Activity 3
BIOTIC AND ABIOTIC FACTORS
FOOD CHAIN AND FOOD WEB
Objectives:
As you complete this lab exercise you will:
1. Draw graphs of seasonal changes in the physical environment.
2. Determine the different biotic and abiotic factors in an environment. 3. Draw graphs
showing the relationship between changes in the physical environment and the
physiological responses of plants and animals.
4. Identify trophic levels in food chains and pyramids of numbers, energy and biomass.
5. Be able to write a scientific paper using the results.
Evolution has allowed life to respond to the variety of environmental conditions found on Earth
and created life-forms that are able to survive in the most extreme conditions. The study of the
relationships between the physiological functioning of an organism and its physical
environment is called ecophysiology. The scope of abiotic and biotic factors spans across the
entire biosphere, or global sum of all ecosystems. Such factors can have relevance for an
individual within a species, its community or an entire population. For instance, disease is a
biotic factor affecting the survival of an individual and its community. Temperature is an abiotic
factor with the same relevance.
Some factors have greater relevance for an entire ecosystem. Abiotic and biotic factors
combine to create a system or, more precisely, an ecosystem, meaning a community of living
and non-living things considered as a unit. In this case, abiotic factors span as far as the pH
of the soil and water, types of nutrients available and even the length of the day. Biotic factors
such as the presence of autotrophs or self-nourishing organisms such as plants, and the
diversity of consumers also affect an entire ecosystem.
Climatic variation
As a general rule, temperature falls with altitude in the atmosphere and with depth in water.
The quantity of sunlight received by the Earth reduces from the equator to the poles due to the
angle of the axis of the planet’s rotation to the sun. Light levels determine the amount of plant
growth that can be sustained and so too does carbon dioxide concentration in the atmosphere.
The differential heating of the surface creates air movements and drives the water cycle
creating rainfall patterns that support a great variety of life-forms in tropical forests while
depriving animals and plants in hot deserts of the water that is essential to their survival.
Climatic differences have caused the development of animal and plant communities that vary
spatially with distance from the equator and vertically as we ascend mountains or descend into
the oceans.
Temperature regulation
Most animal species are poikilothermic and their body temperature is largely determined by
the ambient environmental temperature (Fig. 2.1). When the environment becomes too hot or
too cold they struggle physiologically and become less active or completely inactive.
Behavioural adaptations may allow them to move into the shade to cool down or into the sun
if they are too cold, and to increase their movements when they enter a cold environment in
the hope of finding somewhere warmer. Birds and mammals are homiothermic and are able
to maintain a more or less constant body temperature. This allows them to function even when
the environmental temperature varies considerably on a daily or seasonal basis and has
allowed some species to disperse into extreme environments.
Adaptations to Harsh Environment
Some animal species have evolved mechanisms that allow them to avoid adverse conditions.
Many bear species hibernate to avoid the cold conditions of winter when food is difficult to find.
Some insects enter a period of diapause which allows them to delay their development in
anticipation of a period of adverse conditions. Oxygen is essential to most forms of life but
difficult to obtain in some environments. Some animal species are specially adapted to function
under water where oxygen levels are low or in water that is contaminated with pollution. Marine
species need to remain in osmotic balance while living in a salty environment, and some
achieve this by excreting salt through special glands.
The following exercises provide examples of how physical factors vary between habitats and
illustrate the adaptations that organisms have evolved which enable them to survive.
Exercises
Procedure 1. Climatic variation
Climatic data give the ecologist information about two of the environmental variables that are
most important in determining the nature of the ecosystem which will develop in any particular
part of the world: rainfall and temperature. The table presents such data for two widely
separated parts of the Earth (Savannah and Tropical forest).
1. Draw separate climate graphs for each of the two biotopes (A and B) using the data
from the given table. Each graph should show temperature on the vertical axis on the
left side and rainfall on the vertical axis on the right side. Rainfall should be indicated
using a bar chart and temperature using a line graph. All vertical scales should start at
zero.
2. For each biotope, calculate (a) the mean monthly temperature; (b) the mean monthly
rainfall; and (c) the total annual rainfall received by each area.
3. Describe the main differences in the rainfall pattern in these two biotopes. 4. Why
does the mean monthly rainfall data for these two biotopes give an incomplete picture
of the rainfall differences between them?
5. Identify which set of data is from a tropical forest and which is from a savannah and
explain why.
6. What effect would you expect the rainfall pattern in biotope B to have on the growth
of plants.
Procedure 2. Temperature selection in two fish species
Many species of animals have been shown to prefer a particular temperature range when
placed in a temperature gradient. They are less active at their preferred temperature and thus
arrive there by a type of behaviour known as thermokinesis. Fisher and Elson (1950) examined
temperature preference in the speckled trout (Cynoscion nebulosus) and the Atlantic salmon
(Salmo salar) when placed in a tank with a temperature gradient from 0°C to 22°C. Some of
their observations are presented in the following table.
1. Draw a single graph of the data from the table.
2. What range of temperature was each species able to tolerate?
3. What is the preferred temperature of each species?
4. At what temperatures would you expect each species to be most active? 5.
Why is it important for fish to respond behaviourally to a temperature gradient?
Procedure 3. Living in the Desert: Temperature Tolerance in the Camel
Hot deserts are among the most extreme environments on Earth. Camels have evolved a
number of physiological adaptations that allow them to survive such conditions. The table
below compares the rectal temperature of camels with that of other mammalian species. The
range of body temperatures experienced by camels living under desert conditions and the
amount of water they lose are indicated in the succeeding tables.
