It is almost impossible for an organism to live in isolation. Organisms interact to provide one another with food and shelter and each one influences the distribution of others. Only in highly inhospitable places, like hot springs, do species exist alone, and these are simple organisms such as bacteria that have evolved to survive in these extreme conditions. Even at the bottom of the oceans, where hot vents release geothermal energy from below, there are interactions between organisms.
Removal of habitats
Farmland is not a natural habitat, but at one time hedgerows, hay meadows and stubble fields were important habitats for plants and animals. Hay meadows and hedgerows supported a wide range of wild plants as well as providing feeding and nesting sites for birds and animals.
Intensive agriculture has destroyed many of these habitats; hedges have been grubbed out to make fields larger, a monoculture of silage grasses has replaced the mixed population of a hay meadow and planting of winter wheat has denied animals access to stubble fields in autumn. As a result, populations of butterflies, flowers and birds such as skylarks, grey partridges, corn buntings and tree sparrows have crashed.
Recent legislation now prohibits the removal of hedgerows without approval from the local authority but the only hedges protected in this way are those deemed to be ‘important’ because of species diversity or historical significance.
In Britain, the Farming and Wildlife Advisory Group (FWAG) can advise farmers how to manage their land in ways that encourage wildlife. This includes, for example, leaving strips of uncultivated land around the margins of fields or planting new hedgerows. Even strips of wild grasses and flowers between fields significantly increase the population of beneficial insects.
The development of towns and cities (urbanisation) makes a great demand on land, destroying natural habitats. In addition, the crowding of growing populations into towns leads to problems of waste disposal. The sewage and domestic waste from a town of several thousand people can cause disease and pollution in the absence of effective means of disposal, damaging surrounding habitats.
Extraction of natural resources
An increasing population and greater demands on modern technology means we need more raw materials for the manufacturing industry and greater energy supplies.
Fossil fuels such as coal can be mined, but this can permanently damage habitats, partly due to the process of extraction, but also due to dumping of the rock extracted in spoil heaps. Some methods of coal extraction involve scraping off existing soil from the surface of the land. Soil heaps created from waste rock can contain toxic metals, which prevent re-colonisation of the land. Open-pit mining puts demands on local water sources, affecting habitats in lakes and rivers. Water can become contaminated with toxic metals from the mining site, damaging aquatic habitats.
Oil spillages around oil wells are extremely toxic. Once the oil seeps into the soil and water systems, habitats are destroyed. Mining for raw materials such as gold, iron, aluminium and silicon leaves huge scars in the landscape and destroys large areas of natural habitat. The extraction of sand and gravel also leaves large pits that prevent previous habitats redeveloping.
In response to this increased human activity in 1982 the United Nations developed the World Charter for Nature. This was followed in 1990 by the The World Ethic of Sustainability, created by the World Wide Fund for Nature (WWF), the International Union for Conservation of Nature (IUCN) and the United Nations Environment Programme (UNEP). Included in this charter were habitat conservation and the need to protect natural resources from depletion.
Marine habitats around the world are becoming contaminated with human debris. This includes untreated sewage, agricultural fertilisers and pesticides. Oil spills still cause problems, but this source of marine pollution is gradually reducing. Plastics are a huge problem: many are non-biodegradable so they persist in the environment. Others from micro-particles as they break down and these are mistaken by marine organisms for food and are indigestible. They stay in the stomach, causing sickness, or prevent the gills from working efficiently. Where fertilisers and sewage enter the marine environment, ‘dead zones’ develop where there is insufficient oxygen to sustain life. This destroys habitats.
Oil spills wash up on the intertidal zone, killing the seaweeds that provide nutrients for food chains. Filter-feeding animals such as barnacles and some species of mollusc die from taking in the oil.
Any form of habitat destruction by humans, even where a single species is wiped out, can have an impact on food chains and food webs because other organisms will use that species as a food source, or their numbers will be controlled through its predation.
The removal of large numbers of trees result in habitat destruction on a massive scale.
- Animals living in the forest lose their homes and sources of food; species of plant become extinct as the land is used for other purposes such as agriculture, mining, housing and roads.
- Soil erosion is more likely to happen as there are no roots to hold the soil in place. The soil can end up in rivers and lakes, destroying habitats there.
- Flooding becomes more frequent as there is no soil to absorb and hold rainwater. Plant roots rot and animals drown, destroying food chains and webs.
- Carbon dioxide builds up in the atmosphere as there are fewer trees to photosynthesise, increasing global warming. Climate change affects habitats.
