Patterns of species richness for blackflies (Diptera: Simuliidae) in the Nearctic and Neotropical regions

Patterns of species richness for blackflies (Diptera:Simuliidae) in the Nearctic and Neotropical regions

J O H N W . M C C R E A D IE 1 , PE T E R H . A D L E R 2 and N E U S A

H A M A D A 3 1Department of Biological Sciences, University of South Alabama, U.S.A., 2Department of Entomology,

Soils, and Plant Pathology, Clemson University, U.S.A. and 3Instituto Nacional de Pesquisas da Amazaonia, Entomologia

Manaus, Amazonas, Brazil

Abstract. 1. Patterns of simuliid species richness were examined over a variety ofscales at 532 stream sites in the Nearctic (394) and Neotropical (138) regions. InNearctic streams, species richness of immature blackflies both within and acrossecoregions and over two seasons was examined. Stream variables at each siteincluded seston, width, depth, velocity, discharge, conductivity, pH, dissolvedoxygen, water temperature, dominant streambed-particle size, canopy cover, andriparian vegetation. These variables were subjected to a principal componentanalysis and derived principal components were related back to richness, usingregression analysis. At the level of the stream reach, richness was not highlycorrelated with single-point measurements of stream conditions.

2. Using data from both Nearctic and Neotropical sites, the effect of regionalrichness on local richness was examined. As regional richness increased, localdiversity reached an asymptote in which further increases in regional richnesswere not matched by increases in local richness. Hence, simuliid communitiesare best described as saturated (type II) communities, consistent with the currentview of lotic communities as non-equilibrium systems.

3. The well-established pattern of greater species richness in tropical regions wasnot observed in this study. To the contrary, blackfly richness is higher in temp-erate streams than in tropical streams at both local and regional scales.

Key words. Nearctic, Neotropical, Simuliidae, species richness, streams.

Introduction

Species richness is defined as the number of species in a

specified area, location, or habitat. Because the total num-

ber of species is rarely known, ecologists qualify richness as

the number of species in a particular taxon or group of taxa

within a circumscribed area. This simple concept has occu-

pied the centre stage of community ecology for decades (e.g.

Begon et al., 1996). Although patterns of species richness

have long been known for groups such as mammals

(Simpson, 1964), birds (Cook, 1969), and trees (Currie &

Paquin, 1987), patterns of aquatic insect species richness are

poorly understood (Vinson & Hawkins, 1998). Most studies

have focused on the influence of single factors on the richness

of temperate streams, with multifactor and broad-scale studies

rarely conducted (Allan, 1995; Vinson & Hawkins, 1998).

Several studies have considered species richness in tropical

streams (e.g. Stout & Vandermeer, 1975; Hamada etal., 2002).

Establishing empirical relationships between aquatic

insects and stream parameters has occupied a central pos-

ition in stream ecology, although many investigations have

been limited by the paucity of species-level descriptions (e.g.

McCafferty et al., 1990; Wiggins, 1990; Stewart & Stark,

2002). Because the taxonomy of North American blackflies

(Diptera: Simuliidae) is well developed, integrating morpho-

taxonomic, cytotaxonomic, and ecological approaches

(Adler et al., 2004), these ubiquitous organisms serve as

model subjects for the study of lotic community structure

(Adler & McCreadie, 1997; McCreadie & Adler, 1998).

Correspondence: John W. McCreadie, Department of Biological

Sciences, University of South Alabama, Mobile, AL 36688, U.S.A.

E-mail: [email protected]

Ecological Entomology (2005) 30, 201–209

# 2005 The Royal Entomological Society 201

About 98% and 93% of the 255 North American species of

blackflies are known as larvae and pupae respectively (Adler

et al., 2004). Recent studies also have advanced the taxon-

omy and ecology of Neotropical blackflies (Grillet & Bar-

rera, 1997; Hamada & Adler, 1999; Hamada &

McCreadie, 1999; Hamada & Grillet, 2001; Hamada et al.,

2002; McCreadie et al., 2004). Given their robust taxonomic

status and ubiquitous nature, blackflies are an ideal choice

to examine species richness of stream insects over a spatially

heterogeneous landscape.

