Organisms and Populations

Ecology is the study of the interactions between organisms and their surroundings, both the living and the non-living parts of those surroundings. Ecology is basically concerned with four levels of biological organisation: the organism, the population, the community and the biome. This chapter takes the first two. At the organismic level, ecology is essentially physiological ecology: it asks how an individual survives and reproduces in the physical conditions it is handed. The physical place where an organism lives is its habitat, and the particular set of conditions and resources it actually uses within that habitat is its niche. Two species may share a habitat and still occupy different niches.

Why do different parts of the world hold such different sets of organisms? The earth spins on a tilted axis while it revolves around the sun, and the tilt is what gives us seasons. The result is a regular pattern of variation in temperature and precipitation across the globe and across the year. These patterns produce the major biomes such as desert, rainforest and tundra. Within any one biome, regional and local differences in rock, slope, drainage and shade break the landscape up into many smaller habitats.

Temperature is the most ecologically relevant abiotic factor. Average temperature falls as you move from the equator towards the poles, and as you climb from the plains to a mountain top. It ranges from subzero levels in polar regions and at high altitude to more than 50 degrees Celsius in tropical deserts in summer. There are also unusual habitats such as thermal springs and deep-sea hydrothermal vents where average temperatures cross 100 degrees Celsius. Temperature matters because it controls the rate at which enzymes work, and therefore the whole basal metabolism of the organism. Species that can tolerate a wide range of temperatures are called eurythermal; those restricted to a narrow range are stenothermal. This is why a mango tree does not grow in cold temperate regions, and why some species simply cannot cross a mountain range.

Water is next in importance. Life originated in water and no organism can do without it. On land, the productivity and distribution of plants depends heavily on how much water is available. For aquatic organisms the crucial thing is not the amount of water but its quality, especially its salinity. Salinity is measured in parts per thousand: it is less than 5 in inland waters, 30 to 35 in the sea, and more than 100 in hypersaline lagoons. Some organisms are euryhaline and tolerate a wide range of salinities; others are stenohaline and are restricted to a narrow range. Freshwater animals cannot survive for long in sea water, and marine animals cannot survive in fresh water, because of the osmotic problems each would face.

Light matters because plants need it to photosynthesise. Small herbs and shrubs on a forest floor are overshadowed by tall canopy trees, so they are adapted to photosynthesise efficiently under low light. Many plants also depend on sunlight to meet their photoperiodic requirement for flowering. Animals use light cues for foraging, for reproduction and for the timing of migration. Deep in the sea, sunlight does not reach at all, and organisms there live in permanent darkness. The ultraviolet part of the spectrum, on the other hand, is harmful to many organisms.

Soil varies in nature from place to place because of differences in climate, in the weathering process, in whether the soil is transported or formed in place, and in how soil development proceeded. The composition of the soil, the grain size and the degree of aggregation together determine how easily water percolates through it and how much water it can hold. These properties, along with soil pH, mineral composition and topography, largely decide what vegetation can grow there, and the vegetation in turn decides which animals the place can support. In aquatic habitats, the characteristics of the sediment at the bottom often decide which bottom-dwelling animals can live there.

The environment around an organism keeps changing, but the chemistry inside its body works best within a narrow band of temperature, water content and salt concentration. Keeping that internal environment steady in spite of outside change is called homeostasis. Every successful organism must manage it somehow, and there are four ways of doing so.

Regulate. Some organisms hold their internal conditions constant by physiological and sometimes behavioural means. All birds and mammals, and a very small number of lower vertebrates and invertebrates, can regulate their body temperature. Human beings hold a body temperature close to 37 degrees Celsius whether the day is hot or cold. In summer we sweat profusely, and the evaporation of that sweat cools the body down. In winter we shiver, and the involuntary muscle activity of shivering generates heat that raises body temperature. Plants have no such mechanism for regulating temperature.

Conform. An overwhelming majority of animals, about 99 per cent, and nearly all plants cannot maintain a constant internal environment. Their body temperature changes along with the ambient temperature, and in aquatic animals the osmotic concentration of the body fluids changes along with that of the surrounding water. Such organisms are called conformers. It is fair to ask why thermoregulation did not evolve in all animals, since it is obviously useful. The answer is cost. Heat loss and heat gain both depend on surface area. Small animals have a large surface area relative to their volume, so they lose body heat very quickly when it is cold outside, and would then have to burn a great deal of energy in metabolism just to make that heat back. For a small animal the bill is simply too high, and that is the main reason very small animals are rarely found in polar regions. Some species regulate only over a limited range of conditions and simply conform beyond that range.

