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Annu. Rev. Entomol. 1998. 43:271–93
BIODIVERSITY OF STREAM
INSECTS: Variation at Local, Basin,
and Regional Scales1
Mark R. Vinson
United States Department of the Interior, Bureau of Land Management, Aquatic
Monitoring Center, and Watershed Science Unit, Utah State University, Logan,
Utah 84322-5210; e-mail: [email protected]
Charles P. Hawkins
Department of Fisheries and Wildlife, Ecology Center, and Watershed Science Unit,
Utah State University, Logan, Utah 84322-5210; e-mail: [email protected]
KEY WORDS:
biodiversity, aquatic insects, mayflies, stoneflies, caddisflies
ABSTRACT
We review the major conceptual developments that have occurred over the last 50
years concerning the factors that influence insect biodiversity in streams and examine how well empirical descriptions and theory match. Stream insects appear
to respond to both spatial and temporal variation in physical heterogeneity. At all
spatial scales, the data largely support the idea that physical complexity promotes
biological richness, although exceptions to this relationship were found. These
exceptions may be related to how we measure habitat complexity at finer spatial
scales and to factors that influence regional richness, such as biogeographic history, at broader spatial scales. However, the degree to which local stream insect
assemblages are influenced by regional processes is largely unknown.
INTRODUCTION
Loss of biodiversity in ecosystems has important implications, including diminished resistance and resilience to disturbance, system simplification, and
loss of ecological integrity. Critical to preventing losses of biodiversity is an
1 The
US Government has the right to retain a nonexclusive, royalty-free license in and to any
copyright covering this paper.
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understanding of patterns in taxa richness (a commonly used measure of biodiversity) at a variety of spatial scales. In this review, we examine how taxonomic
richness of stream insects varies in response to environmental factors from local to regional spatial scales and discuss the theory and empirical evidence that
form the foundation for understanding these patterns.
For many ecosystems, patterns in taxonomic richness are well known for a
variety of spatial scales (51, 91, 134, 143). Our knowledge of diversity patterns
in stream insects is less well developed. Our best understanding of these patterns
is at the scale of individual basins, stream reaches, and habitat units (99),
although some stream ecologists have suggested that even at these spatial scales,
consistent or predictable patterns often do not exist (167). Our knowledge of
broad-scale patterns is especially meager (6, 30).
The lack of a strong empirical foundation describing patterns in stream insect
biodiversity is disheartening because streams are among the most threatened
ecosystems on Earth (42, 104). To maintain and restore biodiversity of stream
ecosystems, we need to document patterns in stream insect biodiversity better,
identify the major environmental factors controlling these patterns, and determine if the factors that control stream insect richness are scale dependent. To
address these issues; we review the major conceptual developments that have
occurred over the last 50 years that concern invertebrate biodiversity in streams
and then examine how well empirical descriptions and theory match. We conclude our review by offering a synthesis that may provide a foundation for
understanding patterns at different spatial scales. We also identify knowledge
gaps that we believe must be addressed to develop a richer understanding of
biological richness in streams.
CONCEPTUAL FOUNDATIONS
In summarizing more than 40 years of research, August Thienemann (149),
the noted German limnologist who impacted the stream ecology/aquatic insect
world from about 1910 to the 1950s, concluded that variation in stream invertebrate richness conformed to three ecological principles, which are paraphrased
below:
1. Biotic richness increases with increasing diversity of conditions in a locality.
2. The more the conditions in a locality deviate from normal, and hence from
the normal optima of most species, the smaller is the number of species that
occur there and the greater is the number of individuals of each species that
do occur.
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3. The longer a locality has been in the same condition, the richer and the more
stable is its biotic community.
These principles anticipated, in part, several general ecological and evolutionary
hypotheses that continue to be debated and tested by ecologists today. The first
principle predicts that species richness will increase with increasing spatial
heterogeneity, as observed in many other taxa and as predicted by niche theory
(e.g. 1, 91). Both the second and third principles embody aspects of disturbance
ecology. The second principle makes similar predictions to those of Connell’s
(24) and Greenslade’s (57) models, both of which suggest that only a few
tolerant species will persist under harsh conditions. Although the time span is
not explicitly stated in the third principle, it can be interpreted in both ecological
and evolutionary terms. On one hand, this statement is similar to the timestability hypothesis (131) that argues that extinction rates will be lower and thus
more species will accumulate in climatically stable regions than in variable ones.
