Human activity can alter the flow of spatial subsidy in a number of ways. Habitat fragmentation, for example, can increase subsidy (through small patches having greater 'edge effects'), or can decrease trophic subsidy, by leaving some patches too distant from sources to allow natural flow. Further, human activity can directly block or increase subsidy relative to natural levels. For example, the movement of spatial subsidies between coastal consumers (utilizing aquatic resources) and inland habitats has been dissected in many regions by roads and highways, potentially restricting access to these trophic resources for wide-ranging species such as coyotes or small mammals. This lack of connectivity could have wide-ranging effects on terrestrial communities both by lowering population densities of consumers that are subsidized by aquatic resources and by restricting the flow of aquatic nutrients to inland.
There are numerous other examples: the creation of levees around rivers diminishes the aquatic subsidy to the surrounding terrestrial floodplain; urban development in coastal zones reduces the ability for terrestrial organisms to obtain spatial subsidy from the ocean; and destruction of foraging habitat along migratory bird fly-ways can greatly lessen the subsidy obtained from intermediate habitats. Over 90% of Pacific saltwater and Great Lake freshwater marshes have been lost, largely due to direct human habitat alteration, leading to limitation of aquatic resources available to terrestrial consumers. On southern California beaches in the United States, macrophyte wrack washing up in the intertidal provides a regular subsidy for macrofauna, which in turn provide food for shorebirds. The practice of 'beach grooming' (using heavy equipment to remove macrophytes and debris from sandy beaches) in populated areas directly removes this subsidy and thus mitigates the associated bottom-up effects.
Anthropogenic processes and mechanisms of spatial subsidy can be even more direct, however. In any given year, millions of tons of crops are moved between continents, and some 75 million metric tons of marine biomass onto land worldwide, including 27 million tons of discarded nontarget animals (bycatch), as well as 126 5001 (dry weight) of algae, with ramifications for the entire oceanic ecosystem, as well as subsidizing human populations often quite distant from the ocean. Each of these movements of resources not only represents a spatial subsidy of its own, but also suggests that removing these quantities of organisms from the ecosystem may well be altering natural levels of subsidy occurring there.
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