In the current study, the patterns of species richness were

examined for immature blackflies from 532 stream reaches (of

which 70 were sampled on two occasions, for a total of 602

collections) over a variety of scales. As shown in other studies

(e.g. Vinson & Hawkins, 1998), different sets of factors should

influence species richness at different scales. First, the associa-

tion between species richness and stream conditions in three

adjacent ecoregions is examined. Next, the patterns of richness

across ecoregions are investigated, followed by the relationship

between regional and local richness, using data from 20 distinct

regions. Finally, species richness between Nearctic and

Neotropical streams is compared.

Although the distinction between local and regional

scales of species richness is unclear for streams (Angermeier

& Winston, 1998), the regional unit can be considered as the

species pool that provides colonists for the local scale (Lake,

2000). Accordingly, each stream (reach) collection was con-

sidered the local scale and the regional species pool was

considered as the sum of all species from local collections

in that region.

Methods

Sampling

All sites in this study were sampled using the same pro-

tocol and scale of stream reach. Sites were chosen based on

accessibility and the presence of flow. Each site was sampled

by hand collecting larvae and pupae from all available

substrates. Rather than a specific amount of collecting

time or a specific length of stream, sampling was geared to

collect a minimum of 30 specimens from each site. Dates of

collections for each region are given in Table 1. This type of

sampling, with a minimum of 30 specimens, has proven

successful in other studies (e.g. McCreadie & Adler, 1998;

Hamada et al., 2002; McCreadie et al., 2004). As in previous

studies of this type (Corkum & Currie, 1987; McCreadie &

Colbo, 1991; McCreadie et al., 2004), it was reasonably

assumed that species found in the sample from each site

were representative of local occurrences. Results for pre-

vious studies have shown that the faunistic lists produced

from hand-sampled larvae (McCreadie & Colbo, 1991) are

the same as those produced using more quantitative and

repeatable sampling units such as artificial substrates

(McCreadie & Colbo, 1992, 1993). When the identity of a

specimen could not be confirmed, the entire collection was

not used. Hence, only collections in which all specimens

were identified were included in any analysis.

Identification

Larvae and pupae were fixed in three changes of acetic

ethanol (1:3) and stored at 4 �C until processing. Larvae and

pupae were identified morphologically (Hamada & Grillet,

2001, Adler et al., 2004). Mid- to late-instar larvae of morph-

ologically similar species and of taxa known or suspected

to contain sibling species (Hamada & Adler, 1999; Adler

et al., 2004) were then prepared for examination of silk-

gland chromosomes, following the procedure of Rothfels

and Dunbar (1953). In other words, when any doubt of

species identity existed using traditional taxonomic

methods, chromosomes were analysed. This standard was

applied to all regions.

To make cytospecific identifications, chromosome-

banding patternswere comparedwith standardmaps available

in the literature (e.g. Rothfels et al., 1978; Hamada & Adler,

1999; Adler et al., 2004) or on file in our laboratories.

Voucher specimens (larvae and pupae) were deposited in

the following institutions: Clemson University Arthropod

Collection, Clemson, South Carolina, U.S.A.; Laboratorio

de Biologia de Vectores (MLBV), Instituto de Zoologia

Tropical, Universidad Central de Venezuela, Caracas,

Venezuela; or Instituto Nacional de Pesquisas da Amazonia

(INPA), Manaus, Amazonas, Brazil.

Local richness within and across ecoregions

The aim of this study was to associate habitat conditions

with species richness over two seasons and within and

across ecoregions. Thus, sampling was conducted at two

scales of study. The study area included streams in the

upper two-thirds of South Carolina, U.S.A. (33.25�

�35.20�N, 80.68��83.26�W). Rivers in South Carolina

drain south-eastward from the Appalachian Mountains to

the Atlantic Ocean. Boundaries of the ecoregions were based

on those defined by Myers et al. (1986), Omernik (1995),

and McCreadie and Adler (1998). Thus, the ecoregions

used – Mountains (MT), Piedmont (PD), and Sandhills

(SH) – were established a priori to regression analyses.

