Fig. 3. Example typologies of relationships between sending, receiving, and spillover systems. (A) represents one-one-one relationship and (B) represents select many-many-many relationships. Arrows indicate directions of flows. More arrows are possible between sending, receiving, and spillover systems but are omitted for the sake of simplicity. Spillover systems may be connected to sending and receiving systems in at least three ways: by being in the pathway between the sending and receiving systems (e.g., oil spill by tanker in transit; straight dashed arrow), by being an intermediate stopover between the sending and receiving systems (e.g., migratory bird stopover or airport layover; long-dashed curvy arrow), or by interacting with sending and/or receiving systems in other ways (e.g., third party in transnational land deals; short-dashed curvy arrow).