A similar interpretation can be made for smaller spatial and shorter temporal
scales. If stable habitats have lower rates of local extinction than unstable
or frequently disturbed habitats, they can potentially accumulate more of the
regional taxa pool than unstable ones.
Work by stream ecologists over the last 40 years has been characterized by
refinement and elaboration of many of Thienemann’s ideas and innovative development of essentially new ideas. Like many areas of ecology, however,
development of theory has tended to outpace empirical testing, and the literature contains much speculation regarding the mechanisms controlling stream
invertebrate assemblage structure and richness. Variation in stream insect biodiversity has been hypothesized to be under the proximate control of nearly
any measurable variable depending on the question and on the temporal and
spatial scale of the investigation (5, 73, 161). In this section we briefly review
the role that theory has played in influencing our ideas about insect biodiversity
in streams.
Stream ecologists have often tried to relate differences in taxa richness within
and among streams to differences in habitat heterogeneity, i.e. Thienemann’s
first principle. Most of this work was conducted at small spatial scales and
plowed little new conceptual ground, but it did refine our understanding of how
taxa richness and composition vary with specific differences in habitat structure.
In the last two decades, however, the way we think about habitat heterogeneity
in streams expanded in two important ways. Both developments appear to have
occurred in association with an expansion in the spatial scales at which stream
ecologists were working, i.e. from one primarily focused on the variation that
occurs among small patches within single stream reaches to the variation that
occurs along entire river networks.
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First, Vannote et al (156) argued that variation in biotic richness along rivers
should be positively related to the magnitude of daily variation in water temperature that occurs along stream systems. They argued that predictable daily thermal variation and hence biological richness should peak in midorder streams.
They based this prediction on the idea that a wide daily amplitude in thermal
conditions would allow more species to experience optimum temperatures for
growth and development than would occur in a less thermally variable site.
Stanford & Ward (139) expanded on these ideas and also argued that greatest
species richness in streams should occur in intermediate size streams, but they
suggested that richness tracked the magnitude of the joint annual variation in
thermal and discharge regimes. Both Vannote et al (156) and Stanford & Ward
(139) were essentially arguing that many streams are temporally heterogeneous
and predictable environments and that stream biota have evolved life-history
schedules that allow them to exploit temporally shifting resources and environmental conditions. In animals with intermediate life spans like aquatic insects,
such temporal gradients could promote high taxa richness in the same way that
high spatial heterogeneity does (72). Statzner and others (140, 141) also argued
that physical heterogeneity was important to stream biota, but they developed
the idea that the distribution and richness of stream invertebrates are largely
governed by the hydraulic properties that exist at a site and not by the thermal
regime.
Stream ecologists have also expanded on Thienemann’s third principle by
exploring how differences in physical stability should influence taxa richness.
The main conceptual additions to Thienemann’s original idea were in refining
our understanding of how the frequency and magnitude of physical disturbance
should influence taxa richness and population densities, as well as how disturbance might interact with biotic processes to influence assemblage structure.
Much of this latter work occurred in concert with a growing interest in the importance of biotic interactions in streams and borrowed concepts from Connell
(24) and Huston (71) regarding how disturbance and competition interact to
maintain diversity. The roles that predation, herbivory, and competition play in
structuring stream communities have been reviewed elsewhere (50, 114, 119),
but we discuss some of these ideas here as they apply to understanding variation
in taxa richness in streams.
Hildrew et al (67), Hildrew & Townsend (66), Minshall (99), Poff & Ward
(118), Townsend (153), and Townsend & Hildrew (154) tried to synthesize
many of these ideas by developing conceptual models that more thoroughly
integrated the effects of abiotic and biotic processes on stream communities.
For example, Hildrew & Townsend (66) suggested richness would be highest
at intermediate levels of disturbance but most noticeably so under conditions
of high productivity. Minshall (99), borrowing ideas of Southwood (137) and
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Greenslade (57), speculated that species richness would be greatest in streams
intermediate in both stability and harshness. Poff & Ward (118) argued that
biological structure in streams should be influenced by four hydrologic factors
(intermittency, flood frequency, flood predictability, and flow predictability)
and that invertebrate richness should be highest under conditions of high flow
predictability, intermediate under conditions of high flood predictability, and
low under conditions of either high flood frequency or intermittency. Townsend
(153) developed a “patch-dynamic” framework for understanding stream communities and suggested that few stream ecosystems would be stable enough to
allow development of strong, lasting biological interactions. The implication of
his reasoning is that we should seldom expect competitive interactions to limit
richness in streams. However, Townsend & Hildrew (154) later used the habitat
templet ideas of Southwood (137) to suggest that species richness might exhibit
three trends in streams: increase as spatial heterogeneity increases, decrease
per unit resource with increasing temporal stability, and peak at intermediate
levels of disturbance. Minshall & Petersen (100) explored a variation of these
themes and incorporated an explicit temporal dimension in their conceptual
model. They suggested that stream communities should be viewed in context
of island biogeographic theory and the relative importance of “equilibrium”
and “stochastic” mechanisms operating in streams. They argued that stochastic
processes should be most important immediately following disturbances and
that biotic interactions should gradually become more important as population
densities recover and approach equilibria. How well does the empirical literature support these ideas? In the following sections we try to answer this
question.
