4.1 Species, Communities and Ecosystems

Question 1

What are species?

Answer 1

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

Question 2

What is interbreeding?

Answer 2

When two members of the same species mate and produce offspring.

Question 3

What is crossbreeding?

Answer 3

When members of different species breed together. The offspring of crossbreeding are almost always infertile, which prevents the genes becoming mixed and ensures the species stay separate.

Question 4

What is a population?

Answer 4

A population is a group of organisms of the same species who live in the same area at the same time.

If two populations of species never interbreed then they may gradually develop differences in their characteristics, but even then they are still considered a species until they cannot interbreed and produce fertile offspring.

Question 5

What is autotrophic nutrition?

Answer 5

All organisms need a supply of organic nutrients, such as glucose and aminos acids. They are needed for growth and reproduction. Autotrophic nutrition is a way of gaining these, it means organisms make their own carbon compounds from carbon dioxide and other simple substances. It basically means self feeding.

Question 6

What is heterotrophic nutrition?

Answer 6

All organisms need a supply of organic nutrients, such as glucose and aminos acids. They are needed for growth and reproduction. Heterotrophic nutrition is a way of gaining these. Heterotrophic nutrition is when organisms obtain their carbon compounds from other organisms. It basically means feeding on others.

Question 7

What is mixotrophic nutrition?

Answer 7

All organisms need a supply of organic nutrients, such as glucose and aminos acids. They are needed for growth and reproduction. There are two methods for gaining these, autotrophic (self feeding) or heterotrophic (feeding on others), very few organisms use both methods, however some do. These very few organisms are called mixotrophs, they are mostly unicellular organisms. For example the Euglena gracilis, it has both chloroplasts but can feed on detritus or smaller organisms by endocytosis.

Question 8

What organisms are usually autotrophic?

Answer 8

Plants and algae. Except there are some plants and algae that are exceptions to this, they do not contain chloroplasts and do not carry out photosynthesis. These species grow on other plants, obtain carbon compounds from them and cause them hard. They are therefore parasitic.

Question 9

Which plants are not autotrophic?

Answer 9

Parasitic ones, they do not contain chloroplasts and do not photosynthesise instead they grow on other plants, obtain carbon compounds from them and cause them harm.

Question 10

Do parasitic plants mean that you cannot apply the rule that all plants and algae are autotrophic?

Answer 10

To decide whether parasitic plants falsify the theory that plants and algae are groups of autotrophic species or whether they are just minor and insignificant discrepancies we need to consider how many species there are and how they evolved.

• The number of parasitic plants and algae is relatively small - only about 1% of all plants and algal species.
• It is almost certain that the original ancestral species of plant and alga were autotrophic and that the parasitic species evolved from them. Chloroplasts can quite easily be lost from cells, but cannot easily be developed. Also, parasitic species are diverse and occur in many different families. The pattern suggests that parasitic plants have evolved repeatedly from photosynthetic species.

Because of this evidence, ecologists regard plants and algae as groups of autotrophs, with a small number of exceptional species that are parasitic.

Question 11

What are consumers?

Answer 11

Consumers are heterotrophs that feed on living organisms by ingestion. Consumers ingest their food, this means they take in undigested material from other organisms. They digest it and absorb the products by digestion.

• Primary consumers feed on autotrophs
• Secondary consumers feed on primary consumers
  In practice, most consumers do not fit neatly into one of these groups because their diet includes material from a variety of trophic groups.

Question 12

What are detritivores?

Answer 12

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

Organisms discard large quantities of organic material for example
- dead leaves
- feathers, hairs and other dead parts
- feces
This dead organic matter rarely accumulates in ecosystems and instead is used as a source of nutrition by two groups of heterotroph detritivores and saprotrophs.

Detritivores ingest dead organic matter and then digest it internally.

Question 13

What is the difference between detritivores and saprotrophs?

Answer 13

They both obtain nutrients from dead organic matter however detritivores ingest it and then digest it internally whereas saprotrophs get nutrients by external digestion. They secrete digestive enzymes into the dead organic matter and digest it externally and then absorb the products of digestion.

