BIODIVERSITY OF STREAM INSECTS - Cramer Fish Sciences

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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). Br¨onmark 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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