Biosphere Contents

4. Interdependence

Conclusion

Organisms live together in ecological communities and interact with each other in many different ways. These interactions are sometimes beneficial, sometimes harmful and sometimes neutral. Some of the most fundamental interrelationships are represented in food webs and food chains. The inter-relationships within a food web can be summarised via tropic levels, of which there are typically relatively few in any one food web. Predator prey relationships develop dynamically over time, and where they persist, they may reach a dynamic equilibrium.

Question and answers

1. What is an ecological community?

question 1 answers

2. Give three examples of types of interaction in which one individual benefits and the other is harmed by the interaction.

question 2 answers

3. It what situation might an organism benefit from being eaten?

question 3 answers

4. What are the three basic trophic functions organisms have in a community?

question 4 answers

5. In Elton's pyramid of numbers principle, which would have more individuals, a blackbird or a hawk?

question 5 answers

6. Outline Lindemann's approach to ecosystem organisation.

question 6 answers

7. How does Hutchinson (1959) propose that food chain length is constrained?

question 7 answers

8. Give one reason why only 10% of energy from a given trophic level is available to the next trophic level in a marine food chain.

question 8 answers

9. What are the differences between a predator, a grazer and a parasite?

question 9 answers

10. Why do predators not normally consume every individual of a prey species in a given location?

question 10 answers

11. Give one example of a predator-prey relationship in 'dynamic equilibrium'.

question 11 answers

Question 1 answer:

An ecological community is a group of plant and animal populations that interact together in a particular area.

Question 2 answer

Competition, predation and parasitism.

Question 3 answer:

Plants actively encourage certain insects and birds to feed from their flowers. These insects or birds enable the pollination of the flowers (they carry pollen from one flower to another), allowing the plant to reproduce. This is a mutualistic interaction (++).

Question 4 answer

Primary producers - form organic compounds from simple inorganic
ones (autotrophic).
Consumers - feed upon primary producers or other consumers
(heterotrophic).
Decomposers - feed on dead and decaying matter (saprotrophic).

Question 5 answer

The blackbird will have more individuals as it is a smaller animal. Larger animals tend to have fewer individuals; there is a relationship between the size and abundance of animals.

Question 6 answer

He placed organisms into trophic (feeding) levels:
1 = producers
2 = herbivores
3 = carnivores
4 = top carnivores

Question 7 answer

Hutchinson proposed food chain length is constrained by the limited amount of energy available to higher trophic levels.

Question 8 answer

The transfer of energy is extremely inefficient, with much of the remaining 90% being used for metabolic processes, through respiration.

Even energy used for growth is not all available the next trophic level - some tissues will be indigestible, or individuals will not be caught by predators.

Question 9 answer

A parasite feeds on a very few hosts during its life-time, and does not normally kill the hosts, at least not immediately, whereas a predator eats and typically kills many prey organisms. Grazers feed on parts of many prey individuals, but do not usually kill them.

Question 10 answer

If a species is heavily preyed upon it and its numbers decline, it will become harder to find. Predators normally have a range of different food sources, so they will be likely to seek more accessible alternatives, giving the original species time for its population to stabilise and recover.

Question 11 answer

Populations of the snowshoe hare and the Canada lynx undergo oscillations on an 8-11 year cycle, which appear to represent a dynamic equilibrium.