Detailed descriptions of these ecoregions can be found in

McCreadie and Adler (1998).

Eighty-seven sites were sampled from February to July in

1992 and 1993, of which 39 were sampled on two occasions

for a total of 126 collections. Collections were designated as

either spring (February to April) or summer (May to July).

Both stream conditions and the blackfly fauna in the

‘spring’ differed from that of the ‘summer’ collections

(McCreadie & Adler, 1998). Collections were made in the

Mountain (n¼ 40), Piedmont (n¼ 47), and Sandhills

(n¼ 39). All sampled streams are believed to be permanent.

The protocol of McCreadie and Colbo (1991, 1992) was

followed for measuring the following stream variables at

202 John W. McCreadie, Peter H. Adler and Neusa Hamada

# 2005 The Royal Entomological Society, Ecological Entomology, 30, 201–209

Table 1. Estimates of local and regional species richness of blackflies in the Nearctic and Neotropical Regions.

GPS Number of Regional Mean local Collecting

Region coordinatesa collections richness richness (�SE) date

Nearctic Region

Alaska, U.S.A. 60.51–65.47�N 40 40 4.10� 0.27 July 1994

141.38–150.78�W

Alberta, Canada 53.00–54.80�N 18 29 3.72� 0.37 June–August 1984

114.40–117.28�W

California, U.S.A. 37.90–39.16�N 29 21 4.03� 0.38 March 1990

122.37–123.08�W

Colorado, U.S.A. 38.81–40.05�N 22 21 3.73� 0.47 August 1997

105.62–106.36�W

Florida, U.S.A. 30.55–30.80�N 17 11 2.53� 0.37 February 1998

84.80–85.95�W

Michigan (Isle Royale), 47.90–48.08�N 12 13 3.67� 0.31 May 1997

U.S.A. 88.54–88.88�W

Nevada, U.S.A. 40.75–41.48�N 20 15 2.55� 0.29 March 1997

117.38–118.05�W

New Jersey, U.S.A. 40.98–41.34�N 16 20 3.06� 0.42 March 1998

74.65–75.77�W

Newfoundland, Canada 44.27–45.00�N 28 24 3.96� 0.27 May–July 1987, 1988

109.46–111.06�W

Saskatchewan, Canada 53.17–54.05�N 24 19 2.54� 0.27 April–September 1989–1994

104.07–105.95�W

South Carolina (Mountains) 34.42–35.20�N 22 16 3.18� 0.25 February–April 1992, 1993

U.S.A.b 81.71–83.26�W

South Carolina (Piedmont), 33.75–35.18�N 16 13 3.19� 0.34 February–April 1992, 1993

U.S.A.b 81.55–82.63�W

South Carolina (Sandhills), 33.25–34.13�N 19 16 4.74� 0.37 February–April 1992, 1993

U.S.A.b 80.68–81.65�W

South Carolina (Mountains), 34.42–35.20�N 18 16 3.28� 0.24 May–July 1992, 1993

U.S.A.c 81.71–83.26�W

South Carolina (Piedmont), 33.75–35.18�N 31 16 3.68� 0.24 May–July 1992, 1993

U.S.A.c 81.55–82.63�W

South Carolina (Sandhills), 33.25–34.13�N 20 11 4.00� 0.23 May–June 1992, 1993

U.S.A.c 80.68–81.65�W

Yellowstone National Park, 44.27–45.00�N 55 31 3.27� 0.29 June–July 1992–1994

U.S.A. 109.46–111.06�W

Yosemite National Park, 37.66–37.93�N 40 36 3.63� 0.25 June 1990

U.S.A. 119.33–119.78�W

Neotropical Region

Boa Vista, Brazil 1.98–2.75�N 12 7 2.92� 0.31 October 1996

60.89–61.73�W

La Gran Sabana, Venezuelad 4.45–5.98�N 17 14 3.88� 0.36 October 1996

60.96–61.75�W

Gran Sabana, Venezuelae 4.45–5.98�N 37 14 3.22� 0.26 February 1998

60.96–61.75�W

Manaus, Brazil 1.78–3.32�S 58 11 2.28� 0.13 March–May 1996

58.17–60.89�W

Pacaraima, Brazil 3.62–4.45�N 16 16 3.06� 0.34 October 1996

61.34–61.20�W

Puerto Ayacucho, 5.06–6.54�N 15 7 2.67� 0.32 October 1996

Venezuela 66.98–67.78�W

aGPS coordinates expressed as degrees and decimals of degrees.bSummer collections (May to July).cSpring collections (February to April).dDry season.eWet season.

