live without and occurs in many quantities that are lower than the population. It would require to increase in the population size.
  • Carrying Capacity (K) - The limit on how many individuals an environment is able to sustain without using all their resources.

If a population has limiting resources, they can only have a small carrying capacity. They will not be able to sustain a large population, because they have a limited amount of resources that they all have for their population.

section three


  • Population Growth models - Math equations that are able to be used to predict population sizes at any moment.
  • behavioralbehavioralReproductive Strategies - are behavioural adaptations that improve the chances survival rate.
  • Survivorship Curves - A graph that shows the number or proportion of individuals surviving to each age for a given species or group (example: males or females).
  • Metapopulations - A group of populations that are separated by space but the population consists of the same species.
  • Intrinsic Growth Rate (r) - The maximum potential for growth of a population under ideal conditions with unlimited resources.

Overshoots and die-offs relate to carrying capacity because they can take a population from a high number to a very low number of species and vis versa. Both die-offs and overshoots can lead to them being in a pattern of oscillations around the populations carrying capacity

Predator and Prey can play a big role in many populations. Predation may play a very important role in the populations growth. If there are more predators then there are prey, all the predators will start to die-off.


  • K-Selected Species - Species that have a low intrinsic growth rate that causes the population to increase.
  • R-Selected Species - Species that has a high intrinsic growth rate that leads the populations to undergo overshoots and die-offs.
  • Corridors - Strips of natural habitat that connect populations.

K-selected species and R-selected species are opposites in reproductive strategies. Many different species are in the middle of these two strategies. K-selected species cause the population to grow, in contrast, R-selected species causes a decrease in the growth rate.

Corridors are important to metapopulations because they provide some type of connectedness among populations. Smaller populations are more likely to go extinct more than larger populations. Smaller populations have less genetic variation than larger populations.

This chart shows k-selected and r-selected species with their appropriate traits.
This chart represents k-selected and r-selected species with their appropriate survivorship curves.

Exponential Growth Model

This is an example of Exponential Growth. It is showing the population growing over time.

Logistic Growth Model

This is an example of Logistic Growth model because it represents the rapid growth and then falls to zero.

section four

Species interactions includes competition, predation, parasitism, and herbivory.


  • Symbiotic Relationships - The relationship between two species that live close to each other.
  • Mutualism - An interaction between two species that increases the chances of survival or reproduction for both species (Example: Bees and flowers).
  • Commensalism - A relationship between species in which one species benefits and the other species is not harmed and it doesn't benefit in any way. (Example: Fish using coral reefs to hide in).
  • Parasitism - An interaction in which one organism lives in or on another organism.(Tapeworms that live inside animals intestines and bloodstream).

Competitive exclusion principle leads resource partitioning because two species divide a resource based on their behavior or forms. Natural selection will favor individuals that overlap with other species in their resources.

4 different types of predators:

  • Parasitoids - Animals that lay eggs inside other organisms. They refer other organisms as their host. The benefit would be that the parasitoids would be able to get out and let their eggs hatch. Unfortunately, for the host, it will die.
  • Parasitism - An interaction in which one organism lives in or on another organism. Just like parasitoids, when the parasite is on or in another organism, its called a host. A parasite rarely causes a death to the host because it is so small.
  • Pathogens - A parasite that causes a disease in its host. Pathogens include viruses, bacteria, fungi, protists, and helminths.
  • Herbivory - an interaction in which an animal consumes a producer. Usually they only eat a portion of their host, instead of killing the whole organism.


  • Camouflage makes it very difficult for predators to see.
  • On a porcupine, the spikes on its back protects it from predators.
  • Poison dart frog has toxic skin, that sends predators away.
  • Species mocking other species to do their defense mechanisms, so it acts as if predators cant see them nor harm them.
Keystone Species
  • Keystone Species - A species that plays far more important in its community than its relative abundance might suggest.
  • It is a single centered stone that supports all the other stones, without having the center stone, it would fall. In a population you have to have an even amount of predators and prey so that it wouldn't have an overshoot and be unsustainable.

