2.1. Handling the high dimensionality and complexity challenges
ACS Science provides a comprehensive, complex systems framework that can address the high dimensionality and complexity challenges in sustainability science. Compared to Complex Adaptive Systems [
26] or Agent Societies [
27] to which ACS are similar to, ACS emphasize the pivotal role of individual actors (i.e., agents in ACS) or entities (objects) that make choices (decisions) and/or act in order to pursue a certain goal [
28]. Agents exist within ACS and interact with one another (
Figure 1, dashed arrows) and with the environment. Agents possess different degrees of autonomy, proactivity, and intellectual capabilities, such as memory, knowledge, reasoning, learning, social capital, and adaptation. Computationally, agents are represented as software abstractions that bundle a particular set of attributes (or traits) and methods (or actions). Algorithmically, agents follow rules ranging from very simple “if-then” (reactive decision) rules to sophisticated ones based on evaluating the future consequences of alternative decisions [
29]. This representation builds on a unique ontology (
Figure 1) in which real-world actors are represented as heterogeneous, individual agents that comprise ACS and generate the interactions in question [
30,
31]. This ontology of methodological individualism represents a shift from understanding aggregate agent features and/or relationships to the individuals and micro-level processes (including interactions) that constitute and explain such aggregate features (detail in
Appendix B). Given the features in this ontology (
Figure 1), ACS science offers a comprehensive, complex systems framework, which can guide sustainability scientists and practitioners from the following perspectives.
First, social-environment systems under sustainability challenges can be examined in a hierarchical structure, where agents at one level or location may affect and be affected by agents at other levels or locations. To demonstrate the applicability and usefulness of this hierarchical structure, we performed a literature survey of empirical studies in both ACS and sustainability sciences (detail in
Appendix C). We found that agents affect one another across lower-, focal-, and upper-levels; for example, individual migrants (lower-level agents) affect their households (focal agents) through sending back remittances [
32], wastepaper markets (upper-level agents) affect decisions of wastepaper suppliers and recyclers (focal-level agents) [
33]. For more examples, see
Table S1 in
Appendix C (under the lower-, focal-, and upper-level agent subcategories) and
Appendix E, where individual monkey agents and monkey group agents affect each other across focal- and upper-levels.
Second, the importance of tracking (to a large degree) the behavior of autonomous, heterogeneous, and decision-making agents should be appreciated in SES. For instance, tracking movement of prey animals, predator animals, and hunters makes the ACS related simulation as realistic as possible: they encounter, hunt, or predate on the heterogenous landscape at certain times. This way, the simulation gives rise to meaningful results when alternative behavioral models are plugged in the agent-based model (ABM), testing the reliability of various theories of social behavior of hunter–gatherers behind these behavioral models (
Appendix F).
Third, exploration of sustainability challenges should be administered in a dynamical, progressive way. This suggests that environmental conditions at earlier times may constrain those at current time, which may in turn further constrain those at future times. In this regard, there exist a plethora of case studies regarding the impacts of historic precipitation, disasters, fires, local weather conditions, and land use on the current environment (
Table S1 under historic, focal, and future environment subcategories). Similarly, adjacent or distant environments may affect and be affected by the environment at the same level through various mechanisms such as the telecoupling effect [
34] (
Table S1 under the Same level (adjacent/distant) environment subcategory).
Fourth, decisions or actions of agents at one time or location may influence their own and other agents’ decisions and/or actions, which may translate to system-level events and/or emerging outcomes at later times or other locations. Abundant examples exist regarding how agents affect one another through crop choice, land abandonment, social norm changes, coastal defensive buildings, trading of goods, and other interactions in SES and ACS (see
Table S1 under several agent-agent interaction subcategories); more discussion is in
Section 2.4.
Fifth, at the system level, attention should be paid to mutual influences between SES (or ACS) across different levels, between parallel SES (ACS), or among different times. For instance, to project future human migrations and changes in the environment, Kniveton and associates point out that the interactions between parallel ACS in the future can be assessed by the exchange of information of migration destinations within a social network, which can be viewed as interconnection between the local system of migration origin and outside systems of migration destinations [
35]. More examples about system-level SES/ACS interactions are presented in
Table S1 (under various ACS-ACS interaction subcategories).
Finally—as a result of all the above points—this ontology provides a framework that captures the essence of many SES processes and dynamics (e.g., adaptive decision-making and the co-evolutionary aspect of ACS or SES). It guides sustainability interests in the formulation of goals (e.g., focus on focal-level alone or at focal-, lower-, and upper-levels), data collection (e.g., collect data at one time or multiple times), and analysis and modeling (e.g., perform cross-sectional data analysis, time series analysis, or simulation).
