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.