This graph depicts the hierarchy of mechanisms underlying ecological change (black line) as ecosystems are exposed over time to chronic resource alterations associated with global changes (e.g., elevated CO2, nitrogen deposition, and climate shifts). Fairly modest initial ecosystem responses may reflect relatively rapid individual-level responses (A), with the magnitude and extent of these initial responses limited by traits of the resident species. Larger shifts in ecosystem response are expected with reordering of species (B) in the community (e.g., shifts in relative abundance). The timing and duration of this phase may vary depending on variation in traits and the rate of population turnover or may be attenuated depending on internal interactions. Finally, immigration of new species better suited for altered resource levels may result in further change in ecosystem response (C). Timing may depend on the regional species pool and dispersal limitation. Other responses to chronically altered resources are possible, including gradual linear change (thin grey line) if the magnitude and rate of change were similar for all three mechanisms (A, B, and C). The HRM has potential exceptions. For example, ecosystems dominated by very long lived species with slow turnover rates, such as forests, may appear to be resistant to change (D) as resources accumulate over time. Conversely, ecosystems that become susceptible to invasion by exotic species or pests and pathogens due to resource alterations may bypass changes driven by individual-level responses or community reordering and could experience large shifts in structure and function in a relatively short period of time (E).
Credit or Source: Figure modified from Smith, M. D., A. K. Knapp, and S. L. Collins. 2009. “A Framework for Assessing Ecosystem Dynamics in Response to Chronic Resource Alterations Induced by Global Change,” Ecology, 90(12), 3279-89.
U.S. DOE. 2008. Carbon Cycling and Biosequestration: Report from the March 2008 Workshop, DOE/SC-108, U.S. Department of Energy Office of Science. (p. 73) (website)