Blackfly species richness 203


each site: seston, width, depth, velocity, discharge, conduc-

tivity, pH, dissolved oxygen, water temperature, dominant

streambed-particle size, canopy cover, and riparian vegeta-

tion. These variables are useful predictors of simuliid

distributions among stream reaches (McCreadie & Colbo,

1991; McCreadie & Adler, 1998; Hamada et al., 2002;

McCreadie et al., 2004) and of species richness for lotic

insects (Vinson & Hawkins, 1998).

All statistical tests were performed separately for spring

and summer collections. Stream variables are often highly

inter-correlated (e.g. discharge and width, r¼ 0.893,

P< 0.001). This multicollinearity affects confidence intervals

and significance tests of regression coefficients so that result-

ing models are unreliable (Neter et al., 1990). Principal com-

ponent analysis (PCA) therefore was used to collapse stream

parameters into a smaller number of statistically independent

principal components (PCs) (McCreadie & Adler, 1998;

Quinn & Keough, 2002). The use of PCA allows broader

ecological interpretations of habitat variables (McCreadie &

Adler, 1998; Hamada et al., 2002). Each PC is a linear com-

bination of the original stream variables, with each successive

PC accounting for a smaller percent of the variation in the

original data set. A preliminary examination of the stream

data suggested that a linear description of relationships

among stream variables was reasonable, especially for trans-

formed data. PCs with eigenvalues greater than 1.0 (Norusis,

1985) replaced the original stream variables in all further

analyses. Accordingly, temperature, percent dissolved oxy-

gen, conductivity, depth, width, velocity, discharge, seston,

streambed-particle size, riparian vegetation, and canopy

cover were entered into the PCA. Stream variables that

were not normally distributed were subjected to appropriate

transformations (log10, power, root, inverse negative) before

entering a PCA. Interpretation of each PC was based on rank

correlations between the PC and the original stream variables

(Ludwig & Reynolds, 1988). To provide a rigorous interpre-

tation of PCs, significance for these correlations was set at

P< 0.01 (McCreadie et al., 2004). A PCA was performed for

each combination of season (spring, summer) and ecoregion

(Mountains, Piedmont, Sandhills) for a total of six analyses.

A stepwise multiple regression analysis (ordinary least

squares) was then used to link species richness to stream-

site conditions, with stream conditions now expressed as

PCs. Entrance of each variable (PC) into the model was at

P¼ 0.05. Regressions were calculated for each combination

of season (spring, summer) and ecoregion for a total of six

regression sets.

One-way ANOVA was used to examine the influence of

ecoregion (main effect) on species richness (response vari-

able) for both spring and summer collections. A Tukey

multiple comparison was used to separate means in the

case of a significant analysis of variance (Zar, 1996).

Local vs. regional effects

The relationship between local and regional richness can

be described in two broad patterns (Cornell & Lawton, 1992;

Cornell, 1993). A relationship in which local richness

depends on regional species richness in a simple linear man-

ner is referred to as a type I or unsaturated community. In

contrast, as regional richness increases, local diversity could

reach an asymptote in which further increases in regional

richness are not matched by increases in local richness. This

type of community is referred to as a type II or saturated

community. Hence, local richness would exhibit a curvilinear

relationship with regional richness. Instances in which no

relationship between local and regional richness are found

are also considered type II communities.

Several assumptions and postulations were made for this

analysis which require discussion. First, the distinction

between local and regional scales of species richness is cur-

rently unclear in streams (Angermeier & Winston, 1998).

