Mooney and Drake (1989), in summarizing a text on the ecology of biological invasions, suggested that humans have transformed nature to such a great extent that a "new order" now exists. They list a number of dramatic changes that have occurred due to human population growth and state that the world is now dominated by new systems because of these changes, as is highlighted in the following quote:
All of these alterations are providing a new landscape with an abundance of disturbed habitats favoring organisms with certain traits. This massive alteration of the biosphere has occurred in conjunction with the disintegration of the great barriers to migration and interchange of biota between continents due to the development by humans of long-distance mass transport systems. The introduction of a propagule of an organism from one region to a distant one has changed from a highly unlikely event to a certainty. The establishment and spread of certain kinds of organisms in these modified habitats, wherever they may occur, is enhanced. The net result of these events is a new biological order. Favored organisms are now found throughout the world and in ever increasing numbers. It is evident that these changes have not yet totally stabilized either in the Old or New World. In the former the success of invading species has changed through time with differing cultural practices and new directions and modes of transport. Old invaders are being replaced by new ones (Heywood, this volume). In the New World additional invading species are still being added.
The kinds of disruptions that non-intentionally introduced invading species can play in natural systems have been outlined above and have been the focus of the SCOPE study. These disruptions may in time stabilize on the basis of a new system equilibrium.
This interpretation might be translated as a kind of algebraic equation for understanding exotic species:
Increased disturbance by humans + Increased dispersal by humans = New systems with dominance of exotic species
This equation is useful in illustrating the two main causes of exotic invasions but it especially focuses on the idea that the resulting systems are new. To some this is an exciting concept in that these are systems that have never existed previously, and they are new challenges for science to describe and explain. To others this is an environmental disaster that requires remediation or restoration. While the concept is a philosophical statement, there is a definite reality in the new organization of systems with exotic invasion.
Some have focused on the role of disturbance by humans as a key factor in exotic invasions. Elton (1958) was the first to tie exotics to disturbance, as did Baker (1965) in his definition of weeds. More recently others have discussed the connection (Hobbs, 1989; Hobbs and Huenneke, 1992; Horvitz, 1997; Lepart and Debussche, 1991; Orians, 1986). The notion is that invasions are more likely in disturbed ecosystems because resources are available and competition from resident native
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