C4.1 Populations and Communities

Definitions

• Species: A group of organisms that can be interbreed and produce fertile offspring

• Population: A group of organisms of the same species who live in the same area at the same time

• Community: A group of populations living and interacting with each other in an area

• Ecosystem: A community and its abiotic environment

• Habitat: The environment in which an organism, species, population or community lives

• Habitat can be described by their geographical or physical location and by the type of ecosystem.

• Carrying Capacity: maximum population size that the environment can sustain over a set time

• Population density: number of individuals per unit area/volume :  indication of  how closely packed individuals are within a given space

• Density dependent factors: Factors that change the size of a population are dependent on population density (e.g. spread of disease)

• Endemic species: Species that can only be found in one specific area nowhere else in the world.

• Invasive species: Species that have been introduced into a new area from a distant origin

• Invasive species population grows well that they start to cause problems for the species that are already living there.Herbivores : an animal that feeds on plants.

• Top-down control: Presence/activities of organisms at higher trophic levels influence the abundance or behavior of organisms in lower trophic levels

• Bottom-up control: Availability of resources at lower trophic levels influence the abundance and distribution of organisms at higher trophic levels

Estimation of population size by random sampling → sessile organsisms

Random sample for sessile organisms

• Quadrats

Capture-mark-release-recapture and the Lincoln Index to estimate population size for motile organisms

Motile organisms: organisms that can move around the habitat.

• Method: Capture-mark-release-recapture

• Lincoln index: The population size = (n₁× n₂ ) ÷ n₃

• n₁ is the number of individuals initially caught.

• n₂ is the total number of individual caught in the second sample.

• n₃ is the number of marked individuals in the second sample.

(1st sample markerd x 2nd sample caught ) / 2nd sample marked

capture mark release recapture environmental requirement

1. Must make sure that there is at least 24 hours gap before the second trap was set up.

   1. This is because to allow time for the organisms to randomly distributed in the area again.

2. When marking the organism, one must make sure the markings are not toxic, not permanent and will not make the organism stand out from its surroundings. (losing its natural camouflage.)

3. Assumes that the population is closed, with no immigration or emigration.

4. No death no birth

Assumptions & Limitations of using the Lincoln Index

Assumes that marked individuals are representative of the whole population

Assumes the ratio of marked to unmarked individuals in the second sample accurately reflects the ratio of the population

Assumes there are no births, deaths, immigrations or emigrations during the study period

• Increase sample size and conduct multiple trials of sampling to increase accuracy

Carrying Capacity

• maximum population size that the environment can sustain over a set time

• Influenced by the availability of resource → it varies over space and time.

limiting resources: , Food , Space for breeding , Shelter, Water , Light Soil nitrogen

• When these resources become scarce, competition (interspecific and intraspecific) arises for survival; this will reduce the population.

Population density

Population density: number of individuals per unit area/volume

•  indication of  how closely packed individuals are within a given space

Size of a population is influenced by: 

1) density-dependent and 

2) density-independent limiting factors

Density-dependent factors

• Density dependent factors: Factors that change the size of a population are dependent on population density (e.g. spread of disease)

• Negative feedback in the form of competition for resources & the spread of disease works

  • to control the size of population so that cannot go above it carrying capacity.

• PANDA

  • Predators

  • Availabiltiy of resources

  • Nutrient supply

  • Disease / pathogenic spread

  • Accumulation of wastes

Density-independent factors

Factors are external to the population and can cause sudden and drastic changes to population size. These include:

PAW

• Phenomena - natural disaster

• Abiotic factor (eg. temperature, O2)

• Weather conditions (floods, storms)

• Anthropogenic causes (human)

Population growth curves

Exponential growth (J curve)

• Reflects population growth under ideal conditions where resources are unlimited and abiotic factors are favourable. 

ex. Bacterial populations in lab settings when resources are available.

__

• Exponential Phase: Abundant of food, space, light increases reproduction.

• Transitional Phase: Competition increase

• Plateau Phase: When population reaches carrying capacity (K) there is no longer growth in population size

Populations tend to fluctuate around the carrying capacity and maintained by negative feedback. [density dependent factor]

Population growth curve case study

Eurasian collared doves

• Occupy new niche

• Reproduction → exponential growth in population

Competition versus cooperation in intraspecific relationships

Intraspecific competition : Members of the same species compete for a limited resource such as food, water or space

• → as they have same ecological niche, density dependent factors

Intraspecific competition: Individuals collaborate to increase their chances of survival and reproduction

• Group hunting, defense against predators

Community

Community: A group of populations living and interacting with each other in an area

Many interspecific relationships:

• Herbivory 

• Predation 

• Interspecific competition 

• Mutualism 

• Parasitism 

• Pathogenicity

interspecific competition , Herbivory, predation, Mutualism, parasitism and pathogenicity as categories of interspecific relationship within communities

 Interspecific competition

• Definition: two species using the same resources, thus the resources available is reduced by the other species

• E.g. Red and Grey squirrel in Britain

Herbivory

• Definition: where primary consumers feed on producers

• E.g. Giraffes on tree

Herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity as categories of interspecific relationship within communities

Predation

• Definition: A consumers feed on another consumers

• E.g. Fox and rabbit

Parasitism

• Definition: an organism that lives on or in a host and obtain nutrient from the host, at the expense of the host

• E.g. Tapeworm in humans

Herbivory, predation, interspecific competition, mutualism, parasitism and pathogenicity as categories of interspecific relationship within communities

Pathogenicity

• Definition: microorganism causing disease to another species.

