A population is the total number of individuals of the same
species that live in a certain area.
The size + distribution of a population can be measured using one
or more techniques: Pooters, Sweepnets, difficult number, Pitfall
traps, Quadrats.
When sampling you must make sure you take a big enough
sample, reliable & sample randomly
Estimating Animal
Populations
Animal populations are difficult to sample and
measure because most species are highly
mobile and are not easily visible because they
are usually hiding from predators.
One method is the capture, recapture method
sometimes called the Lincoln index: Animals are
caught humanely, the animals are marked, the
marked animals are released, a second sample is
obtained which will contain so marked animals
and some unmarked. The numbers in each group
are again counted.
The following formula can then be used to
estimate the total population size in the habitat:
No in 1st sample x no in 2nd sample / no in 2nd
sample which were previously unmarked.
Certain assumptions are made when using the
capture/ recapture data. No death, immigration,
emigration. Each sample is collected in exactly
the same way without bias. The marks given to
an animal do not affect their survival rate.
Using Transects
Sometimes an enviromental
scientist may want to look at how
species change across habitat, or
the boundary between them. This
needs more systematic method:
A line such as a tape measures is
laid down, regular intervals marked
A small quadrat is laid next to the line
and the number of plants of the
different species is estimated or
counted. This can sometimes be done
by estimating the percentage cover.
The quadrat is moved along at regular
intervals and the plant populations
estimated and recorded at each point
until the end of the line.
The height of a particular kite shows how
abundant that particular species is at that point.
Zonation
Zonation is described
as a gradual change in
the distribution of a
species across a
habitat. Changes
usually result from a
physical factor
changing.
Keys
Correctly
identifying species
in a sample can be
difficult. Using
keys can help to
identify organisms.
Does it have six
legs?
Important Ecological terms
Population - the number of individuals of a species in a defined area.
Community - the total number of individuals of all the different
populations of organisms that live together in a habitat any one time.
Ecosystem - A physical environment with a particular set of conditions,
plus all the organisms that live in it. The organisms interact through
competition and predation. They are affected by the physical factors
present in the ecosystem. Some ecosystems are still unexplored, with
possible undiscovered species.
Biodiversity - the variety of different species living in a habitat.
Habitat - the part of the physical environment where a plant or
animal lives. There are many types of habitat, each with particular
characteristics. An organism must be well suited to it's habitat to
be able to compete with other species for limited resources.
Organisms that are specialised in this way are restricted to that
type of habitat because their adaptations are unsuitable
elsewhere. Organisms are not normally distributed at random in a
habitat. They may often be clumped into specific areas.
Natural and Artificial
Ecosystems
Natural ecosystems - not
man-made. They change over
time. Examples of natural
ecosystems are native
woodlands or lakes.
Artificial ecosystems - man-made
and carefully controlled to keep the
conditions constant. Examples
include fish farms and foresty
plantations.
Biodiversity in natural ecosystems is
always greater than in artificial ones.
Natural ecosystems have high biodiversity many different species of plants and
animals co-exists in the same environment. Artificial ecosystems are designed for a
particular purpose. A market gardener growing a crop in a greenhouse uses fertilisers,
weedkillers and pesticides to prevent other organisms from growing alongside his crop.
The biodiversity in this ecosystem will be low.
Comparing Ecological Habitats
Two habitats can be compared by sampling using a
quadrat. First the plant and animal species in a 1m2
quadrat must be identified and then the population of
each species must be counted. The process is
repeated several times to get a large, reliable sample.