For anyone who are choosing ‘Option C: Ecology and Conservation’, I hope this blog can help you with this topic! RAHH!! The topics I’ll be talking about are listed down below :3
Limiting Factors
What are limiting factors?
They are factors that puts an upper limit on the size of a population. They affect the growth, abundance or distribution of a population of an organism in an ecosystem. All ecosystems have characteristic abiotic factors and a species exists within these. Abiotic factors are non-living chemical and physical parts of the environment that affects living organisms in a ecosystem. They are temperature, water, light, soil type, pH, salinity and mineral nutrients.
A community of species always lives in an ecosystem that has a certain set of abiotic factors that make it unique. It is best for each species in the community to live in a certain range of environmental conditions. These factors also limit the range of a species because living things can’t handle too many changes from their ideal environment. These edges are called the zones of intolerance. Limiting factors are things that make it hard for a species to live in certain areas.
Example Species:
Marram grass
Polar bears
Panda bears
Transect and limiting factors
Transect is a lines that goes through a habitat or part of a habitat. It is used to find out how the organisms that live there are related to one or more nonliving or abiotic factors. Transects are a sampling method that lets you study a small group of organisms by looking at them (taking a sample) instead of counting all of them. By looking at where there are lots or not many organisms in relation to certain abiotic factors, like where there is a lot or not many marram grass in relation to salinity, they help figure out the zone of tolerance that a species can live in.
Niche in ecology
What is a niche?
It is the sum total of a species’ use of the biotic and abiotic resources in their environment. There are 2 types of niches.
The fundamental niche: represents the broadest possible niche a species could potentially live in within the boundaries of its limiting factors or zones of intolerance, with no external pressures such as competitors or interactions with other species. The idea of the fundamental niche is very much theoretical.
The realised niche: represents the actual mode of existence in which the species is really found, taking into account its zone of intolerance as well as all interactions with other species.
Competitive exclusion
It is when two species occupy fully overlapping niches, and the one with the greater selective advantage outcompetes the other, resulting in extinction from that environment. When talking about competitive exclusion, it’s important to note It’s important to remember that the population of that species, not the species itself, is what is being wiped out. This could be the species, but it’s more likely to be a group of that species.
In this graph you can see that when 2 organisms are placed together in an environment, one is more dominant than the other, and usually one species is more dominating than the other, proving that organisms with similar niches can result in the eventual decline of the population
Interaction between species
Interactions are often considered as competition amongst species in the environment. Usually a prey and predator relationship. Competition are interactions between organisms in which both are harmed. There are 2 types of competition:
Intraspecific – competition occurs between organisms of the same species for resources, such as food and mates.
Interspecific – competition occurs when one species competes with another.
Types of relationships in the ecosystem:
| Interaction | Description | Example |
| Predatory & Prey | When an organism feed on other animals | Lion & Deer |
| Herbivory | When an animal specialises eating leaves | Worm & Leaf |
| Mutualism | When both parties benefit | Clown fish & Anemone |
| Parasitism | When only one party benefits and the other suffers | Mosquito & Human |
| Commensalism | When one party benefits and the other is unaffected | Whales & Barnacles |
Keystone species
Keystone species represent an organism that has a significant impact on community structure despite not being abundant in numbers. A keystone species has a significant impact on an ecosystem that is disproportionate to its population size.
This is a graph showing when Pisaster (starfish) was taken out of the community, most of the other invertebrates and algae were also taken out. So in this context, the removal of Pisaster has caused other species to decrease. This is because some invasive species may take over and become a bigger threat to them than the Pisaster.
Checklist from the syllabus
| The distribution of species is affected by limiting factors. |
| Community structure can be strongly affected by keystone species. |
| Each species plays a unique role within a community because of the unique combination of its spatial habitat and interactions with other species. |
| Interactions between species in a community can be classified according to their effect. |
| Two species cannot survive indefinitely in the same habitat if their niches are identical. |
| Distribution of one animal and one plant species to illustrate limits of tolerance and zones of stress. |
| Local examples to illustrate the range of ways in which species can interact within a community. |
| The symbiotic relationship between Zooxanthellae and reef-building coral reef species. |
| Analysis of a data set that illustrates the distinction between fundamental and realized niche. |
| Use of a transect to correlate the distribution of plant or animal species with an abiotic variable. |
Sample questions
The majority of questions from the open ended questions are analysing data from graphs and table. So you should master questions like the ones below.
What is the relationship of X and Y?
You should answer, they have a positive/ inverse relationship. Then refer back to the data given.
Comment on the change of X and Y?
X has increased in___ and Y has decreased___.
Real life example:
Explain the importance of the following factors in determining community structure.
(taken from: https://studyib.net/biology/page/2853/option-c-ecology)
- Keystone species. (2 marks)
- Limiting factor. (2 marks)
- Competitive exclusion. (2 marks)
Firstly, to approach this question, it is important to define the words given. Next you should underline the question, what is it asking you to do? Here the key word is ‘explain the importance’, so you are expected to describe what the importance is. Remember to not go off topic and make sure to not write too much. 2-3 sentences is fine for a 2 mark question. Lastly, use examples if it will make your explanations better.
Model answer
(If your answers are some-what similar, then you are all good :3)
Explain the importance of the following factors in determining community structure.
- Keystone species. (2 marks)
- Keystone species are species within a community that have a major effect on other populations within a community.
- Predators control the population of herbivorous prey which, if uncontrolled may overgraze the plant species, lowering productivity/example.
- Sea otters reducing sea urchin population. (other relevant example)
- Limiting factor. (2 marks)
- A limiting factor is the single abiotic factor that limits population size of a species.
- Extremes of temperature limit species distribution to those species that can tolerate heat stress.
- Camel blood temperature variation in the day and night is an adaptation to temperature extremes. (other relevant example)
- Water stress/low water availability can exclude some (mesophytic) plants from environments.
- Xerophytes can tolerate water stress (other relevant example).
- Competitive exclusion. (2 marks)
- Two species cannot coexist in the same environment if their niches completely overlap.
- Species have evolved such that their niches do not completely overlap by competing for some resources but not others/evolved to narrow the niche they inhabit to avoid direct competition for one or more resource.
- Wading birds such as herons and egrets may compete for food sources but use different nesting sites. (other relevant example)
*Knowledge of correct terminology and precise examples are important in this topic*
Taken from Biology SL Paper 2 – May 2023
(a) A study was carried out to collect the data from smartphones to estimate different levels
of activity measured in steps per day. The graph below displays the estimates of 4 different
countries using this data
a) Identify the mode for activity in steps for Japan and USA?
Japan: 6000 steps
USA: 4000 steps
(Mode is the most common number in a data base, so using a ruler try to estimate the value)
b) Distinguish the levels of activity in steps for Saudi Arabia and UK
Saudi Arabia had higher probability than UK, 0.17>0.15
UK has had a higher steps per day with lower probability
Resources Used
Kognity & Bioninja
For more resources, check the links down below!
– https://ibbetter.com/data-based-questions/ (How to answer IB test questions)
– https://prodatblog.org/ib-sl-biology-topic-4-1-ecology/ (Another blog about ecology)
– https://www.revisionvillage.com/ib-biology/sl/past-papers/ (Past papers)
and lastly, Free Palestine!
English