26
2.3.4 Resistance and Resilience
Resistance and resilience describe the role of disturbances in creating and maintaining physical and biological aspects of river ecosystems. These concepts are useful
in the context of river management because they describe how a river ecosystem
responds to disturbance and the length of time that changes associated with disturbances are likely to persist.
Interactions between disturbances and river corridor form and erosional resistance govern the diversity and stability of habitats available to aquatic and riparian
organisms. In general, greater habitat diversity equates to greater biodiversity in
natural rivers (Naiman et al. 1988b), although other factors also strongly influence
biodiversity (Ward et al. 1999; Palmer et al. 2010). Resistance can refer to the ability of channel and floodplain substrate to physically resist erosion, but ecologists
use resistance in a different context. In an ecological context, resistance describes
the ability of a river ecosystem to resist changes in form and process caused by
disturbance (Webster et al. 1975). Resilience is the persistence of the river ecosystem and its ability to absorb external changes and maintain the same relationships
between biological populations or physical parameters (Holling 1973). Physical
scientists also use resilience or sensitivity to describe the ability of a river corridor
to return to its pre-disturbance configuration following disturbance (Brunsden and
Thornes 1979; Bull 1991; Wohl 2010; Reid and Brierley 2015; Fryirs 2016). In an
ecological context, a resistant river corridor undergoes little change during disturbance and a resilient river corridor returns to its pre-disturbance configuration and
relationships over a shorter time than the return interval of the disturbance. In other
words, a resistant river segment might exhibit very little change following a large
flood. A resilient river segment might be changed by the flood, but recover within
five to ten years, whereas a flood of that size only occurs once every 50 years.
An important aspect of understanding resistance and resilience is to explicitly
recognize that physical characteristics of the river corridor and biotic communities
are adjusted to the natural range of variability in water and sediment fluxes through
time. Numerous physical characteristics—channel cross-sectional area, the type
and dimensions of bedforms, the grain-size distribution of channel substrate, floodplain surface topography and stratigraphy, riparian water table, and so forth—reflect
the history of fluctuating volumes of water and sediment entering the river corridor
from upstream portions of the river network and from adjacent uplands. Similarly,
the species composition, age distribution of individuals within particular species,
and abundance and spatial distribution of organisms reflect the history of water and
sediment fluxes as well as interactions among organisms. Change in any of these
control variables—water and sediment flux, biotic community structure (e.g., via
introduced species)—creates the potential for changes in the river ecosystem that
can put native species at risk.
Most riverine species are tough: they live in a naturally variable environment and
they have evolved numerous mechanisms to survive disturbances such as floods and
droughts. However, riverine species are vulnerable to sustained changes associated
2 Rivers as Ecosystems
2.3.4 Resistance and Resilience
Resistance and resilience describe the role of disturbances in creating and maintaining physical and biological aspects of river ecosystems. These concepts are useful
in the context of river management because they describe how a river ecosystem
responds to disturbance and the length of time that changes associated with disturbances are likely to persist.
Interactions between disturbances and river corridor form and erosional resistance govern the diversity and stability of habitats available to aquatic and riparian
organisms. In general, greater habitat diversity equates to greater biodiversity in
natural rivers (Naiman et al. 1988b), although other factors also strongly influence
biodiversity (Ward et al. 1999; Palmer et al. 2010). Resistance can refer to the ability of channel and floodplain substrate to physically resist erosion, but ecologists
use resistance in a different context. In an ecological context, resistance describes
the ability of a river ecosystem to resist changes in form and process caused by
disturbance (Webster et al. 1975). Resilience is the persistence of the river ecosystem and its ability to absorb external changes and maintain the same relationships
between biological populations or physical parameters (Holling 1973). Physical
scientists also use resilience or sensitivity to describe the ability of a river corridor
to return to its pre-disturbance configuration following disturbance (Brunsden and
Thornes 1979; Bull 1991; Wohl 2010; Reid and Brierley 2015; Fryirs 2016). In an
ecological context, a resistant river corridor undergoes little change during disturbance and a resilient river corridor returns to its pre-disturbance configuration and
relationships over a shorter time than the return interval of the disturbance. In other
words, a resistant river segment might exhibit very little change following a large
flood. A resilient river segment might be changed by the flood, but recover within
five to ten years, whereas a flood of that size only occurs once every 50 years.
An important aspect of understanding resistance and resilience is to explicitly
recognize that physical characteristics of the river corridor and biotic communities
are adjusted to the natural range of variability in water and sediment fluxes through
time. Numerous physical characteristics—channel cross-sectional area, the type
and dimensions of bedforms, the grain-size distribution of channel substrate, floodplain surface topography and stratigraphy, riparian water table, and so forth—reflect
the history of fluctuating volumes of water and sediment entering the river corridor
from upstream portions of the river network and from adjacent uplands. Similarly,
the species composition, age distribution of individuals within particular species,
and abundance and spatial distribution of organisms reflect the history of water and
sediment fluxes as well as interactions among organisms. Change in any of these
control variables—water and sediment flux, biotic community structure (e.g., via
introduced species)—creates the potential for changes in the river ecosystem that
can put native species at risk.
Most riverine species are tough: they live in a naturally variable environment and
they have evolved numerous mechanisms to survive disturbances such as floods and
droughts. However, riverine species are vulnerable to sustained changes associated
2 Rivers as Ecosystems