Sloping ground is particularly vulnerable to erosion started by the frequent passage of animals. In woodland and savanna areas where trees are present, livestock may eat all the tree seedlings, and thus prevent the normal replacement of the trees. Traditional agriculture was almost entirely subsistence agriculture, in which food crops were raised to be eaten locally. Today most agricultural development projects are for commercial agriculture, where the produce is sold to earn money. Such cash crops are becoming increasingly important in rural agriculture.
Commercial agriculture pays best if it is done on a large scale.
It often requires the use of machines which are expensive to buy and difficult to maintain. It almost always means using fertilizers and pesticides which cost money and may cause health or environmental problems if they are not used with care. The commercial market requires uniform varieties which may be more vulnerable to an agricultural disaster. All of this means a higher investment which may be difficult or beyond the reach of a small farmer, as well as a greater risk of environmental damage. Subsistence agriculture is usually based on traditional methods which have proven themselves over hundreds of years.