EMPIRICAL FOUNDATIONS
A large body of literature now exists describing relationships between taxa
richness and environmental factors in streams, and several generalizations have
emerged regarding the mechanisms that control stream invertebrate richness
(5). Until recently, however, little attention has been given to the potential confounding effects that scale-dependent processes may have on relationships; and,
consequently, few researchers have explicitly stated either the spatial or temporal scales to which their observations apply. In this section we examine relationships between stream benthic insect taxa richness and different environmental
factors that were described by different authors. In a few cases, we have analyzed or reanalyzed data given by an author. Where possible we also examine
how relationships vary with the spatial scale of observation. Because of space
limitations, we do not present an exhaustive review of every paper published
on every topic; rather we present papers that we believe represent the current
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Table 1 Results of empirical studies that describe the effect of some attribute or phenomenon
on stream insect taxa richness at a variety of spatial scales
Scale and factor
Patch to reach a
Substrate
Surface
complexity,
roughness
Heterogeneity
Positive effect
38
—
2b, 41, 53, 64
2b, 38, 40, 46, 56,
113c, 168d
—
—
Light-shading
Food
Disturbance
Biotic interactions
Predation
40k
Substrate type
Substrate area
Habitat type
Competition
Basin
Substrate
heterogeneity
Cation
concentration
pH
Water temperature
Diel range
Annual range
Flow/hydraulics
Discharge
Velocity
Stream flow pattern
intermittency
Altitude
Basin area, stream
size
Basin vegetation
Negative effect
59, 84, 151
2e, 33, 46, 56f,
120, 165, 170
7, 10, 61, 78, 92, 152
37, 59, 84, 145,
151, 158
7, 18, 58, 94, 132,
150, 169
64
41, 52, 116
—
Median particle size
No effect
—
2l, 31
—
—
—
—
78, 90g, 132
—
63, 70
39, 56, 63
45, 107, 123h
—
93
13, 31, 32, 48, 69,
95, 107, 123i,
129, 132j, 135
4, 40, 52, 53, 68,
81, 122, 124
63
—
2l, 56
—
44, 48, 78, 108
78, 105, 136, 155
63, 168
—
—
—
166
19, 79m
75, 87, 89, 96,
97, 115, 138, 139,
160, 162
—
—
—
79m
125
48
64, 84
—
120
11, 12, 34, 87,
146, 148
93, 148
21, 144
58o, 102o
62
103, 121
—
49
47
80
13, 19, 49n, 88, 164
2, 14, 80, 83,
115, 160
17, 18, 78, 86,
116, 160
—
(Continued )
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Table 1 (Continued )
Scale and factor
Positive effect
No effect
Negative effect
Regional-continental
Terrestrial biome
Continenality
Latitude
14, 21, 36, 89, 148
89
8p
—
—
54, 77, 85p, 109, 110
—
—
11, 86, 112, 145
to reach scale: ∼10−1–104 m2; basin: ∼105–107 m2; regional: >107 m2.
concluded richness was positively related to within-patch heterogeneity, but our
inspection of data in his table 5 suggests no effect.
cPeckarsky showed an effect in only one of three trials.
dWise & Molles found mixed substrates had higher richness than uniformly large but not
uniformly small substrates.
eOur analysis of Allan’s data shows a strong positive relationship between richness and
log median particle size.
fOur reanalysis combining seasons shows a strong relationship between richness and log
median particle size.
gLogan & Brooker analyzed 17 published studies and concluded there was no overall effect
of habitat type.
hData for riffles only.
iData for pools only.
jScarsbrook & Townsend found higher taxa richness in riffles than pools in a stable stream
but no differences between the two habitats in an unstable stream.
