19
Methods for Determining
Historical Range of Variability
Hope C. Humphries and Patrick S. Bourgeron
19.1 Introduction
Previous chapters have emphasized the dynamic
nature of ecosystems, including the occurrence of
periodic disturbances. Consequently, current ecosystem composition, structure, and function are
likely to operate within ranges of variability that
arise from climatic variability, disturbance, and the
effects of human activities (Bourgeron and Jensen,
1994; Kaufmann et aI., 1994; Morgan et aI., 1994;
Cisse1 et al., 1998). Understanding the magnitude
and direction of anthropogenic impacts requires
knowledge of the range of fluctuations historically
experienced by ecosystems as a result of variability in climatic conditions, disturbance regimes, and
their interactions (Swetnam and Betancourt, 1998).
Therefore, the determination of the historical range
of variability (HRV) in key ecosystem patterns and
processes is an important part of ecological assessments and results in the characterization of the
range of variability in conditions to which ecosystem components (e.g., species) are adapted (Bourgeron and Jensen, 1994; Morgan et aI., 1994; Swanson et al., 1994). HRV provides a baseline for
evaluating anthropogenic changes and a means for
identifying the potential for surprise events to occur (Holling, 1986). Historical conditions serve as
a model of the functioning of ecosystems under unmodified disturbance regimes and alternative landuse scenarios. Ecosystem patterns and processes
Hope C. Humphries and Patrick S. Bourgeron wish to
acknowledge partial funding provided by a Science to
Achieve Results grant from the U.S. Environmental Protection Agency ("Multi-Scaled Assessment Methods:
Prototype Development within the Interior Columbia
Basin").
operate at multiple hierarchically structured spatial
and temporal scales, and therefore the determination of HRV should be conducted at scales that both
meet the objectives of the assessment and are appropriate for the patterns and processes of interest
(Bourgeron et al., 1994).
In North America, HRV has been reconstructed
for the period prior to significant impact from EuroAmerican settlement (Fule et aI., 1997; Kaufmann
et aI., 1998). The characterization ofHRV includes
species composition and structure, as well as the
disturbance processes that are important determinants of biotic patterns. Such patterns occur at a
variety of spatial and temporal scales. Methods for
determining HRV usually have associated scales of
resolution and spatial and temporal extents. The
appropriateness of these method-specific scales for
meeting the objectives of the assessment should be
taken into account in selecting HRV methods. No
single widely applicable period of time for assessing HRV exists for North America or elsewhere
(Landres et aI., 1999). In addition, HRV methods
are limited in their ability to reconstruct many
ecosystem patterns and processes (e.g., understory
vegetation, animal populations, nutrient cycling);
these limitations should be clearly understood by
those conducting ecological assessments. This
chapter describes methods for determining HRV in
biotic patterns and disturbance processes (Table
19.1), their characteristic temporal and spatial
scales, their advantages and limitations, and selected examples of their use, including studies that
combine more than one HRV method. The methods primarily address HRV in terrestrial ecosystems, but selected methods for aquatic systems are
also included. The examples span a wide range of
geographic and ecological conditions.
273
Methods for Determining
Historical Range of Variability
Hope C. Humphries and Patrick S. Bourgeron
19.1 Introduction
Previous chapters have emphasized the dynamic
nature of ecosystems, including the occurrence of
periodic disturbances. Consequently, current ecosystem composition, structure, and function are
likely to operate within ranges of variability that
arise from climatic variability, disturbance, and the
effects of human activities (Bourgeron and Jensen,
1994; Kaufmann et aI., 1994; Morgan et aI., 1994;
Cisse1 et al., 1998). Understanding the magnitude
and direction of anthropogenic impacts requires
knowledge of the range of fluctuations historically
experienced by ecosystems as a result of variability in climatic conditions, disturbance regimes, and
their interactions (Swetnam and Betancourt, 1998).
Therefore, the determination of the historical range
of variability (HRV) in key ecosystem patterns and
processes is an important part of ecological assessments and results in the characterization of the
range of variability in conditions to which ecosystem components (e.g., species) are adapted (Bourgeron and Jensen, 1994; Morgan et aI., 1994; Swanson et al., 1994). HRV provides a baseline for
evaluating anthropogenic changes and a means for
identifying the potential for surprise events to occur (Holling, 1986). Historical conditions serve as
a model of the functioning of ecosystems under unmodified disturbance regimes and alternative landuse scenarios. Ecosystem patterns and processes
Hope C. Humphries and Patrick S. Bourgeron wish to
acknowledge partial funding provided by a Science to
Achieve Results grant from the U.S. Environmental Protection Agency ("Multi-Scaled Assessment Methods:
Prototype Development within the Interior Columbia
Basin").
operate at multiple hierarchically structured spatial
and temporal scales, and therefore the determination of HRV should be conducted at scales that both
meet the objectives of the assessment and are appropriate for the patterns and processes of interest
(Bourgeron et al., 1994).
In North America, HRV has been reconstructed
for the period prior to significant impact from EuroAmerican settlement (Fule et aI., 1997; Kaufmann
et aI., 1998). The characterization ofHRV includes
species composition and structure, as well as the
disturbance processes that are important determinants of biotic patterns. Such patterns occur at a
variety of spatial and temporal scales. Methods for
determining HRV usually have associated scales of
resolution and spatial and temporal extents. The
appropriateness of these method-specific scales for
meeting the objectives of the assessment should be
taken into account in selecting HRV methods. No
single widely applicable period of time for assessing HRV exists for North America or elsewhere
(Landres et aI., 1999). In addition, HRV methods
are limited in their ability to reconstruct many
ecosystem patterns and processes (e.g., understory
vegetation, animal populations, nutrient cycling);
these limitations should be clearly understood by
those conducting ecological assessments. This
chapter describes methods for determining HRV in
biotic patterns and disturbance processes (Table
19.1), their characteristic temporal and spatial
scales, their advantages and limitations, and selected examples of their use, including studies that
combine more than one HRV method. The methods primarily address HRV in terrestrial ecosystems, but selected methods for aquatic systems are
also included. The examples span a wide range of
geographic and ecological conditions.
273