Organisms and Environment

Population interactions are the ways in which populations of two different species affect each other in a community. Each species may gain $(+)$, lose $(-)$ or stay unaffected $(0)$, and six types are recognised: mutualism, competition, predation, parasitism, commensalism and amensalism. This page explains each type with its examples, from prudent predators and plant defences to the competitive exclusion principle, resource partitioning, brood parasitism, the fig-wasp mutualism and sexual deceit in Ophrys. NEET often asks match-the-list and statement questions on population interactions and their examples.

1. Why Species Interact

• No natural habitat on earth is inhabited by just a single species. Such a situation is inconceivable.
• ★ Exam imp For any species, the minimal requirement is one more species on which it can feed.
• Even a plant, which makes its own food, cannot survive alone. It needs soil microbes to break down organic matter and return inorganic nutrients for absorption.
• Many plants also need an animal agent for pollination.
• So animals, plants and microbes do not and cannot live in isolation. They interact in various ways to form a biological community.
• Even minimal communities have many interactive linkages, although all may not be readily apparent.

2. Types of Population Interactions

• ★ Exam imp Interspecific interactions arise from the interaction of populations of two different species.
• Each interaction may be beneficial, detrimental or neutral (neither harm nor benefit) to one species or both.
• Signs used: $+$ for a beneficial effect, $-$ for a detrimental effect, and $0$ for a neutral effect.

3. Predation

3.1 Predation and energy transfer

• Without animals to eat plants, the energy fixed by autotrophs would not move up the food chain.
• ★ Exam imp Predation is nature's way of transferring the energy fixed by plants to higher trophic levels.
• The tiger and the deer are a familiar predator and prey. But a sparrow eating any seed is no less a predator.
• Animals eating plants are categorised separately as herbivores. In a broad ecological context, they are not very different from predators.

3.2 Roles of predators

• Conduits for energy: predators act as 'conduits' for energy transfer across trophic levels.
• Control of prey: they keep prey populations under control. Without predators, prey could reach very high densities and cause ecosystem instability.
• Species diversity: they maintain species diversity in a community by reducing the intensity of competition among competing prey species.

• Certain exotic species introduced into a new area become invasive and spread fast, because the invaded land lacks their natural predators.
• ★ Exam imp The prickly pear cactus, introduced into Australia in the early 1920s, spread rapidly into millions of hectares of rangeland.
• It was controlled only after a cactus-feeding predator (a moth) from its natural habitat was introduced into the country.
• Biological control methods in agricultural pest control are based on the ability of the predator to regulate the prey population.
• Pisaster, a starfish, is an important predator in the rocky intertidal communities of the American Pacific Coast.
• ★ Exam imp In a field experiment, all starfish were removed from an enclosed intertidal area. More than 10 species of invertebrates became extinct within a year, because of interspecific competition.

3.3 Defences of prey animals

• Prey species have evolved various defences to lessen the impact of predation.
• Camouflage: some insects and frogs are cryptically coloured, so predators cannot detect them easily.
• Poison: some species are poisonous, so predators avoid them.
• ★ Exam imp The Monarch butterfly is highly distasteful to its predator (a bird) because of a special chemical in its body.
• The butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.

3.4 Defences of plants against herbivores

• For plants, herbivores are the predators.
• Nearly 25 per cent of all insects are phytophagous (they feed on plant sap and other parts of plants).
• The problem is severe for plants because, unlike animals, they cannot run away from their predators.
• So plants have evolved an astonishing variety of morphological and chemical defences against herbivores.

4. Competition

• Darwin, speaking of the struggle for existence and survival of the fittest, was convinced that interspecific competition is a potent force in organic evolution.
• It is generally believed that competition occurs when closely related species compete for the same limiting resources. This is not entirely true.
• ★ Exam imp First, totally unrelated species can also compete. In some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton.
• Second, resources need not be limiting. In interference competition, the feeding efficiency of one species is reduced by the interfering and inhibitory presence of the other, even when food and space are abundant.

4.1 Evidence for competition

• In laboratory experiments, Gause and other experimental ecologists showed that with limited resources, the competitively superior species eventually eliminates the other.
• Evidence for such competitive exclusion in nature is not always conclusive, but strong circumstantial evidence exists in some cases.
• ★ Exam imp The Abingdon tortoise of the Galapagos Islands became extinct within a decade after goats were introduced, apparently because the goats browsed more efficiently.
• Competitive release: a species restricted to a small area by a competitively superior species expands its range dramatically when the competitor is experimentally removed.
• Connell's field experiments on the rocky sea coasts of Scotland: the larger, competitively superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chthamalus (also written Chathamalus) from that zone.
• In general, herbivores and plants appear to be more adversely affected by competition than carnivores.

4.2 Competitive exclusion and co-existence

• More recent studies do not support such gross generalisations. They do not rule out interspecific competition in nature.
• But they point out that species facing competition might evolve mechanisms that promote co-existence rather than exclusion.
• ★ Exam imp One such mechanism is resource partitioning: two species avoid competition by choosing, for example, different times for feeding or different foraging patterns.
• MacArthur showed that five closely related species of warblers living on the same tree avoided competition and co-existed because of behavioural differences in their foraging activities.

5. Parasitism

5.1 Features of parasites

• The parasitic mode of life ensures free lodging and meals. So parasitism has evolved in many taxonomic groups, from plants to higher vertebrates.
• ★ Exam imp Many parasites are host-specific: they can parasitise only a single species of host.
• Host and parasite tend to co-evolve. If the host evolves mechanisms to reject or resist the parasite, the parasite must evolve mechanisms to counteract and neutralise them.
• Special adaptations of parasites, in keeping with their life style:

• Loss of unnecessary sense organs.
• Presence of adhesive organs or suckers to cling on to the host.
• Loss of the digestive system.
• High reproductive capacity.

