Populations & Communities (DP IB Biology)

False.

Standard deviation measures the spread or dispersion of data around the mean value.

A small standard deviation indicates that the data points within a data set all lie close to the mean value.

The capture-mark-release-recapture method is used to estimate the population size of motile organisms.

It is important that the marking method does not affect survival of organisms because this would violate one of the assumptions of the Lincoln index and result in an inaccurate population estimate.

The Lincoln index equation is:

Population size = M × 

Where:

• M is marked individuals in the first sample

• N is total captured in second sample

• R is marked individuals recaptured in the second sample

Assumptions made by the Lincoln index when using the capture-mark-release-recapture method include:

• The marked individuals mix fully with the main population

• The marking doesn't affect the survival rates of the marked individuals

• The marking remains visible throughout the sampling and doesn't rub off

• There are no significant changes in population size due to births and deaths

• There are no migrations into or out of the main population

Carrying capacity is the maximum number of individuals of a species that an ecosystem can support.

Examples of abiotic factors that can affect carrying capacity include:

• light availability

• water availability

• temperature

• soil mineral content

• oxygen availability

• carbon dioxide concentration

Examples of biotic factors that can affect carrying capacity include:

• competition for resources

• predation

• disease

Population density is the number of individuals present per unit area of habitat.

Density-dependent factors are those that affect populations differently at different population densities, e.g. disease will spread more easily at higher population densities and will therefore have a larger effect on population size when density is higher.

True.

Density-dependent factors tend to act to keep a population at or below its carrying capacity; this is because they have a negative feedback effect.

The population in the graph is controlled by negative feedback; this is because the population is fluctuating around its carrying capacity.

Negative feedback affects population size when the population is either above or below its carrying capacity as follows:

• above carrying capacity: density-dependent factors cause a reduction in survival and reproduction, decreasing population size

• below carrying capacity: density-dependent factors will allow increased survival and reproduction, increasing population size

During the predator-prey cycle:

• more preymore food, so more predators

• more predatorsmore predation, so fewer prey

• fewer preyless food, so fewer predators

• fewer predatorsless predation, so more prey

The relationship between the Canada lynx and the snowshoe hare is a famous example of a predator-prey interaction.

Top-down control occurs when the size of a population is limited by other species feeding on it, e.g. prey populations are controlled by the presence of predators.

Bottom-up control occurs when the size of a population is limited by the availability of resources, e.g. predator populations are controlled by the availability of prey.

True.

While top-down and bottom-up factors are often both in play at the same time, one is likely to be dominant in any given ecosystem.

Both allelopathy and antibiotic secretion are strategies that involve secretion of harmful chemicals into the environment to outcompete other species.

Phases A-C in the sigmoid curve are:

• A = exponential phase

• B = transition phase

• C = plateau phase

Fast growth occurs during the exponential phase because there are no factors that limit population growth, so the population increases exponentially.

False.

Limiting factors start to act on a population during the transition phase of growth.

During the plateau phase limiting factors cause the death rate to equal the birth rate, and population growth stops at the carrying capacity.

Scientists use models because they allow experiments to be carried out and predictions to be made for biological systems that are otherwise difficult to study, e.g. due to their complexity or the time scales involved.

True.

Models are useful, but they are not perfect representations of biological systems.

It is possible to test for exponential population growth using a logarithmic scale as follows:

• plotting population size (y-axis) against time (x-axis)

• using a logarithmic scale on the y-axis

• an exponentially growing population will appear as a straight line

False.

Population data can be collected from yeast or duckweed in the lab to model sigmoid population growth.

Intraspecific relationships involve interactions between individuals of the same species.

Intraspecific cooperation is a relationship where members of the same species work together to aid the survival of the group.

Intraspecific competition is a relationship in which individuals of the same species compete for the same resources.

Plants of the same species may compete for:

• light

• minerals

• water

• space

Animals of the same species may compete for:

• food

• mates

• territory

A community is all the populations of different species living and interacting in the same area.

False.

Communities include populations from all groups of living organisms, including plants, animals, fungi and bacteria.

Interspecific relationships are the interactions between different species that occur within a community.

Herbivory is a feeding relationship where an organism feeds on a plant.

Predation is a relationship where one organism catches and consumes an animal, or consumes a recently dead animal.

Interspecific competition occurs when organisms of different species compete for the same resources.

Mutualism is an interaction where organisms of different species work together for the benefit of both.

False.

Parasitism benefits the parasite while harming the host.

Pathogenicity occurs when an infectious microorganism (pathogen) lives inside a host organism, causing disease.

Examples of mutualistic relationships include:

• bacteria living in the root nodules of legume plants

• mycorrhizal relationships between fungi and orchid plants

• coral polyps and zooxanthellae algae

The benefit to each organisms in a mutualistic relationship between root nodule bacteria and legume plants is:

• root nodule bacteria convert nitrogen into nitrates, providing mineral ions for the plant

• the plants carry out photosynthesis, providing carbon compounds for the bacteria

An invasive species is a non-native species that causes harm to the environment to which it has been introduced.

An introduced species might become invasive if it has no natural predators or competitors. As a result the species is able to increase in number at a rapid rate.

Invasive species can affect native species as follows:

• outcompete them for resources

• displace them

• transmit disease

• cause extinction

The presence of interspecific competition can be indicated if the removal of one species results in improved success for another species.

False.

The competitive exclusion effect does not prove the presence of interspecific competition, but indicates that it could be occurring.

A laboratory experiment can be used to test for interspecific competition by culturing bacteria species on their own and together and measuring how this affects population size or colony distribution.

A field study could be used to test for interspecific competition by observation using:

• random sampling: quadrat samples to record the presence/absence of one or both species at different locations

• manipulation: removing one species and measuring the effect that this has on the second species

A hypothesis is a proposed explanation for an observation that can be tested by scientific investigation.

Hypothesis can be tested using experiments and/or observation.

True.

A chi-squared test can determine whether an apparent association between two species is statistically significant.

If there is no statistically significant association between two species then there is no interaction between them, and any apparent association is due to chance.

A statistically significant association might indicate an important interaction, such as a symbiotic relationship, or competition between the species.

A null hypothesis is a testable statement that states that there is no significant association between data sets.

If a chi-squared test rejects the null hypothesis, it means there is a significant difference/association between the variables being tested.

True.

If an association is not statistically significant, it suggests the species' distributions are independent.

The purpose of constructing a contingency table is to record the number of quadrats that contain one, both, or neither species.

A p-value of 0.05 corresponds to a 95 % certainty that any association is not due to chance.

If the chi-squared value is larger than the critical value there is a statistically significant association between two sets of results.

If the chi-squared value is smaller than or equal to the critical value this means that there is no statistically significant association between data sets, and the null hypothesis can be accepted.