IB Categories Archives: Topic 4: Ecology

4.4 – Evolution

4.4 – Evolution

4.4.1 – Define evolution

Evolution is the cumulative change in the heritable characteristics of a population.

It is the development of new types of living organisms from pre-existing types by accumulation of genetic differences over long periods of time. All the forms of life on the Earth today are the result of divergence from a common ancestor over billions of years.

Sub-Theories of evolution include: 

1. Evolution – all life is perpetually changing, in contrast with the idea that all forms of life are fixed and unchanging

2. Common Descent – all living things share a common ancestor if traced back far enough

3. Gradualism – Evolutionary change takes place slowly and gradually

4. Multiplication of Species – diversity of life is a consequence of speciation, where populations adapt to locations and become reproductively isolated from other populations

5. Natural Selection – a two-stage process involving genetic variation and selection of the most suitable to the location

4.4.2 – Outline the evidence for evolution provided by the fossil record, selective breeding of domesticated animals and homologous structures

Evolution requires evidence showing that organisms change over time, even resulting in production of new species

Fossil Records

Fossils are ancient remains of organisms that have been preserved though a rare, chance event. Scientists can determine the age of fossils from the age of the rock (using 14C, 40K or 40Ar radiometric dating). Fossil sequences show change over time, indicating how species have evolved. However, gaps often occur in records.

Fossils can be formed through a number of processes:

  • Petrification – Organic matter is replaced by mineral ions
  • Mould – Organic matter decays, space left becomes a mould, filled by mineral matter
  • Trace – Impression (footprint or leaf) hardens in the layers
  • Preservation – The organism is preserved, such as in amber or in anaerobic, acidic peat

Homologous Structures

These show that all life is connected, having derived traits from a common ancestor. Organisms with closer connections have similar structures

Above: pentadactyl limb of the vertebrate – humerus, radius, ulna. Bones are adapted to the animal’s locomotion. It shows divergence, which is adaption and modification from a limb structure found in their common ancestor. Convergence, on the other hand, is when organisms with different ancestors have structures fulfilling the same function, but that have evolved from different origins. i.e. Insect wings and bird wings

Selective Breeding

Useful or valuable characteristics in an organism lead to selection for breeding, such as size or colour. These characteristic will be present in the next generation in higher frequency, leading to gradual improvement. Weaker or deformed organisms may be culled.

Similarly, natural populations show phenotypic variation, are subject to natural selection pressures for advantageous characteristics, such as resistance to disease.


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

By producing more offspring than the environment can support, the chances of survival are increased for the population as a whole. A single death would be less devastating to a population of 1000 than of 10. The increased population causes intraspecific competition for the limited resources. Those with advantageous characteristics are ‘fitter’ and are more likely to successfully reproduce, causing the advantageous characteristics then become more frequent in the next generation. These characteristics have a genetic basis: the alleles for them are more frequent in the population.


4.4.4 – Explain that the consequence of the potential overproduction of offspring is a struggle for survival

The ‘struggle for survival‘ is a result of overpopulation, leading to competition for limited resources. Individuals with beneficial traits that increase their chances of survival will be selected for, and will be able to breed. Thus, these advantageous alleles are present in a higher frequency in the next generation. The consequent change in heritable characteristics is evolution.

4.4.5 – State that the members of a species show variation

All populations of a species will show variation, or a difference in phenotypes, which occurs in two different patterns:

Discontinuous variation

When there are distinct classes of individuals. This indicates the condition is controlled by one or two genes. For example, gender, eye colour and blood group.


Continuous variation

This shows no distinct classes, but a complete range of the characteristic. This normally indicates a polygenic condition or multiple alleles. Examples include skin colour, height and mass.



4.4.6 – Explain how sexual reproduction promotes variation in a species

Sources of Genetic Variation
Some organisms may contain a mutated gene. This may be beneficial, harmful, lethal or have no effect. This can occur in both asexual and sexual reproduction. Also, migration may cause genes that were not previously present in the population to be present in later generations, as individuals from another population bring the different genes.