The undesirable effects of deforestation on the environment
Forests have a profound effect on climate, water supply and soil maintenance. They have been described as environmental buffers. For example, they intercept heavy rainfall and release the water steadily and slowly to the soil beneath and to the streams and rivers that start in or flow through them. The tree roots hold the soil in place.
At present, we are destroying forests, particularly tropical forests, at a rapid rate (1) for their timber, (2) to make way for agriculture, roads and settlements and (3) for firewood.
The Food and Agriculture Organisation, run by the United Nations, reported that the overall tropical deforestation rates in the decade up to 2010 were 8.5% higher than during the 1990s. At the current rate of destruction, it is estimated that all tropical rainforests will have disappeared in the next 75 years.
Removal of forests allows soil erosion, silting up of lakes and rivers, floods and the loss for ever of thousands of species of animals and plants.
Trees can grow on hillsides even when the soil layer is quite thin. When the trees are cut down and the soil is ploughed, there is less protection from the wind and rain. Heavy rainfall washes the soil off the hillsides into the rivers. The hillsides are left bare and useless and the rivers become choked up with mud and silt, which can cause floods.
For example, Argentina spends 10 million dollars a year on dredging silt from the River Plate estuary to keep the port of Buenos Aires open to shipping. It has been found that 80% of this sediment comes from a deforested and overgrazed region 1800 km upstream, which represents only 4% of the river’s total catchment area. Similar sedimentation has halved the lives of reservoirs, hydroelectric schemes and irrigation programmes. The disastrous floods in India and Bangladesh in recent years may be attributed largely to deforestation.
The soil of tropical forests is usually very poor in nutrients. Most of the organic matter is in the leafy canopy of the tree tops. For a year or two after felling and burning, the forest soil yields good crops but the nutrients are soon depleted and the soil eroded. The agricultural benefit from cutting down forests is very short-lived, and the forest does not recover even if the impoverished land is abandoned.
Forests and climate
About half the rain that falls in tropical forests come from the transpiration of the trees themselves. The clouds that form from this transpired water help to reflect sunlight and so keep the region relatively cool and humid. When areas of forest are cleared, this source of rain is removed, cloud cover is reduced and the local climate changes quite dramatically. The temperature range from day to night is more extreme and the rainfall diminishes.
In North Eastern Brazil, for example an area which was once rainforest is now an arid wasteland. If more that 60% of a forest is cleared, it may cause irreversible changes in the climate of the whole region. This could turn the region into an unproductive desert.
Removal of trees on such a large scale also reduces the amount of carbon dioxide removed from the atmosphere in the process of photosynthesis. Most scientists agree that the build-up of CO2 in the atmosphere contributes to global warming.
Factors affecting the distribution of plant species
Organisms are said to live in communities. A community may be described by the geographical area it occupies ( a lake community, for example) or by the dominant plant species present (coniferous forest, for instance). The organisms present in a community depend on the other organisms living there, as well as on the non-living, abiotic aspects, such as soil or climate. The distribution of plants in communities depends on a number of these abiotic factors.
No plant can survive freezing conditions for very long because, to grow and reproduce, plants must carry out chemical reactions within their cells that require enzymes. In arctic climates, plant growth is often very slow because enzymes work slowly at the low temperatures, but seasonal because the rate of growth picks up during the relatively short summer. In tropical areas, like rainforests, growth is usually rapid because temperatures are warm, and continuous because there is little seasonal variation in temperatures.
All plants require water. It is the universal solvent in their cells, the substrate for photosynthesis, and their transport medium. However, many plants have evolved a variety of mechanisms to survive periods of drought. Some remain dormant, some (such as cacti and succulent plants) store water, and others complete their life cycle in a brief rainy season.
Plant need light for photosynthesis. Many use the changing day lengths of the different seasons to trigger flowering. Where light intensity is high, as in a desert, plants have evolved mechanisms to prevent damage to their chlorophyll, such as dense spines or white hair that reflects light.
Where light levels are low, as they are at ground level in a deciduous forest in the northern hemisphere, some plants grow and complete their annual life cycle in the early part of the year, before overshadowing trees have come into leaf.
Most plants prefer a pH of 6.5-7.0 because nutrients are easily available in this range. Some soils are slightly alkaline because they are based on chalk. Chrysanthemum and lavender are two examples of plants that tolerate alkaline soils well and are found in chalky areas. Other soils are acidic, beech, spruce and camellia can grow here. Peat bogs are very acidic because they are composed of decomposing organic material. Very few plants can grow here, although heathers can survive in acid soils.
