Introduction

Our living world is fascinatingly diverse and complex. One way to make sense of that complexity is to study it at different levels of biological organisation — organisms, populations, communities, and biomes.

At any of these levels, questions come in two flavours. A "how-type" question asks about the underlying mechanism: how does a bird produce its morning song? A "why-type" question asks about the significance of what we observe: why does the bird sing at all? For the bulbul singing at dawn, the "how" answer lies in its voice box and vibrating membrane; the "why" answer lies in its need to communicate with a mate during the breeding season. Looking at nature with this dual curiosity throws up plenty of interesting questions of both kinds — why are night-blooming flowers usually pale or white, how does a bee know which flower is holding nectar, why does a cactus grow so many thorns, how does a newly hatched chick recognise its own mother?

Ecology is the branch of biology that studies these kinds of interactions — among organisms themselves, and between organisms and their physical (abiotic) surroundings — and it is organised around four levels: organisms, populations, communities, and biomes. This chapter takes up ecology at the population level: how a group of individuals of one species, sharing a habitat, behaves as a unit with its own properties that no single individual has on its own.