2.2. Providing an effective platform for systems integration
The modeling advances of ACS Science point to its potential in addressing the aforementioned high dimensionality, complexity, and other problems of SES and sustainability given the following considerations:
Agents: what agents (or actors in sustainability science; see
Appendix A), attributes and/or traits, and behaviors of the agents should be included at each level of the corresponding ACS or SES?
Environment: what attributes and processes should be included (especially those affected by and feed back to affect agents) at each level? In ACS, the environment can be broadly defined to be the context other than the agent under consideration, such as the space (land) and/or other agents can be the environment.
Agent-agent and agent-environment interactions: what relationships (expressed as rules, influences, or actions) among agents or between agents and the environment govern system dynamics at each level? What cross-level (e.g., from upper- to focal level) relationships are needed to account for systems dynamics and complexity?
Systems-level complexity (e.g., emergence): what emerging patterns may arise from the interactions? Such patterns, often not the sum of the system’s parts, cannot be analytically solved by examination of the system’s parts alone
i. This complexity includes surprises, path dependence, nonlinearity, self-organization, contingency, emergence, multifinality, and equifinality (for definitions see Liu et al. [
13] and An [
37]).
Sustainability science examines human-environment relationships in which actors/agents are people or people groups and the environment is the biophysical world. It seeks to understand the interactions between the two subsystems, which, more often than not, requires attention to components and interactions within or between subsystems. It is also open to applications of various methods and models, especially those that can handle integration among the components of SES [
38]. ACS science, in contrast, examines any kind of relationships, agents, and subsystem interactions (e.g., bacteria and their hosts) and has heavily leveraged the use of ABMs, although cellular automata [
39], partial differential equations [
40,
41,
42], cell-based stochastic modeling [
43], and structural equation modeling [
44] are not uncommon (for detail see
Table S1). Regardless of the range of agents entertained, ACS science provides a platform for systems integration applicable for sustainability science topics, including integration of data, information, and knowledge gained from case studies, stylized facts, role-playing games, and laboratory experiments (e.g., the four empirical approaches for social science research by Jansen and Ostrom [
45]). Significantly, agent-based modeling (ABM), as a prime ACS method and tool (e.g., credited to do “a new kind of science” [
46]), provides a way to fuse the deductive-mechanistic and the inductive-empirical approaches that pervade different pathways toward understanding and envisioning ACS, earning it the moniker of a “third way of doing science” [
47] (see endnote
ii for more discussion).
Perhaps the most advantageous feature of ABM is its capacity to provide a platform and tool for systems integration, a major goal of sustainability science [
16]. Mimicking the realistic (though tailored and simplified) structure and processes of the system under investigation (
Figure 1), ABM seeks to “translate” real-world actors, environment (e.g., forestland), and constraints (e.g., land use regulations;
Figure 1) into virtual agents, virtual environment (e.g., land pixels), and computerized rules (e.g., if A then B else C), offering opportunities for integrating heterogenous data, knowledge, models/methods that cross spatial, temporal, and organizational scales, disciplines, and borders (e.g., political) [
49] (see the exemplar ABM in
Appendix E). ABMs are powerful when modeling learning and adapting processes [
31,
50,
51], accounting for heterogeneity, bounded rationality and incomplete knowledge/information, and nonlinearities [
52,
53], and exploring many complexity features such as path-dependence, abrupt changes, and critical thresholds, among others [
13,
37].
ABMs have been widely developed and used in ACS studies to address problems confronting social, environmental, and social-environmental systems since the 1990s [
55,
56]. These endeavors have generated a rich legacy of ABM methodology, such as the Overview, Design concepts, Details (ODD) protocol for model documentation [
57] and the Pattern-oriented Modeling (POM) approach [
58] for model validation. At the same time, ABM endeavors have enriched the literature in sustainability science in terms of modeling human behavior [
24,
31] (e.g., the frameworks for Belief-Desire-Intentions and physical, emotional, cognitive, and social factors [
27,
59]), exploring how adaptive behavior, abrupt changes, crises or disasters, and critical transitions may generate surprising patterns in the corresponding SES [
13,
53,
60], life cycle assessment [
61,
62], and modeling emergent macro-level outcomes and pathways under various policies or interventions [
49,
53,
63,
64].
A milestone in the sustainability science and ABM nexus was a 2006 special issue of
Ecology and Society [
45] addressing various empirical methods by which ABMs were empirically tested for SES. Subsequently, ABMs applied to sustainability problems have significantly increased, although they comprise only about 1.24% of all sustainability science publications in 2021 (
Figure 2). Among the 29 ACS cases in our literature survey, 22 use ABM as the major method, while among the 32 sustainability science cases, only nine use ABMs (
Table S1 in
Appendix C). Aside from a variety of challenges in developing and employing ABMs (e.g., sharp learning curve, high data demand, programming difficulties) [
31,
52,
55], the relative unfamiliarity of ACS science and ABMs in the sustainability science community highlights the timeliness and importance of this article.