Question 14

What are saprotrophs?

Answer 14

Saprotrophs are heterotrophs that obtain organic nutrients from dead organic matter by external digestion. Saprotrophs secrete digestive enzymes into the dead organic matter and digest it externally. They then absorb the products of digestion. Many types of bacteria and fungi are saprotrophic, they are also known as decomposers because they break down carbon compounds in dead organic matter and release elements such as nitrogen into the ecosystem so that they can be used again by other organisms.

Question 15

Give an example of a detritivore and then a saprotroph?

Answer 15

Detritivore - earthworms, unicellular organisms with vacuoles, larvae of the dung beetle.

Saprotroph - bacteria and fungi (decomposers)

Question 16

What is a community?

Answer 16

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

The relationships between organisms are complex sometimes they benefit each other, for example when a humming bird feeds on nectar from a flower and helps the plant by pollinating it, other times it is not beneficial for example a parasite on a host.

All species are dependant on relationships for survival. For this reason a population of one species can never live in isolation. Groups of populations live together. A group of populations living together in an area and interacting with each other is known in ecology as a community. Typical communities consist of hundreds or even thousands of species living together in an area.

Question 17

How do you randomly position a quadrat?

Answer 17

• A base line is marked out along the edge of the habitat using a measuring tape. It must extend all the way along the edge of the habitat.
• Random numbers are obtained using either a table or a random number generator on a calculator.
• A first random number is used to determine a distance along the measuring tape. All distances along the measuring tape must be equally likely.
• A second random number is used to determine a distance out across the habitat at right angles to the tape. All distances across the habitat must be equally likely.
• The quadrat is placed precisely at the distances determined by the two random numbers.

THIS METHOD IS ONLY VALID FOR PLANTS AND OTHER ORGANISMS THAT ARE NOT MOBILE.

Question 18

What is a quadrat?

Answer 18

Quadrats are square sample areas, usually marked out using a quadrat frame. Sampling with a quadrat usually involves repeatedly placing a quadrat frame at random positions in a habitat and recording the number of organisms present each time.

Question 19

How, in practise, would you test the association between two species?

Answer 19

Sample your environment by using random quadrat samples.

You start with the two hypothesises.
H0: Two species are distributed independently (the null hypothesis)
H1: The two species are associated (either positively so they tend to occur together, or negatively so they tend to occur apart).

We can test these using chi-squared. It is only valid if all the expected frequencies are 5 or larger and the sample was taken at random from the population.

1) Draw up a table of observed frequencies, which are the number of quadrats containing or not containing the two species. Have species A present and not present at the top for two columns and then species B present and not present for 2 rows.
2) Add up the column and row totals. So how many species A absent and not absent, and how many species B absent and not absent.
3) Then what you do is you calculate ‘expected frequency’. You divide (row total x column total) by the (grand total).

4) Then calculate the ‘degrees of freedom’.
degrees of freedom = (number of rows -1) x (number of columns - 1)

5) Then find the critical region for chi-squared from a table of chi-squared values. Using the degrees of freedom that you calculated and 0.05 or 5% as the significance. The critical region is any value of chi-squared larger than the value in the table. You google this value. E.g. what is the critical value for 1 degree of freedom at 0.05.

6) Then use the chi-squared equation:
The sum of the values from each square of your table:
((observed frequency - expected frequency)^2/(expected frequency)

7) Compare the calculated value of chi-squared with the critical region. If the calculated value is in the critical region then we reject H0, if it is equal or below then we accept it.

Question 20

What is chi-squared used for?

Answer 20

To test association.

Question 21

When is chi-squared valid?

Answer 21

Chi-squared is only valid if the expected frequencies are 5 or larger and the sample was taken at random from the population.

Question 22

What does statistical significance mean?

Answer 22

Biologists often use the phrase ‘statistically significant’ when discussing results of an experiment. This refers to the outcome of a statistical hypothesis test. There are two alternative types of hypothesis.