kPredation effects were observed only after 2 weeks and not at the end of the 4-week
experiment.
lAllan concluded there was no relationship between richness and between-site substrate
heterogeneity, but our analysis suggests a strong positive relationship.
mKalmer reported a negative relationship for stoneflies and a positive one for mayflies.
nFeminella found flow duration was related to total richness in one of six comparisons and
to Ephemeroptera+Plecoptera+Trichoptera richness in four of six comparisons.
oRichness peaked within midorder streams.
pWithin Australia only.
aPatch
bAllan
state of our knowledge regarding invertebrate taxa richness in streams. We have
largely organized this section to treat factors that operate at progressively larger
spatial scales, from fine scales (patch to reach) to basin to regional scales.
Substrate
Stream ecologists have spent considerable effort studying the effect substrate
has on benthic invertebrates (98). Like most ecologists, many stream ecologists
generally assume biological richness will be correlated with environmental
heterogeneity (5); however, the empirical evidence supporting this generality
for stream substrates is equivocal (Table 1). At the scale of individual stones, 3
of 4 studies showed that richness was higher on stones with complex surfaces
than on stones with simple surfaces, but at the scale of individual substrate
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patches, only 4 of 10 studies found higher richness in more heterogeneous
patches (Table 1). Furthermore, Hawkins et al (64) found the highest number
of taxa in trays of small (5 cm) stones that had substantial (≥75%) amounts of
sand filling the interstitial volumes between stones, and Dudley et al (41) found
that the presence of macroalgae on stones increased taxa richness within habitat
patches. Even fewer data exist for larger spatial scales. Cowie (31) reported that
richness was highest at stream sites with the most substrate heterogeneity but
did not give data on heterogeneity. Our reanalyses of data given in two papers
show that richness can be positively related [r 2 = 0.80, n = 7 after omitting
one extreme outlier (2)] or unrelated [n = 15 (56)] to among-site differences in
substrate heterogeneity.
Although richness increases with area sampled regardless of spatial scale
(37, 59, 84, 145, 151, 158), the average size of stones within patches does not
appear to affect taxa richness consistently; studies show that more taxa occur on larger than on smaller substrates (33, 46, 56, 120, 165, 170) but also that
more taxa occur on smaller than on larger substrates (63, 168). These differences do not appear to be a function of differences in the range of substrate
sizes examined. In contrast, each of the six studies we reviewed that examined relationships between richness and substrate type showed that different
types of substrates support different numbers of taxa; physically complex substrate types (leaves, gravel or cobble, macrophytes, moss, wood) generally
support more taxa than structurally simple substrates such as sand and bedrock
(7, 10, 61, 78, 92, 152).
Inferences regarding the mechanisms that underlie these substrate relationships must be made with caution because richness tends to increase with the
number of organisms collected in a sample (60, 84, 158) and few investigators
have attempted to standardize their richness estimates to adjust for this passive
sampling phenomenon. Although there are strong biological reasons why richness should increase with increasing substrate heterogeneity, we suspect that
part of the problem in interpreting both individual studies and understanding
why studies differed in their results is likely caused by how substrate heterogeneity has been defined.
Habitat Type
The most common habitat delineations in stream studies are pools and riffles.
Like substrate, the pattern is not as clear as one might expect. Scarsbrook &
Townsend (132) found higher taxa richness in riffles than pools in a stable stream
but no differences between the two habitats in an unstable stream. In a review
of 17 studies that sampled both pool and riffle habitats, Logan & Brooker
(90) found no significant differences in richness at several taxonomic levels
between the two habitats. Similarly, Jenkins et al (78) showed that there were
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no substantial differences in richness among eight habitat types. In contrast, the
other studies we evaluated did find significant interhabitat variation in richness
(7, 18, 58, 94, 150, 169).
Light
Most of the studies that have evaluated the effect of light on stream insect
assemblages have looked at production or guild composition and not richness.
The few studies that we located found no or minor effects of light levels on insect
taxa richness (63, 64, 70). Of these three studies, only Hawkins et al (64) found
a relationship between taxa richness and light. They found more taxa in the
herbivore-shredder and collector-gatherer guilds in streams with open canopies
than in streams with heavy riparian canopies. However, the total number of
taxa across all guilds did not vary among streams. They also found that highest
overall taxa richness in experimental substrate trays tended to occur under open
canopy conditions, but only when estimates were adjusted for differences in
current and substrate conditions.