Saline (salty) soils present a particular problem to plants, because they make it difficult for them to take up water and minerals. Some plants absorb salt in the soil, secrete it in their leaves and then drop these leaves to remove the salt. A few plants, such as marram grass and lyme grass, can survive in saline conditions.
Soil that are rich in minerals can support a diverse community of plant species, including trees and shrubs. Plants that survive in mineral-poor soils often have special adaptations to supplement their needs. Carnivorous plants such as sundew and Venus flytraps live in very peaty soils that are deficient in nitrogen.
Factors affecting the distribution of animal species
Just as for plants, the distribution of animals is affected by the abiotic factors in the environment.
Animal enzymes are influenced by temperature in much the same way as those of plants. However, animals have the advantage that they can move to avoid the harshest of conditions. In hot, arid areas like deserts, many animals avoid the heat of the day and burrow underground. The jerboa (Jaculus jaculus) has long legs that keeps its body off the hot sand and its ears have a large surface area, enabling the animal to lose heat efficiently. Birds an mammals can control their internal temperatures but other species use behaviour and other adaptions to maintain theirs.
Some animals, such as the hedgehog, hibernate to overcome the rigours of cold winters. Many bird species migrate during wintry seasons to warmer climates.
Most animals need to drink water to survive. Very few have evolved to be independent of water. Some desert animals like the jerboa have done this, however. Jerboas eat seeds and, as the stored carbohydrate is respired in their cells, it produces all the water these animals need – they do not actually drink any liquid water.
Lack of water in certain seasons may change the distribution of animals. Herds of wildebeest and zebra in Africa undertake huge migrations to find new supplies of water and, therefore vegetation. Carnivorous species often follow these herds, which are their source of food.
Animals need to find appropriate sites to express mating behaviour and then rear young. These sites may be chosen for safety away from predators, or because they provide rich feeding grounds so the young may benefit. Different species have their own requirements. Many frogs and toads live almost entirely on land but must return to water to breed.
Unlike plants, which are autotrophic, animals need a source of food. Herbivores need plants and carnivores need other animals to feed on. The availability of food will determine the distribution of different types of animal. Some animals are restricted to a particular area because it supplies their food – so, for example, rabbits are usually found on grasslands. Others, such as lions, have huge territories and may cover many kilometres searching for food. Animals that have a varied diet are generally more successful and have a wider choice of habitats. If one source of food becomes scarce, they can move on to another.
Herbivores that exist in large herds, such as wildebeest, graze on large areas of grassland and, when the dry season arrives, they migrate to find fresh grass. Some birds, such as the European robin, live in smaller numbers or singly and have less need for space but males defend their territories vigorously because they contain food and a nesting area. Carnivores, such as wolves, that live in packs require a large area to hunt in. They may mark their territory with scent and defend it from other packs. Others, like eagles and other raptors, live solitary lives and have a large hunting territory because their prey is hard to find.
Random sampling of communities
When ecologists want to understand the distribution of a species or to compare the distribution of one species with another in a different location, it is usually impossible to do so by a direct counting method. In most cases, ecologists take a sample of the population and, if the sample is random, it should provide a good representation of the whole population. Random sampling assumes that every organism has an equal chance of being sampled. There are a number of methods available to sample not only what species are present, but how many.
One of the simplest and easiest sampling techniques involves using a quadrat. A quadrat is a square made of metal or wood that is place on the ground so that the organisms present inside the square can be counted.
The size of the quadrat will largely be determined by what is being measured. To estimate the number of different trees in a wood may require quadrats of 10 m by 10 m, but a 1 m quadrat would be the best size for studying wild flowers in grassland. Very small 10 cm quadrats might be used for sampling lichens on walls or tree trunks.
If you just place a quadrat on the ground, you introduce personal bias, because even without meaning you might place it in a spot that you think will be more interesting, perhaps, or easier to work in. To ensure that your sampling within the survey area is completely random, you should follow these steps.
- Divide the area to be surveyed into a grid of 1 m squares. Each square can be identified within the grid by numbered coordinates. For example, the square in column 3 (from the left) and row 4 (from the bottom of the grid) would be (3,4) see Figure.
- Use a random number generator programme or table to choose which grid squares to use. For example, in a grid of 9 m by 9 m , the randomly generated numbers 7 and 2, 6 and 0, and then 3 and 8 would select squares (7,2), (6,0) and (3,8).
- Now put a quadrat on each of the selected squares in turn, and count the number of individuals of the species being sampled that occur within the quadrat.
- Finally, find the average number of individuals found in 1 m2 , and multiply this by the total number of squares in the survey area to obtain an estimate of the numbers present in the whole area.
