Why we classify

Walk into a big sweet shop or clothes shop. You can find what you want only because things are sorted into groups. Scientists sort living things for the same reason.

The same animal can go in different groups, depending on what you sort by. An owl could be grouped by:

• where it lives (in trees);
• when it is active (at night);
• what covers its body (feathers).

Scientists use seven criteria:

• External features: what it looks like.
• Mode of nutrition: makes its own food (autotrophic) or depends on others (heterotrophic).
• Internal structure: its organs and tissues.
• Cell structure: one cell or many; with or without a true nucleus; with or without a cell wall.
• Ecological role: producer, consumer or decomposer.
• Reproduction: how it makes young.
• Genetic similarity: how alike their DNA is.

When organisms share many features, it suggests they evolved from common ancestors, like cousins sharing grandparents.

"Why do we need classification?" Imagine millions of books thrown on a library floor. You could never find anything. Biological classification arranges living things in an orderly system. It helps us:

• study them in an organised way;
• see how they are similar and related;
• name new organisms;
• protect threatened species;
• share one system with scientists all over the world.

An example from Arunachal Pradesh. Pakke Tiger Reserve has nearly 300 of India's roughly 1,300 bird species, including four hornbills: the Rufous-necked, Oriental Pied, Great and Wreathed Hornbill. They look alike in some ways but feed and nest differently, and all nest only in large, old trees. Classifying them helps us understand what each one needs to survive.