Populations & Communities DP IB Biology: HL

Flashcards covering key definitions, concepts, methods, and examples related to populations, communities, sampling, population growth, inter/intraspecific relationships, and statistical analysis (Chi-Squared Test) based on the provided IB Biology HL notes.

How is a population defined in biology?

A group of organisms of the same species living in an area at one time, interacting with each other and capable of breeding together.

What is meant by 'reproductive isolation' in the context of populations?

Two populations of the same species that are separate from each other, meaning they are isolated and not interbreeding.

Why is sampling used to estimate population size in large habitats?

It is often not possible to find, identify, and count every organism, so sampling provides an estimate from smaller, representative portions.

What is the key characteristic of random sampling?

Sampling points are selected at random, which helps to avoid bias by the person carrying out the sampling.

When is systematic sampling preferred over random sampling?

When researchers want to investigate the effect of environmental features on species distribution, often using a transect.

What is a sampling error?

The difference between an estimated population size and a true population size, occurring when a sample is not truly representative of the whole population.

What tool is commonly used to study the distribution of sessile organisms?

A frame quadrat, a square frame placed within the study area to provide a sample.

Name two types of data that can be recorded from a frame quadrat.

Presence or absence of a species, species frequency, species abundance (ACFOR scale), or percentage cover.

What does a small standard deviation indicate about data in a quadrat study?

The results lie close to the mean, indicating little variation and a more even distribution of the population.

What method is used to estimate the population size of motile organisms?

The capture-mark-release-recapture method, using a statistical measure known as the Lincoln Index.

What is the formula for the Lincoln Index?

Population size (N) = (Number of marked individuals in 1st sample (M) × Total number of individuals in 2nd sample (N)) / Number of marked individuals recaptured in 2nd sample (R).

Name one assumption made when using the Lincoln Index.

Marked individuals disperse and mix back in fully with the main population; marking doesn't affect survival rates; marking remains visible; population size stays constant during the study.

What is carrying capacity (K) in an ecosystem?

The maximum number of individuals of a species that an ecosystem can support.

Provide an example of an abiotic factor that affects carrying capacity.

Light availability, temperature, or soil mineral availability.

Provide an example of a biotic factor that affects carrying capacity.

Competition for resources, predation, or disease.

What is a density-dependent factor?

Factors that have a different effect at different population densities, acting more strongly in dense populations (e.g., pathogens, competition, predation).

How do density-dependent factors act to keep a population at or below its carrying capacity?

Through a negative feedback effect; if population increases, factors like disease or competition cause a reduction, and if population decreases, these factors lessen, leading to increase.

What characterizes a predator-prey cycle?

The population sizes of predators and prey rise and fall in a linked cycle, with predator numbers increasing after prey, and prey numbers decreasing after predator increases.

What is top-down population control?

When a population is limited by predators or herbivory (e.g., snowshoe hare limited by lynx, plants by herbivores).

What is bottom-up population control?

When a population is limited by the availability of resources (e.g., lynx limited by hare numbers, plants limited by light intensity).

What are secondary metabolites?

Harmful chemicals secreted by some species into the environment to decrease the survival chances of competing species, as opposed to primary metabolites essential for survival.

What is allelopathy?

A strategy in which organisms secrete secondary metabolites that harm other organisms into their surroundings, reducing a competitor's survival.

How does antibiotic secretion relate to allelopathy?

It is a form of allelopathy found in microorganisms where antibiotics are secreted to kill bacteria, reducing interspecific competition for the producer species.

What are the three main phases of a sigmoid (s-shaped) population growth curve?

Exponential phase (rapid growth), Transition phase (growth slows), and Plateau phase (growth stops, population stabilizes around carrying capacity).

How can exponential population growth be identified on a graph with a logarithmic scale?

When population size (y-axis, logarithmic scale) is plotted against time (x-axis, non-logarithmic scale), exponential growth appears as a straight line.

Name one organism suitable for modelling population growth curves in laboratory conditions.

Yeast or Duckweed.

How is yeast population growth commonly monitored in a laboratory setting?

By measuring the turbidity (cloudiness) of the broth culture at fixed time intervals, as turbidity increases with cell number.

What are intraspecific relationships?

Interactions between individuals of the same species, which can involve cooperation or competition.

Give an example of intraspecific cooperation.

Orcas cooperative hunting, meerkats dividing roles (e.g., lookout), or ants working together to build nests.

Give an example of intraspecific competition in animals.

Male red deer fighting for access to females, or robins defending their territory against other robins.

How is a community defined in an ecosystem?

Multiple populations of different species living and interacting in the same area.

What are interspecific relationships?

Interactions that occur between different species within a community.

Define mutualism and provide an example.

An interaction where organisms of different species work together for the benefit of both; e.g., pistol shrimp and goby fish, oxpecker birds and large mammals, bacteria in root nodules of plants.

Define parasitism and provide an example.

A relationship where a parasite organism lives in or on a host organism, causing its host harm; e.g., mistletoe plants on trees, fleas on mammals.

What is nitrogen fixation in the context of root nodules?

The conversion of nitrogen gas in the air into ammonia and then nitrates by bacteria (like Rhizobium) living in plant root nodules, making nitrogen useful for plants.

How do mycorrhizae benefit plants?

Fungal hyphae in mycorrhizae greatly increase the surface area of plant root systems, enhancing the absorption of water and mineral ions.

What is an invasive species?

A non-native species that causes harm to the environment to which it has been introduced.

Name two ways invasive species can harm a native ecosystem.

They can outcompete native species for resources, act as successful predators, or introduce new diseases to which native species have no immunity.

What makes endemic species particularly vulnerable to invasive species?

Endemic species are found only in a particular location, so local extinction due to invasive species means the entire species goes extinct.

What is competitive exclusion?

When one species is forced out of its fundamental niche into an alternative (realised) niche or becomes locally extinct due to interspecific competition.

What is the purpose of the chi-squared test in ecological studies?

To determine whether or not there is a significant association between the distributions of two species within a habitat, or if any observed association is due to chance.

What does a null hypothesis state in the context of species distribution?

That there is no significant association between the distributions of two species, and any apparent association is due to chance.

What happens if the calculated chi-squared value is larger than the critical value?

The null hypothesis can be rejected, indicating a statistically significant difference or association between the data sets.