Box 1. Spatial concordance – a basis for resolving intra- and inter-ecosystem patterns
Ecosystems can be distinguished by predicting their component features (Table S1) and the spatially structured relationships between them. These complex relationships span biotic and abiotic domains, scales, and levels of ecological organization. To resolve these complexities, and how they vary across landscapes, we focus on variations in spatial concordance within and among assemblages of biotic (B) and abiotic (A) variables.
Spatial concordance is a condition where two or more ecosystem features show similar scale-specific responses (e.g., presence, abundance) across shared segments of a geographic gradient (plots A and B – right panel). Concordant spatial relationships can arise between individual biotic (e.g., local species populations – O’Neill 2001) and abiotic (e.g., inorganic elements – Austin 2013) ecosystem constituents. Furthermore, concordances can occur across structural (e.g., vegetation physiognomy – Yang et al 2017), compositional (e.g., community – Pinto-Ledezma and Cavender-Bares 2021), and functional (e.g., productivity – O’Neill 2001) ecosystem features. Most studies have focused on concordance among biotic properties (Soranno et al 2019) and particularly how they relate to community variations (e.g., identity – Austin 2013) or to landscape heterogeneity (Pickett and Cadenasso 1995). Instead, we emphasize spatial concordance within and between biotic and abiotic domains (plot AB – right panel). This integrative approach reflects the reciprocal roles (Gignoux et al. 2011, Richter and Billings 2015, Halvorsen et al 2020) of biotic-abiotic constituents and processes in ecosystem assembly (Keith et al 2022). It also provides a means to parse individual and aggregate (Table S1) sources of ecosystem variation and to identify the strongest co-varying biotic-abiotic relationships.
Variations in spatial concordance reveal relationships among ecosystem features and how they scale up to form different ecosystems and the boundaries separating them. To resolve these relationships and their roles in shaping intra- and inter-ecosystem patterns, we distinguish areas where spatial concordance levels are relatively uniform and continuous, from adjacent areas where these collective properties show pronounced change. The latter correspond with boundaries – some narrow and relatively discrete, others wider and more diffuse (Cadenasso et al 2003) – between neighbouring ecosystems. Distinguishing adjacent ecosystems and their common boundaries requires resolution of joint distinctions in biotic-abiotic composition and structure.
To illustrate these patterns, random point occurrences of biotic and abiotic variables were independently simulated across a categorical landscape. The map (left panel) summarizes how these occurrences relate to one another, showing landscape areas marked by: (1) high biotic and abiotic; (2) moderately high abiotic, low biotic; (3) moderately high biotic, low abiotic; and by; (4) low biotic, low abiotic patterns of spatial concordance. These geographic areas are depicted in plots showing individual (plots A and B – right panel) and shared (plot AB – right panel) biotic and abiotic responses (Y axes) along a spatial gradient (X axis).