Pest compete with humans for food or shelter, transmit pathogens, feed on humans or otherwise threaten human health, comfort or welfare.
A pest species is any species which are considered undesirable. The use of the term pest is therefore subjective. A more complete definition of pest is that pests compete with humans for food or shelter, transmit pathogens, feed on humans or otherwise threaten human health, comfort or welfare. Weeds may be included in this definition as they represent a plant which competes with other plants which have food, timber or amenity value. There are many species of pest representing a wide range of taxonomic groups. It’s difficult to attribute certain general characteristics which are features of all pests. Pests have very varied life histories. Some pests have explosive bursts of population increases, rapidly reaching a level where vast damage is caused (for example rats and locusts).
However, other pest species cause huge damage and yet their population growth rate is relatively small (e.g. the codling moth, Cydia pomonella which lays only 40-50 eggs per year but is the most important pest of apples). One important characteristic of pests is the degree to which they are normally regulated by their natural enemies (parasites and predators). Pests are frequently species that have evaded their natural enemies, possibly due to their importation to new regions of the world, having left their natural enemy complex behind or because their natural enemies have been eliminated by man.
Weeds are often plant species which are adapted to the high-disturbance, low-competition environment that characterizes agricultural systems, having a ruderal life history strategy.