AP Biology Unit 8: Ecology

Stimulus

Stimulus

A change in the environment that triggers a response is called this. Some are communicated among organisms, who send signals to each other in response to changes in their environment. These signals can trigger changes in the behavior of other organisms. This communication among organisms can occur through different types of signals: audible signals, chemical signals, electrical signals, tactile signals, and visual signals.

Audible Signals

Birds use these to send warnings to other birds and to attract mates. Some primates use these to assert dominance or to warn of the presence of predators.

Chemical Signals

Pheromones are these which are released by some plants and animals to illicit a response in other organisms. Skunks release pungent chemicals to scare off potential predators. Female insects release chemicals that males of the same species can detect to initiate mating. Some plants release chemicals to warn neighbors when an herbivore is damaging them.

Electrical Signals

Sharks and rays send these through the water to locate prey species.

Tactile Signals

Touching between primates can be used to express affection or to indicate dominance. Some plants curl up the delicate parts of their body to shield them away from touch.

Aposematism

Biological means by which an organism advertises its dangerous nature to a potential predator.

Visual Signals

Some species use warning coloration, a form of aposematism, to scare off potential predators.

Signaling

This among organisms can help them find mates, determine social hierarchies, and find needed resources. Natural selection will favor signals and responses that increase survival and the chances of a successful reproduction. Over time, this selection can lead to changes in the population and evolution.

Cooperative Behaviors

Can lead to increased fitness of individuals and populations.

Endotherms

Use thermal energy generated from their metabolism to maintain their body temperature. Mammals and birds are these.

Ectotherms

Do not have internal mechanisms for maintaining body temperature and obtain heat from their environment. They must change their behaviors to regulate their body temperature. Fish, reptiles, and amphibians are these.

Metabolic Rate

The total amount of energy an organism uses per unit time. As the size of an organism increases, this decreases. One reason for this is that smaller organisms have a greater surface area to volume ratio and therefore lose more heat to their environment, so they need higher rates of this to compensate for the heat loss.

Access to Energy

Is key to maintaining the health of an organism. Organisms are constantly expanding energy to survive and obtaining energy from the food they eat or the carbon-containing molecules they produce if they are photosynthetic. A net gain in energy can result in energy storage or the growth of an organism. A net loss of energy can result in the loss of mass or even the death of the organism.

Changes in Energy Availability in an Ecosystem

This can result in changes in population size. If energy becomes less available in an ecosystem, the producers’ ability to perform photosynthesis will be reduced, and some will die. If the population size of producers in an ecosystem decreases, the entire ecosystem will be disrupted.

Trophic Levels

Represent steps in the food and energy transfers between organisms in an ecosystem. Organisms are classified into these based on their food and energy sources. Energy is captured by and then moves from producers to herbivores to carnivores and omnivores. As energy moves between these, it is lost. This energy is either lost as heat or it is consumed by the necessary metabolic processes that the organisms in these use. Because less energy is available as you move up these, the higher ones necessarily will have smaller population sizes.

Herbivores

Primary consumers.

Carnivores and Omnivores

Secondary, tertiary, and quaternary consumers.

Food Chains

Show the transfer of energy between these trophic levels.

Food Webs

Most organisms do not rely on just one food source and are part of multiple food chains. These show the interconnections between organisms in different food chains and provide a more complete representation of energy transfers in ecosystems than food chains.

Autotrophs (Producers)

Get energy from physical or chemical sources in their environment.

Photoautotrophs

Get energy from sunlight; plants are these.

Chemoautotrophs

Obtain energy from small inorganic molecules in their environment. Most are bacteria found in extreme environments.

Heterotrophs

Get energy from carbon compounds made by other organisms. They can obtain energy from carbohydrates, lipids, or proteins by breaking down these macromolecules using hydrolysis reactions; animals are these.

Decomposers

Break down dead organic material, allowing the nutrients in dead organisms to be recycled through ecosystems; many fungi and bacteria are these.

Detritivores

Organisms that obtain energy by consuming the organic waste of dead plants and animals; millipedes, centipedes, and earthworms are examples of these.

Kleptoplasty

When a heterotroph consumes an autotroph that it uses as a food source but removes the chloroplasts from the autotrophs’s cells and incorporates them into its own cells.

Bottom-Up Regulation of Ecosystems

If the population size of the producers decreased, there may not be sufficient food or energy for the remaining trophic levels, and the food web may collapse.

Top-Down Regulation of Ecosystems

Animals at higher trophic levels may help limit the population sizes at lower levels. If these top predators are removed from an ecosystem, the population sizes of other trophic levels may exceed the producers’ ability to produce enough food to support them, and the food web may collapse.

Reproductive Strategies

The availability of food and the energy it provides organisms affects these for organisms’. Different organisms use different ones in response to energy availability. Organisms that live in unstable environments that have less access to energy-containing compounds will produce large numbers of offspring at a time. Because there is less access to energy-containing food in their environment, the survival rate of the offspring is lower than that of organisms living in more stable environments.

Population Growth

Depends on population size (N), birth rate (B), and death rate (D).

