IB Biology/Communities

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Topic 4: Ecology and Evolution[edit]

Communities and Ecosystems[edit]

Define ecology, ecosystem, population, community, species and habitat.

  • Ecology - the study of relationships between living organisms and between organisms and their environment.
  • Ecosystem - a system made up of organisms as well as the abiotic factors in the area.
  • 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
  • Species - a group of organisms which can interbreed and produce fertile offspring
  • Habitat - the environment in which a species normally lives or the location of a living organism

Explain how the biosphere consists of interdependent and interrelated ecosystems.

  • In an ecosystem, organisms feed off of each other. This relation or interaction of organisms can be in the form of a food chain or a food web. The food chain is a linear and simple feeding relation, where one organism has one type of food and is eaten by one type of organism. However, a food web is more complex and it includes more variety of organisms, each of which can feed on a variety of other organisms and is fed upon by a variety of organisms. These are not the only interactions that compose the biosphere, however. A remarkable diversity of animal interactions, as well as the work of plants, bacteria, fungus, and protists combine to influence the biosphere. Also, organic cycles such as the water cycle, the recycling of the respiratory products of animals (carbon dioxide) in photosynthesis, and the transpiratory return of water to the atmosphere in plants all play major roles as well.

Define autotroph (producer), heterotroph (consumer), detritivore and saprotrophs (decomposer).

  • Autotrophs - also known as producers, they can make their own food - main producers are photosynthesizers, which utilize the sun's energy and convert it into chemical energy, which they use to build their bodies. Considered net producers of O2.
  • Heterotroph - are consumers, they feed on ready made organic material, they cannot synthesize their own food, and they are considered net producers of CO2.
  • Detritivore - organisms that feed on the detritus (thus detritivore)and decomposing organic material of living organisms (eg. dung beetle).
  • Saprotrophs or decomposers- organisms that feed on dead organisms and products of living organisms. They secrete enzymes on these materials that cause decomposition, and then they absorb the resulting simple compounds into their bodies. So they do not ingest whole food, but rather, they absorb decomposed and digested food. Examples are bacteria and fungi.

Describe what is meant by a food chain giving three examples, each with at least three linkages (four organisms).

  • A food chain is a linear and simple feeding relation, where one organism has one type of food and is eaten by one type of organism. For example:
  1. Mosquito larva --->beetle --->mouse--->snake
  2. Plankton---->krill---->mullet--->shark
  3. Earwig---->lizard--->shrew-->owl
  4. Clams---->starfish--->sea otters--->orcas
  5. Periwinkle plant--->moth--->frog--->snake---->hawk
Tip: It's not necessary to memorise this, instead you can think logically.

Describe what is meant by a food web

  • A food web is more complex than a food chain and it includes a larger variety of organisms. Each of which feed on a variety of other organisms and they are in turn fed on by more organisms. Therefore, if one species becomes extinct the ecosystem will still be able to exist. A drawing will be inserted at a later date of a food web.

Define trophic level.

  • Trophic Level: Position in the food chain, determined by the number of energy-transfer steps to that level; A functional classification of taxa within a community that is based on feeding relationships (eg, plants make up the first trophic level, herbivores make up the second).

Deduce the trophic level of organisms in a food chain and a food web.

Construct a food web containing up to 10 organisms, given appropriate information.

State that light is the initial energy source for almost all communities.

  • Light is the initial energy source for almost all communities.

Explain energy flow in a food chain. • Energy losses between trophic levels include material not consumed or material not assimilated and heat loss through cell respiration. Also, energy flows (basically) from the sun to producers to herbivores to carnivores.

State that when energy transformations take place, including those in living organisms, the process is never 100% efficient, commonly between 10-20%.

  • When energy transformations take place, including those in living organisms, the process is never 100% efficient, commonly between 10-20%.

Explain what is meant by a pyramid of energy and the reasons for its shape.

  • A pyramid of energy shows the flow of energy from one trophic level to the next in a community. The units of pyramids of energy are therefore energy per unit area per unit time.

Explain that energy can enter and leave an ecosystem, but that nutrients must be recycled.

  • Energy can enter and leave an ecosystem but nutrients must be recycled. Sun light is the main source of energy on this planet. It is absorbed by photosynthesizing organisms, which convert light to chemical energy. Nutrients must be recycled by obtaining them from other organisms or products of organisms.

Draw the carbon cycle to show the processes involved.

  • See Carbon Cycle
  • Five major processes involved: Photosynthesis, cellular respiration, feeding, fossilization and combustion.

Explain the role of saprotrophic bacteria and fungi (decomposers) in recycling nutrients.

  • These organisms feed on dead organisms and products of living organisms. They secrete enzymes on these materials that cause decomposition, and then they absorb decomposed and digested foods. They absorb only what they need to survive, the remainder of the nutrients are reabsorbed into the ecosystem. They are essential as they unlock these nutrients, readying them for reabsorbtion. Examples include many species of bacteria and fungi. These are essential organisms to an ecosystem, since they cause recycling of materials between biotic and abiotic parts of the ecosystem.