The number of quadrats that are counted can be determined in one of two ways. To compare populations of a species in two areas, it may be appropriate to sample 5% of the quadrats in each survey area grid. Or, if you are studying the number of different species present in your survey area, you might continue to count quadrats until, after five consecutive samples, no new species is found.
Another commonly used sampling method in ecology is a transect. A transect can show the distribution of a species in relation to a particular abiotic factor or it can give and idea of successions or changes in communities of organisms across a habitat. Transects can be used to sample the distribution of plants on a beach or in a field or to study the different vegetation or the changing plant distributions as soil or moisture varies. Transects provide a method of systematic, rather than random sampling.
To take samples along a transect, follow these steps.
- Stretch a tape or rope from a fixed point for a selected distance across the changing habitat you are interested in. If you are studying a salt marsh or sand dunes above a beach, a distance of 100 m would be appropriate.
- At intervals of 10 m, or another suitable distance, along the tape put down a quadrat and count the organisms inside it. A series of samples like this provides information about the changes in density and community composition along the transect.
- Measure the abiotic factor of interest – such as temperature, salinity, soil pH or light intensity – at each quadrat location.
The best type of transect to carry out depends on the terrain and on the organisms present. It may be better to carry out a point transect, where organisms are recorded at specific sampling points along the tape. On the other hand, a continuous ‘belt’ transect where all species in a 1 m zone along the transect are recorded, might be more helpful in providing a detailed picture of the area.
Niches and habitats
A niche is the particular environment and ‘lifestyle’ that is adopted by a certain species. It is the place where the organisms lives and breeds, and includes its food and feeding method, as well as its interactions with other species. A niche is unique to each species because it offers the exact conditions that the species needs or has become adapted to.
A habitat is a wider area offering living space to a number of organisms, so a habitat comprises a number of inches and includes all the physical and abiotic factors in the environment. An example might be a woodland habitat, which contains niches for a huge variety of species, from burrowing invertebrates at ground level to nesting birds in the tree canopy.
Every organism has its own space in an ecosystem, which is known as its spatial habitat. The surroundings are changed by the presence of the organism – for example, a woodpecker lives inside hollow trees, adapting them to provide nesting places and shelter, while a rabbit burrowing underground affects the soil and plant species growing there.
As an organism feeds within its niche, it affects the other organisms that are present. For example, an owl feeding on mice in woodland helps to keep the population of mice at a stable level, and rock limpets grazing on small algae control the degree of algal cover.
Interaction between organisms
Organisms interact with other organisms living in the same area. The interaction include competition, herbivory, predation, parasitism and mutualism. Almost all organisms influence the lives of others.
Competition occurs when two organisms require the same resource. As one uses the resource, less is available to the other so there is competition for a limited supply. If a pride of lion kills an antelope, they must protect this source of food from scavenging hyenas and vultures that will compete with them for prey.
Plants also compete for resources such as light and space. Fast growing birch trees quickly become established in areas of cleared land, but they require high light levels. Slower-growing species such as oak begin to grow up around them and for a while, they form a mixed woodland. Eventually the birch trees are over-shadowed and out-competed by the more dominant oaks.
Loss of habitat, often caused by human activities such as farming or deforestation, severely limits vital resources such as food, water and breeding sites for the species that live there. When two different species require the same limited resources in the same area, they may find themselves in competition for the same niche. If they are prey species, they may become susceptible to the same predators as well. The principle of competitive exclusion states that no two species can occupy the same niche. The species cannot exist together because one will come to dominate and exclude the other. The oak and birch trees described above are an example of competitive exclusion. Both compete for soil resources and light but eventually the oak shades out the light and the birch die off.
In 1934, a classical study on competition was conducted by G G Gause (1910-1986), a Russian ecologist. He experimented with two species of Paramecium, a large protozoan that is common in fresh water – P. aurelia and P.caudatum. If the two species were allowed to grow in separate cultures on a food source of bacteria, both species grew well. When the two species were cultured together with an identical food source, P.aurelia survived while P. caudatum died out. Both species had similar needs in the culture but P. aurelia had an advantage that enabled it to outgrow P. caudatum.
A single plant may provide leaves for herbivorous animals, fruits and seeds for birds, and roots for burrowing animals. The horse chestnut leafminer (Cameraria ohridella) is a moth that lays its eggs on horse chestnut leaves. As the larvae hatch, they burrow inside to feed on the tissues of the leaf. The nuts from the horse chestnut tree also provide food for squirrels and deer. Other leafminer species feed on different tree species around the world, such as oak, birch and holly.
A well studied example of predation is that of the Canadian lynx, which feeds on the arctic hare. The numbers of the predator and prey fluctuate over the years with changes in the bare population being followed by corresponding changes in the numbers of lynx.