ECOSYSTEM ENGINEER - A keystone species that creates or maintains habitat for other species. (Example: Alligators digging deep "gator holes" in the summer.

section five

Ecological succession can occur over multiple time spans varying from decades to even centuries. For instance, over time soil develops on the surface that is bare and this leads to the environment becoming hospitable for plants with deep root system


  • Ecological succession - The predictable replacement of one group of species by another group of species over time.
  • Primary succession - Ecological succession occurring on surfaces that are initially devoid of soil. (Example: temperate forest biomes)
  • Secondary succession - The succession of plant life that occurs in areas that have been disturbed but have not lost their soil. (Example : forest fires)

Soil Formation Process in Primary Succession:

Algae and early arriving plants can begin to form over time. Over time distinct communities develop. In the picture above it shows that rocks start out and over time mosses and weeds grow upon it. After that the mosses and weeds will slowly turn into shrubs and later a forest will appear after a long time.

Weeds, grasses, and wildflowers make good mid-successional plants because they have wind-borne seeds and they have light.

Pioneer species - A species that can colonize new areas rapidly and grow well in full sunshine. Pioneer species increase the number and grow larger. (Example: the seeds of aspen trees are carried on the wind, and cherry tree seeds are carried by birds that consume the fruit and excrete the seeds on the ground).

Aquatic succession occurs from time to time by storms that turn over rocks or clear their surfaces of living things. These bare rocks that are left can be further colonized through the process of primary succession. This process takes around 15 months. If the rock is then not touched for about another 3 years, the area may become dominated by the species that are clinked onto mussels and barnacles.

Section six

Latitude, time, and area affect the richness of species within a community. The temperature affects where the animals are, due to their ability to adapt to that place. In the Southern United States, they carry 12,000 more species of plants than northern regions. Canada only supports 1,700 plant species. For many years, scientists have been studying from plant species to animal species on why this is happening. No one has a set reason; the answer remains unclear.

The longer a population can exist, the more speciation and extinction can occur. Time has a lot to do with affecting the animals habitat. For example, there is a lake in Siberia called Lake Baikal and it is more than 25 million years old. It holds more than 580 different kind of invertebrate species. In a lake in northern Canada, Great Slave Lake, they only have 4 species invertebrates. The two lakes are roughly around the same size. Great Slave Lake is only a few tens of thousands years old, in comparison to Lake Baikal which its been around for a very long time.

Theory of Island Biogeography - A theory that demonstrates the dual importance of habitat size and distance in determining species richness. The factors that tag along to this are the size of the habitat and the distance of the habitat from a source of colonizing species.

Larger habitats contain more species because of the following reasons:

  • Dispersing species are more likely to find larger habitats than smaller habitats
  • At different latitudes, large habitats are capable of supporting larger populations of any kind of species.
  • Larger populations are less prone to extinction
  • Larger habitats often contain a wider range of environmental conditions, which provides more niches that support a large number of species

Species richness increases as the size of the habitat increases. The source of colonizing species is the second factor that affects the species richness of communities. For example, if there were two islands that were the same size, but one was closer than the other, the island that is closer would accumulate more species than the island that is farther. This is because it has a higher rate of immigration by newer species.

All the effects of colonization, speciation, and extinction on the species richness of communities have important roles for conservation. The theory of island biogeography was applied to oceanic islands originally. it has changed and been applied to "habitat islands" within a continent, such as the "islands" of habitats that are represented by national parks. These habitats are usually surrounded by less hospitable habitats that have been altered dramatically by human activities. If we wanted to set aside natural habitat for a given species or a specific group of species, then we would have to consider the size of the protected area and the distance between the protected areas and other areas that could provide colonists; people.


Friedland, Andrew, and Rick Relyea. "Chapter 2." Environmental Science for AP. 2nd ed. W.H. Freeman, 2015. 31-65. Print.


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