0610 · 20.2Habitat destruction

Habitats and biodiversity

0:00Imagine a woodland being cleared to make space for a farm. The obvious change is that the trees disappear. But the effects can reach birds, insects, soil, nearby rivers and even the atmosphere. Why does removing trees cause so many different problems? By the end of this video, you will understand how habitat destruction changes living conditions, disrupts feeding relationships and affects the environment beyond the cleared land. First, we need two connected ideas.

0:25A habitat is the place where an organism lives. It provides conditions and resources that the organism needs, such as food, water, shelter and places to reproduce. A food chain shows feeding relationships, while a food web connects several food chains. These connections matter because damaging the habitat of one species can affect other species that depend on it. Biodiversity is the number of different species that live in an area.

0:49Notice that this definition counts different species, not the total number of organisms. A field containing thousands of wheat plants could have lower biodiversity than a small woodland containing many different plant and animal species. Having many individuals of one species is not the same as having many different species. Here is a simple worked example using two imaginary areas. The first contains grass, rabbits and foxes, while the second contains grass, rabbits, foxes and oak trees.

1:15Count the different species in each area, rather than counting individual organisms. The first contains three species and the second contains four, so the second has greater biodiversity. Adding more rabbits to the first area would not increase its number of different species.

Causes of habitat destruction

1:30Habitat destruction happens when an area is changed so that it no longer provides suitable living conditions for the organisms that lived there. This does not always mean that the area becomes completely empty. A forest replaced by a crop field still contains living organisms, but many woodland species can no longer survive there. The important change is the loss of their original habitat. One major reason is the increased area needed for housing, crop plant production and livestock production.

1:56Housing replaces habitats with buildings and roads. Growing food crops may involve clearing natural vegetation and replacing it with one crop species. Livestock production may require land for grazing or for growing animal feed. In each case, land that previously supported wild species is converted for human use. Another reason is the extraction of natural resources. Natural resources are materials obtained from the environment, such as timber, minerals and oil.

2:20Cutting trees for timber removes woodland habitat, while mining can remove vegetation and soil to reach minerals underneath. The damage comes from the changes needed to obtain the resource, not simply from the fact that humans use materials. Freshwater and marine pollution can also destroy habitats by making them unsuitable for organisms. Freshwater habitats include rivers and lakes, while marine habitats include seas and oceans. Toxic chemicals entering water can poison organisms, and oil pollution can harm marine wildlife.

2:49The habitat may still look like a river or a coastline, but its conditions have changed. Pollution does not have to remove the physical place to damage the habitat.

Food webs and deforestation effects

2:58Humans can also have a negative impact on habitats by altering food chains and food webs. Think of the organisms as connected through their food supplies. If a human activity reduces one population, organisms that eat it may have less food. Other populations may change because fewer organisms are eating them. A disturbance can therefore spread through feeding relationships rather than remaining limited to the first species affected. For example, suppose caterpillars feed on oak leaves and insect-eating birds feed on those caterpillars.

3:26Removing the oak trees reduces the caterpillars' food supply, so their population may decrease. The birds then have fewer caterpillars available to eat, so their population may also decrease. In a real food web, alternative foods may change the outcome. Even so, removing one food source can affect several connected species. Now let us bring these ideas together through deforestation, the removal of forest. Forests provide much more than a collection of tree trunks.

3:52Their leaves provide food, branches provide nesting sites, and the shaded ground provides particular living conditions. Removing the trees changes several resources and conditions at once. That is why deforestation can have several undesirable effects. The first effect is reduced biodiversity. Some species lose food, shelter or suitable places to reproduce, so they may die or leave the area. As species disappear from that area, the number of different species living there decreases.

4:19Replacing the forest with a single crop does not replace all those lost habitats. The crop may support some organisms, but not the same range of woodland species. Deforestation can also contribute to extinction. Extinction means that every individual of a species has died, so the species no longer exists anywhere. Losing a species from one forest is not necessarily extinction if it survives elsewhere. But if a species depends on a habitat that is destroyed throughout its entire range, it may have nowhere suitable left to survive and reproduce.

4:49Next, consider loss of soil. Tree roots help hold soil in place, and the canopy intercepts rainfall, reducing the direct impact of raindrops on the ground. After deforestation, the soil is less protected. Rainwater can wash soil away, and wind can blow exposed soil away. This removal of soil is called soil erosion, and it can carry away the fertile upper layer needed for plant growth. Deforestation can also increase the risk of flooding.

5:14Trees absorb water through their roots, and some rain is intercepted by leaves before reaching the soil. After clearance, less water is taken up by trees. If the exposed soil becomes compacted, less water may soak into it and more may flow over the surface. Water can then reach rivers more quickly, causing river levels to rise and increasing flood risk. There is also an effect on carbon dioxide in the atmosphere.

5:35Trees remove carbon dioxide during photosynthesis and store carbon in their organic materials. Fewer trees mean less carbon dioxide is removed by photosynthesis. If the cleared wood is burned, carbon dioxide is released into the atmosphere. Decomposition of dead plant material can also release carbon dioxide through respiration by decomposers.

Applying deforestation effects

5:55Let us work through a complete situation. Imagine a forest on a slope being cleared for cattle grazing, with a village beside the river below. First, identify the reason for habitat destruction, which is increased area for livestock production. Next, identify what has been removed, namely the trees, their roots and the habitats they provided. Those changes let us predict several effects rather than simply recall a list. Animals that depend on the trees may lose food and nesting sites, so biodiversity may decrease.

6:23Without living roots and protective vegetation, soil is more easily washed downhill. Increased surface runoff may make the river rise more quickly after heavy rain, increasing the village's flood risk. Finally, fewer trees remove carbon dioxide by photosynthesis. Each prediction follows from a specific change caused by clearing the forest. Watch out for three common mistakes. Biodiversity does not mean the total number of organisms, because it counts different species.

6:49A species disappearing locally does not necessarily mean extinction, because extinction requires the loss of every individual everywhere. Also, trees do not increase atmospheric carbon dioxide simply because they have been cut down. Explain reduced removal by photosynthesis and, where relevant, release through burning or decomposition. In an examination, the command word matters. If asked to describe reasons for habitat destruction, identify the human activities involved. If asked to explain an undesirable effect, connect the change to its consequence.

7:20For example, an exam-style question might ask you to explain how deforestation causes loss of soil. Begin by thinking about what roots and vegetation did before they were removed. A precise answer would be this. Tree roots bind soil particles together, and removing trees removes this support. The loss of vegetation exposes soil to rain and wind. Rainwater can wash the exposed soil away, and wind can blow it away.

7:45Each sentence contributes a clear point, and the answer explains how deforestation causes soil loss instead of merely repeating that soil is lost. Let us recap the connections. Habitat destruction removes or damages the conditions and resources organisms need, often through housing, farming, resource extraction or pollution. Changing populations can disrupt food chains and food webs. Deforestation can reduce biodiversity and contribute to extinction, while removing soil protection and increasing flood risk.

8:13It can also increase atmospheric carbon dioxide through reduced photosynthesis and the release of carbon dioxide from cleared vegetation.