Migrate. Instead of fighting the stress, the organism can leave. Migration means moving away temporarily from the stressful habitat to a more hospitable area and returning when the stressful period is over. Every winter, large numbers of migratory birds travel thousands of kilometres from colder northern regions to the wetlands of India, and fly back when spring returns.

Suspend. The fourth option is to shut down and wait. In bacteria, fungi and lower plants, thick-walled spores are formed which help them survive unfavourable conditions; these germinate when a suitable environment returns. In higher plants, seeds and certain other vegetative reproductive structures do the same job, tiding the plant over a stressful period while also helping it disperse. They do this by sharply reducing their metabolic activity and entering a state of dormancy, and they germinate when moisture and temperature become favourable again. Animals have their own versions of this. Bears go into hibernation during winter. Some snails and fish go into aestivation to escape summer heat and the drying out that comes with it. Under unfavourable conditions, many zooplankton species in lakes and ponds enter diapause, which is a stage of suspended development.

Keep the three suspension terms straight, because they are easy to confuse. Hibernation is winter dormancy. Aestivation is summer dormancy, taken up to avoid heat and desiccation. Diapause is not tied to a season by definition at all; it is a suspended stage of development, and the standard example is the zooplankton of lakes and ponds.

An adaptation is any attribute of an organism, whether it is a structural feature, a physiological process or a piece of behaviour, that enables the organism to survive and reproduce in its habitat. Most adaptations are genetically fixed and have evolved over many generations, though a few responses are short-term adjustments made within an individual's lifetime.

The great problem in a desert is water. The kangaroo rat of the North American deserts never drinks water. It meets all its water requirement through the internal oxidation of fat, a process in which water is produced as a by-product. It also has the ability to concentrate its urine strongly, so that only a minimal volume of water is spent in flushing out excretory products. Read those two halves as one strategy: make water from food, and lose as little of it as possible.

Desert plants face the same problem from the other side, because they must open their stomata to take in carbon dioxide and lose water whenever they do. They have a thick cuticle on the leaf surface and their stomata are arranged in deep pits, both of which cut water loss by transpiration. They also use a special photosynthetic pathway called the CAM pathway, which allows their stomata to remain closed during the daytime, when evaporation would be worst. Some desert plants, such as Opuntia, have no leaves at all; the leaves are reduced to spines, and the job of photosynthesis has been taken over by the flattened green stems.

In cold climates the problem reverses: the organism must hold on to heat rather than water. Allen's Rule states that mammals from colder climates generally have shorter ears and limbs, which minimises heat loss. The logic is the same surface-area argument as before, but applied to the projecting parts of the body: long ears, long tails and long limbs offer a lot of surface for the heat inside to escape through, so animals of cold regions tend to have them short and stubby. Be careful not to state Allen's Rule as a claim about overall body size; it is specifically about the ears and limbs. A second cold-climate adaptation is insulation: aquatic mammals of the polar seas, such as seals, carry a thick layer of fat called blubber below the skin, which acts as an insulator and cuts the loss of body heat.

At high altitude the problem is oxygen. The atmospheric pressure is low there, so the body does not get enough oxygen, and a person who goes up quickly may feel altitude sickness, with symptoms such as nausea, fatigue and heart palpitations. Over a period of time the body adjusts, or acclimatises, by increasing the production of red blood cells, by decreasing the binding affinity of haemoglobin for oxygen so that oxygen is released more readily to the tissues, and by increasing the breathing rate. Note the direction of the haemoglobin change carefully: the affinity goes down, not up, because a looser grip on oxygen is what lets the tissues get it.

Some organisms show biochemical adaptations that let them live where almost nothing else can, such as the archaebacteria that thrive in hot springs and deep-sea hydrothermal vents where the temperature exceeds 100 degrees Celsius. Others solve the problem by behaviour. Desert lizards do not have the physiological ability of mammals to deal with high temperature, yet they manage to keep their body temperature fairly constant. When their body temperature drops below the comfort zone they bask in the sun and absorb heat; when the surrounding temperature starts rising they move into the shade; and some species burrow into the soil to escape the heat above ground.

A population is a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources, and capable of interbreeding. Population ecology is the level at which ecology connects directly to population genetics and evolution, because natural selection acts through differences in birth and death among individuals of a population.

The central idea is that a population has attributes that an individual does not. An individual is born, and an individual dies, but only a population has a birth rate and a death rate. These rates are expressed per capita, that is, per individual of the population. Likewise, an individual is either male or female, but only a population has a sex ratio. And an individual has an age, but only a population has an age distribution.