Sexual Reproduction and Variation

Meiosis and the independent assortment of chromosomes creates 2n new combination of chromosomes in the next generation

n = haploid number of chromosomes

Random fertilisation increases variation to 22n again. Two haploid gametes are unified during fertilisation, leading to greater variation due to the mixing of genes. The number of genetic variations is increased further by cross-over in meiosis by 23 in addition to the above

With so many possible combinations of genes, the variation amongst a population is greatly increased. Whilst variation can still occur through asexual reproduction, during sexual reproduction, the genes of two random individuals will be mixed, which gives rise to greater variation still.

4.4.7 – Explain how natural selection leads to evolution

Not every species that has ever developed is still present on the Earth today. Many have become extinct due to changes in biotic and abiotic factors that caused them to die off. Those species that remain have been able to adapt and respond to these pressures but selecting for favourable characteristics.

Individuals with favourable heritable variations tend to have better survival and reproductive rates, which influences the types of genes that will be passed on to the next generation.

Natural selection happens in the stages:

  • Overproduction – the organisms have more offspring than the environment can
  • Variation – Mutations, random assortment of chromosomes and random fertilisation
    lead to a range of characteristics amongst the population, some more beneficial than
  • Competition – Limited resources, including habitat and food, mean that not all individuals will survive to a reproductive age
  • Survival of the fittest phenotype – Individuals with more beneficial characteristics will
    have an advantage in obtaining food and finding a mating partner to pass their genes on to the next generation
  • Increase in the frequency of favourable genes – These beneficial alleles will become more frequent as they are more likely to be passed on, whilst individuals with unfavourable characteristics will die off before they have the chance to reproduce.

Evolution happens through the cumulative change in heritable characteristics of a population. Natural selection can still occur without speciation happening. The genetic profile of the population is adapting to changes in local conditions. Every phase is affected by variation and selection.


For example, the heights of a certain population may be distributed as follows:
However, if environmental conditions only favour those individuals at the top of this range, then those at the other end will die off due to competition.



4.4.8 – Explain two examples of evolution in response to environmental change; one must be antibiotic resistance in bacteria

Staphylococcus Aureus

This bacterium is associated with skin and lung conditions.
It is commonly found in hospitals, and has two forms:


Methicillin-resistant Staphylococcus Aureus (MRSA) – This is resistant to the antibiotic Methicillin

Methicillin-Susceptible Staphylococcus (MSSA) – This can still be controlled by the use of Methicillin

This antibiotic used to be used to control the spread of golden staph; however, from the 1980’s to the 2000’s, MRSA became more frequent as the resistance gene became dominant.

MRSA evolved because antibiotics selectively kill only the susceptible forms of bacteria, putting selective on the population. DNA mutation would have produced a resistant gene, which could be passed on via plasmids. MSSA bacteria would have the disadvantage, and be readily killed off by the antibiotic. MRSA would survive to reproduce, making the gene more frequent in the population.

This bacterium is of concern to health professionals because it cannot be controlled by methods used previously, and can lead to infection.

New Zealand Kaka

These became isolated from their parrot family ancestor by the Tasman sea. When mountains formed in New Zealand, the environment changed and became more alpine, placing selection pressure on the birds. As a result, two new species developed: the Alpine Kea and the Lowland Kaka. Later, New Zealand split into two islands, causing further divergence of the species to produce the North Island Kaka and the South Island Kaka.

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4.3 – Populations

4.3 – Populations

4.3.1 – Outline how population size is affected by natality, immigration, mortality and emigration

Population can be affected by:



Or birth. This is because as the birth rate increases, the population increases. This
increase is exponential, so as the population increases, the birth rate increases accordingly.


This is the arrival of organisms to the population from another area, adding to the numbers
of the total population


Or death. The mortality rate also increases as the population increases.


This is when a part of the population migrates to another area. Along with mortality, this helps to stabilise the population growth.

A population is stable when:

Natality + Immigration = Mortality + Emigration

4.3.2 – Draw and label a graph showing a sigmoid (S-shaped) population growth curve

At the beginning, the is a short lag phase, as the population adapt to their environment

Exponential Phase

Also called the log phase. This is when there are low or reduced limiting factors, allowing the population to expand exponentially in the habitat. It may be increasing by 2n where n = number of generations. In these circumstances, the rate of natality and immigration is higher than mortality and emigration. However, emigration and mortality will not = 0. During this time, little waste product accumulates, and there is adequate nutrients.

