Introduction and Definition

There is an increasing appreciation in ecology for the fact that even seemingly insular and discrete habitats are often deeply interconnected. This connectivity can arise from a number of mechanisms, including the provision of trophic resources from outside the focal habitat. This subsidization from across the habitat boundary is known as 'spatial subsidy'. Synonyms and related terms include allochtho-nous inputs, trophic subsidy, and cross-boundary subsidy.

Spatial subsidy arises when nutrients, food resources (detritus, primary producers, or prey items), or consumers move across boundaries, thus 'subsidizing' local food resources in the 'recipient' habitat with those from other 'donor' habitats or ecosystems.

The conceptual origin for the integration of spatial subsidy into our understanding of ecosystems goes back at least to the late 1800s, when the strong trophic interactions between terrestrial and limnetic ecosystems were recognized. Spatial subsidy gradually became better integrated into ecology, in particular with respect to oceanic, riverine, and limnological ecosystems. More recently, we have seen an expansion of recognition of spatial subsidy due to the work of Gary Polis and colleagues, who integrated landscape, population, and food web ecology by focusing on the contributions of spatial subsidy to ecosystem dynamics.

Subsidies can occur between any two habitats, from those that are relatively similar (e.g., different water masses within the ocean) to those that are strikingly dissimilar (e.g., benthic and pelagic or terrestrial and aquatic habitats). Further, these subsidies can occur across a vast range of temporal and spatial scales, and can be driven by a range of biotic or abiotic processes, with ramifying effects throughout the ecosystem.

An understanding of spatial subsidies is critical to basic, theoretical, and conservation ecology. So despite the fact that ecologists often tend to focus tightly on a specific habitat or subdiscipline, external forcing due to spatial subsidies can often be the dominant factor driving the dynamics within a given habitat. Subsidies can have effects at the population level for individual species; at the community level, through changing species interactions; and at the ecosystem level, altering biodiversity and stability patterns.

This article presents examples of spatial subsidies that result from the movement of nutrients, food resources, and consumers. Along the way, key aspects of the mechanisms (e.g., unidirectional vs. bidirectional; biotic vs. abiotic) and scale (temporal and spatial) of subsidies are addressed, as well as the types of effects these subsidies have on the recipient ecosystem. Finally, the implications of understanding spatial subsidies for basic, theoretical, and conservation ecology are presented.

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