4.2 The greenhouse effect[edit]

Draw and label a diagram of the carbon cycle to show the processes involved.

Analyse the changes in concentration of atmospheric carbon dioxide using historical records.

Explain the relationship between rises in concentrations of atmospheric carbon dioxide, methane and oxides of nitrogen and the enhanced greenhouse effect.

'As levels of carbon dioxide, methane, and nitrous oxides increase, more radiation is reflected back to Earth instead of being lost to space.

Outline the precautionary principle.

'better safe than sorry'- When an activity raises threats of harm, measures should be taken, even if a cause-and-effect relationship has not been established scientifically.

Evaluate the precautionary principle as a justification for strong action in response to the threats posed by the enhanced greenhouse effect.

Outline the consequences of a global temperature rise on arctic ecosystems.

•Effects include increased rates of decomposition of detritus previously trapped in permafrost, expansion of the range of habitats available to temperate species, loss of ice habitat, changes in distribution of prey species affecting higher trophic levels, and increased success of pest species, including pathogens.

4.3 Populations[edit]

Outline how population size can be affected by natality, immigration, mortality, and emigration.

  • If (natality + immigration) > (mortality + emigration) then a population is increasing. These factors determine whether a population is increasing or decreasing.

Draw a graph showing the sigmoid (s-shaped) population curve.

Explain reasons for the exponential growth phase, the plateau phase, and the transitional phase between these two phases

  • During the Exponential phase the population increases exponentially because the natality rate is higher than the mortality rate. The resources needed by the population such as food and space are abundant, and diseases and predators are rare.
  • During the Transitional Phase, the birth rate begins to decrease. Natality is still larger than mortality, but the difference between them is slowly decreasing.
  • During the Plateau phase, available resources become so low that no further reproduction can take place. Mortality starts to become larger than natality. A species may have reached its Carrying Capacity.

Define carrying capacity

  • Carrying Capacity: The maximum number of organisms of a species, or the maximum population size which an environment is able to support.

List three factors which set limits to population increase.

  • Available resources.
  • Disease
  • Space available
  • Predators

Define random sample

  • A sample where every individual in a population has an equal chance of being chosen.
  • No Bias

Describe one technique used to estimate the population size of an animal species based on a capture-mark-release-method.

  • One method of estimating the population size of an animal species is the capture-mark-release method.
  1. As many individuals of a population are caught. These individuals are marked.
  2. The individuals are released back into their environment.
  3. After a while, as many individuals of a population are caught in the same area again.
  4. The total number is taken note of and the number of those caught which are marked
This is known as the "Lincoln Index"
  • The formula (n1 × n2) / n3 is then used where:
  • n1 = the number of individuals caught in the first trial.
  • n2 = the number of individuals caught in the second trial.
  • n3 = The number of individuals in the second trial which were marked.

Describe one method of random sampling used to compare the population numbers of two plant species, based on quadrant methods

Calculate the mean of a set of values

  • Mean= Total sum of values / number of values.

State the term standard deviation is used to summarize the spread of values around the mean and that 68% fall within 1 standard deviation of the mean

Explain how standard deviation is useful for comparing the means and spread of ecological data between two or more populations.


Define evolution.

  • Evolution is the accumulation of changes in the heritable characteristics of a population.

State that populations tend to produce more offspring than the environment can support.

This increases the chance of survival of the population as a whole--a single death is less disastrous in a population of 1,000 than it is in a population of 10.

Explain that the consequence of the potential overproduction of offspring is a struggle of survival.

  • Populations of living organisms tend to increase exponentially.
  1. More offspring are produced than the environment can support. There is a struggle for important resources such as food and space. Intraspecific competition. Some individuals survive and others die.
  2. Characteristics in organisms differ from one another. Some have characteristics which make them better suited to survive in their environment. These are the most likely to survive.

State that the members of a species show variation.

  • Members of a species show variation.

Explain how sexual reproduction promotes variation in a species.

  • Variation is essential for natural selection and therefore for evolution. Although mutation is the original source of new genes or alleles, sexual reproduction promotes variation by allowing the formation of new combinations of alleles. Two stages in sexual reproduction promote variation.
  1. Meiosis allows a huge variety of genetically different gametes to be produced by each individual
  2. Fertilization allows alleles from two different individuals to be brought together in one new individual.

Explain how natural selection leads to the increased reproduction of individuals with favourable heritable variation.

The much better-adapted individuals pass on their characteristics to more offspring than the less well adapted individuals. The results of natural selection therefore accumulate. As one generation follows another, the characteristics of the species gradually change, the species evolve.

Discuss the theory that species evolve by natural selection.

