Cross-boundary subsidies are caused by organisms or materials that cross or traverse habitat patch boundaries, subsidizing the resident populations. The transferred organisms and materials may provide additional predators, prey, or nutrients to resident species, which can affect community and food web structure. Cross-boundary subsidies of materials and organisms occur in landscapes composed of different habitat patch types, and so depend on characteristics of those patches and on the boundaries in between them. Human alteration of the landscape, primarily through fragmentation, has the potential to alter important cross-boundary subsidies to increasingly isolated habitat patches. Understanding how processes that occur outside of habitat patches can affect populations within them may be important to habitat management.
The concept of cross-boundary subsidies developed out of a merging of ideas from the studies of landscape ecology and food web ecology. The ideas from landscape ecology allow the study of population, community, and food web dynamics to incorporate spatial relationships between landscape elements into an understanding of such dynamics (Polis et al. 1997).
Janzen (1986) first defined cross-boundary subsidies as a process whereby organisms that disperse from one patch into another impact resident organisms by providing increased food resources or opportunities for reproduction, thus serving as a subsidy to the residents. By this definition, only the cross-boundary movement of organisms is considered, but broader definitions of cross-boundary subsidies can also include materials such as nutrients and detritus (i.e. Marburg et al. 2006, Facelli and Pickett 1991).
Cross-boundary subsidies are a subset of the more general process of spatial subsidies (see Polis et al. 1997). Cross-boundary subsidies acknowledge the presence and role of the boundary between different habitat patches in mediating flows of organisms and materials. In contrast, spatial subsidies require only that external inputs of materials and organisms originate from outside the patch of interest.
Few attempts have been made to combine landscape and food web ecology in such a way that explicitly recognizes the importance of cross-boundary subsidies and spatial features of the landscape on food web dynamics. Often, spatial subsidies are treated as subsidies that simply arrive from outside the patch of interest, not addressing the landscape patterns and processes that may affect the movement of these inputs, such as boundary characteristics and patch connectivity. Polis et al. (1997) published a thorough review of spatially subsidized food web dynamics, focusing on the effect of subsidies on population, community, consumer-resource, and food web dynamics. One of the main conclusions was that subsidies of consumer species (organisms that eat other organisms to obtain energy) resulted in declines of food resources in the recipient patch. Callaway and Hastings (2002) built off Polis et al.’s conclusion with a model to show that subsidized consumers may not always drive down the resource in the recipient patch if consumers move between patches frequently. This might occur because consumers often move for reasons other than food resource acquisition.
Cadenasso et al. (2003) developed a framework for studying ecological boundaries, which has implications for understanding the dynamics of specific cross-boundary subsidies. The boundary is defined as the zone of the steepest gradient of change in some characteristic from one patch to another, such as rapidly decreasing light levels as habitat transitions from a field to a forest. In this framework, flows across variable landscapes are characterized by the type of flow (materials, energy, organisms, etc.), patch contrast (architecture, composition, process), and boundary structure (architecture, composition, symbolic and perceptual features). Considering a cross-boundary subsidy in terms of this framework shows how the boundary itself can mediate the subsidy. For example, Cadenasso and Pickett (2001) found that the decreased lateral vegetation at the boundary between a forest and field increased the amount of seeds transferred into the forest interior.
Another conceptual model that specifically considers cross-boundary subsidies is a model developed by Rand et al. (2006) of spillover from agriculture to wildland patches by predatory insects. The edge is permeable to insects that are habitat generalists and therefore capable of easily crossing the boundary between agriculture and wildland patches, whereas it is considered impermeable to insects that specialize on a particular patch type and cannot cross the boundary. In this model, edge permeability (habitat specialists vs. generalists), patch productivity, and complementary resource use (use of resources obtained in both agriculture and wildland patches) determine the expected impact of cross-boundary subsidies by predatory insects (Fig. 1).