No organism stands alone; its genes and their resulting phenotype are all interacting with the environment. Ecology – meaning the ‘study of the home’ in Greek – is the scientific field that concerns the impact of those interactions. Ecologists are not simply biologists; they must also take into account aspects of geology, oceanography and climate science.
As if that were not enough, the real-life study of ecology is also exceedingly complex, if not impossible. Therefore, ecologists form models of wildlife communities. This could be for a specific habitat, or on a global scale – as espoused by the Gaia Hypothesis (which looks at the way that the Earth might function as a single self-regulating ecological system). The goal of the ecological models is to understand the factors that influence the success or failure of wildlife communities – and crucially to predict how they would be affected by human activities, such as habitat destruction and pollution.
The term ecosystem is perhaps familiar but can be somewhat nebulous. Scientifically, an ecosystem is a way of describing a community of wildlife living in a particular habitat. However, in the real world, such ecosystems do not really exist in a meaningful sense, since there is no clear boundary between one community and the next. Nevertheless, the concept of an ecosystem is a good way of understanding the ecological factors at play in a certain habitat.
Every ecosystem is characterized by its ecological factors. These can be either biotic (living) or abiotic (non-living). Biotic factors are the interactions between different species present in the habitat, as they compete for food and resources, or prey and parasitize one another. Abiotic factors concern things such as soil chemistry, water supply and changing weather conditions. Ecologists model how an ecosystem would respond to changes in one of these factors – or the addition of a new one.
Every species occupies an ecological niche within an ecosystem. A niche represents the opportunity for a life form to exploit the resources available in a habitat. As a result, natural selection adapts a species to survive in that niche by using a unique collection of anatomical features and behaviours. Speciation is driven by the presence of empty ecological niches, appearing either due to changes in abiotic factors (such as climate change), or when a population finds its way to a new habitat yet to be fully exploited by similar life.
The best known example of how organisms fill a niche are Darwin’s finches. These birds (actually tanagers, not finches) live on the Galápagos Islands, and were seen by Darwin during his visit aboard the Beagle. The birds all descend from a common ancestor that arrived from South America, but have since evolved to occupy different niches in the islands’ ecosystems. This is illustrated by bill shapes adapted to exploit a particular food supply, from insects to ripe seeds and fallen fruits.
Put simply, a habitat is the place where an organism lives. One could list dozens of distinct types of habitat and increase that list innumerably by making more specific descriptions of each one. Nevertheless, in general terms a habitat is an area where ecologists can construct a meaningful ecosystem. It could be a coral reef, a grassland or a tropical forest. There are common features within all these habitats irrespective of where they appear on Earth – and also commonalities among the organisms that comprise their wildlife.
Habitats are not constant. When a tree falls in a forest, for example, an empty gap is created. Organisms race to occupy this gap, with fast-growing plants arriving first, and then being gradually replaced by a succession of larger plants that are able to slowly but surely take control of the available resources. Eventually, the gap is filled completely and the habitat returns to its stable, or climax, state.
The largest unit in ecology is the ‘biome’. Each biome is a broad grouping of habitats that exist around the world. The number of possible biomes varies depending on scientific opinion, but this list is a good starting point: aquatic, forest, grassland and desert. Adding in the effect of different climates around the world, the list can be extended: deserts all have very low levels of liquid water, but can be split into hot deserts, semideserts and polar regions. Forests appear in areas of high rainfall and divide into tropical, temperate (home to deciduous trees) and boreal (conifer). Grasslands are places where the climate is too dry for trees to grow but not so dry that they are desert. They can be subdivided into three biomes: savannah, steppe or prairie, and tundra. Lastly, aquatic habitats can be divided between the saltwater marine biome and freshwater lakes and rivers. There are other ways of listing and defining biomes, but each results in a means to divide up the surface of the planet into large-scale regions for which the broad sweep of ecological factors is the same.
One of the most dominant factors within an ecosystem is what organisms eat. Relationships can be made into a food chain, or more pertinently, a food web, where many organisms are a possible meal for more than one other species. Food chains contain all the organisms in the ecosystem, and an almost universal feature is that the first point in the chain is a photosynthetic organism, such as a plant. These organisms are called primary producers because they collect energy from an abiotic source (sunlight) and make it available as a biotic resource. All other organisms in the chain are known as consumers. Herbivores, which eat only plant material, are called primary consumers. In turn they are eaten by secondary consumers and so on. Many secondary consumers are likely to be omnivores, meaning they eat both plant and animal foods. Tertiary consumers are almost certainly carnivores, restricted to a meat diet. Further up the chain we reach detritivores, such as dung beetles or fungi, which consume the waste and remains of other organisms.
A food chain illustrates routes taken by nutrients and energy through an ecosystem. The nutrients follow cycles, collected from the surroundings by plants, passed through the consumers and finally returned to the environment by detritivores. Energy does not work in the same way, but enters the system via primary producers and is then steadily lost as it moves along the chain. This is where the concept of trophic levels arises.
‘Trophic’ derives from the Greek word for ‘feeder’, and every step in a food chain moves up a trophic level. When all the trophic levels are presented according to the amount of energy – or more simply, by their biomass, or weight of living material – the food chain forms a pyramid. This is because only about 10 per cent of the energy from one level is passed to the next one up. As a result, the mass of plant material is far greater (in land ecosystems, at least) than the mass of animal material. This also explains why only a few large predators can survive in an ecosystem – hardly any energy makes it up to their niche.
