IB DP Biology Topic 4: 4.1 Species, communities and ecosystems: Study Notes

In the Species, Communities and Ecosystem unit we will look at how almost the entire surface of the Earth is home to organisms of one kind or another. The will see that there are as many as 10 million different species on Earth and understand where and how they live and interact is a branch of biology. We will also look at how  humans are not the most numerous species on Earth, but humankind is having a disproportionate effect on the world’s ecosystems as damage is caused by pollution, rainforest destruction and global warming.

This unit will last 3 class lesons

​Essential idea:

• The continued survival of living organisms including humans depends on sustainable communities.

Nature of science:

• Looking for patterns, trends and discrepancies—plants and algae are mostly autotrophic but some are not. (3.1)
  • State the trend found in the nutritional patterns of plants and algae.
  • Describe the discrepancy in the nutritional pattern of parasitic plants and algae.

4.1.U1 ​Species are groups of organisms that can potentially interbreed to produce fertile offspring.

• Describe limitations of the biological species concept.
• Define species according to the biological species concept.

When two members of the same species mate and produce an offspring it is called interbreeding. Their offsprings it is often fertile.

When two different species breed together it is called cross-breeding. This type of breeding happen once in awhile, but the offspring is not fertile, which does not let this to reproduce, because it is infertile.

Interbreeding: When 2 members of same species mate
Cross-breeding: When 2 members of different species mate

• Happens sometimes but cross-bred offspring is usually infertile so genes of 2 species do not mix

4.1.U2 ​Members of a species may be reproductively isolated in separate populations.

• Define population.
• Outline how reproductive isolation can lead to speciation.

A population is a group of organisms of the same species that are living in the same area at the same time

Members of a species may be reproductively isolated in separate populations

Organisms that live in different regions (i.e. different populations) are reproductively isolated and unlikely to interbreed, however are classified as the same species if interbreeding is functionally possible

4.1.U3 ​Species have either an autotrophic or heterotrophic method of nutrition (a few species have both methods).

• Define autotroph and heterotroph.​

Living organisms obtain chemical energy in one of two ways:

• Organisms that make their own carbon compounds from carbon dioxide and other simple substances – autotrophic (self-feeding)
• Organisms that obtain their carbon compounds from other organisms – heterotrophic (feeding on others)
• Some unicellular organisms use both methods of nutrition
• Eg Euglena gracilis – has chloroplasts and carries out photosynthesis when there is sufficient light. Can also feed on detritus or smaller organisms by endocytosis.
• Organisms that are not exclusively autotrophic or heterotrophic are mixotrophic

4.1.U4 ​Consumers are heterotrophs that feed on living organisms by ingestion.

• Describe the feeding behaviors of consumers.
• List three example consumer organisms.​

Heterotrophs obtain organic molecules from other organisms via different feeding mechanisms and different food sources. Consequently, heterotrophs can be differentially classified according to their feeding pattern

• Heterotrophs divided into two groups according to the source of organic molecules that they use and the method of taking them in.
• One group of heterotrophs is called consumers
• Consumers feed off other organisms (either still alive or have already been dead for a short time)
  • A mosquito sucking blood from a larger animal is a consumer on an organism that’s still alive
  • A lion feeding of a gazelle that it has killed is a consumer
• Consumers ingest their food – they take in undigested material from other organisms. They digest it and absorb products of digestion.
• Consumers are sometimes divided into trophic groups according to what other organisms they consume.
  • Primary consumers feed on autotrophs
  • Secondary consumers feed on primary consumers Lions

4.1.U5 ​Detritivores are heterotrophs that obtain organic nutrients from detritus by internal digestion.

• Describe the feeding behaviors of detritivores.
• List two example detritivore organisms.

• Organisms discard large quantities of organic matter
  • Dead leaves and other parts of plants
  • Feathers, hair, and dead parts of animal bodies
• Feces from animals
• This dead organic matter rarely accumulates in ecosystem – is used as a source of nutrition by two groups of heterotroph-detritivores and saprotrophs.
• Detritivores ingest dead organic matter then digest it internally and absorb products of digestion
  • Large multicellular detritivores such as earthworms ingest dead matter into their gut
  • Unicellular organisms ingest into food vacuoles. organic matter discarded by organisms
• Rarely accumulate in ecosystem → used as source of nutrition for detritivores & saprotrophs detritivores
• Ingest dead organic matter → digest internally → absorb products of digestion
  • example ; earthworms (large multicellular): ingest dead matter into gut

4.1.U6 ​Saprotrophs are heterotrophs that obtain organic nutrients from dead organisms by external digestion.

• Describe the feeding behaviors of saprotrophs.
• List two example saprotroph organisms.

Saprotrophs secrete digestive enzymes into dead organic matter and digest it externally – absorb products of digestion. Also known as decomposers – they break down carbon compounds in dead organic matter and release elements such as nitrogen into ecosystem so they can be used again by other organisms secrete digestive enzymes into dead organic matter → digest externally → absorb products of digestion

• example;  bacteria & fungi

4.1.U7 ​A community is formed by populations of different species living together and interacting with each other.

