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).