Ecology and the Levels of Life

Ecology is the branch of biology that studies the interactions among organisms and between an organism and its physical, abiotic environment. It gives biology a holistic perspective — a way of seeing how living things, while remaining individuals, behave together as organised wholes.

Ecology works at four levels of biological organisation: organisms, populations, communities and biomes. This chapter focuses on just one of these — the population level.

'How' and 'Why' Questions

At any level of organisation, nature invites two kinds of question. A 'how-type' question asks about the mechanism behind a process; a 'why-type' question asks about its significance.

Hear a bulbul singing in the morning and you might ask how it sings — an answer about the voice box and vibrating bones — or why it sings, an answer about communicating with a mate in the breeding season. Both are valid scientific questions, and keeping them apart helps clarify what we are really trying to explain.

What Is a Population?

In nature we rarely find single, isolated individuals. Most live in groups within a well-defined geographical area, where they share or compete for similar resources and potentially interbreed — and such a group is a population. Although "interbreeding" implies sexual reproduction, a group produced even by asexual reproduction is also treated as a population for ecological study.

All the cormorants in a wetland, the rats in an abandoned building, the teak trees in a forest tract, the bacteria in a culture plate and the lotus plants in a pond — each is an example of a population.

Why the Population Level Matters

An individual organism is the one that must cope with a changed environment, but it is at the population level that natural selection operates to evolve the desired traits. This is why population ecology is so important: it links ecology to population genetics and evolution.

That connection is the thread running through this chapter. The attributes of a population, the way its density changes and grows, and the ways populations of different species interact all ultimately feed back into how species evolve.

Quick Recap

  • Ecology studies interactions among organisms and between organisms and their abiotic environment.
  • It works at four levels: organisms, populations, communities and biomes; this chapter covers the population level.
  • 'How' questions seek the mechanism of a process; 'why' questions seek its significance.
  • A population = a group of one species in a defined area, sharing/competing for resources and potentially interbreeding (asexual groups count too).
  • Natural selection acts at the population level, so population ecology links ecology to population genetics and evolution.

Solved Examples — Section 1

Q1. Define ecology.

Answer: The study of interactions among organisms and between organisms and their physical (abiotic) environment.

Q2. Name the four levels of biological organisation that ecology deals with.

Answer: Organisms, populations, communities and biomes.

Q3. What is the difference between a 'how' and a 'why' question in ecology?

Answer: A 'how' question seeks the mechanism behind a process; a 'why' question seeks its significance.

Q4. Define a population.

Answer: A group of individuals of a species living in a well-defined geographical area, sharing or competing for similar resources and potentially interbreeding.

Q5. Give two examples of a population.

Answer: Any two of: all the cormorants in a wetland, rats in an abandoned building, teak trees in a forest, bacteria in a culture plate, lotus plants in a pond.

Q6. At which level does natural selection operate to evolve traits, and why does this matter?

Answer: At the population level; this makes population ecology important because it links ecology to population genetics and evolution.