Conceptual foundation
The premise of our modelling strategy is that ecosystems can be
predicted by resolving patterns of relative spatial concordance (Box 1)
among biotic and abiotic features (Table S1). The particular combination
of features found along a geographic gradient, and the spatial
relationships among them (Figure 2), provide an integrated basis for
distinguishing conterminous ecosystems and their respective boundaries
(Figure 2). Such distinguishing properties are not absolute, for
ecosystems intergrade and vary continuously in space and time (Leroux
and Loreau 2012, Newman et al 2019, Soranno et al 2019). These
gradations occur because individual biotic-abiotic features, intrinsic
to a particular ecosystem, may transcend boundaries of locally adjacent
ecosystems. However, ecosystem-level organizational patterns can be
identified by relative differences in the collective strength of
concordance among constituent features. For example, in boreal
landscapes, tree species composition may be relatively concordant across
adjacent upland forest and treed bog ecosystems. Here, key differences
are expressed in lower vegetation layers, and in soil, organic matter,
hydrology, and topographic properties. Under this approach, different
ecosystems can be discriminated by relatively unique and consistent
combinations of features, that recur predictably across landscapes.
Landscapes of heterogeneous ecosystems are composed of these different
ecosystems. Here, each ecosystem manifests at characteristic spatial
scales (Massol et al 2011, Soranno et al 2019, Bullock et al 2022) and
has characteristic levels of integration (Rowe 1961, Loreau 2020),
uniformity (Pickett and Candenasso 1995, Halvorsen et al 2021), and
distinction (Dor-haim et al 2019, Simensen et al 2020).
A nuance of our interpretation is that we do not seek a means to
reconcile patterns of low spatial concordance between or among biotic
and abiotic ecosystem constituents. Instead, we emphasize levels of
collective spatial concordance within and across biotic-abiotic domains
(Box 1). Furthermore, we suggest collective levels of concordance are a
relative, continuous, and changeable ecosystem property. Some
ecosystems have inherently higher concordance, whereas others are
characterized by lower relative levels of this emergent ecosystem
property (Loreau 2020). Holling (1992) attributed differences in overall
concordance to keystone variables. Keystone variables are individual
ecological processes or constituents with a disproportionate influence
on ecosystem spatial organization and variation (Holling 1992). Over
time, these variables, many of which operate across scales, can catalyze
developmental changes in other aspects of ecosystem structure,
composition, and function (Nash 2014, Newman et al 2019). Such changes
often lead to more consistent expressions of landscape pattern and
spatial grain (Peterson 2002), and potentially to higher levels of
ecosystem integration (Rowe 1961, Loreau 2020).