These conditions are typical of a population of germinating annual plants in a new season. Also, a bacterial population during the initial phases of an infection, or of any species occupying a previously unoccupied habitat (succession)

Transition Stage

Also called the linear growth phase. During this time, resources are reduced and the growth of the population becomes limited. This comes as a result of increased competition as the population grows. Some individuals will survive, who obtain the resources, while others will not. This produces a lower rate of population growth than that observed in the exponential phase.

Population Plateau

Also called the stationary phase. This is when the population remains constant over time or generations. The population is determined by the carrying capacity of the habitat at that point in time.

Natality + Immigration = Mortality + Emigration

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

The exponential growth phase comes as a result of the fact that the population has already begun to grow, and rises quickly because there are no limiting factors, and the resources exist in unlimited amounts.

The plateau phase happens once the habitat is supporting the maximum number of organisms at that time.

The transitional phase happens because limiting factors in the environment slow the increase.

4.3.4 – List three factors that set limits to population increase

  • Availability of nutrients
  • Number of predators and parasitism
  • Accumulation of waste materials
  • Disease
  • Shortage of space or territory

In other words, populations tend to be naturally self-regulating.

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4.2 – The Greenhouse Effect

4.2 – The Greenhouse Effect

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

Carbon can be found in four ‘pools,’ and moves between these four pools through a variety of biological, geochemical or industrial processes.

  • Biosphere
  • Oceans
  • Atmosphere
  • Sediments

Photosynthesis – By terrestrial plants and algae in which atmospheric (and dissolved) carbon dioxide is removed and fixed as organic compounds such as carbohydrate, lipid and protein

Respiration – This is done by all organisms in which they metabolise organic molecules, releasing carbon dioxide

Feeding – The carbon of organic molecules is moved from one link in the food chain to another

Fossilisation – Carbon, as organic molecules, becomes trapped in sediment as coal, gas and oil

Combustion – This happens during the burning of fossil fuels and biomass


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

The trends in atmospheric gases are studied as indicators of potential climate change. Those studied include carbon dioxide, methane and oxides of nitrogen, the greenhouse gases. In Mauna Loa, atmospheric carbon dioxide has been studied since 1958. There are many other labs around the world these days, adding to the database of carbon dioxide levels.

Carbon dioxide is released unevenly around the world, which is in part due to the distribution of vegetation. Therefore, collective data allows us to see what has happened after there is mixing of the atmospheric carbon dioxide. The basic trend is an increase in atmospheric carbon dioxide levels. Longer term estimates of global CO2 levels have been determined by a variety of sources including gases trapped in ancient ice cores.

Bubbles of atmospheric gases are trapped within the ice formed thousands of years ago. Taking cores of the ice and then analysing the gases allows CO2 levels to be determined. The temperature can be determined from the ratio of O16 to O18. From this, in has been concluded that there is a clear correlation between atmospheric CO2 and temperature. Of course, correlation does not mean causation.

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

The greenhouse effect is a natural phenomenon that creates moderate temperatures on Earth to which life has adapted. The Earth has relatively little CO2 in its atmosphere compared to planets like Venus. The enhanced greenhouse effect, however, is the idea

The enhanced greenhouse effect, however, is the idea that the activities of humans are increasing the levels of CO2 and other greenhouse gases in the atmosphere. This in turn may lead to increased global temperatures and climate change. Gas molecules in our atmosphere with three or more atoms can capture

Gas molecules in our atmosphere with three or more atoms can capture outgoing infrared energy and warm the planet, and are called greenhouse gases. These include H2O, O3, CO2 and CH4. Chlorofluorocarbons, CFCs, also have a disproportionately large effect.

The increase greenhouse gases means that more infrared light will be absorbed, scattered and retained as heat. The average global temperature will rise. The enhanced greenhouse effect is predicted to cause global climate changes, often referred to as global warming, although local effects may vary greatly.



4.2.4 – Outline the precautionary principle

The precautionary principle holds that, if the effects of a human-induced change would be very large, perhaps catastrophic, those responsible for the change must prove that it will not do harm before proceeding. This is the reverse of the normal situation, where those who are concerned about the change would have to prove that it will do harm in order to prevent such changes going ahead.

