Ecology and Environment

1From organism to biosphere

Ecology asks its questions at six nested levels. An organism belongs to a population — all the individuals of one species in one area; populations of different species sharing that area make a community; the community together with the non-living surroundings, temperature and water and light and soil, is an ecosystem; ecosystems of one broad climatic type make a biome; and the sum of them all is the biosphere. The step that carries marks is community to ecosystem: adding the abiotic half is the whole difference.

At every level the abiotic factors set the terms, and an organism facing a hostile one has four options. It can regulate, holding its internal state steady whatever the outside does, as birds and mammals hold body temperature almost constant. It can conform, letting the internal state track the external, as most fish and invertebrates do. It can migrate away for the bad season. Or it can suspend — a seed, a bacterial spore, a hibernating bear, an aestivating snail. Regulation is the expensive option, which is why the great majority of animals are conformers.

Figure. Each level contains the one before it. Only the ecosystem box takes in the non-living surroundings — which is exactly why a community and an ecosystem are not the same thing, and why questions like to ask about that one boundary.

2What a population is made of

A population has properties no individual has: density, natality, mortality, sex ratio and age distribution. Over any interval its size obeys a balance sheet, N₁ = N₀ + (B + I) − (D + E), where B is births, D deaths, I immigrants and E emigrants. Only when B + I beats D + E does the population grow.

Natality and mortality are quoted per capita, per individual already present, so that populations of different sizes can be compared; their difference b − d is r, the intrinsic rate of natural increase. Notice what r leaves out — migration. A population whose births comfortably beat its deaths can still shrink, if enough individuals walk out.

The age distribution, drawn as an age pyramid, forecasts the interval after this one. A broad base of pre-reproductive individuals means growth is already committed whatever happens to this year's birth rate; a narrow base means decline is.

Figure. Two arrows in on the left, two out on the right. r is built from the natality and mortality pair alone, so a population can be losing individuals over the year while r stays comfortably positive.

3Exponential growth and doubling time

Give a population unlimited resources and every individual contributes the same per-capita surplus, so dN/dt = rN and N_t = N₀eʳᵗ. The graph is the J-shaped curve: not merely rising but steepening, because each year's increment is proportional to a number that is itself growing.

The most useful thing to pull out of it is the doubling time, t_d = ln 2 / r ≈ 0.693/r. Notice what is missing from that expression: the starting size. A population of a hundred and a population of ten thousand, growing at the same r, take exactly the same time to double.

Figure. Both curves climb for ever; only the pace differs. Halving r does not halve the population — it doubles the doubling time, so the two marked points sit at the same height and 9.2 years becomes 18.5.

4Logistic growth and carrying capacity

No habitat has unlimited resources, so the per-capita surplus is not a constant: it shrinks as the population fills the space. The Verhulst–Pearl logistic equation puts that shrinkage in one factor, dN/dt = rN(K − N)/K, with K the carrying capacity. While N is small the bracket is nearly 1 and growth is all but exponential; as N approaches K the bracket collapses towards zero and growth stops.

The result is the sigmoid, S-shaped curve, and the feature most asked about is not the plateau but the middle. The number of individuals added per unit time is greatest at N = K/2, where it equals rK/4 — small populations grow slowly because there are few breeders, nearly full ones because there is little left to grow into. It is also why a fishery is managed down to roughly half its carrying capacity rather than left untouched.

Figure. One population under two assumptions. The curves are indistinguishable at first; the dashed one keeps accelerating and runs off the top of the frame, while the solid one turns over at K/2 and settles under the ceiling without ever crossing it.

5Energy flow and the ten per cent law

Nutrients cycle; energy does not. Sunlight fixed by producers passes to herbivores, to carnivores, to top carnivores, and then out of the ecosystem as heat — one direction, no return, which is why an ecosystem needs a fresh delivery of sunlight every day while the atoms in it can be used over and over. Lindeman's rule of thumb is that roughly a tenth of the energy at one trophic level appears at the next. Measured efficiencies vary, but ten per cent is the figure NEET expects and the order of magnitude is right.

The other ninety per cent is not destroyed. Most is spent on respiration and leaves as heat, some departs unassimilated in faeces, and the rest is biomass that dies uneaten and enters the detritus food chain. Only what is both eaten and assimilated moves up.