• E.g. HIV in human

 Mutualism

• Definition: two different species live together in a close relationship, where both species benefits from the relationships.

• E.g. Zooxanthellae and coral reef

 Mutualism as an interspecific relationship that benefits both species → Root nodules in legume plants and nitrogen-fixing bacteria

Rhizobium Bacteria → nitrogen fixing bacteria → convert nitrogen gas to nitrate ion (into a form that plant can use)

Live in root nodules in legume plant → mutualistic relationship

1. Receive carbohydrate from legume plants + protection

2. provide supply of nitrogen compounds

 Mutualism as an interspecific relationship that benefits both species → Mycorrhizae between non-photosynthetic orchid and fungus

• Mycorrhizae(fungus) forms a symbiotic relationship with roots of orchid

• Plant provides carbohydrate from photosynthesis

• Fungi absorb and provides nutrient from soil → increase plants ability to acquire nutrient by increasing SA for absorption of water and minerals

  • aiding in nutrition acquisition

Mutualism as an interspecific relationship that benefits both species → coral polyps and algae zooxanthellae

Coral polyps and algae zooxanthellae

• Coral polyps are small animals with tentacles.

• Photosynthetic algae zooxanthellae are embedded in the tissue of their tentacles.

• Coral polyps give algae Co2, → algae need to photosynthesize and grow

• algae provide carbon-based energy molecules → made through photosynthesis

Endemic and invasive species

1. endemic species : only be found in one specific area and nowhere else

2. invasive species : introduced in a new area from a distant origin → cause problem to original species

   1. Traits of invasive species

      1. rapid reproduction, fast growing

      2. no natural predators in new area

      3. high tolerance to a wide range of environmental factor

      4. live of a wide range of food type

      5. associate with human

Resource competition between endemic and invasive species

• Endemic or native species have evolved and adapted to a specific region over time 

• Invasive species are introduced to new environments where conditions are favourable for them to rapidly establish and spread

• Lead to the rapid decline and even extinction of endemic species

case study

1. Giant tortoise in galapagos island → endemic species

2. Over many decades, the populations of giant tortoises decreases

3. Reasons for the decline of these species 1) Humans eating them 2) Competition with introduced invasive species such as goats

4. Humans brought goats → some escaped and formed feral populations, which their population eventually grew. 

5. Goats compete with tortoises for grazing space and tend to destroy their habitats. 

6. Goats breed more rapidly than tortoises

7. To correct the mistake, Culling (reducing population numbers by killing off a certain percentage of the population) have been implemented

Testing for interspecific competition

Aim: To see if there is any indication that the population of one species changes in the presence of another.

• Population of the remaining species increases → competition

• If the population of the remaining species does not change → no competition

• Correlation does not provide competition

Null hypothesis (Ho) = There is no association between the species. They are distributed independently in the area.

Alternative hypothesis (H1) = There is an association between the species. They are not distributed independently in the area (this could be positive or negative - both always present or both always absent)

DF =1

P= 0.05

calculated value > criticla value → reject null, accept alternative

Predator-prey relationships as an example of density-dependent control of animal populations

• Density-dependent factor & negative feedback loop

• Canada lynx and the snowshoe hare are an example of a predator-prey interaction.

• Changes in the numbers of the lynx (predator) population are followed by changes in the numbers of the hare (prey) population.

• Limiting factor for lynx population: food availability

• Limiting factor for snowshoe hare: number of predator

Top-down and bottom-up control of populations in communities

Types of population control: both possible in a community, but one is more likely dominant 

Top Down Control: Presence/activities of organisms at higher trophic levels influence the abundance or behavior of organisms in lower trophic levels

• predation

Bottom Up Control: Availability of resources at lower trophic levels influence the abundance and distribution of organisms at higher trophic levels 

• nutrient availability

Allelopathy and Antibiotic Secretion

Both involve organisms releasing chemical substances into the environment to deter potential competitors.

Allelopathy : release of chemicals by one organism

Allelopathy: Black Walnut tree

• Releases a chemical compound into the soil

• Inhibitor → suppresses the growth of nearby competing plants

• by inhibiting key physiological processes : root development and photosynthesis

Antibiotic Secretion: Penicillin fungus

• secrets antibiotic chemical Penicillin

• inhibit growth of bacteria near the fungus → reduce competition for resources

• Penicillin is used as medicine → inhibit bacterial growth