Abundance/Occupancy Relationships
The abundance-occupancy (A/O) relationship refers to the prevalent pattern among macroorganisms, whereby more abundant species occupy more locations within a given range (Gaston et al. 2000). It has been shown both within and among species. Most A/O relationships report positive correlations but fail to investigate the potential reasons why this relationship forms. There are a handful of studies that show weakly positive, or even negative trends amongst plants (Boeken & Shachak 1998), fish (Marshall & Frank 1994), and birds (Blackburn et al.1999b). In addition, evidence of positive A/O relationships have been found for microorganisms, with protists (Warren & Gaston 1997), bacteria (Holt et al. 2004), and freshwater diatoms (Heino & Soininen 2006; Spatharis et al. 2009; Soininen & Teittinen 2019).
In general, the mechanisms resulting in the A/O pattern are not well understood. The A/O relationship can occur when extinction and colonization are density-dependent (Warren & Gaston 1997). In experimental microcosms with constant resources, when dispersal was allowed, the relationship tracked the carrying capacity hypothesis (i.e. locally abundant species have lower extinction and higher colonization rates than species with low abundance; Holt et al. 2002). However, this study found that the relationship was not contingent on dispersal; without dispersal, the A/O relationship arose through extinction (Holt et al. 2002). Moreover, the A/O relationship does not always depend on environmental resource consistency. For example, the relationship held in heterogeneous environments, though to a lesser degree than in homogeneous environments (Holt et al.2004). In field-based observations, the A/O relationship has been observed with soil fungal assemblages where dispersal was shown to have less of a role (Kivlin et al. 2014). Mechanisms behind this pattern were attributed to soil nutrients, though the relationship was weak (Kivlin et al. 2014).
Niche-based processes can also create A/O relationships among organisms. In microcosm experiments with protists, biotic interactions strengthened A/O relationships (Holt et al. 2002). Likewise, in natural systems Rocha et al. (2018) found that niche-based variables were the primary predictors for the positive A/O relationship they observed in freshwater diatoms and that the effect of niche breadth on the relationship was less important than local environmental niche position. In animal host-associated intestinal bacteria, Green et al.(2016) found support for the A/O relationship that they attributed to multiple factors, including dispersal limitation, host selection, historical contingency, and stochastic processes.