• Define species, population and community.
• Give an example of a community of organisms.

Sometimes interaction between two species is of benefit to one species and harms the other (eg parasite and host). Sometimes interaction benefits both species (eg hummingbird and flower – hummingbird feeds on nectar and pollinates it)

One species can never live in isolation

​A group of populations living together and interacting with each other – community

4.1.U8 ​A community forms an ecosystem by its interactions with the abiotic environment.

• Define abiotic and ecosystem.​

A community is composed of all organisms living in an area. Community of organisms in an area and their nonliving environment – ecosystem is a highly complex interacting system)  The system has both abiotic and biotic factors. The organisms depend on the non-living surroundings of air, water, soil, rock. –> abiotic environment.​

Many interactions take place between organisms and the abiotic environment

• Rainforest: Each layer is a different habitat as there are different species living in each layer.
  • Light intensity affects each habitat – there will be different species of autotrophs and will therefore provide different food for different consumers.
• Rocky intertidal zone: All in the same ecosystem but have multiple habitats because of the amount of water available – how much time they spend under water
  • Depends a lot on the abiotic factors
  • Temperature also affects the different habitats

4.1.U9 ​Autotrophs obtain inorganic nutrients from the abiotic environment.

• Define nutrient.
• List the common nutrients needed by organisms.
• Outline how nutrients enter living systems.

Autotrophs synthesise organic molecules from simple inorganic substances​

• Living organisms need a supply of chemical elements:
  • Carbon, hydrogen, oxygen (make carbohydrates, lipids, other carbon compounds)
  • Nitrogen and phosphorus needed to make compounds
    • comes from the soil (for plants taking up nutrients from the ground)
  • 15 other elements are needed too
• Autotrophs obtain all of the elements that they need as inorganic nutrients from the abiotic environment (including carbon and nitrogen)
• Heterotrophs obtain these as part of the carbon compounds in their food.
  • Obtain inorganic nutrients from abiotic environment (sodium, potassium, calcium)
• *NUTRIENT CYCLE*: plant takes nutrients from the soil -> primary consumer eats the plant and gets the nutrients from the plant -> secondary consumer gets nutrients from primary consumer and so forth

• State that chemical elements can be recycled but energy can not.
• Outline the generalized flow of nutrients between the abiotic and biotic components of an ecosystem

Nutrients refer to the material required by an organism, and include elements such as carbon, nitrogen and phosphorus. The supply of inorganic nutrients on Earth is finite – new elements cannot simply be created and so are in limited supply

• Chemical elements can be endlessly recycled
• Organisms absorb the elements they require from abiotic environment, use them, then return them to the environment with atoms unchanged
• Passed from organism to organism before it is released back into abiotic environment
• Nutrient = an element that an organism needs (in this context)

4.1.U11 ​Ecosystems have the potential to be sustainable over long periods of time.

• Define sustainability.​
• Give an example of an unsustainable practice.
• Outline three requirements of a sustainable ecosystem.

Ecosystems describe the interaction between biotic components (i.e. communities) and abiotic components (i.e. habitat). They are largely self-contained and have the capacity to be self-sustaining over long periods of time

There are three main components required for sustainability in an ecosystem:

• Energy availability – light from the sun provides the initial energy source for almost all communities
• Nutrient availability – saprotrophic decomposers ensure the constant recycling of inorganic nutrients within an environment
• Recycling of wastes – certain bacteria can detoxify harmful waste byproducts (e.g. denitrifying bacteria such as Nitrosomonas)Sustainable: it can continue indefinitely

4.1.S1 ​Classifying species as autotrophs, consumers, detritivores or saprotrophs from a knowledge of their mode of nutrition.

• Use a dichotomous key to identify the mode of nutrition of an organism.​

Species can be classified according to their mode of nutrition

• Autotrophs produce their own organic molecules using either light energy or energy derived from the oxidation of chemicals
• Heterotrophs obtain organic molecules from other organisms via one of three methods:
• Consumers ingest organic molecules from living or recently killed organisms
• Detritivores ingest organic molecules found in the non-living remnants of organisms (e.g. detritus, humus)
• Saprotrophs release digestive enzymes and then absorb the external products of digestion (decomposers)

4.1.S2 ​Setting up sealed mesocosms to try to establish sustainability. (Practical 5)

• Outline why sampling must be random.
• Explain methods of random sampling, including the use of a quadrat.
• State the null and alternative hypothesis of the chi-square test of association.
• Use a contingency table to complete a chi-square test of association.