2.3. Handling alternative pathways or theories in sustainability
ACS science has been wrestling with “finality” related challenges, which also abound in sustainability problems. Equifinality—a macro-level pattern can be generated through different pathways from micro-level processes [
65]—confronts the search for mechanistic processes. In ACS science, for instance, cooperation or betrayal in the Prisoner’s Dilemma can emerge from tit-for-tat retaliation [
66], strong reciprocity [
67], and group selection [
68], among other strategies [
69]. As a double-edged sword, equifinality may offer more explanatory pathways, but also question the validity of explanations because different theories can reproduce very similar or even the same macro-patterns. In contrast, multifinality—the same causes and/or starting conditions lead to very different outcomes—also poses challenges to our understanding for mechanistic approaches [
55].
The Pattern-Oriented Modeling approach [
58,
70], overlapping with Approximate Bayesian Computing [
71] in ACS, offers a possible means to address the “finality” challenges. It is based on the multi-criteria design, selection, and calibration of models by requiring that models can simultaneously reproduce an entire set of patterns characterizing an ACS. Often a set of broad, general patterns can more effectively reduce finality issues than trying to force a model to reproduce a single pattern, such as a time series of a single variable. Given the high synergy between ACS and sustainability sciences hitherto discussed, we posit that despite its rare application in sustainability science, POM may prove useful to uncovering many sustainability related mechanisms. We refer to the example of foraging behavior model for theory testing using ABM (
Appendix F).
Given the reflexivity of human agents, the social sciences tend to approach the dynamics of the social subsystem in multiple, probabilistic ways, commonly applying both quantitative and qualitative methods. Empirical models use evidence to explore outcomes and plausible, inductively derived explanations. These “top-down” models reproduce macro-level patterns that lend themselves to explanatory interpretations. For example, empirical models can accurately reproduce flight patterns of birds, even emergent ones, in the absence of theory explaining the patterns (but offering insights about the outcome to be explored). Mechanistic or “bottom-up” models, common in the biophysical sciences and some parts of the social science (e.g., economics), rely on theory-based deductive approaches. ACS science supports both approaches because its ontology explicitly represents the behavior of agents, for which theory exists and can be tested, while also providing environmental responses to that behavior and agents’ responses to the changes in the environment (
Figure 1). This mechanistic and empirical blend opens opportunities to identify and explore integrated human-environment theory[
38]. ACS science has empowered computational social science, allowing researchers to explore social phenomena and test hypotheses by virtue of computer-based simulations of agents and their interactions [
72], nurturing a generative social science in which the dynamics are “grown” in the assessment stages [
73].
2.4. Enabling and evaluating processes and temporal progression
Revealing temporal progression in the variable of interest (e.g., amount and spatial distribution of a certain resource or wildlife habitat) is important as projected patterns, if reliable, provide insights about the system’s sustainability. For instance, the dynamic habitat maps in
Figure S2 (
Appendix E) may inform whether the conservation policy is effective. A “byproduct” of such temporal progression information is its usefulness for model evaluation. Many investigations evaluate models (mostly statistical models) based on their goodness of fit or the maximum likelihood. Modelers strike a balance between fitting the data (e.g., by adding more parameters or equations) and keeping the explanation as simple as possible [
74], reflecting the long-time trade-off between generalizability and context [
45]. Evaluation of ACS models, however, does not depend extensively on statistical performance. Rather, the ACS may provide insights into the viability of the mechanistic (e.g., cognitive, institutional, and/or social) processes accounting for ACS dynamics. In this case, the ACS informs us if the processes are justifiable or not.
ACS science assists in assessing outcomes, which represent states of agents and the environment at a certain level or temporal stage, and evaluate processes and temporal progression [
16], asking whether the direction, magnitude, and significance of certain parameters are supported by existing theories. In essence, all the elements and arrows in
Figure 1 and
Table S1 can be check points for SES documentation, assessment, or model evaluation. As the “new kind of science”, ACS science can leverage the patterns or trajectories (“data”) generated by ABM simulations, evaluating whether and how much such “data” qualitatively and quantitively agree with empirical observations or theory. For instance, sustainability researchers may consider whether the univariate and bivariate statistics or regression coefficients based on such “data” are reasonable and supported by existing theory. Furthermore, the POM approach can escalate our confidence about our understanding of the ACS and its behaviors. Finally, the ACS ontology (
Figure 1) facilitates the development of new tools, platforms, or models, a high-priority research area in sustainability research [
16]. For instance, An and colleagues [
54] followed this ontology and developed a model to explain space-time dynamics among monkey behavior, habitat degradation, human resource collection activities, and nature reserve management policies in a Chinese nature reserve (
Appendix E).