Population Growth Rates

The change in the population size over change in time:

dN/dt

One way to calculate it is to compare the birth rate and the death rate:

dN/dt = B - D

Exponential Growth

If there are no limiting factors on the growth of a population (there is abundant food and habitat, no predators are present, etc), a population will experience this and can be calculated with the following equation:

dN/dt = r max N

N - population size.

r max - max per capita growth rate of the population.

Density-Dependent Factors

Factors that limit the growth of populations that will exceed the resources available in their environment, and their growth will be limited by resource availability. Some examples are disease, predation, and competition for food, habitat, or mates.

Density-Independent Factors

Factors that limit the growth of populations that will exceed the resources available in their environment, and their growth will be limited by resource availability. Some examples are temperature, precipitation, and natural disasters.

Logistic Growth

These limitations result in this for a population. It can be described by the following equation:

dN/dt = r max N (K - N/K)

N - population size.

r max - max per capita growth rate of the population.

K - carrying capacity of the environment.

Carrying Capacity

The max population that can be supported by the available resources in an environment.

Logistic Growth Curves

Are S-shaped; they start with a relatively flat lag phase, followed by a period of exponential growth called the log phase which slows as the population reaches the carrying capacity of the environment. They stabilize at or near the carrying capacity of the environment.

K-Selected

Populations that live in more stable environments and have more energy available tend to have these reproductive strategies. These populations possess relatively stable population sizes at or near the carrying capacity of the environment. These populations usually reproduce more than once per lifetime, with few offspring per reproductive cycle; they invest greater levels of parental care in their offspring, resulting in higher survival rates in their offspring. These populations experience logistic growth and are sensitive to density-dependent factors.

R-Selected

Populations that live in unstable environments and have less energy available have these reproductive strategies. These populations reproduce at a younger age, often only once in their lifetime. Each reproductive cycle produces large numbers of offspring; however, these populations invest little or no parental care in their offspring, leading to much lower survival rates in the offspring. These populations experience boom or bust cycles, with periods of exponential growth leading to populations that far exceed the carrying capacity of an environment (booms), followed by rapid decreases in the population size (busts). The size of these populations is usually not sensitive to population densities.

Community

A group of interacting populations living in the same habitat. They can be described by their species composition and species diversity.

Species Composition

The number of species that live in an area.

Species Diversity

Reflects the number of species in an area and the number of members of each of those species in the area. It gives a more accurate assessment of the variety of organisms found in an area.

Simpson’s Diversity Index

One way of representing species diversity is this equation:

1 - (n/N)^2

n - total number of organisms of a particular species.

N - total number of organisms of all the species.

The higher this is, the more diverse the community.

Relationships within Communities

Interactions and relationships among members of a community are important for the survival of organisms. The interactions among populations within a community can change over time. These changing interactions can influence how members of the community access the matter and energy they need to survive. The different types of relationships are competition, predator/prey, niche partitioning, trophic cascades, parasitism, commensalism, and mutalism.

Competition

Organisms compete for resources, such as food, habitats, and mates. It can occur between two different species (interspecies competition) or between members of the same species (intraspecies competition). It can lead to the demise of organisms in an ecosystem.

Predator/Prey

Predator species eat prey species and depend on prey populations for food. Insufficient numbers of prey species will lead to declining numbers of predator species. As the number of prey increases, predator numbers will follow with an increase in numbers. Fluctuations in the numbers of predators generally follow fluctuations in the numbers of prey.

Niche Partitioning

Competing species may coexist if they use the resources available in their habitat differently; which is known as this.

Trophic Cascades

This refers to the far-reaching effects of the reduction of one trophic level in a food web.

Parasitism

This is a symbiotic relationship where one species benefits from the relationship but the other species is harmed.

Commensalism

In this, one species benefits and the other neither benefits nor is harmed.

Mutualism

These relationships benefit both species.

Biodiversity

It refers to the variety of living organisms in an ecosystem. Ecosystems with this that is greater are usually more resilient and adaptable to changes in their environment. It depends on both abiotic (nonliving) and biotic (living) factors. Abiotic factors, such as climate and water availability, will influence the types of species and the number of organisms of each species that are found in an ecosystem. Biotic factors, such as the number of producers, will limit how many consumers can survive in an ecosystem. This part of an ecosystem will influence the structure of the food chains and food webs found in that ecosystem.

Keystone Species

They have a disproportionally large effect on an ecosystem compared to their numbers.

Invasive Species

Species that are not native to a habitat. If one has no predators in a habitat, it can out-compete native species in the area, leading to the extinction of the native species.

Human Impacts

These can cause disruptions to ecosystems. Habitat destruction (as new cities are built) can lead to decreased biodiversity. When humans move into a previously uninhabited area, new diseases may be introduced into the ecosystem. Humans may also come into contact with previously unknown diseases when they move into new areas. Humans also generate pollution that can make water sources less habitable for other species.

Geological Events

These can disrupt ecosystems, leading to changes in biodiversity. Severe weather events, such as hurricanes, can decrease plant and animal diversity in an area. Prolonged droughts can change the biodiversity of an ecosystem.