Density. Population size, more technically called population density and written as N, is the number of individuals per unit area or per unit volume. But a raw count is not always the most useful figure. Imagine a plot with 200 Parthenium plants and another with a single huge banyan tree: by headcount the banyan plot scores 1, though its effect on the community is enormous. In such cases percent cover or biomass is a more meaningful measure of density. In other situations the total number is difficult or simply unnecessary to obtain, and a relative density serves the purpose, for example the number of fish caught per trap, which tells us about fish density in a lake without counting every fish. Sometimes density is estimated indirectly without ever seeing the animal, as in the tiger census in national parks and tiger reserves, which is often based on pug marks and faecal pellets.

Birth rate and death rate. Take a worked example for each. If a pond had 20 lotus plants last year and reproduction added 8 new plants, taking the current population to 28, then the birth rate is 8 divided by 20, which is 0.4 offspring per lotus per year. Notice that the denominator is the population you started with, not the new total. For death rate, if 4 individuals died in a laboratory population of 40 fruit flies during one week, the death rate is 4 divided by 40, which is 0.1 individuals per fruit fly per week. Both rates are per capita rates and both must carry a time unit.

Sex ratio. This is simply the proportion of males and females in the population, usually stated as a percentage, for example a population in which 60 per cent of the individuals are female and 40 per cent are male.

Age pyramids. A population at any moment contains individuals of many different ages. If you plot the percentage of individuals falling in each age group, the resulting figure is called an age pyramid. For human populations the pyramid usually shows males and females side by side in a single figure. Read it as a stack of horizontal bars: the bottom bar is the pre-reproductive age group, the middle bar is the reproductive group and the top bar is the post-reproductive group, and the length of a bar is proportional to the number of individuals in that group. Now the three shapes make sense without any picture. In an expanding population the bottom bar is much longer than the one above it and each bar above is shorter still, so the outline is a broad-based triangle: there is a large crop of young individuals still to reach reproductive age, so the population will grow. In a stable population the pre-reproductive and reproductive bars are of roughly equal length and only the top tapers, giving a bell-shaped outline, and the population size will stay about the same. In a declining population the bottom bar is shorter than the middle one, so the figure is pinched in at the base and looks like an urn: fewer young are coming up than there are adults, and the population will shrink.

Population density in a habitat is not fixed; it goes up and down with food availability, predation pressure and weather. Whatever the cause, only four processes can change it. Natality, written B, is the number of births during a given period that are added to the initial density. Mortality, written D, is the number of deaths during that period. Immigration, written I, is the number of individuals of the same species that have come into the habitat from elsewhere during the period. Emigration, written E, is the number that have left the habitat and gone elsewhere during the period. Putting them together:

N(t+1) = N(t) + [(B + I) - (D + E)]

Here N(t) is the population density at time t and N(t+1) is the density one time unit later. The population grows if B plus I is greater than D plus E. Under normal conditions births and deaths are the most important of the four; immigration and emigration matter only in special situations, for example when a new habitat is just being colonised, where immigration may contribute more to growth than births do.

Exponential growth. Suppose food and space are unlimited. Then every species has the innate potential to grow in number in an exponential, or geometric, way. If b is the per capita birth rate and d is the per capita death rate in a population of size N, then the rate of change of N with time is

dN/dt = (b - d) N

Writing r for the quantity (b - d), this becomes

dN/dt = rN

The r here is called the intrinsic rate of natural increase. It is a very important parameter, because ecologists use it to judge the impact of any biotic or abiotic factor on population growth. Its value is characteristic of the species and the conditions: for the Norway rat r is 0.015, for the flour beetle it is 0.12, and for the human population of India in 1981 it was 0.0205. A positive r means the population is growing, r equal to zero means it is stationary, and a negative r means it is shrinking.

The same relationship in integral form is

N(t) = N(0) ert

where N(t) is the population density after time t, N(0) is the population density at time zero, r is the intrinsic rate of natural increase, e is the base of natural logarithms and equals 2.71828, and t is time. If you plot N against t for a population growing this way you get a curve shaped like the letter J, rising slowly at first and then climbing almost vertically. Any species growing exponentially under unlimited resources can reach enormous densities in a short time, which tells you at once that the model cannot describe nature for long.

Logistic growth. No population in nature has unlimited resources at its disposal. Competition between individuals for the limited resources follows, and the fitter individuals survive and reproduce. A given habitat has enough resources to support only a maximum possible number of individuals, beyond which no further growth is possible. That limit is nature's carrying capacity, written K, for that species in that habitat. A population growing in a habitat with limited resources first shows a lag phase, then a phase of acceleration, then a phase of deceleration, and finally an asymptote, where the density has reached the carrying capacity and stops rising. Plotting N against t now gives an S-shaped, or sigmoid, curve. This is called the Verhulst-Pearl logistic growth and is described by

dN/dt = rN [(K - N) / K]

where N is the population density at time t, r is the intrinsic rate of natural increase and K is the carrying capacity of that habitat for that species.

