Organisms and Populations

Ecology is the study of how living things relate to one another and to their surroundings. It begins with the single organism, looking at how it copes with its environment. Then it rises to the population, the group of the same kind among which that organism lives. Understanding adaptations, population features, and species interactions is the foundation of all environmental study.

Ecology is studied at four levels of organisation, each larger than the last:

• Organism: a single living being and how it handles its physical environment.
• Population: a group of individuals of the same species living in a given area.
• Community: all the populations of different species living and interacting in an area.
• Ecosystem: a community together with its non-living (abiotic) surroundings.

Above all these lies the biosphere, the thin global zone where all life exists.

An adaptation is any feature of body, behaviour, or chemistry that helps an organism survive and reproduce in its habitat. Adaptations build up over long periods of evolution.

Familiar examples include:

• The kangaroo rat of North American deserts. It lives on water made internally from its food and produces very concentrated urine to save water.
• Desert plants with a thick waxy cuticle, sunken stomata, and leaves reduced to spines. These features cut water loss.
• Mammals of cold regions that are larger and have shorter limbs to conserve body heat.
• Hibernation in some animals to escape the cold, and aestivation to escape the summer heat.
• Insectivorous plants such as the pitcher plant and the sundew. They grow in soils that are very poor in nitrogen. To make up the shortfall, they trap and digest insects and draw their nitrogenous nutrition from this prey rather than from the soil.

A population has features that a single organism does not. These attributes describe the group as a whole:

• Birth rate (natality) and death rate (mortality): the number of births and deaths per individual over a time period.
• Sex ratio: the proportion of males to females.
• Age structure: the proportion of individuals of different ages. It is often shown as an age pyramid, which reveals whether a population is growing, stable, or declining.
• Population density: the number of individuals per unit area.

Populations grow and shrink as births and immigration are balanced against deaths and emigration.

No species lives alone. Each interacts with others in its community. The main interactions are classified by whether each species is helped (+), harmed (−) or unaffected (0):

• Competition (− / −): two species struggle for the same limited resource.
• Predation (+ / −): one species (the predator) kills and eats another (the prey). Predators keep prey numbers in check. They are vital to the ecosystem.
• Parasitism (+ / −): one species (the parasite) lives on or in a host and harms it.
• Mutualism (+ / +): both species benefit, as in the partnership of a flower and its pollinator.
• Commensalism (+ / 0): one benefits while the other is unaffected.
• Amensalism (− / 0): one is harmed while the other is unaffected.

These interactions weave the species of a community into a single web of life.

A parasitoid is a special kind of parasite. Its larva grows on or inside a single host and, unlike an ordinary parasite, almost always kills that host in the end. Parasitoids are common in some animal groups but absent from others. Among small ground-living creatures, flies, wasps, and carabid beetles (the ground beetles) all include parasitoid species. Centipedes and termites do not: centipedes are free-living predators that hunt their prey outright, and termites are plant-feeding social insects. So of these five groups, only three contain parasitoids.

When a partnership is close and long-lasting, it is called symbiosis. Such partnerships turn up across very different branches of life:

• Cnidarians: reef-building corals shelter tiny algae called zooxanthellae in their tissues. The algae make food by photosynthesis and share it, while the coral gives them a safe home.
• Fungi: many fungi sheathe plant roots as mycorrhizae, trading soil minerals for the plant's sugars. Others join with algae to form lichens.
• Protozoa: single-celled protozoa live in the guts of termites and grazing cattle, where they help break down tough plant fibre.

So cnidarians, fungi, and protozoa all contain symbiotic species.

The most famous mutualism of all is between the fig tree and the fig wasp. Each kind of fig is pollinated by its own single species of wasp and by no other. The wasp, in turn, can lay its eggs only inside that fig. Neither partner can complete its life cycle without the other, which makes their bond unusually tight. Other trees, such as the mahua, sandalwood, and silk cotton, rely on more general pollinators and have no such exclusive insect partner.

Life on Earth is sorted into great groups, and a few of those groups appear again and again in study of ecology. Knowing which familiar organisms sit in which group helps you place any unfamiliar one.

The pea family, known formally as Fabaceae or the legumes, is one of the most important plant families. Its members fix nitrogen through bacteria in their roots and enrich the soil. Several everyday crops belong to it:

• Groundnut: the peanut plant, a legume despite its underground pods.
• Horse-gram: a hardy pulse grown in dry regions.
• Soybean: a protein-rich legume used for oil and food.

All three of these crops, the groundnut, the horse-gram, and the soybean, are true legumes of the Fabaceae. Each bears its seeds in a pod and carries nitrogen-fixing bacteria in its root nodules. None of them sits outside the family.

Insects are the largest group of animals, marked by six legs, a three-part body, and usually wings. Many small creatures with odd names are in fact insects. Cicadas (loud sap-sucking bugs), froghoppers (whose young make spittle-like foam), and pond skaters (which walk on the surface of water) are all insects, grouped among the true bugs.

Fungi form a kingdom of their own, separate from plants and animals. Mushrooms, the fruiting bodies of certain fungi, show a wide range of properties:

• Medicinal: some, such as reishi, are used in traditional remedies.
• Psychoactive: some contain psilocybin and alter the mind.
• Insecticidal: some produce compounds that kill insects.
• Bioluminescent: some, like the ghost fungus, glow in the dark.

Diversity also appears within familiar animal groups, where habits vary far more than students expect:

• Herbivorous turtles: the green turtle grazes on sea grass and algae.
• Herbivorous fish: many fish feed mainly on algae and water plants.
• Herbivorous marine mammals: the dugong and the manatee graze on sea grass.
• Viviparous snakes: several, such as the boa, give birth to live young rather than laying eggs.

Behaviour is the set of actions an animal uses to find food, avoid danger, and live with others. Some behaviours are so refined that they rival human skill, and two famous examples are favourites in exams.

The waggle dance is performed by honeybees. A returning forager moves in a figure-of-eight pattern on the comb. The angle of the dance shows the direction of the food relative to the sun, and the length of the waggling run shows how far away it lies. In this way one bee tells the rest of the hive exactly where to fly.

Tool use was once thought to be human alone, but several animals make and use tools. The orangutan, a great ape of Southeast Asian forests, strips and shapes a stick and pokes it into holes in trees or logs to scrape out insects to eat. This shows planning and problem-solving of a high order.