Ecology & Biodiversity in Depth

Every interaction between two species can be written as a pair of signs: + (benefits), − (is harmed) or 0 (unaffected). Once you write the signs, the names stop being a memory test.

Gause's competitive exclusion principle states that two species competing for exactly the same limiting resource cannot coexist indefinitely — the weaker competitor is eventually eliminated. Nature often escapes this through resource partitioning: Robert MacArthur showed that five closely related species of warblers living in the same tree avoided competition by feeding in different parts of it. Same address, slightly different profession.

A population has attributes that an individual cannot have: density, birth rate (natality), death rate (mortality), immigration, emigration, sex ratio and age structure. Births and immigration add to it; deaths and emigration subtract. Its age pyramid tells its future — a broad base means an expanding population, a bell shape a stable one, and a narrow base a declining one.

• r-selected species maximise the growth rate (r): many small offspring, little or no parental care, short life, early maturity — insects, weeds, many fish. They thrive in unstable, newly opened habitats.
• K-selected species live close to the carrying capacity (K): few large offspring, heavy parental care, long life, late maturity — elephants, great apes, whales. They suit stable environments but recover slowly from losses, which is exactly why so many of them are threatened.

Primary production is the amount of organic matter (biomass) made by producers per unit area over a period, usually expressed in g m⁻² yr⁻¹ or kcal m⁻² yr⁻¹. Gross primary productivity (GPP) is the total rate of photosynthesis. Producers burn part of it in their own respiration (R); what remains is net primary productivity: NPP = GPP − R. NPP is the only energy available to herbivores and decomposers. The rate at which consumers form new organic matter is secondary productivity.

Productivity per unit area is highest in tropical rainforests, coral reefs, estuaries and marshes, where warmth, light, water and nutrients all coincide, and lowest in deserts and the open ocean. The open ocean is a desert of nutrients rather than of water: sunlight reaches only the top layer while nutrients sink below it. That is why the oceans, though they cover about 70 per cent of the Earth's surface, account for only about a third of the biosphere's net primary productivity.

Some pollutants travel up a food chain instead of being broken down. Bioaccumulation is the build-up of a substance inside one organism over its lifetime, because it takes the substance in faster than it can excrete it. Biomagnification is the rise in concentration of that substance at each successive trophic level, because every predator eats many contaminated prey. The substances that behave like this share three properties: they are persistent (not easily degraded), fat-soluble (stored in fatty tissue) and not readily excreted — DDT and other organochlorine pesticides, mercury (as methylmercury) and some PCBs.

Carbon and nitrogen cycles are gaseous: their main reservoir is the atmosphere, so they move fast and self-correct quickly. Sedimentary cycles keep their reservoir in rocks and sediments and move on geological time — which is why a nutrient lost from a sedimentary cycle (washed into the deep sea, for instance) is effectively lost for a very long time.

Atmospheric nitrogen (N₂) has a triple bond that very few organisms can break. Those that can, do it with the enzyme nitrogenase, which works only in the near-absence of oxygen. In legume root nodules, a pink pigment, leghaemoglobin, mops up oxygen to protect it. Learn the fixers by where they live:

The whole sequence of communities that replace one another at a site is a sere, and each stage is a seral stage. Succession driven by the organisms themselves — each community changing the soil, shade and moisture so that the next can move in — is autogenic; succession driven by outside forces such as fire, flooding or a changing climate is allogenic. Frederic Clements saw every region heading towards a single climax set by its climate (monoclimax); Arthur Tansley argued that soil, drainage and disturbance can hold different patches at different climaxes (polyclimax).

• Total biomass and organic matter increase as succession proceeds; soil gets deeper and richer in humus.
• Species diversity generally rises, niches become narrower and more specialised, and food chains turn into complex webs.
• In the early stages production outruns respiration, so biomass accumulates; at climax the two come close to balance, and the community stops growing in mass.

A biome is a large natural region defined by its climate — chiefly temperature and rainfall — and the dominant vegetation that climate allows. Read the table across: once you know the climate, the vegetation is predictable, and the animals follow the vegetation.

Aquatic ecosystems are classified by salinity and movement. Freshwater is lentic (still — ponds, lakes) or lotic (flowing — streams, rivers). A deep temperate lake stratifies in summer into a warm upper epilimnion, a lower cold hypolimnion, and between them a zone of rapid temperature change called the thermocline. Aquatic life is grouped by how it lives: plankton drift, nekton swim actively against currents, and benthos live on or in the bottom. In the sea, the sunlit photic zone is where photosynthesis happens; below it lies the dark aphotic zone.