This means that, if it cannot be proven that no harm will come of the action, it must not go ahead.

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

Burden of Proof

Those making the claims must prove, with sufficient evidence, that it is true before others change their understanding or behaviour. Therefore, the environmentalists need to provide conclusive evidence that the actions of the polluters are causing harm to the environment.

Precautionary Principle

Those who are accused of being responsible for causing the enhanced greenhouse effect are required to demonstrate that their actions do not cause harm. This would fall upon governments, industries, communities and individuals to show this.

The General Effects include:

  • Increased frequency and intensity of droughts
  • Flooding due to higher rainfall, increased snowmelts, rising sea levels
  • Declines in food production
  • Increased disease, as warmer temperatures allow for increased numbers of pathogens
  • More extreme weather
  • Loss of biodiversity


4.2.6 – Outline the consequences of a global temperature rise on arctic ecosystems

Decomposition of Detritus

The significant decay by microorganisms of the accumulated detritus (dead organic matter), once released from its permafrost state, leads to huge releases into the atmosphere of methane and carbon dioxide, which was previously locked away in the dead organic matter. This contributes further to global warming.

Range of Habitats

More areas with soil rich in humus are formed. As a result, more plant life, including conifers, appear and grow. As these plant absorb radiant heat energy and contribute further to global warming, since they replace ice, snow and frozen tundra. A wider range of flora is appearing. More insect-eating species have also appeared, such as birds, which are taking advantage of the increasing numbers of insects. The appearance of small mammals, which are taking advantage of the expanding range of plant biota and habitats

Loss of Ice Habitat

This temporarily leads to extensive flooding of surrounding low-lands

Distribution of Prey Species

Predators are appearing to prey on the expanding vertebrate populations, such as birds of prey that can fly on when winter returns

Pest Species

There is an increased presence of pathogens that parasitise the expanded range of animal and plant life that the changing habitat supports.


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4.1 – Communities and Ecosystems

4.1 – Communities and Ecosystems

4.1.1 – Define species, habitat, population, community, ecosystem and ecology

Species – A group of organisms that can interbreed and produce fertile offspring.

They are a group of individuals of common ancestry that closely resemble each other and that are normally capable of interbreeding to produce fertile offspring.

Habitat – The environment in which a species normally lives or the location of a living organism.

If this area is extremely small, we call it a microhabitat, such as the crevices in the bark of a tree in which some insects live. Conditions in a microhabitat are different to those of the surrounding habitat.

Population – A group of organisms of the same species who live in the same area at the same time.

The members of a population have a high chance of interbreeding, assuming the species concerned reproduces sexually. The boundaries of populations are often hard to define.

Community – A group of populations living and interacting with each other in the same area.

Ecosystem – A community and its abiotic environment.

It is a stable, settled unit of nature consisting of a community of organisms, interacting with each other and with their surrounding physical and chemical environment. These vary greatly in size.

Ecology – The study of relationships between living organisms and their environment

4.1.2 – Distinguish between autotroph and heterotroph

Autotroph – An organism that synthesizes its organic molecules from simple inorganic substances

Heterotroph – An organism that obtains organic molecules from other organisms

4.1.3 – Distinguish between consumers, detritivores and saprotrophs

Consumers – An organism that ingests other organic matter that is living or recently killed

Detritivore – An organism that ingests non-living organic matter, also known as a decomposer.

Saprotroph – An organism that lives on or in non-living organic matter, secreting digestive enzymes into it and absorbing the products of digestion

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

A food chain is a representation of the relationships between organisms based on their diet. A → B indicates that A is ‘eaten’ by B (that is, the arrow indicates the direction energy flow). Each food chain should include a producer and consumers, but not decomposers. Named organisms at either species or genus level should be used. Common species names can be used instead of binomial names. However, general names such as ‘tree’ or ‘fish’ should not be used.



4.1.5 – Describe what is meant by a food web

A food web is a diagram that shows how food chains are linked together in to more complex feeding relationships. Advantages of a food web include:

Advantages of a food web include:

  • Shows the much more complex interactions between species within a community or ecosystem
  • There is more than one producer supporting a community, which this shows
  • It shows that a single producer can be a food source for a number of primary consumers
  • A consumer may have a number of different food sources on the same or different trophic levels
  • A consumer can be an omnivore, feeding as a primary consumer and at also at higher trophic levels

Food webs, as they are very detailed and complicated, often reflect the interest of its author. Species of interest are detailed by name, whereas less important or interesting species are grouped into a large family.