Food
Few studies have examined how taxa richness varies with abundance of food
resources. Of the studies we found, variation in the abundance of potential food
sources (algae, detritus) appears to influence taxa richness under some conditions (41, 52, 116) but not others (39, 56, 63, 93). Where observed, differences
in taxa richness were associated with parallel differences in the total number of
individuals found at a location.
Disturbance
Naturally occurring disturbance has been largely defined in terms of the predictability or variability in magnitude of discharge in streams (118, 126). Richness appears to be consistently and negatively related to disturbance intensity or
frequency over the range of disturbances examined (13, 32, 48, 69, 95, 123, 129,
132, 135), although three other studies (45, 107, 123) showed no effect of small
scale disturbances on richness within coarse substrates. None of these studies
either alone or collectively support the idea that intermediate levels of disturbance promote high taxa richness in streams.
Biotic Interactions
Predation and competition are known to influence the relative abundance of
taxa in streams (82, 114), and some have suggested these interactions may
be strong enough to have pervasive effects on the taxa richness of streams
(55). However, our review suggests there is little evidence that either predation
(4, 40, 52, 53, 68, 81, 122, 124) or interspecific competition (63) strongly affects
the number of invertebrate taxa in streams. Of the studies we reviewed, only
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Dudgeon (40) has shown predators to influence richness (reduction). The effect
was significant 2 weeks after the start of his experiment but disappeared by the
experiment’s end (4 weeks).
Water Chemistry
Taxa richness appears to normally increase with increasing cation concentration
(44, 48, 78, 108) and pH (78, 105, 136, 155). One study reported that richness
was not correlated with pH (range 3.5–8.1) in naturally acidic humic streams
in New Zealand (166). However, it is unclear if cation concentration directly
affects the ability of a stream to support different numbers of taxa. For example,
Erman & Erman (48) found richness in springs to be positively correlated
with calcium, magnesium, pH, conductance, and alkalinity, but they suggested
that values of all of these factors were positively related to spring hydrologic
permanence—the factor they believe was actually influencing richness (see
Flow/Hydraulics section below).
Temperature
The annual water temperature regime of a river has several components, including the absolute minimum and maximum temperature, the total annual and diel
variation in temperature, the time of the year when the minimum and maximum
water temperatures occur, the rate of seasonal change, and the number of annual
degree days that can influence stream insects (159, 163). Rarely have these factors been evaluated simultaneously. Laboratory studies are typically restricted
to analyzing the effects of several constant temperatures on a single species, and
no one has correlated the various attributes of water temperature regimes to invertebrate assemblage structure for a range of stream types or regions. Most of
the research on the effects of water temperature on aquatic insects has evaluated
physiological and behavioral responses to natural and altered thermal regimes
(147, 161, 163) or in relation to species distribution patterns across latitudinal
(15, 22, 157) and elevational (16, 34, 79, 160) gradients.
Despite the worldwide influence that Vannote et al’s (156) ideas have had
on the stream ecology literature, few studies have tested their predictions about
how variation in water temperature affects aquatic insect richness. We found
only two studies that examined the relationship between diel water temperature
and richness (19, 79). Kamler (79) found that Ephemeroptera richness was
positively correlated and Plecoptera richness was negatively correlated with diel
temperature variation in central Europe mountain streams. Brussock & Brown
(19) reported that invertebrate richness was negatively related to the amplitude
of diel temperature variation. Most authors who have explored relationships
between richness and temporal variation in temperature have focused on annual
temperature range.
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Several studies have found insect taxa richness to increase within the same
stream system with annual water temperature variation (75, 87, 89, 96, 97, 138,
139, 160, 162). One study (125) showed that taxa richness in a small spring decreased downstream as the annual amplitude of temperature variation increased.
We found that, after dropping the degraded lowest elevation site from the analysis, the richness estimates for 12 sites given by Perry & Schaeffer (115, 133)
were correlated with both mean summertime temperature (r 2 = 0.68) and an
estimate of annual temperature range (mean summer temperature + two standard deviations, r 2 = 0.40). However richness estimates (rarefied) adjusted for
numbers of individuals in each collection were unrelated to either measure of
temperature.
