26
Assessment of Biotic Patterns
in Freshwater Ecosystems
Christopher A. Frissell, N. LeRoy Poff, and Mark E. Jensen
26.1 Introduction
Whether curse or blessing, scientists and managers
have developed a multitude of approaches to the
assessment of patterns in the biota of freshwater
ecosystems. Several rather independent traditions
in aquatic ecology have focused on the relationship
of ecological pattern to physical pattern and
process. Approaches that focus on broad-scale, regional patterns may completely miss aspects of the
ecosystem that are critical to the survival of rare
taxa in special habitats, just as approaches that focus on rare species or endangered popUlations may
provide little information about the broader biotic
community and its response to an ecological
change (Table 26.1). Today's natural resource
laws, societal expectations, limited fiscal resources,
and declining biotic resources demand that these
once separate strands converge in a more holistic
approach to ecological assessment. This brief review cannot aspire to map a clear path toward meeting that major challenge, but is intended only to till
the ground for future synthesis.
This chapter reviews selected examples from the
literature that illustrate varied approaches to assessing and understanding the underlying causes of
biological patterns in streams, rivers, and lakes.
Some theoretical concepts are examined to help to
explain the complementary capabilities and purposes of the various approaches employed in these
studies. Most of our examples are drawn from the
peer-reviewed scientific literature, as these are the
best-documented cases.
Human actions, which affect terrestrial and atmospheric processes inexorably, affect the freshwater ecosystems that are nested in the same landscape. Thus assessment of ecosystems at the scale
of whole watersheds can provide crucial informa390
tion for understanding the dynamic responses of the
diverse freshwater ecosystems residing within
them.
26.2 Biotic Patterns in Streams
26.2.1 Temporal and Spatial Scales
of Variation
To date, most efforts to assess responses of stream
biota to environmental stresses rest on unexamined,
implicit assumptions about the scales of ecological
function and response. Scaling of response in freshwater ecosystems depends on two major and partly
independent aspects of their biophysical organization. First, physical morphology-of the catchment, the stream network, and the lakes and other
water bodies within that network--determines
where different kinds of environmental stresses or
disturbance events originate and how such events
are translated or propagated across space and time
through the network. In running-water ecosystems,
strong physical connectivity downstream and laterally (and biotic connectivity upstream and laterally; Pringle, 1997) affords ample opportunity for
rapid, direct transfer of disturbance effects. Second,
the biological consequences of disturbances or
stresses also depend on the life histories, interactions, and spatial distribution of the species present in the aquatic ecosystem (Southwood, 1977;
Townsend and Hildrew, 1994). Both the physical
and biotic aspects of scaling are crucial to understanding and predicting biotic response to environmental change (Statzner and Higler, 1986; Poff
and Ward, 1989; Naiman et aI., 1992; Townsend
and Hildrew, 1994; Biggs, 1995; Ebersole et aI.,
1997; Poff, 1997). Seldom, however, have both as-
Assessment of Biotic Patterns
in Freshwater Ecosystems
Christopher A. Frissell, N. LeRoy Poff, and Mark E. Jensen
26.1 Introduction
Whether curse or blessing, scientists and managers
have developed a multitude of approaches to the
assessment of patterns in the biota of freshwater
ecosystems. Several rather independent traditions
in aquatic ecology have focused on the relationship
of ecological pattern to physical pattern and
process. Approaches that focus on broad-scale, regional patterns may completely miss aspects of the
ecosystem that are critical to the survival of rare
taxa in special habitats, just as approaches that focus on rare species or endangered popUlations may
provide little information about the broader biotic
community and its response to an ecological
change (Table 26.1). Today's natural resource
laws, societal expectations, limited fiscal resources,
and declining biotic resources demand that these
once separate strands converge in a more holistic
approach to ecological assessment. This brief review cannot aspire to map a clear path toward meeting that major challenge, but is intended only to till
the ground for future synthesis.
This chapter reviews selected examples from the
literature that illustrate varied approaches to assessing and understanding the underlying causes of
biological patterns in streams, rivers, and lakes.
Some theoretical concepts are examined to help to
explain the complementary capabilities and purposes of the various approaches employed in these
studies. Most of our examples are drawn from the
peer-reviewed scientific literature, as these are the
best-documented cases.
Human actions, which affect terrestrial and atmospheric processes inexorably, affect the freshwater ecosystems that are nested in the same landscape. Thus assessment of ecosystems at the scale
of whole watersheds can provide crucial informa390
tion for understanding the dynamic responses of the
diverse freshwater ecosystems residing within
them.
26.2 Biotic Patterns in Streams
26.2.1 Temporal and Spatial Scales
of Variation
To date, most efforts to assess responses of stream
biota to environmental stresses rest on unexamined,
implicit assumptions about the scales of ecological
function and response. Scaling of response in freshwater ecosystems depends on two major and partly
independent aspects of their biophysical organization. First, physical morphology-of the catchment, the stream network, and the lakes and other
water bodies within that network--determines
where different kinds of environmental stresses or
disturbance events originate and how such events
are translated or propagated across space and time
through the network. In running-water ecosystems,
strong physical connectivity downstream and laterally (and biotic connectivity upstream and laterally; Pringle, 1997) affords ample opportunity for
rapid, direct transfer of disturbance effects. Second,
the biological consequences of disturbances or
stresses also depend on the life histories, interactions, and spatial distribution of the species present in the aquatic ecosystem (Southwood, 1977;
Townsend and Hildrew, 1994). Both the physical
and biotic aspects of scaling are crucial to understanding and predicting biotic response to environmental change (Statzner and Higler, 1986; Poff
and Ward, 1989; Naiman et aI., 1992; Townsend
and Hildrew, 1994; Biggs, 1995; Ebersole et aI.,
1997; Poff, 1997). Seldom, however, have both as-