Historical Range of Variability

Historical range of variability (HRV) describes the conditions of a natural system prior to intensive human alteration of that system. In this context, a natural system can be an ecosystem or a particular component of an ecosystem. Ecosystem components can be quite diverse, including the population of a species or geographic range of a species; an aspect of the disturbance regime, such as the frequency, severity, and spatial extent of wildfire or drought; or physicochemical parameters, such as water chemistry. Regardless of the component to which HRV is applied, the intent is to understand the range of variations in relevant parameters in the absence of human influence on the system. With respect to environmental science, deviations from HRV are taken as evidence of human influence on the system under consideration, and resource management is designed to maintain systems within HRV or within a socially preferred range based on HRV. The concept of HRV was initially developed by ecologists working in North America, and it was applied to understanding ecosystem characteristics prior to European settlement of a region. Although HRV was first mentioned in the early to mid-1990s, the idea of using historical conditions as a reference for ecosystem management goes back much further. HRV built on this earlier work by explicitly considering spatial and temporal variability of system components and processes. Systems exhibit variability through time because conditions change in response to disturbances. The concepts of Disturbance Regime and HRV are thus closely coupled. The assumption underlying HRV is that a system exhibits characteristic behavior and complexity when disturbances occur with a characteristic behavior through time. If human activities alter the disturbance regime, the system changes so as to exceed the bounds described by HRV. HRV is now used by scholars in diverse disciplines, and it is sometimes defined for very different time periods. HRV has been used, for example, to refer to variability during the recent past and intensive human alteration of the system. Consequently, it is important to understand the manner in which HRV is being defined in any particular study. HRV is also referred to as range of natural variability and reference variability. Regardless of the phrase used, the underlying concept represents some of the most fundamental questions we can ask in environmental science: What is the natural range of variability in a system, and in what manner have human activities altered this range?

Place, Memory, and Climate Change

Scientists warn about the difficulty of predicting ecological relationships as climate conditions for many places begin to move well outside their historical range of variability. In recent years, ecologists have identified “no-analog” communities, associations of species in the past that arose because of novel climate conditions not found at present. They have suggested that the planet is heading toward a similar period of disappearing climates and “ecological surprises.” What role, if any, can history play as Americans enter that new world?