There are many ways of being alive, and millions of distinct species that each do it in a unique way. However, they all split into two neat groups: autotrophs and heterotrophs. Primary producers belong in the first group – the term autotroph means ‘self feeder’, and refers to organisms that can harness a non-living sources of energy to power their bodies.
The most obvious examples are photosynthetic plants, seaweeds, and a whole host of microscopic organisms, including many bacteria. These are phototrophs, or ‘light feeders’. However, some microbial autotrophs are chemotrophs, using chemicals as a supply of energy (see here). What an autotroph can do, and a heterotroph cannot, is ‘fix carbon’, taking inorganic forms of the element – chiefly carbon dioxide – and converting them into organic sugars. They do this using a chemical process called reduction – the exact opposite of the oxidation that releases metabolic energy during respiration.
The word ‘heterotroph’ means ‘other feeder’ – a reference to the way that heterotrophs are unable to fix carbon by themselves. Instead, they must get all their sugars and other raw materials by consuming the bodies of other organisms. All animals and fungi are heterotrophs, and many microorganisms live this way as well, including amoeba and protozoa. (Euglena, a kind of single-celled flagellate, is able to photosynthesize and consume at the same time.)
Heterotrophs rely entirely on autotrophs for their survival; even though a lion never eats any green vegetables it only survives by eating gazelles that have done. Consumers are not limited to predators and plant-eating prey. The simplest animals, the placozoa, are little more than mobile mats of cells, absorbing whatever organic particles touch the body. Meanwhile fungi (which include the largest organisms on Earth, sometimes spreading across 10 square kilometres of soil) exude digestive enzymes directly onto their foods and then absorb the results.
Until the late 1970s, it was assumed that all food chains began with phototrophic producers harvesting the energy from sunlight. Then, deep-sea submersibles discovered the hydrothermal vents known as ‘black smokers’. These volcanic outlets release chemical-rich water on ocean floors far too deep for sunlight to reach. Yet despite the darkness, black smokers harbour an amazing ecosystem of giant worms, shellfish and crabs. The producers at the root of these food chains are prokaryotic (bacterial and archaean) chemotrophs.
Chemotrophs use the chemical energy of minerals in the vent water (or other volcanic sources, even inside rocks) to ‘fix’ carbon – some biologists believe they mght have been the first forms of life on Earth. The larger animals have evolved to live in harmony with these chemotrophs: some filter bacteria from the water, while the enormous tube worms host bacteria in their body tissue, providing safe harbour and a supply of minerals in return for sugars and other fixed carbon molecules.
As their name suggests, extremophiles love extreme environments. The vast majority of life lives in a temperature range around 0–40°C (32–104°F), but the discovery of deep-sea hydrothermal vents revealed that bacteria and archaea could survive in superheated water above 100°C (212°F). They also thrive in hot volcanic springs at the surface, often creating a stunning rainbow of seemingly unnatural colours in the water.
There are extremophiles – almost always prokaryotes – that survive in regions that are very dry, super salty or acidic. There are even organisms called cryptoendoliths that live hidden inside rocks, occupying the tiny spaces between crystal grains. All of the above are certainly extreme to us, but in many ways these extreme habitats are more stable than our own, which is prone to all kinds of rapid and unpredictable changes. And when we look back into the conditions on the young Earth billions of years ago, today’s extremes look rather normal.
The Grand Prismatic Spring in Yellowstone National Park in Wyoming, USA is a haven for extremophile bacteria.
Animals can be masters of disguise, using camouflage to blend in with their surroundings, using a body shape that looks like a leaf or a twig, or having a body pattern that makes them indistinguishable from a tree trunk or rock. But other animals hide in plain sight by mimicking the appearance of another member of their ecosystem – or even their smells, calls or behaviours.
There are two basic types of mimicry. The most common, called Batesian mimicry, involves a harmless mimic modelling itself on a dangerous neighbour. Thus, a hover fly has the stripes of a stinging wasp or bee, and many butterflies have dark eyespots on their hindwings, which resemble the face of a much bigger beast when opened up. The second type, Müllerian mimicry, is more nuanced, and involves species that do harm to attackers if eaten, such as poisonous butterflies. Predators learn to avoid toxic prey, and by evolving to look the same, both mimics are able to benefit from this lesson more effectively.
It is hard to imagine in this age of unrestrained habitat destruction and transformation, but some ecosystems have survived more or less unchanged for many millions of years. In all that time species have evolved in concert with each other through a phenomenon called coevolution, where a change in one species triggers a change in a second in reaction to the first. This creates a complex of interlacing adaptations that allow an ecosystem to support a large capacity for life. However, that strength is also a weakness because sudden changes from outside the system – typically human activities – are able to easily disrupt the fine balance between life forms.
Classic examples of coevolution are the arms race between predators and prey, and the adaptations of flowering plants and insects or birds that work in symbiosis. By evolving a co-dependent relationship, the plants increase the chances of their pollen being delivered to others of their species, while the animals find themselves with a reliable food supply.
