The movement of organisms between habitats is a fundamental process that occurs within all ecological disciplines. Once the organism has entered a recipient habitat, its behavior generates a movement pattern as it may remain, move on, return, or constantly move between, producing different population dynamics and environmental changes. Originally, “spillover” was one of these distinct patterns - the uni-directional movement from a donor habitat to a different, adjacent recipient habitat. However, ecological definitions of spillover have become overly generalized to include any cross-habitat movement. As spillover research has nearly doubled since 2018, it is imperative to have universal nomenclature and methods that can quantify the term. This will allow us to advance our understanding of organism behavior in dynamic landscapes. To assess how spillover has been addressed within empirical literature, we reviewed 337 studies and organized them into the ecological disciplines of agriculture, disease, fisheries, and habitat fragmentation. Each study’s definition of spillover and the methods used to quantify the term was analyzed using four definition criteria that differentiates different cross-habitat movement patterns. We then determined the specificity of the definition and if the methods used matched the definition provided. Authors often define spillover as a movement (45%) but assess the location of organisms instead (96%). Additionally, 98% of studies assume direct movement out of the donor habitat, which can lead to an over-estimation of movement distance and spillover effect within the recipient habit. Overall, few studies (12%) included methods that matched their own definition, revealing a distinct mismatch. This was particularly noticeable within the fisheries discipline, likely the result of studies incorporating 1.5-fold more criteria within their definitions. There is much theory of the potential impact of organism movement, yet different movement patterns are often not differentiated empirically. Thus, the actual impact within natural systems is unclear. This ambiguity additionally limits communication and collaboration that need universal definitions and methodology. Techniques that quantify movement directly, such as tracking and capture-mark-recapture, are more suited to understanding effective long-term management implications and the impacts of disturbance on populations and communities.

We compiled a database of spillover studies that were found in the Web of Science (http:www.webofknowledge.com) records up to March 31, 2021. We used the search terms “spillover” and “spill over”. We then excluded categories outside of the biology discipline (e.g., economic or chemical spillover) as well as human epidemiology literature, which uses language very distinct from ecological work. As the remaining publications included several hundred papers disparate from organismal spillover, we further refined our search using the terms “organism”, “biodiversity”, or “population”. Articles included in our database 1) specifically mentioned “spillover” or “spill over” within the text, 2) used living organisms as the propagule of movement, and 3) empirically assessed spillover or the consequences of spillover. In this way, we collected a broad spectrum of papers that included ecological, behavioral, evolutionary, applied, and basic science articles.

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