Flow/Hydraulics
Hydraulic properties of streams can influence the quantity and quality of available habitats at all spatial scales, and many insects have anatomical and behavioral adaptations for living in slow or fast water microhabitats (73). Statzner &
Higler (141) concluded from a review of 15 published studies of stream invertebrate zonation patterns from Alaska to New Zealand that stream hydraulics was
the major factor determining invertebrate species richness worldwide, but they
did not describe how taxa richness varied with flow. In the studies we reviewed,
richness was negatively correlated with either current speed or discharge in two
studies (47, 80) and positively correlated with one of these variables in three
others (48, 64, 84). However, in an extremely comprehensive study, Quinn &
Hickey (120) found no relationship between taxa richness and mean or median
discharge among 88 sites that they sampled throughout New Zealand. They also
found richness to be uncorrelated with current velocity in a subset of these sites.
Stream Flow Patterns
In spite of the persuasive arguments by Poff & Ward (118) that flow patterns
should influence taxa richness in streams, few studies have examined how richness varies in response to different flow conditions, and all of the studies we
found compared richness in permanent streams with intermittent ones. Usually authors reported lower richness in intermittent streams than in permanent
ones, and they often noted increasing richness with increasing flow duration
(13, 19, 85, 88, 164). Feminella (49) showed that streams can vary in flow permanence from year to year, and these differences can sometimes obscure trends
between invertebrate richness and average flow duration. Feminella also found
that total taxa richness was positively related to differences in flow duration
among six streams in only one of six comparisons, whereas combined mayfly,
stonefly, and caddisfly richness was related to flow duration in four of six comparisons.
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Altitude
Altitude is a primary factor associated with species richness for many taxonomic groups (9), including stream insects (73, 161). Factors influencing
local stream insect richness that change with respect to altitude include water temperature (both range and annual degree day accumulations), riparian
vegetation, instream habitats, and the quality and quantity of organic matter.
Within individual basins, stream insect taxa richness has been found to decrease
(2, 12, 14, 80, 83, 87, 115, 146, 160), change irregularly (21, 144), or increase
(11, 34, 148) with increasing altitude. Large and sometimes nonlinear changes
in richness can occur along this gradient as streams cross biomes (14, 21, 36),
zones of hydraulic transition (117), or differences in land use (14, 21).
Stream Size
Basin area or length downstream from the stream source are often correlated
with taxa richness (75, 102, 165). Although taxa richness in fish usually increases downstream (6), the generality of such patterns in invertebrate taxa richness and their environmental correlates are less clear. Invertebrate taxa richness
has been reported to both progressively increase downstream (18, 78, 86, 160)
or to peak in midorder reaches along the stream profile (58, 102). Brönmark
et al (17) found that invertebrate taxa richness was positively associated with
basin area (r 2 = 0.47) among 22 small coastal island streams near southern
Sweden. One study (148) found taxa richness to decrease with basin area.
Studies of lake outlet streams have found stream size to be both unrelated (116)
and positively correlated (107) with invertebrate taxa richness.
Basin Vegetation
We found only three studies that examined taxa richness in relation to the surrounding basin vegetation. In the Southern Rocky Mountains (United States),
Molles (103) found similar numbers of Trichoptera species in streams draining
coniferous (18 species) and deciduous forests (20 species). Reed et al (121)
found similar numbers of species in pasture and natural forested sites in three
Victorian (Australia) streams. Hawkins (62) showed that taxa richness was
greatest in streams near Mount Saint Helens, Washington (United States), that
had intact forest in the basin and lowest in streams in which the forest had been
removed by the 1980 eruption.
Regional Factors (Latitude, Biome, and Continentality)
A pervasive broad-scale pattern noticed by many well-traveled benthologists
has been the remarkable worldwide similarity among stream insect assemblages
(74). Indeed, many of the same families and genera of aquatic insects are
found worldwide. Patrick (111) even suggested that aquatic insect taxa richness
varies little with geographic diversity. However, most of our understanding of
stream invertebrate assemblages within and across continents, physiographic
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regions, or biomes is based on the study of species distributions (43, 76, 130),
assemblage associations (25–28), or ecosystem level parameters (101) and not
explicit comparisons of taxa richness.
We found six studies that examined how richness varied across biomes or
continents. Donald & Anderson (36) found that richness varied considerably
among streams draining montane (52 taxa), subalpine (35 taxa), and plains (18
taxa) biomes. Brewin et al (14) reported that tundra streams had fewer taxa than
forested, terraced, or alpine streams and that richness in alpine streams was less
than in forested sites. Tait & Heiny (148) found more taxa in mountain streams
than in plains streams, whereas Carter et al (21) found highest taxa richness at
the transition between montane and valley sites. Lillehammer (89) reported that
Plecoptera species richness increased from coastal to inland regions in concert
with an increase in minimum temperature.