However, the regional unit harbours the species pool that

provides the colonists at the local scale (Lake, 2000). Each

stream (reach) collection in a particular region therefore was

considered a sample of local richness, with mean richness

among streams the best estimate of local richness. Secondly,

it was assumed that all species used in the analysis were

capable of reaching any stream site within a region (i.e.

species were not precluded from sites as a result of dispersal

abilities). Accordingly, the regional species pool was esti-

mated as the sum of species in all local collections for that

region. The assumption about dispersal is reasonable given

the widespread distribution of the species in the study (Adler

et al., 2004), the distance among sites, and the known dis-

persal abilities of many blackflies (Crosskey, 1990).

Either implicit or explicit is the idea of equal sampling

effort (in terms of area and time) among local collections.

This ideal situation is not possible in most cases, even in

studies that imply equal sampling effort. For example,

many studies estimate local richness from field guides

(Caley & Schluter, 1997). Even a well-studied area such as

North America cannot claim equal sampling across the con-

tinent for any group. This is evident by the large number of

new county, state, provincial, or regional records reported

for a variety of taxa, especially insects. For stream habitats,

the situation is even more difficult. For example, with equal

timed collections among sites, an investigator would be able

to sample more habitat (and thus get more specimens) in a

shallow, slow-moving, 1-m wide stream than in a fast-flow-

ing, silt-bottomed, 30-m wide stream with the dominant

substrate being snags. Furthermore, there is no artificial

substrate that will work equally well among all stream

types for blackflies (Colbo, 1988; Adler et al., 2004).

Given the problem of equal sampling, we elected to base

our estimates of local richness on a mean and regional richness

on the sum of species found in the region’s local streams. Such

estimates are also partially self-correcting for bias due to

unequal sampling effort and the problem of rare species. By

definition, a rare species would occur with a low frequency.

Given that our estimate of local richness is based on a number

of sites, missing a rare species would have little effect on the

calculation of our mean, especially if sample size (i.e. number

of streams sampled) is high. Furthermore, if a rare species was

missed in all collections, the regional richness would be

reduced by one.



Fifteen regional collections were made in the Nearctic

Region, with a total of 447 local stream collections from

coast to coast and from Alaska to Florida (Table 1). The

number of local stream samples for the Nearctic regional

collections varied from 12 to 55. Five regions and 155

stream collections were taken in the Neotropics (Brazil

and Venezuela). The number of local stream samples for

Neotropical regional collections varied from 12 to 58.

The relationship between local and regional species rich-

ness was examined using regression analysis. Both simple

linear and curvilinear (second-order polynomial) models

were tested (Caley & Schluter, 1997). Given that local spe-

cies richness will be zero when regional richness is zero,

regression through the origin (i.e. y intercept¼ 0) was

used. Two sets of regressions were calculated. The first set

included Nearctic and Neotropical sites; the second set

included only Nearctic samples. Too few regional collec-

tions were available from the Neotropics (n¼ 5) to perform

a separate analysis for this area.

A standard method for testing whether results were con-

sistent with species saturation in local communities was

applied. A significant simple linear relationship between

local and regional richness was taken as evidence of non-

saturated communities. Alternatively, either a significant

curvilinear (concave down) or non-significant relationship

was taken as support of a saturated community (Cornell &

Lawton, 1992; Cornell, 1993; Caley & Schluter, 1997).

Comparison of Nearctic and Neotropical species richness

Differences in mean local stream richness and mean

regional species richness between Nearctic and Neotropical

streams was examined with a t-test assuming unequal var-

iances. When comparing local richness, each stream was

considered the sampling unit, and when comparing regional

richness, each region was considered the sampling unit.

Results

Of the six regressions calculated for local richness and

stream conditions, only one, from the spring set of collec-

tions in the Sandhills, was significant

(y¼ 4.74þ 0.590PC1þ 0.611PC5, F2,16¼ 11.39, P¼ 0.001,

r2¼ 53.6%). This regression showed a highly significant

positive association with two PCs. Correlation analysis of

PC1 with the original stream variables indicated that high

scores of this PC were associated with large streams with

high pH and conductivity (Table 2). High values of the

second PC indicated streams with lower oxygen than

streams with lower PC values.

Species richness also varied significantly among the three

ecoregions examined in South Carolina during the spring.

