Pests are any organisms that are considered, from the perspective of humans, to be undesirable in some ecological context. For example, pests could be insects that compete with humans for some common resource, such as agricultural production or timber. Other pests might be associated with diseases of humans, livestock, or agricultural plants. Pests could also be unwanted weeds that compete with agricultural plants for necessary resources. Or pests may merely have aesthetics that are viewed as undesirable, as is the case of weeds in a lawn.
When pests are abundant enough to cause damage that is considered to be unacceptable, the abundance of the pests may be managed in some way. For example, if wolves are considered to be an important predator of livestock or wild ungulates, they may be killed by shooting them or using poisons. The most important reason for plowing in agriculture is to reduce the abundance of weeds. Often pesticides are used, for example, to protect crops and livestock from diseases and depredations by fungi, insects, mites, nematodes, or rodents, to protect crop plants from competition with weeds, or to protect humans against the insect vectors of disease-causing pathogens. There are substantial benefits to humans of the use of most pest-management strategies.
It is important to understand, however, that the very species that are considered pests may have desirable attributes in other contexts, or their values may at least be neutral. Therefore, whether an organism is viewed as a pest is entirely a matter of judgement, and the criteria generally focus on the needs and perspectives of humans (this is known as an anthropocentric viewpoint).
The notion of contextually varying merits of organisms can be illustrated by considering the case of plant pests, or so-called weeds. Weeds can be defined as any plant that interferes with the productivity of a desired crop plant, or with some other human purpose. Weeds can severely decrease the productivity of crop plants by competing with them for light, water, nutrients, and more broadly, space. Many studies in agriculture and forestry have demonstrated that weeds can significantly reduce the production of crops. For example, on unweeded plots in Illinois there was an average reduction of yield of corn of 81%, and a 51% decrease was observed in Minnesota. Weeds can also reduce the yields of wheat and barley, typically by 25-50%. Weeds similarly cause decreased yields of other agricultural plants, and they can also be important in forestry, where they may interfere with the growth of desired species of trees.
However, sometimes a particular species is a weed, and sometimes it is not. Consider, for example, the case of the red raspberry (Rubus strigosus). This species is considered an important weed in forestry in parts of North America, because it can quickly overtop young conifer plants and interfere with their growth. This reduces the length of time it takes until the next crop of trees is large enough to harvest from the site. Because its competition with conifers can be interpreted in terms of substantial economic damage, the red raspberry is considered to be an important silvicultural weed, and its abundance may be managed using herbicides or manual weeding treatments. However, this same species can produce large quantities of tasty fruits on the same site on which it is regarded, from the forestry perspective, to be a pest. The raspberry fruits can be beneficial as food to humans and to wild animals. The lush growth of red raspberry also helps to quickly restore improved aesthetics to recently harvested forests, and the plant helps to reduce erosion and nutrient leaching from disturbed sites. In any event, the red raspberry has intrinsic value, regardless of any perceptions of its worth by humans. There are many other examples that demonstrate the fact that pests are only considered as such in certain contexts, and from certain perspectives.