The principles outlined are not limited to temperatures or elevation gradients. They should apply whenever two or more species comprise a competitive hierarchy over any gradient. For example, we would expect a salt-tolerant species to occur more frequently in nonsaline habitats, surrounding a saline source population, when there is an optimal level and pattern of disturbance in the surrounding matrix. Understanding the nature of the realizable niche and the resulting determinants of range boundaries offers rich material for further investigations.
Ecological theorists have helped identify how competitive exclusion can be reduced; in return I propose disturbance and competitive hierarchies on environmental gradients merit attention from theorists. The simple theory advanced in Figure 2 and illustrated in Figure 3 has consequences for local and regional species coexistence and diversity. Where fundamental niches overlap, an intermediate disturbance-type behavior will operate. This behavior refers to the intermediate disturbance hypothesis, meaning that species coexist and richness is maximized at some intermediate level of disturbance, i.e., too little allows exclusion and too much prevents one or more species from maintaining positive population growth (Connell 1978, Sheil and Burslem 2003, Shea et al. 2004). This deceptively simple idea involves many complexities. Various aspects have been discussed in detail elsewhere (see, e.g., Wilson 1994, Chesson 2000, Miller et al. 2011, Fox 2013, Sheil and Burslem 2013, Huston 2014). Nonetheless, the influence of extended gradients and competitive hierarchies upon such relationships appears unexplored, although I speculate that these may reflect significant aspects of many natural systems. The gradient can help stabilize the presence of many species by ensuring that each maintains population growth somewhere in the wider landscape. As long as such populations persist, many species may occur largely as sink populations in the wider landscape (Pulliam 2000). The generation and persistence of such source populations would be the focus of any conservation interventions; this would be a topic in which theoretical ecology again offers potential guidance (e.g., Chesson 2000, Siepielski and McPeek 2010) and would require more attention to spatial processes (e.g., Acevedo et al. 2015, Zelnik et al. 2015) and to temporal processes (such as seed persistence) than I have attempted here.
and intermediate levels of disturbance are difficult to define.
Miller, A. D., S. H. Roxburgh, and K. Shea. 2011. How frequency and intensity shape diversity-disturbance relationships. Proceedings of the National Academy of Sciences 108(14):5643-5648.
Issues affecting the measurement of disturbance …
Exclusion is well studied in simple diversity-disturbance models in which all but the most competitive species are considered fugitive species; these persist only by avoiding the sustained competition that would otherwise eliminate them. Under some disturbance regimes species coexist in a stable manner, whereas in others species are lost. The theory of such coexistence mechanisms have been rigorously examined elsewhere (e.g., Chesson 2000). Persistence for the fugitives requires both sufficient opportunities to avoid competition and a stabilizing mechanism to ensure small populations can grow. Often this stabilizing mechanism involves a trade-off between competitive abilities and each species’ colonization and dispersal abilities, which confer a short-term advantage to the fugitive in terms of access to low competition sites (Amarasekare 2003).
Define disturbance | Dictionary and Thesaurus
Fox, J.W. (2013a) The intermediate disturbance hypothesis is broadly defined, substantive issues are key: a reply to Sheil and Burslem. Trends in Ecology & Evolution, 28, 572-573.
Intermediate Disturbance Hypothesis ":
The last 15 years of ecological research has partly justified Lawton’s skepticism because progress in community ecology has largely rested on local studies and local generalizations. One illustration of the difficulty of devising generalities is the controversy over the intermediate disturbance hypothesis (Schwilk, Keeley & Bond 1997; Wilkinson 1999; Fox 2013a; Fox 2013b; Kershaw & Mallik 2013; Sheil & Burslem 2013). In their recent review Kershaw and Mallik (2013) concluded that confirmation of the intermediate disturbance hypothesis for all studies was around 20%. For terrestrial ecosystems only, support was about 50%. What should we do with hypotheses that fail as often as succeed? That is perhaps a key question in community ecology. Kershaw and Mallik (2013) adopt the approach that states that the intermediate disturbance hypothesis will apply only to grassland communities of moderate productivity. The details here are not important, the strategy of limiting a supposedly general hypothesis to a small set of communities is critical. We are back to the issue of generality. It is certainly progress to set limits on particular hypotheses, but it does leave the land managers hanging. Kershaw and Mallik (2013) state that the rationale for current forest harvesting models in the boreal forest relies on the intermediate disturbance hypothesis being correct for this ecosystem. Does this matter or not? I am not sure.