The whole behaviour of the logistic model sits in the bracket (K - N)/K, which is the fraction of the carrying capacity still unused. When N is very small compared with K, that fraction is close to 1, the equation reduces to something very near dN/dt = rN, and growth looks exponential. As N climbs towards K, the fraction shrinks towards zero and growth slows down. When N equals K, the fraction is exactly zero, dN/dt is zero, and the population stops changing in size. If N were somehow pushed above K, the fraction would be negative, dN/dt would be negative, and the population would fall back towards K. Since resources for most populations become limiting sooner or later, the logistic model is considered the more realistic of the two.

Populations evolve to maximise their reproductive fitness, also called Darwinian fitness, in the habitat they live in. Fitness here means a high r value, that is, a high per capita rate of increase. Under a particular set of selection pressures, organisms evolve towards the reproductive strategy that is most efficient in that habitat, and the strategies differ enormously from species to species.

One axis of difference is how often an organism breeds. Some organisms breed only once in their lifetime, such as the Pacific salmon fish and bamboo, both of which put everything they have into a single massive reproductive effort. Others breed many times over their lifetime, as most birds and mammals do. Ecologists call the first pattern semelparity and the second iteroparity.

A second axis is the number and size of the offspring. Some species produce a very large number of small-sized offspring, as oysters and pelagic fishes do. Others produce a small number of large-sized offspring, as birds and mammals do. The two axes are usually linked, because the total energy an organism can gather in a lifetime is finite. Energy that goes into one clutch cannot also go into growth, into the parent's own survival, or into a later clutch. So there is a genuine trade-off: many tiny offspring with almost no parental investment and a low chance of individual survival at one end, and a few large, well-provisioned offspring with a much higher chance of individual survival at the other.

Neither strategy is better in the abstract; each is the better bet in a particular kind of habitat. In a habitat that is unpredictable and where whole cohorts are frequently wiped out, flooding the environment with vast numbers of offspring quickly is what pays. In a stable, crowded habitat where competition for every resource is intense, a small number of well-equipped offspring that can hold their own will do better. Ecologists still work on the general question of why different organisms behave so differently, and the answer, as far as it goes, lies in the constraints imposed by the abiotic and biotic components of the habitat that a species evolved in.

Watch one common confusion here. A high r value does not mean a large body or a long life. It means a high per capita rate of increase, and by that measure bacteria and insects are far ahead of elephants and whales. Life history traits such as the age at first reproduction, the number of broods per lifetime, the number and size of offspring, and lifespan itself are shaped by natural selection in exactly the same way as any structural feature of the body.

No organism can live alone. Even a plant that makes its own food needs soil microbes to break down organic matter and return nutrients to it, and often needs an animal to move its pollen. Interactions between the populations of two different species are called interspecific interactions, and each interaction can be beneficial, marked plus, detrimental, marked minus, or neutral, marked zero, for each of the two partners. The six named interactions and their sign pairs are these. Mutualism is plus for both species. Competition is minus for both. Predation is plus for the predator and minus for the prey. Parasitism is plus for the parasite and minus for the host. Commensalism is plus for one species and zero for the other. Amensalism is minus for one species and zero for the other.

Notice that predation and parasitism carry exactly the same sign pair, plus and minus. Signs alone will not separate them, so learn the mechanism: a predator normally kills its prey outright and eats it, and is usually the larger of the two; a parasite lives on or in its host over a long period, draws nutrition from it, is usually much smaller, and does not normally kill the host at once. Notice also that commensalism and amensalism are the two interactions with a zero in them, and they differ only in whether the affected species gains or is harmed.

Predation. Predation is nature's way of transferring the energy fixed by plants to higher trophic levels, so predators act as conduits for energy transfer across those levels. In this broad sense a herbivore eating a plant is a predator too. Predators keep prey populations under control. The prickly pear cactus introduced into Australia in the early 1920s spread over millions of hectares of rangeland until it was finally brought under control by introducing a cactus-feeding moth from its natural habitat. All biological control of agricultural pests works on this principle. Predators also help maintain species diversity in a community by reducing the intensity of competition among competing prey species. In a well-known field experiment on the rocky intertidal coast of the American Pacific, all the starfish Pisaster, an important predator there, were removed from an enclosed area; within a year more than 10 species of invertebrates had disappeared from it because of interspecific competition. The other side of the coin is that a predator which is too efficient may overexploit its prey to extinction, and then go extinct itself for want of food.