The largest predatory fish is the spectacular great white shark (Carcharodon carcharias). Its prey include dolphins, porpoises and seals.
Parasites are organisms that live entirely on or in a host species and cannot survive without it.
Exoparasites, such as fleas and ticks, live on the outside of a host. One economically important example is the southern cattle tick (Boophilus microplus) which lives on cattle, feeding on their blood and weakening the animals. It causes significant losses to farmers all over the world.
Endoparasites, such as tapeworms, roundworms and malarial parasites, live inside their host. One example, the barber’s pole worm (Haemonchus contortus) is a roundworm that lives in the stomachs of sheep in warm, humid climates all over the world. It causes anemia and progressive weakness as it feeds on blood in the sheep’s stomach. If present in large numbers, this parasite can kill young animals.
Sometimes two organisms co-exist and benefit each other, forming what is known as a mutualistic relationship.
Lichens such as common orange lichen (Xanthoria parietina), which grows on twigs and branches, are the result of a union between a fungus and an alga. The alga carries out photosynthesis and provides sugars for both organisms, The fungus protects the alga from intense sunlight and drying out and absorbs minerals for the benefit of both organisms.
Another mutualistic relationship occurs between the Egyptian plover (Pluvianus aegyptius) and the Nile crocodile. The bird feeds on parasites and food particles left around the crocodiles mouth, keeping its teeth clean and healthy. The crocodile openly invites the birds to hunt on its body, even allowing them to enter its mouth.
Fundamental and realised niches
We have described a ‘niche’ as the special space and ‘lifestyle’ inhabited by a particular plant or animal. This is the fundamental niche for that species. It is the potential mode of existence of the species, given its adaptations.
Often the environment will change through natural phenomena, competition or human intervention. So a species may find that its niche becomes more restricted or begins to overlap with that of another species. This more restricted life pattern is known as the realised niche. The realised niche is the actual mode of existence of a species resulting from its adaptions as well as competition from other species. A realised niche can only be the same size as or smaller than the fundamental niche.
Biomass is biological material, living or dead, that can be used as an energy source. Since living material also contains water, which is not organic and does not contain energy, biomass is usually measured as dry mass of organic matter in organisms.
Biomass the total amount of living, or recently living, material in a given habitat, population, or sample; it is usually expressed in dry mass (after removal of all water from the sample) per unit area of land or unit volume of water
Measuring biomass is not easy, and may be quite destructive. In a terrestrial ecosystem, a sample area that is representative of the whole area must be chosen. This may be relatively straightforward for studying the biomass of plants, but trapping and measuring the animal life might prove difficult. The presence or absence of some plants or animals might be seasonal, and there might be population explosions at certain times of year.
Biomass does not include biological material that has been changed over time into coal or oil. There is much interest currently into using biomass as fuels in place of fossil fuels, because they are renewable. Plants such as perennial grasses, hemp and sugar cane are undergoing trials as sources of industrial biomass.
To measure the biomass of a forest, you would need to follow the steps.
Tropic level 1 (producers)
- Select one or more areas to study, using the same random sampling method used in population studies with quadrats.
- Choose a small area and measure the height and diameter of all the trees and shrubs.
- In this small area, cut down all vegetation to ground level and dry the specimens of each type of tree and shrub in an oven at 90∘Ⅽ.
- Measure the dry mass of each specimen using an accurate electronic balance.
- Use the masses found for the dried specimens, along with their original height and diameter measurements, to construct tables showing the biomass contained in fresh specimens of particular dimensions, for each plant species.
- Sample other areas in the forest by measuring the heights and diameters of all the plants present and using your tables to calculate their biomass.
Other tropic levels (consumers)
- Set a variety of traps in a measured area to capture the different types of animals present.
- Sort the organisms into trophic levels.
- Dry and weigh a sample of each species caught and calculate the biomass (or use published data to provide this information).
- Estimate the total population of each species in the sample area.
- Use this information to calculate the total biomass for each sampled species by the number in the population.
- Carefully release the unused, captured animals back into the ecosystem.
The combined data – biomass of plant life and of animal life – can then be used to calculate the biomass of the entire ecosystem.
Clearly this procedure is not ideal because it destroys wildlife as results are obtained. Such interference with an ecosystem may be harmful to its survival. In addition, a single set of measurements may not reflect seasonal or annual changes, so repeats are necessary, and this causes further destruction. Ecologists often use tables providing previously calculated information on organisms to estimate biomass and so avoid the need to destroy plants and animals, which can instead simply be counted.