Mesocosms: small experimental areas set up as ecological experiments

• Fenced-off enclosures in grassland or forest could be used as terrestrial mesocosms
• Tanks set up in the laboratory can be used as aquatic mesocosms

Making a Self-Sustaining Terrarium
A terrarium can be created using a glass or plastic bottle with a lid, according to the following steps:

1. Building a verdant foundation

• Add a bottom layer of pebbles, gravel or sand – this layer exists for drainage (smaller vessels require thinner rock layers)
• Add a second thin layer of activated charcoal – this will prevent mold and help to aerate the soil
• Spread a thin cover of sphagnum moss (or use an organic coffee filter) to create a barrier between the lower layers and soil
• The final layer is the pre-moistened growing medium (i.e. potting mix)

2. Selecting the right plants

• Ideally, choose plants that are both slow growing and thrive in a bit of humidity (e.g. most ferns, club moss, etc.)
• Inspect the plant thoroughly for any signs of disease or insects before introducing to the terrarium

3. Maintaining appropriate conditions

• Ensure the terrarium is placed in a location that provides a continuous source of light
• Locate the terrarium in a place that does not experience fluctuating temperature conditions (i.e. avoid direct sunlight)
• Do not initially over-water the plants – once the right humidity is established, a terrarium can go months without watering
• Occasional pruning may be required – however, as level of soil nutrients decrease, plant growth should slow down

4.1.S3 ​Testing for association between two species using the chi-squared test with data obtained by quadrat sampling.

• Calculate a chi-square statistic based on observed and expected values.
• State the null and alternative hypothesis of statistical tests.
• Determine if the null hypothesis is supported or rejected given a critical value and a calculated statistic.
• State the minimum acceptable significance level (p value) in published research.
• Explain the meaning of a “statistically significant” result, including the probability of chance having a role in the result

If two species are typically found within the same habitat, they show a positive association

• Species that show a positive association include those that exhibit predator-prey or symbiotic relationships

If two species tend not to occur within the same habitat, they show a negative association

• Species will typically show a negative association if there is competition for the same resources
  • One species may utilise the resources more efficiently, precluding survival of the other species (competitive exclusion)
  • Both species may alter their use of the environment to avoid direct competition (resource partitioning)

If two species do not interact, there will be no association between them and their distribution will be independent of one another

Quadrat Sampling
The presence of two species within a given environment can be determined using quadrat sampling

• A quadrat is a rectangular frame of known dimensions that can be used to establish population densities
  • Quadrats are placed inside a defined area in either a random arrangement or according to a design (e.g. belted transect)
  • The number of individuals of a given species is either counted or estimated via percentage coverage
  • The sampling process is repeated many times in order to gain a representative data set

Quadrat sampling is not an effective method for counting motile organisms – it is used for counting plants and sessile animals

• In each quadrat, the presence or absence of each species is identified
• This allows for the number of quadrats where both species were present to be compared against the total number of quadrats

Chi-Squared Tests
A chi-squared test can be applied to data generated from quadrat sampling to determine if there is a statistically significant association between the distribution of two species
A chi-squared test can be completed by following five simple steps:

• Identify hypotheses (null versus alternative)
• Construct a table of frequencies (observed versus expected)
• Apply the chi-squared formula
• Determine the degree of freedom (df)
• Identify the p value (should be <0.05)

4.1.4S ​Recognizing and interpreting statistical significance.

• Define mesocosm.
• List three example mesocosms.
• Outline requirements of setting up a mesocosm.

• 6 quadrats = both species  ;  15 quadrats = king scallop only  ;  20 quadrats = queen scallop only  ;  9 quadrats = neither species

• Null hypothesis (H0): There is no significant difference between the distribution of two species (i.e. distribution is random)
• Alternative hypothesis (H1): There is a significant difference between the distribution of species (i.e. species are associated)

Step 2:  Construct a table of frequencies
A table must be constructed that identifies expected distribution frequencies for each species (for comparison against observed)
Expected frequencies are calculated according to the following formula:

• Expected frequency = (Row total × Column total) ÷ Grand total

These calculations can be broken down for each part of the distribution pattern to make the final summation easier

Based on these results the statistical value calculated by the chi-squared test is as follows:

• 𝝌2  =  (2.20 + 2.38 + 1.59 + 1.73)  =  7.90

Step 4:  Determine the degree of freedom (df)
In order to determine if the chi-squared value is statistically significant a degree of freedom must first be identified

• The degree of freedom is a mathematical restriction that designates what range of values fall within each significance level

The degree of freedom is calculated from the table of frequencies according to the following formula:
df = (m – 1) (n – 1)
Where:  m = number of rows  ;  n = number of columns

• When the distribution patterns for two species are being compared, the degree of freedom should always be 1

Step 5:  Identify the p value
The final step is to apply the value generated to a chi-squared distribution table to determine if results are statistically significant

• A value is considered significant if there is less than a 5% probability (p < 0.05) the results are attributable to chance

When df = 1, a value of greater than 3.841 is required for results to be considered statistically significant (p < 0.05)

• A value of 7.90 lies above a p value of 0.01, meaning there is less than a 1% probability results are caused by chance
• Hence, the difference between observed and expected frequencies are statistically significant

As the results are statistically significant, the null hypothesis is rejected and the alternate hypothesis accepted:

• Alternate hypothesis (H1): There is a significant difference between observed and expected frequencies
• Because the two species do not tend to be present in the same area, we can infer there is a negative association between them