4.1.6 – Define trophic level

The trophic level of an organism defines the feeding relationship of that organism to other organisms in a food chain. In a food web, a consumer can occupy a number of different trophic levels, depending on which organism is the prey.


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

Looking at the food web, assess the trophic level by looking at how many previous organisms it feeds off. You should also be able to identify them with the levels of producer, primary consumer, secondary consumer, and so on. The terms herbivore and carnivore are not always applicable.

4.1.8 – Construct a food web containing up to 10 organisms, using appropriate information

Where possible, identify the trophic levels for each organism.


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

To maintain food chains, food webs, communities and all their interactions, energy is required. Sunlight is the source of this energy for most communities, both aquatic and terrestrial. The principle trap of sunlight energy is the protein molecule chlorophyll, found in the chloroplasts of producers cells, mainly green plants.

4.1.10 – Explain the 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. Essentially the loss of heat from respiration

Photosynthesis converts light into energy. Not all solar energy will come into contact with chlorophyll and will therefore not be trapped in the synthesis of organic compounds.

Death and the consumption of dead organisms by detritivores, or as food not assimilated because of incomplete digestion.

Energy loss can occur in undigested food, which is used by saprophytes (decomposers), or in the reactions of respiration. Ultimately, all energy will be lost as heat.


4.1.11 – State that energy transformations are never 100% efficient

The transfer of energy from one trophic level to the next in inefficient. Only 10-20% of the energy on one trophic level will be assimilated at the next higher level.

In extreme environments like the arctic, the initial trapping of energy by producers is low, making the food chains much shorter. Likewise, in tropical rainforests, where the trapping of energy is more efficient, the food chains are longer, and the food webs are more complex.

This explains why larger predators at the top of the food chain are so rare. The energy loss throughout the food chain means that the number of organisms decreases at each step. In the higher trophic levels, organisms become less and less common. They are also more prone to extinction as they rely on the organisms below them. Any decrease in numbers in these organisms cause a chain reaction, resulting in possible extinction.

4.1.12 – Explain reasons for the shape of pyramids of energy

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, such as kJ m-2 yr-1

In a typical pyramid of energy, the initial solar energy is not shown. The narrowing shape shows the gradual loss of energy as you move up the food chain to the higher trophic levels. The scale to which it is drawn (energy/area/unit time) is written at the base of the pyramid.


4.1.13 – Explain that energy enters and leaves ecosystems, but nutrients must be recycled

Energy Flows

At every trophic level, energy is lost as heat. The narrowing of the energy pyramid shows that all energy is eventually radiated into space as heat.

Matter Cycles

New matter is not created, nor is it lost the way energy is. Instead, producers (autotrophs) take organic molecules and convert them into organic compounds, helping them to be recycled and re-used. Consumers then take in this organic matter as they feed and use it for their own growth. Such cycles of matter include the carbon, nitrogen and oxygen cycles.


4.1.14 – State that saprotrophic bacteria and fungi (decomposers) recycle nutrients

Nutrients, unlike energy, are not lost, but are recycled and re-used. Decomposers (saprotrophic bacteria and fungi) recycle organic molecules (nutrients) found in dead organisms. This is a complex process, serving many functions. These include the formation of soil, recycling nutrients stored in organic molecules, and reduction of high energy carbon compounds. Mineral elements are absorbed by plants as ions from the soil solution. This cycling process is called biogeochemical cycles, in which all essential elements take part. The biological process of decomposition begins when saprotrophic bacteria and

The biological process of decomposition begins when saprotrophic bacteria and fungi secrete extra-cellular digestive enzymes onto the dead organism. The enzymes hydrolyse the biological molecules that the dead organism is composed of. The hydrolysed molecules are soluble, and are absorbed by the fungi or bacteria. Organic molecules are oxidised to release carbon dioxide back into the atmosphere, and nitrogen in the form of nitrate, nitrite and ammonium. This also produces energy for the saprophyte, but returns in the various forms of matter to the abiotic environment.

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