C4.1 - Biology Populations and communities

Definition population

An interacting group of organisms of the same species living in an area

Populations are often determined by a geographical barrier

Way to estimate population, general

Random sampling removes bias and is more effective and efficient than counting every individual

Definition sampling error

The difference between estimate and true size of population

How to estimate sessile populations

Random quadrant sampling

1) place quadrant and count how many are present

2) then find the mean and multiply to get an estimate of true population size

A quadrant is a grid with random co-ordinates

Definition sessile organisms

Organisms that stay in one place (e.g. coral and rooted plants)

Definition motile organisms

Organisms that are capable of moving independently

Types of population distributions

Uniform (lower standard deviation), clumped and random (higher standard deviation and variation)

How to estimate the population of motile populations

Capture-mark-release and recapture with the Lincoln index

What is the Lincoln Index?

Population size estimate = (M*N)/R

M is number of organisms captured and marked for the 1st sample

N is total captured 2nd capture

R is number of organisms recaptured so that were marked in 2nd capture

What are the assumptions made with the Lincoln Index?

• No migration out of the population, no births and no deaths

• Marked individual mix naturally back and behaviour / likelihood of caught again not influenced

• Marks remain visible

• Ratio in the 2nd population is representative of the entire population

Population growth curve

• Population graphed over time

• Sigmoid growth curve with ups and downs at plateau

• K is carrying capacity

What is the exponential growth phase?

• Beginning part of population growth curve, J shape

• Reproduction means exponential growth, at an accelerating rate

• Positive feedback

• Little inhibiting growth, there are plenty of resources and space

What is the transition phrase?

• 2nd part of population growth curve

• Population growth is slowing down

• Resources start to have a limiting effect

What is the plateau phase?

• Last part of population growth curve

• Population stabilises (up and down slightly)

• Because of carrying capacity

Carrying capacity definition

Maximum size of population that an environment can support

Space, food, disease, water, light, soil (plants) and predation can limit population and determine carrying capacity

Definition density-independent limiting factors

Factors that have the same effect no matter the size of the population

Density-independent limiting factors examples

Forest fires kill plants and animals no matter how many were present before

Also seawater flooding

Density-dependent factors

Factors that increase the effect as population sizes increase

• Negative feedback

• Reduce larger populations and smaller populations thrive

Examples of density-dependent factors

• Competition for limited resources like water, food and light

• Predation (significant factor): cyclical processes (e.g. fox and rabbit, cheetah and gazelle)

• Infectious diseases / parasitism / prey infestation, with an increase in the density there is an easier spread in the disease

Intraspecific relationships definition

Relationships between individuals of the same species

Types: competition and cooperation

How does competition as an intraspecific relationship work?

• Organisms share an ecological niche

• Compete for resources

• Natural selection

E.g. plants compete for light, flowers for pollination, bohemian waxwings migrate south for food and berries, guillemos compete for breeding sites on cliffs

How is cooperation an interspecific relationship?

• More present with animals and less in plants

• Strong benefit for all

E.g. all birds and mammals conserve body heat by huddling together, chimpanzees hunt in groups, fish form tightly packed bait ball, birds share parental care

Definition community

A group of populations living and interacting together in an area

Definition herbivory

Primary consumers feeding on producers (plants)

Definition predation

Consumer (predator) killing and eating another consumer (prey)

Definition mutualism

2 species living together in close association, both species benefit

Examples of mutualism

• Root nodules and Fabaceae (legumes), bacteria called Rhizobium fix nitrogen from the air for the plant, providing carbs and receives a place to live

• Mycorrhizae (fungi) in orchids, the fungi penetrate the seed and provide minerals and water, orchid provides carbs

• Zooxanthellae (algae) in hard corals, algae photosynthesise and provide carbs and oxygen for coral, coral provides a safe place for carbon dioxide

Definition paratism

One species (a parasite) lives inside or on a surface of another species, obtaining nutrients

The host is harmed but the parasite benefits

Examples of paratism

E.g. tapeworms and ticks

Definition pathogenecity

One species (pathogen) living inside another species (host) causing a disease in it

E.g. potato fungus

Examples of interspecific competition

Barnacles compete for space and food on rocky shores

Trees compete for light in mixed woodlands

Interspecific competition

Two species competing for a limited resource

There is an endemic species and an invasive species

If endemic and invasive species are competing for resources, for a realised niche, usually endemic species is reduced, sometimes to extinction

Definition endemic and invasive species

Endemic is the species that naturally occur in an area

Invasive species are alien to that environment, introduced artificially and spread rapidly

Example of interspecific competition

Grey squirrel (invasive, only 30 were introduced) and red squirrel (endemic)

Red squirrel extinct in England, only present in North Wales and Scotland.

How to investigate interspecific competition

• Field observations, random sampling over time

• Field manipulation, remove 1 competing species and observe consequences

• Laboratory experiments, controlled conditions, observe interactions and isolation

Allelopathy

Some plants release secondary metabolites into soil, impact the soil negatively, inhibiting root growth and photosynthesis

E.g. eastern black walnut tree releases chemical juglone into soil

Antibiotics

Secreted by some microorganisms to kill other competing microorganisms

E.g. fungus penicillium secrete penicillin (antibiotic) kills bacteria in soil

Top-down control