Unit 3
Topic 3 Sampling techniques and measurement of abiotic and biotic factors
Pupil Notes
Biodiversity
Biodiversity is the range of different living things in an area.
Biotic factors and abiotic factors can affect the biodiversity. Biotic factors are living while
abiotic factors are non-living features of the environment.
Abiotic Factors
Abiotic factors are the non-living things present in an ecosystem
An organism is only able to survive in an ecosystem if the correct combination of abiotic
factors are present. Some examples of abiotic factors are:
•
Light intensity
•
Soil pH
•
Temperature range
•
Humidity
Biotic Factors
Biotic factors are the living factors in an ecosystem and include:
•
Diseases
•
Competition
•
Predators
•
Grazing
•
Prey (how easy it is to get find and catch them)
Biotic factors can affect the biodiversity in the ecosystem.
Sampling organisms in an ecosystem
There are several techniques that can be used to sample organisms in an ecosystem. These
include:Pitfall Traps
Tree Beating
Quadrats
Pitfall Traps:
Small animals moving across the surface of a piece of ground can be sampled using a pitfall
trap.
A container is placed in the ground, making sure the top is level with the ground (See
diagram)
Pitfall trap
Possible errors when using a pitfall trap can include:
•
Predators such as birds eating the contents – this is prevented by covering trap with
raised lid
•
Large animals caught in trap may eat smaller ones - this is prevented by adding a
2cm depth of ethanol to the carton
•
Animals that can fly can escape from the trap
Tree Beating:
Randomly select trees. Hit one branch on each tree three times with a padded stick, over a
tray or sheet and quickly count all the organisms that fall onto it.
Stick to shake
branches
Tray or sheet to
collect organisms
Quadrats:
A quadrat can be used to sample plants as well as animals that don’t move very much.
A quadrat is a frame divided into squares that marks off a small piece of ground but it can be
used to estimate the distribution of plants in a larger area. The number of squares in the
quadrat containing each type of plant is counted. This is the abundance estimate.
Possible errors can include:
•
Non-random sampling– quadrats must be thrown randomly to make sure results are
valid
Identifying
 Number
Organisms
of samples – A single quadrat may not be representative of the area. Increasing
the number of quadrats leads to a more representative sample
After samples have been taken, each organism found can be identified using a biological key.
Identifying organisms
A biological key is used to identify organisms.
There are two main types of keys
•
Branch keys - These are in the form of a branching tree but have the problem that
they become very large for more than a few unknown living things
•
Paired statement key– These ask questions and send you to the next question based
on your answer. These can fill huge books and involve large numbers of unknown
living things but are much harder to use
The example below illustrates a branched key and a paired statement key for 4 animals:
Paired statement key
Branching key
1. Does animal have legs _______ go to 2
Animals
No legs ___________________ go to 3
2.
Has legs
No legs
Does animal have eight legs __ spider
3.
6 legs
8 legs
has shell
Ladybird
spider
snail
Does animal have six legs _____ ladybird
no shell
Does animal have a shell ____ snail
No shell ________________ earthworm
earthworm
Measuring abiotic factors
Abiotic factor
Light intensity
Measured using
Light meter
Soil moisture
Moisture meter
Soil pH
pH meter
Possible sources of error
Shading the light meter with
your body
Not drying the probe between
readings
Not putting the probe deep
enough into the soil
Not cleaning the probe
between readings
Not putting the probe deep
enough into the soil