Prey species are not passive. Many use cryptic colouration, or camouflage, so that they are not easily spotted. Others are distasteful or poisonous. The Monarch butterfly is highly distasteful to its predator because of a special chemical in its body, and it acquires that chemical during its caterpillar stage by feeding on a poisonous weed. Plants defend themselves against herbivores with thorns, as in Acacia and cactus, and with chemicals. Calotropis produces highly poisonous cardiac glycosides, which is why cattle and goats are never seen browsing on it. Nicotine, caffeine, quinine, strychnine and opium are all produced by plants as chemical defences against grazers and browsers.

Competition. Competition is usually pictured between closely related species fighting over the same limiting resource, but totally unrelated species can compete as well: in some shallow South American lakes, visiting flamingoes and resident fishes compete for the same food, the zooplankton. Competition need not always be about a scarce resource either. The feeding efficiency of one species may be reduced simply by the interfering and inhibitory presence of another even when resources are abundant, and that is called interference competition. The clearest evidence for competition is that removing one species allows the other to increase in distribution and abundance. Connell's field experiments on rocky sea coasts showed that the larger and competitively superior barnacle Balanus dominates the intertidal zone and excludes the smaller barnacle Chthamalus from it. On one of the Galapagos islands, the Abingdon tortoise became extinct within a decade of goats being introduced, apparently because the goats browsed more efficiently.

Gause's competitive exclusion principle states that two closely related species competing for the same resources cannot coexist indefinitely, and that the competitively inferior one will eventually be eliminated. The important qualification is that this holds only when the resource is limiting. Species can also evolve their way around competition. MacArthur showed that five species of warblers living on the same tree were able to coexist because of behavioural differences in their foraging activities, each working a different part of the tree in a different way. That mechanism is called resource partitioning.

Parasitism. Parasites are usually host-specific, and parasite and host tend to co-evolve so that the parasite becomes ever better at exploiting that particular host. Parasites show a characteristic set of adaptations: loss of unnecessary sense organs, the presence of adhesive organs or suckers for clinging to the host, loss of the digestive system, and a very high reproductive capacity. Many life cycles pass through one or two intermediate hosts or vectors; the human liver fluke, for example, depends on two intermediate hosts, a snail and a fish, to complete its life cycle. Parasites reduce the survival, growth and reproduction of the host, lower its population density, and may leave it more vulnerable to predation. Ectoparasites feed on the external surface of the host, as lice do on humans, ticks on dogs and copepods on marine fish; the plant parasite Cuscuta, which grows on hedge plants, has lost its chlorophyll and its leaves and draws all its nutrition from the host. Endoparasites live inside the host at various sites; their life cycles are more complex, their form and internal structure greatly simplified, and their reproductive potential greatly enhanced. A special case is brood parasitism: the cuckoo lays its eggs in the nest of a crow, and the cuckoo's eggs have evolved to resemble the host's eggs in size and colour closely enough that the host does not throw them out.

Commensalism, plus and zero, has several standard examples. An orchid grows as an epiphyte on a mango branch, gaining a place in the light while the mango tree is unaffected. Barnacles grow attached to the back of a whale, which gains and loses nothing. The cattle egret forages beside grazing cattle, catching the insects that the moving cattle stir up, without affecting the cattle at all. The clown fish lives among the stinging tentacles of a sea anemone and gains protection from predators, while the anemone appears to derive no benefit from hosting it.

Mutualism, plus and plus, includes some of the most intimate associations known. A lichen is a close mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. Mycorrhizae are associations between fungi and the roots of higher plants, where the fungus helps the plant absorb essential nutrients from the soil and the plant supplies the fungus with carbohydrates. Plant and animal mutualisms are everywhere: plants offer nectar, pollen or edible fruit and in return animals pollinate their flowers or disperse their seeds. The fig and its wasp are the classic one-to-one case. A given species of fig can be pollinated only by its partner species of wasp. The female wasp uses the fig not only as a site to lay her eggs but as a nursery, since her larvae feed on some of the developing seeds inside it, and while she moves about searching for suitable egg-laying sites she pollinates the fig inflorescence. Each partner gets what it cannot get elsewhere.

Amensalism, minus and zero, is the interaction in which one species is harmed while the other is neither helped nor harmed. The example usually given is a mould that releases a chemical into its surroundings which inhibits or kills the bacteria growing nearby, while the mould itself is unaffected by the presence or absence of those bacteria.