No clear latitudinal gradient has been established for lotic benthic assemblages (5, 30). The best known temperate-tropical comparison to date is Stout
& Vandermeer’s (145); however, their conclusion was based on the extrapolation of species-area curves, the accuracy of which is suspect (23). Species
richness was only higher in their projected number of species and not in their
actual collections, which implies higher regional richness but not local richness.
The potentially serious methodological problems notwithstanding, the results
of Stout & Vandermeer are intriguing because if their curves accurately describe
real differences in richness, their results imply that local richness is saturated
and limited by local conditions (20, 29, 128) and not strongly influenced by the
total number of taxa in the regional pool. Results from other studies, however, are equivocal in supporting Stout & Vandemeer’s conclusions. Bishop
(11), Lake et al (86), and Pearson et al (112) suggested tropical streams may
be more diverse than temperate ones. However, Patrick (109, 110) concluded
that there were no differences in richness between temperate North American
streams and headwater streams of the Amazon basin, and Illies (77) concluded
that there were more species of stoneflies in Europe than in South America,
although the number of genera was similar. Arthington (8) found more taxa in
temperate than tropical Australian streams. Lake et al (85) presented data that
suggest tropical and temperate Australian streams have on average about twice
the richness of North American streams. Flowers (54) argued that richness
in both tropical and temperate streams is highly variable and that no strong
latitudinal trend exists in richness.
Multifactor Studies
Few studies have simultaneously examined how richness varies with more than
one factor, although such studies are critical for understanding the relative importance of different factors and how they interact. Two types of studies have
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been conducted—analyses based on comparative studies (21, 56, 64, 78, 84, 120)
and experimental manipulations of 2 or more factors (40, 53, 63, 64). Friberg
et al (56) examined correlations between richness and 10 environmental factors (substrate, flow, and food factors) and reported significant correlations
with depth, median particle size, algae abundance, and moss cover. However,
our reanalysis of their data (combining seasons) showed that median particle
size was the only factor strongly (+, i.e. positively) associated with richness
(r2 = 0.67). Similarly, of 17 factors examined by Quinn & Hickey (120), including maximum temperature, only median particle size (+) and flooding (−)
were related to richness. Hawkins et al (64) compared richness among high- and
low-gradient streams with and without well-developed riparian canopies. They
found richness within the scraper guild to increase with stream gradient, but
it was unrelated to amount of canopy over the stream. In contrast, richness of
collector-gatherers and herbivore-shredders was highest under open canopies
but unrelated to stream gradient. Richness within all other guilds and total
richness were unrelated to either of these factors. The analysis of Jenkins et al
(78) revealed that four variables were positively related (r 2 = 0.73) to taxa richness (number of habitat types, river width, pH, and water hardness). Kuusela’s
(84) data are interesting in that two (moss and velocity) of the three significant
relationships he described with richness (r 2 = 0.54) are not significant if invertebrate abundance is included in the model. In this model, abundance and stone
area account for 61% of the variation in richness.
Experimental studies are generally consistent with these comparative studies
in identifying the importance of substrate attributes, and they further show that
biotic interactions appear to be unimportant. Hawkins et al (64) found highest
richness in experimental trays under conditions of no shading, high current
velocity, and high amounts of sand, whereas lowest richness occurred under
high shading, low current velocity, and low amounts of sand. Flecker & Allan
(53) found richness was related to the amount of interstitial spaces but not the
presence of fish, and Hawkins & Furnish (63) found richness increased with
decreasing substrate size but was unaffected by the presence of a potentially
dominant competitor. Only Dudgeon (40) found the presence of fish to affect
richness. However, the effect of fish was weak relative to the effect of season,
and he detected no effect of substrate complexity on richness. Although many
factors appear to affect richness in streams, drawing strong generalities about
the relative importance of different factors is impossible at this time because so
few multifactor studies report effects on richness.
SUMMARY AND SYNTHESIS
Two potential values of reviewing findings from independently conducted studies are that the collective information offers a far more powerful test of theory
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than any single study. Collective analysis of several studies also may reveal
either previously undiscovered trends and relationships or problematic areas
where clear-cut empirical generalizations have not emerged. In this section we
consider the consistency of the empirical information and how well the compiled literature supports both Thienemann’s principles and current theory. We
then propose a conceptual framework that we believe aids in understanding the
diverse patterns and relationships that emerged from this review.
