Spatially segregated root systems have been documented among conspecifics and among species at the scale of whole root systems and individual fine roots. Root segregation is often caused by architectural constraints, proliferation in particular microsites and plastic responses to competition for resources, but there is also evidence to suggest that allelopathy and non-toxic signals contribute to active root segregation. Root segregation appears to provide competitive advantages for water and nutrients for some species, as well as advantages of space itself. Plant growth and photosynthesis decreases when space is physically restricted, even when other resources are abundant. Moreover, plants appear to be able to compete for space independently of nutrient, water or light resources. Species that utilise resources efficiently and conservatively may particularly benefit from active root segregation because more profligate neighbouring species would not be able to take resources that were being utilised slowly. Stressful conditions, produced by adverse physical conditions and herbivory, have been shown to enhance the production of secondary metabolites and increase root exudation, mechanisms that can affect spatial root segregation. Resource availability may also determine the relative importance of root segregation in plant communities. A large portion of the evidence for root segregation comes from arid and semi-arid environments, where resources are often low. In resource-rich communities the defence of space may be less important. For animals, the defence and exclusive use of space is considered to be evidence for territoriality and suggests that organisms that exhibit such behaviour are avoiding the costs and uncertainties of "scramble" competition. Active root segregation and the defence of space by plants indicates that plants also may be territorial and opens the possibility of a level of taxonomic generality in population biology that is not currently recognised.