Priority effect

In ecology, a priority effect is the impact that a particular species can have on community development due to prior arrival at a site. In ecology, a priority effect is the impact that a particular species can have on community development due to prior arrival at a site. There are two basic types: inhibitory priority effects occur when a species that arrives first at a site negatively impacts a species that arrives later by reducing the availability of space or resources. Facilitative priority effects occur when a species that arrives first at a site alters abiotic or biotic conditions in ways that positively impact a species arriving later. Priority effects are a central and pervasive element of ecological community development. These effects have important implications for natural systems as well as ecological restoration efforts. Early in the 20th century, Frederic Clements and other plant ecologists suggested that ecological communities develop in a linear, directional manner towards a final, stable end-point: the climax community. Clements indicated that a site's climax community would reflect local climate. He conceptualized the climax community as a 'superorganism' that followed a defined developmental sequence. Early ecological succession theory maintained that the directional shifts from one stage of succession to the next were induced by the plants themselves. In this sense, succession theory implicitly recognized priority effects; the prior arrival of certain species had important impacts on future community composition. At the same time, the climax concept implied that species shifts were predetermined. A given species would always appear at the same point during the development of the climax community, and always have the same impact on community development. This static view of priority effects remained essentially unchanged by the concept of patch dynamics, which was introduced by Alex Watt in 1947. Watt conceived of plant communities as dynamic 'mechanisms' that followed predetermined succession cycles. Although Watt questioned the idea of a stable endpoint to community development, he seemed to agree with Clements that each particular species had a predetermined role to play in community development. They viewed succession as a process driven by facilitation, in which each species made local conditions more suitable for another species. In 1926, Henry Gleason presented an alternative hypothesis in which plants were conceptualized as individuals rather than components of a superorganism. Gleason suggested that the distribution of various species across the landscape reflected species-specific dispersal limitations and environmental requirements rather than predetermined associations among species. Gleason set the stage for future research on priority effects by explaining that initially identical ponds colonized by different species could develop through succession into very different communities. Thus, Gleason contested the idea of a predetermined climax community and recognized that different colonizing species could produce alternative trajectories of community development. Frank Egler (1954) built on Gleason's hypothesis by developing the Initial Floristic Composition model to describe community development in abandoned agricultural fields. According to this model, the set of species present in a field immediately after abandonment had strong influences on community development and final community composition. Although rooted in succession theory, this approach foreshadowed the development of alternative stable state and community assembly theory. In the 1970s, it was suggested that natural communities could be characterized by multiple or alternative stable states. In accordance with the conclusions of Gleason and Egler, multiple stable state models indicated that the same environment could support several different combinations of species. Theorists argued that historical context could play a central role in determining which stable state would be present at any given time. Robert May explained, 'If there is a unique stable state, historical accidents are unimportant; if there are many alternative locally stable states, historical accidents can be of overriding significance.' The development of assembly theory followed from the emergence of alternative stable state theory. Assembly theory explains community development processes in the context of multiple stable states. It asks why a particular type of community developed when other stable community types were possible. In contrast to succession theory, assembly theory was developed largely by animal ecologists and explicitly incorporated historical context.