Ecosystem modeling, a pillar of the systems ecology paradigm (SEP), addresses questions such as, how much carbon and nitrogen are cycled within ecological sites, landscapes, or indeed the earth system? Or how are human activities modifying these flows? Modeling, when coupled with field and laboratory studies, represents the essence of the SEP in that they embody accumulated knowledge and generate hypotheses to test understanding of ecosystem processes and behavior. Initially, ecosystem models were primarily used to improve our understanding about how biophysical aspects of ecosystems operate. However, current ecosystem models are widely used to make accurate predictions about how large-scale phenomena such as climate change and management practices impact ecosystem dynamics and assess potential effects of these changes on economic activity and policy making. In sum, ecosystem models embedded in the SEP remain our best mechanism to integrate diverse types of knowledge regarding how the earth system functions and to make quantitative predictions that can be confronted with observations of reality. Modeling efforts discussed are the Century ecosystem model, DayCent ecosystem model, Grassland Ecosystem Model ELM, food web models, Savanna model, agent-based and coupled systems modeling, and Bayesian modeling.

The systems ecology paradigm (SEP) emerged in the late 1960s at a time when societies throughout the world were beginning to recognize that our environment and natural resources were being threatened by their activities. Management practices in rangelands, forests, agricultural lands, wetlands, and waterways were inadequate to meet the challenges of deteriorating environments, many of which were caused by the practices themselves. Scientists recognized an immediate need was developing a knowledge base about how ecosystems function. That effort took nearly two decades (1980s) and concluded with the acceptance that humans were components of ecosystems, not just controllers and manipulators of lands and waters. While ecosystem science was being developed, management options based on ecosystem science were shifting dramatically toward practices supporting sustainability, resilience, ecosystem services, biodiversity, and local to global interconnections of ecosystems. Emerging from the new knowledge about how ecosystems function and the application of the systems ecology approach was the collaboration of scientists, managers, decision-makers, and stakeholders locally and globally. Today’s concepts of ecosystem management and related ideas, such as sustainable agriculture, ecosystem health and restoration, consequences of and adaptation to climate change, and many other important local to global challenges are a direct result of the SEP.

Emerging from the warehouse of knowledge about terrestrial ecosystem functioning and the application of the systems ecology paradigm, exemplified by the power of simulation modeling, tremendous strides have been made linking the interactions of the land, atmosphere, and water locally to globally. Through integration of ecosystem, atmospheric, soil, and more recently social science interactions, plausible scenarios and even reasonable predictions are now possible about the outcomes of human activities. The applications of that knowledge to the effects of changing climates, human-caused nitrogen enrichment of ecosystems, and altered UV-B radiation represent challenges addressed in this chapter. The primary linkages addressed are through the C, N, S, and H2O cycles, and UV-B radiation. Carbon dioxide exchanges between land and the atmosphere, N additions and losses to and from lands and waters, early studies of SO2 in grassland ecosystem, and the effects of UV-B radiation on ecosystems have been mainstays of research described in this chapter. This research knowledge has been used in international and national climate assessments, for example the IPCC, US National Climate Assessment, and Paris Climate Accord. Likewise, the knowledge has been used to develop concepts and technologies related to sustainable agriculture, C sequestration, and food security.