1. How do camels differ from the other animals listed in the table with regard to the
range of body temperature experienced?
2. In what circumstances does the camel store excess heat in its body?
3. How does this help the camel to conserve water?
4. How does the maintenance of a high body temperature reduce heat gain from the
environment?
5. How is heat lost during the night without the use of water?
FOOD CHAINS AND FOOD WEBS
The functioning of all ecosystems depends upon the flow of energy from one organism to
another and the passing of nutrients in a cycle between the physical environment and living
things (DeAngelis,1980). The energy in most organisms has ultimately been derived from the
sun. Carnivores eat herbivores; herbivores eat green plants; and the green plants
photosynthesise using light from the sun to make carbohydrates from carbon dioxide and
water (using chlorophyll):
Organisms that obtain their energy from the physical environment rather than by eating other
organisms are known as autotrophs. Green plants are a type of autotroph known as a
phototroph, because they use light energy. Some bacteria obtain energy from chemical
compounds and are known as chemotrophs. All organisms obtain energy from their food,
principally by breaking down carbohydrates during respiration. However, some of the energy
is lost as heat. It is the heat generated by respiration that ultimately restricts the length of
food chains.
When a herbivore eats a plant, some of the energy contained in the tissues of the plant is used
by the herbivore, but some of it is lost as the heat of respiration. When the herbivore is eaten
by a carnivore, once again, some of the energy is used by the carnivore but some is lost when
the carnivore respires. If we consider a parcel of energy captured by a green plant and then
passed along a food chain to a herbivore and then a carnivore, at each stage in this transfer
of energy some is lost as the organisms respire. This means that the parcel of energy is
much smaller by the time it reaches the carnivore.
Food chains may be studied in a number of ways. Simple observation will allow us to
determine what each species is feeding on in some ecosystems. An alternative is to examine
the stomach contents of animals which, of course, involves killing the animals and depends
on the ability to identify partly digested animal and plant remains. A third method uses
radioactive isotopes. For example, if phosphorus-32 is injected into grass its presence in
other species sometime later would allow us to trace its path from one species to another
(Marples, 1966).
Procedure 4. A Steppe Ecosystem: Biotic and Abiotic Factors
A steppe is a mid-latitude grassland consisting of a flat, generally treeless plain extending
across Eurasia. Characteristically, steppes have a semi-arid continental climate with
extremes of temperature occurring in summer and winter, high temperatures during the day
and low temperatures at night. Table 3.2 contains some of the major components of this type
of ecosystem, including a wild equid known as the kiang (Equus kiang).
1. From an ecological point of view these components may be divided into two
fundamentally different categories. Give a name to each of these two categories and
three examples of each.
2. Group the components of the table into five distinct categories of ecological significance.
Give a name to each of these categories; four of the categories should be based on
positions in the food chain (counting carnivores and top carnivores as a single group).
3. A taxonomist might classify the organisms in Table 3.2 into mammals, birds, reptiles,
insects, grasses, etc. Why is this type of classification of less interest to an ecologist
than that produced in answer to question number 2?
Procedure 5. Food Webs in an English Woodland
The feeding relationships between organisms in an ecosystem may be illustrated by the
construction of food chains. A food chain is simply a linear sequence showing, for example,
green plants collecting energy from light and then, for each animal species, what it eats and
what eats it. A simple food chain might look like this:
Light → greenplant → rabbit → fox → eagle
Each organism in this food chain represents a different trophic level (Fig. 3.4) In a real
ecosystem, most animals feed on more than one other species. If we draw a diagram of all of
these interrelated food chains we construct a food web. For complex ecosystems it is almost
impossible to construct a complete food web. Apart from the difficulty of establishing all of the
feeding relationships at any particular point in time, the situation is made even more complex
by seasonal differences (e.g., in ecosystems where migratory species are present for only
part of the year).
Table 3.3 is a very simplified representation of the food web in Wytham Woods in England.
This woodland has been widely studied by ecologists because of its proximity to the University
of Oxford and includes a variety of common species such as titmice (e.g., blue tits (Cyanistes
caeruleus) and great tits (Parus major) (and small mammals including the wood mouse
(Apodemus sylvaticus).
1. Draw a food web for Wytham Woods using the information on feeding relationships in
Table 3.3.
2. Complete Table 3.4, indicating the trophic level to which each organism belongs:
primary producer, primary consumer, secondary consumer, tertiary consumer. 3.
Populations of which taxa are likely to benefit if the numbers of voles and mice
increased?
Procedure 6. Ecological pyramids
Pyramids of numbers, biomass and energy are used to examine standing crop and energy
flow in ecosystems. Silver Springs is a freshwater ecosystem in Florida, United States. Data
for this system are presented in Table 3.5.
1. Draw a pyramid of biomass to scale using the data in Table 3.5. You may find it
useful to draw a vertical line down the centre of the page before you begin as this will
help you to keep the pyramid symmetrical (Fig. 3.7).
2. Explain why there are differences in the efficiency of energy transfer between trophic
levels.
3. Explain what happens to the energy which is not transferred to the next trophic level
in a food chain when one organism feeds on another.
4. Calculate the efficiency with which energy is transferred from one trophic level to the
next by completing Table 3.6.
Pyramids of numbers representing parasitic food chains have unusual characteristics. The
data in Table 3.7 relate to a food chain in which bacteria are parasitising nematode worms
which in turn are fish parasites.
5. Draw a food chain showing the relationships between these organisms, indicating the
direction of the flow of energy.
6. Which organism is at the top of this food chain?
7. Calculate the standing crop biomass in each trophic level (C = A × B).