• Estuaries — where a river meets the sea and fresh water mixes with salt; brackish, nutrient-rich, among the most productive ecosystems, and nurseries for many fish and prawns.
• Mangroves — salt-tolerant trees (halophytes) of tropical tidal coasts. They breathe through upward-growing roots called pneumatophores, many show vivipary (the seed germinates while still on the parent tree), and they shield coasts from storm surges. The Sundarbans, shared by India and Bangladesh, is the largest mangrove forest in the world.
• Coral reefs — built by colonies of tiny coral polyps that secrete calcium carbonate. Reef-building corals live in symbiosis with algae called zooxanthellae, which photosynthesise inside the coral tissue and supply most of its food — so corals need warm (roughly above 20 °C), clear, shallow, sunlit, saline water low in sediment. Reefs are fringing (attached to the shore), barrier (separated from it by a lagoon — the Great Barrier Reef of Australia) or atolls (a ring of reef around a lagoon — the Lakshadweep islands). India's reefs lie in the Gulf of Mannar, the Gulf of Kachchh, Lakshadweep and the Andaman and Nicobar Islands.

Counting species is only half of diversity. Species richness is the number of species; evenness is how equally individuals are spread among them. A forest with ten species where one makes up 95% of the trees is less diverse than one where all ten are equally common. Ecologists also measure diversity at three spatial scales, a scheme introduced by Robert Whittaker:

• Latitudinal gradient — diversity generally falls from the equator towards the poles. The usual explanations: the tropics were not repeatedly glaciated, so species had a long, undisturbed time to diversify; the tropical environment is less seasonal and more predictable, which favours niche specialisation; and more solar energy means higher productivity.
• Species–area relationship — Alexander von Humboldt observed that within a region, species richness rises with the area explored, but only up to a limit. On a log scale the relation is a straight line: log S = log C + Z log A, where S is species richness, A is area, C is the intercept and Z the slope. For smaller areas Z typically lies between 0.1 and 0.2, almost regardless of the group of organisms or the region; across very large areas such as whole continents, the slope is much steeper.
• Why diversity matters to function — Paul Ehrlich's rivet popper hypothesis compares an ecosystem to an aircraft and species to its rivets: losing a few may not matter, but every lost rivet weakens the whole, and losing a critical one can bring it down. David Tilman's long-term field plots found that plots with more species showed less year-to-year variation in total biomass — diversity buys stability.

India's best-known invasive alien species came from the Americas: Lantana camara, a thorny shrub that now smothers the understorey of many forests; Parthenium hysterophorus ("congress grass"), a weed believed to have arrived with imported food grain in the 1950s and a cause of allergies; Eichhornia crassipes (water hyacinth), which chokes ponds and rivers and deoxygenates them; and Prosopis juliflora (mesquite, vilayati babool), planted to check desertification and now crowding out native grassland. The illegal introduction of the African catfish (Clarias gariepinus) for aquaculture threatens native fish in Indian rivers.

Norman Myers proposed the hotspot idea in 1988. Conservation International later fixed two quantitative tests: a region must hold at least 1,500 species of vascular plants found nowhere else (endemics), and it must have lost at least 70% of its original natural vegetation. Richness alone is not enough; a hotspot is rich and under threat. There are 36 hotspots worldwide (the North American Coastal Plain was the latest, added in 2016), and four of them extend into India.

Megadiversity is a different, country-level idea. Conservation International identified 17 megadiverse countries — India among them — which together hold the large majority of the world's species. In 2002 the Cancún Declaration set up the Group of Like-Minded Megadiverse Countries, of which India was a founding member, to press for the rights of countries of origin over their genetic resources.

For planning protected areas, the Wildlife Institute of India (W.A. Rodgers and H.S. Panwar) divided the country into ten biogeographic zones — regions that differ in their geological history, climate and the species they hold. The Deccan Peninsula is by far the largest; the Trans-Himalaya is a cold desert; and the Western Ghats, though small, are among the richest.

Endemics worth knowing, with the place that defines them: the Asiatic lion survives in the wild only in the Gir landscape of Gujarat; the sangai (brow-antlered deer) lives on the floating vegetation mats (phumdis) of Loktak Lake in Manipur; the Narcondam hornbill is confined to Narcondam Island in the Andamans; the hangul (Kashmir stag) is the deer of Dachigam in Kashmir; and the lion-tailed macaque and Nilgiri tahr belong to the Western Ghats.

The International Union for Conservation of Nature (IUCN), founded in 1948, is an unusual body: its members are both governments and non-governmental organisations, and its headquarters is at Gland, Switzerland. Its Red List of Threatened Species, begun in 1964, is the world's most widely used assessment of extinction risk. A species is placed in a category using five quantitative criteria — (A) a reduction in population size, (B) a small or shrinking geographic range, (C) a small and declining population, (D) a very small or very restricted population, and (E) a quantitative analysis of the probability of extinction. Meeting any one criterion is enough.