In many cases, food resources themselves (in the form of prey or detritus, the latter including carrion and dead plant material) are passively or actively moved between habitats. A thorough discussion of detrital inputs is available elsewhere in this volume, so here we will touch on that topic only briefly, and focus instead on the movement of prey.
Detritus represents a significant food resource for many systems, both terrestrial and aquatic, and spatial subsidies through detritus are common across many natural boundaries. Terrestrial coastal areas receive as much as 2000 kgm~ of shoreline per year in the form of dead algae, seagrass, and carrion; leaf and litter fall represent massive inputs of detritus into stream ecosystems; benthic communities in aphotic zones are entirely dependent on detritus and food resources coming from the photic zone of the ocean, and new volcanic islands often have food webs based entirely on detrital input aerially deposited from other terrestrial ecosystems. Spatial subsidy through movement of detritus is a common phenomenon occurring across a range of spatial and temporal scales, and is generally driven by abiotic forcing (e.g., wind, waves, gravity, or currents).
The movement of prey items similarly impacts a vast number of habitats. A clear example is illustrated by the movement of emerging aquatic insects, where one study found as much as 97% of the biomass of emerging insects from a stream was transported to riparian terrestrial consumers including birds, herpetofauna, and arthropods. Other groundbreaking research on stream systems clearly showed a strong seasonal and reciprocal dynamic to spatial subsidy - during the summer, terrestrial invertebrates falling into the stream subsidized aquatic consumers, representing 44% of the total energy budget for fishes. Conversely, during the spring emergence of aquatic insects, allochthonous inputs from the aquatic environment provided the terrestrial ecosystem (specifically birds) with more than 25% of their total energy, and as much as 98% for a species of wren. This biotically mediated reciprocal flux between the forest and stream can alter community structure by causing top-down effects from subsidized predators. The dynamic and reciprocal nature of this spatial subsidy, while rarely experimentally tested, is likely common in nature, particularly wherever adjacent habitats undergo asynchronous pulses of productivity.
Spatial subsidies through the movement of prey items are, however, not confined to those that cross the aquatic-terrestrial boundary as in the examples above, but are commonplace in many environments and at many spatial scales. In particular, organisms which undergo 'ontogenetic shifts' in foraging or habitat-use patterns often mediate these subsidies. Cicadas, termites, and other emerging insects provide a subsidy from below-ground to aboveground ecosystems; ontogenetic shifts in foraging and movement patterns of California killifish (Fundulus parvipinnis, a small wetland resident fish) subsidize predators in subtidal ecosystems with energy these fish derived from foraging as juveniles in high intertidal pools; migrations of potential prey items, from Monarch butterflies to caribou, provide consumers in distant habitats with substantial trophic subsidies. Taken as a whole, subsidies resulting from the movement of prey (or detritus) can range in distance from a few centimeters to many thousands of kilometers, and over timescales from a matter of hours to decades.
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