Of the factors we examined, we found the most consistent patterns of richness
to exist with substrate size, disturbance regime, predation, annual temperature
range, flow intermittency, and biome type. Relationships with substrate heterogeneity, habitat type, food, altitude, and latitude were more equivocal. We
suspect the main reason authors have reported equivocal results in these cases
may be caused by differences in how ecologists measure the environment (e.g.
substrate heterogeneity, habitat type) or the range of conditions they examined (e.g. altitude, latitude, also mean particle size). These exceptions should
prompt stream ecologists to consider two questions: Are there specific ecological or evolutionary reasons why consistent relationships should not hold under
some situations? Have we always measured habitat conditions in a way that
reflects how organisms perceive them? Given the above exceptions, the data
as a whole largely support Theinemann’s first principle that physical complexity should promote biological richness. Stream insects seem able to “perceive”
their environment at several scales, from the surface texture of individual stones
to the temporally varying availability of suitable thermal conditions.
The generality of Thienemann’s second principle is less clear because it is
difficult to define what “normal” conditions are for any collection of stream
invertebrates. If “non-normal” is defined as streams having extreme physicochemical conditions (e.g. thermal springs or severe cold), this principle approaches an ecological truism, but it is less obvious how much streams must
deviate from mean regional conditions before richness declines.
The applicability of Thienemann’s third principle is generally consistent
with the trend for richness to be highest in the physically most-stable streams.
The recent extensions of theory, especially those that predict highest richness
at intermediate levels of disturbance as defined by discharge attributes, appear to offer little explanatory power beyond Thienemann’s original prediction.
This conclusion is consistent with recent studies designed explicitly to test
disturbance related hypotheses of these models. With few exceptions, richness in real streams did not behave as predicted by these new models (35, 127,
142).
There are also patterns where neither Thienemann’s principles nor the newer
models are needed to explain patterns. For example, the tendency for taxa
richness to increase with basin area or stream length may reflect the wellknown tendency for larger habitats to have more taxa. Such relationships
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are especially likely to occur in streams because downstream areas may act
as sinks for individuals washed out of upstream source areas. This phenomenon is apparent from the skewed (downstream) longitudinal distributions
exhibited by many stream invertebrates (2, 3, 115), which results in a high degree of species addition from up- to downstream rather than replacements of
taxa.
A relationship that stream ecologists have seldom considered (65, 106) is
the degree to which the surrounding taxa pool influences richness at a location
(5, 128). These ideas are important because they have implications for understanding the degree to which local assemblages are organized by local factors or
are largely the consequence of broader scale historical processes (20, 29, 128).
Palmer et al (106) hypothesized that assemblages such as stream insects, which
live in frequently disturbed environments and exhibit high dispersal, will be
controlled by regional rather than local processes. To our knowledge, however,
empirical observations describing relationships between region, basin, and local
richness have not been reported for stream insects.
How should regional and local richness be related? We believe a feedback
exists between taxa pools at different scales that influences richness at each
scale. Over evolutionary time, regional taxa pools should grow or shrink as
local populations evolve and others become extinct. For highly dispersive taxa
such as aquatic insects, the waxing and waning of richness at larger scales should
define the maximum number of taxa that can potentially occur at smaller areas
or locations. The taxa that exist within basins or regional pools can potentially
colonize any local site through dispersal; however, environmental conditions
at finer spatial scales limit which specific taxa establish from these larger taxa
pools. These ideas imply that invertebrate richness in streams is jointly structured by historical events and by the physical and chemical conditions unique
to each location. We do not believe the evidence presented in this review supports the idea that assemblages at smaller scales are largely unstructured, as
was suggested by some (167).
Presently, our current understanding of stream insect biodiversity patterns is
heavily biased toward local studies of small temperate forested streams. Few
of the studies cited in this review were conducted in tropical, polar, or desert
streams; in large rivers; or across broad spatial scales. Additionally, most stream
benthologists seem to have assumed that local patterns are primarily determined
by local processes (106). The role of regional and historical processes in
structuring local assemblages remains virtually unstudied (65). We believe
much could be learned by examining the extent to which local and regional
processes interact to control taxa richness at a variety of spatial scales. We
also believe that we need to examine if the relationships observed in temperate
streams hold in other stream types worldwide.
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ACKNOWLEDGMENTS
We thank Ted Angradi for commenting on an early version of the manuscript.
Visit the Annual Reviews home page at
http://www.AnnualReviews.org.
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