• Life cycles of parasites are often complex, with one or two intermediate hosts or vectors that help them reach the primary host.
• ★ Exam imp The human liver fluke (a trematode) depends on two intermediate hosts, a snail and a fish, to complete its life cycle.
• The malarial parasite needs a vector (the mosquito) to spread to other hosts.
• Most parasites harm the host. They may reduce its survival, growth and reproduction, and so reduce its population density.
• They may also make the host physically weak and so more vulnerable to predation.
• A question to think about: an ideal parasite would thrive in the host without harming it, so why has natural selection not produced totally harmless parasites?

5.2 Ectoparasites and endoparasites

• Cuscuta has lost its chlorophyll and leaves in the course of evolution. It derives its nutrition from the host plant it parasitises.
• The female mosquito is not considered a parasite, although it needs our blood for reproduction.

5.3 Brood parasitism

• ★ Exam imp Brood parasitism in birds: the parasitic bird lays its eggs in the nest of its host and lets the host incubate them.
• The eggs of the parasitic bird have evolved to resemble the host's eggs in size and colour. This reduces the chance that the host detects the foreign eggs and ejects them.
• Example: the cuckoo (koel) and the crow, seen during the breeding season (spring to summer).

6. Commensalism

• In commensalism, one species benefits and the other is neither harmed nor benefited.

7. Mutualism

• Mutualism confers benefits on both the interacting species.
• ★ Exam imp Lichens: an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria.
• Mycorrhizae: associations between fungi and the roots of higher plants.
• The fungi help the plant absorb essential nutrients from the soil. The plant in turn gives the fungi energy-yielding carbohydrates.

7.1 Plant-animal mutualism and co-evolution

• The most spectacular and evolutionarily fascinating examples of mutualism are found in plant-animal relationships.
• Plants need animals to pollinate their flowers and disperse their seeds.
• Plants pay 'fees': pollen and nectar for pollinators, and juicy, nutritious fruits for seed dispersers.
• The system must be safeguarded against 'cheaters', such as animals that steal nectar without aiding pollination.

7.2 Fig and wasp

• ★ Exam imp Many species of fig trees have a tight one-to-one relationship with a pollinator species of wasp. A given fig species can be pollinated only by its 'partner' wasp species.

1. The female wasp searches the fig inflorescence for suitable egg-laying sites.
2. While searching, she pollinates the fig inflorescence.
3. She uses the fruit as an oviposition (egg-laying) site.
4. Her larvae are nourished by some of the developing seeds within the fruit: the fig's payment for pollination.

7.3 Orchids and bees

• Orchids show a bewildering diversity of floral patterns. Many have evolved to attract the right pollinator insect (bees and bumblebees) and ensure guaranteed pollination.
• Not all orchids offer rewards.
• ★ Exam imp The Mediterranean orchid Ophrys employs 'sexual deceit' to get pollination done by a species of bee.

1. One petal of the Ophrys flower bears an uncanny resemblance to the female bee in size, colour and markings.
2. The male bee is attracted to what it perceives as a female.
3. It 'pseudocopulates' with the flower and is dusted with pollen during the process.
4. When the same bee pseudocopulates with another flower, it transfers pollen to it and so pollinates the flower.

• This shows how co-evolution operates.
• If the female bee's colour patterns change even slightly during evolution, pollination success will fall, unless the orchid flower co-evolves to keep its petal resembling the female bee.

8. Pairs to Match, Examples and Numbers

Pairs to Match

Examples to Remember

Numbers to Remember

• Six types of interspecific interaction.
• Nearly 25 per cent of all insects are phytophagous.
• Prickly pear cactus: introduced into Australia in the early 1920s; spread over millions of hectares.
• Pisaster removal: more than 10 invertebrate species extinct within a year.
• Abingdon tortoise: extinct within a decade of goats being introduced.
• MacArthur: five closely related warbler species on one tree.
• Parasite life cycles: one or two intermediate hosts or vectors; liver fluke: two (snail and fish).

9. Quick Revision

• No species lives alone; animals, plants and microbes interact to form a biological community.
• Interspecific interactions: mutualism $(+,+)$, competition $(-,-)$, predation and parasitism $(+,-)$, commensalism $(+,0)$, amensalism $(-,0)$.
• Predation, parasitism and commensalism: the interacting species live closely together.
• Predators transfer energy, control prey (biological control; prickly pear and moth) and maintain diversity (Pisaster); they are prudent.
• Prey defences: camouflage, poison; the Monarch butterfly takes its chemical from a weed as a caterpillar.
• Plant defences: thorns (Acacia, cactus); chemicals (Calotropis cardiac glycosides; nicotine, caffeine, quinine, strychnine, opium).
• Competition: lowers $r$ of one species; occurs between unrelated species and even with abundant resources (interference).
• Evidence: Abingdon tortoise and goats; competitive release; Connell's Balanus and Chthamalus.
• Gause's competitive exclusion principle may hold when resources are limiting; resource partitioning (MacArthur's warblers) allows co-existence.
• Parasites: many are host-specific; they co-evolve, lose unnecessary sense organs and the gut, gain suckers and high reproduction; liver fluke needs a snail and a fish.
• Ectoparasites: lice, ticks, copepods, Cuscuta; endoparasites: simplified bodies, complex life cycles.
• Brood parasitism: cuckoo (koel) eggs in the crow's nest, resembling the host's eggs.
• Commensalism: orchid on mango, barnacles on whale, cattle egret and cattle, clown fish and anemone.
• Mutualism: lichens, mycorrhizae; fig and wasp (one-to-one); orchids and bees co-evolve, though Ophrys offers no reward and uses sexual deceit (pseudocopulation).

10. Solved Examples

Common Mistakes to Avoid