DP BIO Populations and communities C4.1

population

A population consists of one species in one location at one time. All members of a population are the same species, and are capable of interbreeding to produce viable offspring.

To be considered ‘populations’, different groups of the same species must be reproductively isolated

Reproductive Isolation

 occurs when there is a barrier which prevents individuals from reproducing. Can be used to distinguish populations.

Random sampling benefits

Time, avoids desstructive sampling, easier to count as animals move in and out of ecosystems

why random sample

representation, removal of bias, and generalization

Random sampling nature of sceince

sampling error will always be present-it is an estimate

quadrat sampling

can be used to estimate population sizes of plants and sessile animals.

Capture-Mark-Release-Recapture

method of estimating the population size of motile species. Data is collected by capturing, marking, releasing and recapturing individuals , and the population is estimated using the Lincoln index formula:

Lincoln index formula:

(mxn)/r

carrying capacity

 maximum population size of a species that can be supported long term in an environment.

factors affecting carrying capacity

• Availability of food

• Availability of water

• Space

• Shelter

• Disease

• Predators

• Climate

Density independent factors

Density independent factors limit population growth but are not dependent on population density. Density dependent factors are usually abiotic.

Density independent factors examples

• Climate events and natural disasters, such as extreme weather: can increase mortality within a population

• Habitat destruction: reduces the resources available to a population

• Seasonal changes:  the availability of resources within an ecosystem changes with the seasons.

Density dependent factors

ause a population's growth to change due to population density. Density dependent factors tend to be biotic. Typically, as the population increases, density dependent factors will decrease populations back to carrying capacity. This is an example of negative feedback.

Density dependent factors include:

competition for resources, risk of predation, disease

competition for resources

As the population increases, competition for resources leads to reduced availability of resources for each individual.

Better-adapted individuals are more likely to survive and reproduce.

Members of the same species require the same resources to survive and reproduce.

Risk of Predation

 target abundant populations in an ecosystem. High populations of prey animals result in an increase in predator populations, as there is more food available. and vice versa

Disease

High population densities facilitate the spread of pests and diseases which are spread by contact and vectors. The spread of pathogens can result in increased mortality, and a decrease in the population.

factors affecting popilation growtg

• Natality

• Mortality

• Immigration

• Emigration

chnage in pop. size

(N + I) - (M + E)

sigmoid population growth

exponential phase, transitional phase, plateau phase

exponential phase,

There is then an exponential increase in numbers, as natality greatly exceeds mortality.

Mortality is low due to abundant resources and low competition.

Transitional Phase:

resources become limited due to competition as the population continues to grow.

Mortality increases as natality decreases.

Plateau Phase:

Carrying capacity is reached as natality equals mortality due to limiting factors (such as food, predators, and disease).

The population will fluctuate around the carrying capacity.

Intraspecific competition

Competition for resources between members of the same species

will compete for: territory, mates, social dominance

Intraspecific cooperation

Intraspecific cooperation benefits all members of a population as it increases access to resources and protection. 

Includes: group hunting, foraging defense against predators and parenting

Communities

is formed by populations of different species living and interacting with each other in an ecosystem.

Communities include plants, animals, fungi and bacteria.

Interspecific interactions

Interspecific interactions are interactions between members of different species.

• Herbivory

• Predation

• Interspecific competition

• Mutualism

• Parasitism

• Pathogenicity.

Herbivory

Act of eating plants

Predation

preying on other animals

interpspecific competition

Interspecific competition occurs when individuals from different species compete for the same resources. Interspecies competition leads to competitive exclusion, or species avoiding competition by having different ecological niches.

Mutualism

a close relationship between two organisms of different species, in which both individuals benefit. from the activity of the other. 

Symbiosis

is a close interaction between two different species

Parasitism

is a form of symbiosis where one species benefits, but the other species is harmed. like ticks and dogs

Pathogenicity

Pathogenicity occurs when an organism infects another species, causing a disease.

The disease malaria in humans is caused by the malarial parasite entering the human bloodstream.

Root Nodules in Fabaceae 

Plants in the legume family, including soybeans, have nodules containing nitrogen-fixing bacteria.

Plants require nitrogen compounds, but are unable to synthesise these compounds.

The nitrogen-fixing bacteria provide the plant with a supply of nitrogen compounds.

The bacteria receives carbohydrates and other organic compounds, produced by photosynthesis, from the plant.

Mycorrhizae 

symbiotic relationship between roots of a plant and a fungus. Orchids benefit by increasing surface area for absorption of water and minerals.

The fungi greatly enhance the orchids’ ability to acquire nutrients.

The fungus gains as it receives carbohydrates and protection from the plant.

Zooxanthellae 

Zooxanthellae algae have a mutualistic relationship with coral polyps.

The photosynthetic zooxanthellae algae produce nutrients such as glucose used by the corals, as well as removing wastes and producing oxygen.

The coral polyps provide protection for the zooxanthellae, as well as providing them with carbon dioxide for photosynthesis.

Alien species

rganisms that have been introduced to an ecosystem, and do not occur there naturally.

invasive species

lien species become invasive species if they cause harm to the natural ecosystem.

Invasive Species and Competitive Exclusion

Invasive species often replace endemic (native) species by competitive exclusion.

The alien species may become an invasive species if they have a better chance of survival than the native species due to:

• The absence of predators in the new habitat

• The absence of diseases in the new habitat.

• Faster rate of reproduction

• Larger size/more aggressive

• Outcompeting for food and other resources

Interspecific Competition and science

Interspecific competition is a reasonable hypothesis if one species is more successful than another species in a habitat. 

However, there may be other factors for the change in populations. 

Scientists have a range of approaches to determine if competition is the reason for the changes in population, such as:

• Laboratory experiments 

• Field studies where one species is removed

• Field observations using tests of association between two species.

Positive associations:

• hese occur when two species are more likely to be found together in an ecosystem. Species in mutualistic relationships have positive associations in ecosystems.

Negative associations:

• hese occur if two species are not likely to be found together. Species competing for resources tend to have a negative association as they avoid direct competition by being in different niches in the ecosystem.

Chi-Squared (χ²) Tests of Association

• Null hypothesis (H₀): There is no significant difference between the distribution of two species (i.e. distribution is random) – Accept if P > 5% 

• Alternative hypothesis (H₁): There is a significant difference between the distribution of two species (i.e. species are associated) Accept if P < 5% 

chi squared formula

(row total x colum total)/grand total

Predator-Prey Relationships

Density dependent factors cause a population's growth to change due to population density. Density dependent factors tend to be biotic.

Predator-prey relationships is an example of a density dependent factor.

predator prey cyles

increase prey increase predator decrease prey decrease predator

Top Down Controls

Top down factors are pressures applied at higher trophic levels to control dynamics in an ecosystem.

The top predator prevents overpopulation of its prey.

This has an impact on many species within the ecosystem

Bottom Up Controls

Bottom up factors are resources (such as nutrients) available to producers, affecting the growth of producers.

Population growth at higher trophic levels is impacted by the availability of producers.

Allelopathy

 is the release of chemicals by one organism which influences the germination, growth, survival or reproduction of another organism.

Secretion of Antibiotics

The Penicillium fungus secretes the antibiotic chemical penicillin.

Penicillin inhibits the growth of bacteria near the fungus, reducing competition for resources.

Penicillin is used as a medicine to inhibit bacterial growth.