Figure. Ten per cent moves right at each arrow; the ninety per cent heading downwards is respiration heat, faeces and uneaten biomass. Three transfers take 10 000 J down to 10 J, which is the real reason nothing lives above the fourth link.

6Ecological pyramids, and which ones invert

A pyramid stacks the trophic levels with the producers at the base and plots number, biomass or energy at each. The pyramid of energy can never be inverted: every transfer loses energy as heat, so each tier must be smaller than the one below it, and there is no exception anywhere.

Numbers and biomass carry no such guarantee, because they count standing crop rather than flow. One big tree is a single organism feeding thousands of insects, so the pyramid of numbers inverts. In the open sea the phytoplankton present at any instant weigh less than the zooplankton and fish they support, because they are eaten and replaced so fast, so the pyramid of biomass inverts. Both are entirely consistent with an upright pyramid of energy over the very same chain.

Pyramids also flatter an ecosystem in two ways worth remembering: they assume each species sits at exactly one trophic level, which a food web denies, and they leave the decomposers out altogether.

Figure. The same chain can give an upright pyramid of energy and an inverted pyramid of numbers or biomass, because the first measures a flow through time and the others measure a snapshot.

7Nutrient cycles: what comes back

Matter, unlike energy, is finite and therefore recycled. A biogeochemical cycle shuttles an element between a large reservoir and the living pool, and where the reservoir sits decides the cycle's character: carbon and nitrogen have atmospheric reservoirs and are called gaseous cycles, while phosphorus sits in rock and is sedimentary. That single fact is why phosphorus so often limits productivity — rock weathers slowly and there is no aerial shortcut.

Nitrogen shows the pattern most clearly. Nearly four-fifths of the atmosphere is N₂ and almost nothing can use it: the triple bond has to be broken — by nitrogen-fixing bacteria, by lightning, or industrially — before nitrogen enters biology at all.

Figure. Read the row left to right for the forward path, then the two long arrows for the returns: decay sends nitrogen back to ammonium, denitrification sends it all the way back to the air. Nitrification is drawn as one arrow but really runs via nitrite, as the steps say.

The nitrogen cycle, one turn

1. FixationFree-living and symbiotic bacteria — Rhizobium in root nodules, Azotobacter in soil — reduce atmospheric N₂ to ammonia.
2. NitrificationNitrosomonas oxidises ammonium to nitrite, then Nitrobacter takes the nitrite on to nitrate, the form roots take up most readily.
3. Assimilation and decayPlants build nitrate into amino acids and proteins; when they and their consumers die, decomposers return the nitrogen to the soil as ammonium.
4. DenitrificationIn waterlogged, oxygen-poor soil, bacteria such as Pseudomonas reduce nitrate back to N₂, and the atmosphere gets its nitrogen back.

8Biodiversity: measuring it and keeping it

Species richness is not spread evenly. It climbs steeply from the poles to the equator — the tropics have had a longer uninterrupted run without glaciation, more constant conditions and more solar energy — and it climbs with area, regularly enough to be written down.

Alexander von Humboldt's species–area relationship is a straight line on log–log axes: log S = log C + Z log A, with Z the slope. Within a region Z sits near 0.1 to 0.2 whatever the taxon, so doubling a species list takes roughly a hundredfold increase in area. Compare whole continents and Z climbs to 0.6–1.2, because you are then sampling separate evolutionary histories rather than a bigger slice of one.

What we protect follows from that. It is habitat area that is being lost, so conservation is first of all about keeping habitat intact. In-situ methods protect a species where it lives, with its whole community around it — national parks, wildlife sanctuaries, biosphere reserves, sacred groves, and the biodiversity hotspots that concentrate effort where endemism is highest, among them the Western Ghats, the Eastern Himalaya and Indo-Burma. Ex-situ methods take the organism out — zoos, botanical gardens, seed banks, cryopreserved gametes — and are the fallback for when the habitat has already gone.

Figure. Straight lines, because the relationship is a power law; both are drawn from a common intercept so that only the slope differs. The shallow line is what you meet inside a region — a hundredfold area for twice the species. The steep one appears only when whole continents are compared.