  • There is a struggle for existence in populations. There are a limited amount of resources to suffice a population of organisms. They must face both interspecific and intraspecific competition in order to obtain these resources.
  • Some organisms have more useful characteristics than others that makes them more adapted to their environment, and give them a better ability at obtaining these limited resources.
  • These creatures will survive because of this trait, reproduce, and there is high chance their offspring will also survive as well. Overtime, more members of the population will acquire this trait and thus overtime the whole species may have evolved.

'Explain two examples of evolution in response to environmental change; one must be multiple antibiotic resistance to bacteria.

  • Before Penicillin was invented, bacteria was the leading cause of death. However, once it began to be used, since it's an antibiotic, some individuals of bacteria may carry the gene Penicillinase, which codes for an enzyme that deactivates Penicillin, making them resistant to an antibiotic such as Penicillin. Thus, when it is indeed used, they will be the only ones left to reproduce and new bacteria will also be resistant to the antibiotic.
  • The Peppered Moth is another example of evolution in response to environmental change. When Britain began industrialising, soot would come from factories and land on trees. A species of peppered moth with a lighter colour vanished and those with a darker colour flourished because they could hide themselves easily.


Define species.

  • A species is a group of organisms with similar characteristics, which can interbreed and produce fertile offspring.

Describe the values of classifying organisms.

  • Species Classification:It is easier to find out which species an organism belongs to when you have other organisms to compare it to.
  • You can make assumptions about characteristics of a species in general.
  • Evolutionary links, you can make assumptions about traits of a common ancestor. You can also predict how they evolved.

Outline the binomial systems of nomenclature.

  • Called binomial because two names are used.
  1. First name is genus, with first name being a capital.
  2. Second name is species, with no capital.
  3. Italics are used when the name is printed.
  4. The name is underlined if it is handwritten.

State that organisms are classified into the kingdoms, Prokaryotae, Protoctista, Fungi, Plantae, and Animilia.

  • Prokaryotae- bacteria
  • Protoctista- Including unicellar organisms like Amoeba and Algae
  • Fungi- Including moulds and yeasts
  • Plantae- Including conifer, ferns, mosses and flowering plants
  • Animalia- Multicellular and locomotive, including sponges, corals, birds and mammals

List the seven levels in the hierarchy of taxa- kingdom, phylum, class, order family genus species- using an example from two different kingdoms for each level. (King Philip Comes Over For Good Soup)

  • Kingdom: Animalia | Plantae
  • Phylum: Chordata | Conferophyta
  • Class: Mammalia | Pinopsida
  • Order: Cetacea | Pinales
  • Family: Balaenopteridae | Taxodiaceae
  • Genus: Baleaenoptera | Sequoia
  • Species: musculus | sempervirens

Apply and/or design a key for a group of up to eight organisms. Shape, number, color, etc.
Keys are most commonly used to identify plants, insects, and birds. These are often area specific, for example, the Plants of Northern Europe. Keys are usually constructed in the following ways:

  • The key consists of a series of numbered stages
  • Each stage consists of a pair of alternative characteristics
  • Some alternatives give the nest stage of a key to go to
  • Some alternatives give the identification

For an in depth analysis of constructing a Dichotomous Key see: Dichotomous_Key

Human Impact[edit]

Outline the two local or global examples of human impact causing damage to an ecosystem or the biosphere. One example must be the increased greenhouse effect.

  • Air Pollution: The warmth of the earth can be attributed to radiation from the sun. Some of the radiation is absorbed by the earth. Many gases aid in these absorption such as carbon dioxide, methane, and water vapour. They withhold the radiation and form a blanket warming the earth. The effect is like a greenhouse, hence the name: Greenhouse Effect.
  • However, there has been in increase in carbon dioxide and methane because of combustion of fossil fuels. This has led to an increased greenhouse effect and thus global warming.
  • Deforestation: In some areas, rainforests are cut to make way for farmland- slash and burn techniques. This has generally a negative effect because rainforest soil is not generally good for farming and can only be used temporally. Moreover, this is a catalyst for soil erosion. Almost everywhere, forested land is cleared for construction and for resources (wood).

Explain the causes and effects of the two examples before, supported by data.

  • See before.

Discuss measures which could be taken to contain or reduce the impact of the two examples with reference to the functioning of the ecosystem.

  • In reference to the Greenhouse Effect: Cleaner fuels that emit less carbon dioxide and other gases, especially in motor vehicles, could ease pressure on the environment, allowing natural effects to work the problem out. Photosynthesis should be encouraged to reduce carbon dioxide from the air, and also emissions from the burning of fossil fuels should be reduced.
  • In reference to deforestation: People having fewer children (2 at most) would stop the increase in the human population. If the population were to remain the same, new housing construction would be unnecessary. Also, re-planting deforested areas and giving them time to grow would serve to rectify the problem to an extent.