H0: this is the null hypothesis, that there is no relationship. The expected frequency and the observed frequency are really similar.

H1: is the alternative hypothesis and is the belief that there is a relationship, for example that two means are different or that there is an association between two variables.

The usual procedure is to test the null hypothesis, with the expectation of showing that it is false. A statistic is calculated using the results of the research and is compared to a range of possible values called the critical region. If the calculated statistic exceeds the critical region, the null hypothesis is considered to be false and is therefore rejected, though we cannot say that this has been proved with certainty.

When a biologist states that results were statistically significant it means that if the null hypothesis (H0) was true, the probability of getting results as extreme as the observed results would be very small. A decision needs to be made about how small this probability needs to be. This is known as the significance level. It is the cut off point for probability of rejecting the null hypothesis when in fact it was true. A level of 5% is often chosen, so the probability is less than one in twenty. That is the minimum acceptable significance level in published research.

• If there is a difference between the mean results for the two treatments in an experiment, a statistical test will show whether the difference is significant at the 5% level. If it is, there is a less than 5% probability of such a large difference occurring by chance, even when the population means are equal. We say that there is statistically significant evidence that the population means differ.
• In the example of testing for an association between two species, described on previous pages, the chi-squared test shows whether there is a less than 5% probability of the difference between the observed and the expected results being as large as it is without the species being either positively or negatively associated.

Question 23

In my basic explanation what does the Chi-squared test do? And what does statistically significant mean?

Answer 23

It means that there is a less than 5% chance that the results occurred by chance, it is extremely unlikely to happen unless they are associated.

• In the example of testing for an association between two species, described on previous pages, the chi-squared test shows whether there is a less than 5% probability of the difference between the observed and the expected results being as large as it is without the species being either positively or negatively associated.

Question 24

How are inorganic nutrients obtained by organisms?

Answer 24

Autotrophs and heterotrophs obtain inorganic nutrients from the abiotic environment.
Living organisms need a supply of chemical elements:
- Carbon, hydrogen and oxygen to make carbohydrates, lipids and other carbon compounds on which life is based.
- Nitrogen and phosphorus are also needed to make lots of these compounds.
- Approximately fifteen other elements are needed by living organisms. Some of them are used in minute traces only, but they are nonetheless essential.

Autotrophs obtain all of the elements that they need as inorganic nutrients from the abiotic environment, including carbon and nitrogen. Heterotrophs on the other hand obtain these two elements as part of the carbon compounds in their food. They do however obtain other elements as inorganic nutrients from the abiotic environment including sodium, potassium and calcium.

Question 25

What is the nutrient cycle?

Answer 25

The supply of inorganic nutrients is maintained by nutrient cycling.
Chemical elements can be endlessly recycled. Organisms absorb the elements that they require as inorganic nutrients from the abiotic environment, use them and then return them to the environment with the atoms unchanged.

Question 26

What is the sustainability of ecosystems?

Answer 26

Ecosystems have the potential to be sustainable over long periods of time. If something is sustainable it means it can continue indefinitely. Human use of fossil fuels is an example of an unsustainable activity. Supplies of fossil fuels are finite and are not currently being renewed and cannot therefore carry on indefinitely.

There are 3 requirements for sustainability in ecosystems:
- nutrient availability
- detoxification of waste
- energy availability
Nutrients can be recycled indefinitely and if this is done there should not be a lack of the chemical elements on which life is based. The waste products of one species are usually exploited as a resource by another species. For example, ammonium ions released by decomposers are absorbed and used for an energy source by Nitrosomonas bacteria in the soil. Ammonium is potentially toxic but because of the work of this bacteria it does not accumulate.

Energy cannot be recycled, so sustainability depends on continued energy supply to ecosystems. Most energy is supplied to ecosystems as light from the sun.

Question 27

What are mesocosms?

Answer 27

Mesocosms are small experimental areas that are 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. Ecological experiments can be done in replicate mesocosms, to find out the effect of varying one or more conditions.
Another possible use of mesocosms is to test what types of ecosystems are sustainable.