Populations and communities

Species

Organisms that can interbreed to produce fertile offsprings

Population

The number of organisms of one species, living in the same area at the same time

Community

All the interracting organisms within an area without counting abiotic factors

Habitat

The place where an organism lives, considering abiotic and biotic factors

Ecosystem

All the organisms and non-living environment living in one place

What is reproductive isolation?

Reproductive isolation is used to distuinguish a population of a species from another

Types of reproductive isolation

• Geographical isolation: There is a physical separation between two population.

• Beheavioral isolation: When difference in beheaviour (mating rituals) prevent individuals from breeding.

• Temporal isolation: When two populations have different mating seasons

What happens if two isolated population never interbreed?

• Over time, two separate populations of the same species may develop differences in their characteristics.

• When these become so great that individuals cannot intebreed to produce fertile offsprings, two different species are created.

What is the advantage of random sampling

• In ecological investigations, the study sites are often too big and complex, so not every organism can be examined.

• Samples are taken that form a representation of the study site.

• Randomness is needed when sampling to avoid bias and avoid areas being selected that don’t reflect the whole study site.

What organisms can be sampled using quadrat sampling?

Sessile animals or plants, where the number of individuals can be counted.

What equipment is needed for quadrat sampling?

Quadrant, random number generator, tape measure, identification key

How many samples would need to be taken?

Enugh for the mean to stabilise (around 30)

What calculations need to be done to measure the variation and how evenly the population is spread

Standard deviation

What is meant by sampling error?

The difference between the estimate of the population size and the true size of the whole population

Formula for population size (quadrat sampling)

Mean no. of individuals per quadrant * total area / area of one quadrant

Capture-Mark-Release-Recapture

1. Capture and mark a significant sample of the population

2. Allow marked organisms to fully integrate

3. Capture a second sample of the population

4. Record the number of marked individuals.

Formula for population size (Capture-Mark-Release-Recapture)

(M / R) * N
M = No. of marked individuals captured the 1st time

R = No. of marked individuals captured the 2nd time

N = No. of individuals captured in the 2nd sample

What are the assumptions of Capture-Mark-Release-Recapture technique

Marking:

• The marking technique doesn’t have any influence on the beheaviour and survival of the organism

• Marks are not removed between capture and recapture

Mixing:

• The marked individuals fully reintagrate into the populations and have equal chances of being captured

Population size:

• There are no births, deaths, immigrations or emigrations during the study period.

Carrying capacity

The maximum number of individuals of a species which can be supported by a given environment

Negative feedback

Refers to the return of a population to its carrying capacity

Density dependent factors

• Competition for resources

• Predation

• Disease

Density independent factors

• Natural disturbances

• Habitat destuction

• Pollution

In what conditions might exponential growth in population size occur?

• Limiting factors are not restricting the growth of the population

• There are plentiful resources and lack of competition with other species

• There are favourable abiotic factors

Population growth curve

1. Exponential growth: birth rate exceeds death rate. No limiting factors, abundant resources, high reproduction rates

2. Transition: Death rate increases and approaches birth rate. Density dependent limiting factors begin to have an effect, resources become scarce & competition increases.

3. Plateau: Birth rate and death rate are equal, so population size fluctuates around the carrying capacity, kept in check by the negative feedback.

Competition

Interaction between organisms in which both parties strive to gain a limited resource or achieve a particular goal

What causes competition within the same species?

Competition for:

• Food

• Partners

• Territory

• Water

• Space

Intraspecific competition example

• Two oak trees growing very close to each other and competing for water and sunlight

• Two deers competing for mates

Intraspecific interaction example

Wolfs hunt in packs

Community

Formed by populations of different species living together and interacting with each other

Community example

Pond

• Fish, frigs, daphnia, Daphnia

• Reeds, water lilies, Elodea

• Paramecium, Chlorella

• Fungi

• Bacteria

Interpecific interactions

The relationship between different species in the same ecosystem

Herbivory

The interaction where an organism (herbivor) feeds on plants

Predation

The interaction where one organism (the predator) captures and feeds on another organism (the prey)

Interspecific competition

Occurs when different species compete for limited resources within an ecosystem

Parasitism

A type of symbiotic interaction where one organism (the parasite) benefits at the expense of its host.

Pathogen

• Microorganism capable of causing disease to its host.

• Often have a direct and immediate detrimental effect on the host's health

• Easily spread from one host to another.

Mutualism

A symbiotic interraction between two or more species, where both parties obtain benefits.

Examples of mutualism

• Root nodules in Fabaceae: the root nodules of legume plants contain many nitrogen-fixing bacteria which convert atmospheric nitrogen into usable form. Plant gains nitrogen, bacteria gain carbohydrates and other compounds.

Invasive species

An group of organisms that is not indigenous or native to an area and can potentially cause harm to the ecosystem.

Endemic species

A species that is naturally found in a a geographical region and is not found naturally elsewhere.

How do humans contribute to the introduction of new species to environments?

Trade, transport, intentional or unintentional releases.

Example of invasive species

Caulerpa taxifolia, also known as killer algae is originally native to the indian ocean, but was introduced to the Mediterrenean sea. They secrete a toxin that deters molluscs, herbivorous fish and sea urchins, and they lack a natural predator.

Chi squared test

The chi-squared test of independence is used to determine whether there is a significant association between two or more categorical variables

How does the Chi squared test work?

It compares observed values to the expected values to establish whether there is a significant association.

Method for Chi squared test

1. Construct a contingency table and input obeserved values

2. Calculate each expected frequency

3. Calculate the number of degrees of freedom

4. Determine the critical region by reffering to table of Chi-squared values

5. Calculate the Chi-squared statistic

6. Interpret the results

Chi-squared expected frequency formula

Row total * column total / grand total

Chi-squared degrees of freedom formula

(N rows -1)(N columns -1)

Chi-squared test limitations

• Sensitive for sample size

• Can establish a statistical but not causal association

• Assumes samples are independent and randomly collected

Population control

• The regulation of the size and growth of a population in an ecosystem

• Prevents resource depletion and over exploitation.

Main population control factor

Predators

Top-down control

Control of a population by predation or other factors from higher trophic levels.

Examples of top down control

Introduction of grey wolves to the Yellostone National Park in 1990s:

• Decreased population of elk and increased woody plants and saplings

Bottom-up control

Control of a population by factors originating from lower trophic levels such as availability of resources and primary activity.

Allelopathy

The process by which organisms release biochemical compounds into the environment, influencing the growth, survival or reproduction of other organisms.

Example of allelopathy

Black walnut tree releases juglone into the soil, suppressing the growth of nearby plants.