mechanisms. Dispersal of organisms is a critical process for understanding both geographic isolation in evolution through gene flow and the broad patterns of current geographic distributions (biogeography). A distinction is often made between natal dispersal where an individual (often a juvenile) moves away from the place it was born, and breeding dispersal where an individual (often an adult) moves away from one breeding location to breed elsewhere. Costs and benefits In the broadest sense, dispersal occurs when the fitness benefits of moving outweigh the costs. There are a number of benefits to dispersal such as locating new resources, escaping unfavorable conditions, avoiding competing with siblings, and avoiding breeding with closely related individuals which could lead to inbreeding depression. There are also a number of costs associated with dispersal, which can be thought of in terms of four main currencies: energy, risk, time and opportunity. Energetic costs include the extra energy required to move as well as energetic investment in movement machinery (e.g. wings). Risks include increased injury and mortality during dispersal and the possibility of settling in an unfavorable environment. Time spent dispersing is time that often cannot be spent on other activities such as growth and reproduction. Finally dispersal can also lead to outbreeding depression if an individual is better adapted to its natal environment than the one it ends up in. In social animals (such as many birds and mammals) a dispersing individual must find and join a new group, which can lead to loss of social rank. Dispersal range "Dispersal range" refers to the distance a species can move from an existing population or the parent organism. An ecosystem depends critically on the ability of individuals and populations to disperse from one habitat patch to another. Therefore, biological dispersal is critical to the stability of ecosystems. Environmental constraints Few species are ever evenly or randomly distributed within or across landscapes. In general, species significantly vary across the landscape in association with environmental features that influence their reproductive success and population persistence. Spatial patterns in environmental features (e.g. resources) permit individuals to escape unfavorable conditions and seek out new locations. This allows the organism to "test" new environments for their suitability, provided they are within animal's geographic range. In addition, the ability of a species to disperse over a gradually changing environment could enable a population to survive extreme conditions. (i.e. climate change). As the climate changes, prey