Deforestation

Reduced biodiversity

A forest provides food, shelter, and nesting or breeding sites across many different microhabitats. Removing the forest removes all of that at once, and species that can’t survive anywhere else decline. Fewer species surviving in the cleared area means lower biodiversity by the same definition used throughout this chapter — the number of different species present, covered in full on biodiversity and habitat destruction.

Extinction

If enough of a species’ habitat is destroyed across its whole range, its population can fall too low to survive and reproduce successfully. Pushed far enough, that’s the difference between a local habitat problem and the permanent loss of a species — deforestation is capable of causing both.

Loss of soil

Tree roots physically bind soil together, and the canopy and ground vegetation absorb much of the direct force of rainfall before it ever reaches bare ground. Take the trees away and both of those protections go with them: fewer roots means soil is held together far less effectively, so exposed soil is eroded by wind and water much more easily, and valuable topsoil is lost.

Increased flooding

Standing vegetation intercepts rainfall, and plants take up water from the soil and release it back into the air as water vapour through transpiration. Once trees are removed, both of these processes shrink: less rain is intercepted, and less water is taken up by roots that are no longer there. More water is therefore left free to run across the surface, reaching rivers faster than it would have with the forest still standing — and that faster, larger surface run-off is what raises the risk of flooding downstream.

Increased atmospheric carbon dioxide

The precise phrasing matters here more than in almost any other point in this chapter. Trees remove carbon dioxide from the atmosphere by photosynthesis, so a standing forest acts as a continuous carbon sink. Deforestation raises atmospheric carbon dioxide through two separate mechanisms working together, not one: cleared vegetation that is burned or left to decompose releases the carbon dioxide it had stored, and — just as importantly — fewer trees remain to keep removing carbon dioxide from the atmosphere by photosynthesis in the first place. The second mechanism is a lost sink, not a new source, and it’s worth keeping that distinction precise in an exam answer rather than describing deforestation itself as directly “releasing” carbon dioxide through photosynthesis loss.

Because carbon dioxide is a greenhouse gas, this is exactly where deforestation connects to the enhanced greenhouse effect and climate change — and to the wider carbon cycle from Chapter 19, where photosynthetic uptake and respiration or combustion normally stay roughly balanced over long timescales. Deforestation shifts that balance, rather than introducing an entirely new process.

Two terms not to confuse

Soil erosion means the physical removal of soil by wind or water. Loss of soil fertilitymeans a reduced ability of the remaining soil to support plant growth. The two are related — eroded soil often takes fertile topsoil with it — but they are not the same process, and a question asking specifically about one shouldn’t be answered with the other.