The mean number of species during the spring was signifi-

cantly higher (P< 0.05) in the Sandhills than in either the

Piedmont or Mountains (Table 3). In contrast, no signifi-

cant difference was found among ecoregions in the summer.

Regression analysis between local and regional species

richness showed significant polynomial terms (Table 4).

This relationship did not change when either the complete

data set or only the Nearctic data set was considered. This

result was interpreted to mean that blackfly communities

are saturated. Support for this interpretation can be found

in the low variation of mean local species richness

(2.28–4.74), regardless of regional richness (7–40) (Table 1)

and the scatter plot (Fig. 1) of the relationship between local

and regional richness.

t-test analysis showed that mean local species richness of

larval blackflies was significantly higher (t329¼ 5.07,

P< 0.001) in Nearctic streams (n¼ 447) than in Neotropical

streams (n¼ 138). Mean richness (� SE) was 3.53� 0.08

across all Nearctic streams as opposed to 2.85� 0.11 for

all Neotropical streams. Although there was no significant

difference (t6¼ 0.13, P¼ 0.903) in the mean number of local

sites sampled per region for Nearctic (24.8� 2.6) and Neo-

tropical streams (25.8� 7.4), regional species richness also

was significantly higher (t19¼ 3.52, P¼ 0.002) in the

Nearctic (20.44� 2.0) than in the Neotropics (11.50� 1.6).

Discussion

The current study is the most taxonomically detailed and

geographically encompassing analysis of simuliid species

richness, with 532 stream sites in two zoogeographic

regions. Because factors influencing species richness are

scale dependent (e.g. Vinson & Hawkins, 1998), scales

from stream reach to zoogeographic region were examined.

At the scale of the reach (and habitat), numerous factors are

associated with species richness, including substrate hetero-

geneity (Cowie, 1985), water chemistry (Townsend et al.,

1983; Jenkins et al., 1984; Erman & Erman, 1995), water

temperature (Ward & Dufford, 1979; Ward, 1986), stream

flow and size (Jenkins et al., 1984; Erman & Erman, 1995;

Malmqvist & Eriksson, 1995; Tate & Heiny, 1995), disturb-

ance (Erman & Erman, 1995; Palmer et al., 1995), and

altitude (Tate & Heiny, 1995). Biotic interactions seem to

have little influence on species richness (Townsend, 1989;

Vinson & Hawkins, 1998), although models of lotic commun-

ity structure have been advanced that, under specific condi-

tions, suggest biotic interactionsare important (e.g.Minshall&

Petersen, 1985). A variety of factors (size, temperature,

velocity, streambed, and riparian vegetation), acting in

concert, influences blackfly richness in Amazonas (Hamada

et al., 2002).

Of the six data sets from three ecoregions in South

Carolina, only the spring collection in the Sandhills yielded

a significant relationship between site conditions and rich-

ness. Combined regression and PCA analyses suggested

that species richness was highest in larger streams with

higher pH and conductivity and lower dissolved oxygen.

Had the traditional approach been taken, limiting the

study to a restricted geographic area and one season, the

interpretation of results would have depended on where and

when sampling occurred. If sampling only occurred in the



Sandhills during the spring, for example, it would have

concluded that blackfly species richness at the scale of

the basin was highly predictable. Conversely, if all three

ecoregions (Mountains, Piedmont, and Sandhills) had

been sampled during the summer, the conclusions would

have been the opposite. It is not being suggested that season

influenced local species richness, but rather that the ability

to predict local richness, based on stream conditions, varied

across seasons. It is suspected that conclusions reached in

many previous studies of lotic insect richness would be

considerably different if the studies had extended over a

wider range of habitat conditions. Resh and Rosenberg

(1989) have stressed the need to conduct lotic studies over

a variety of temporal and spatial scales.

The lack of significance for five of the six regressions can

be reconciled with an interpretation of streams existing as

non-equilibrium systems. The traditional paradigm of com-

munity structure has been one of the systems in equilibrium,

with biotic interactions acting as the primary determinants

of species diversity (Begon et al., 1996) and the importance

of disturbance or other environmental fluctuations being

minimal. However, lotic ecosystems are viewed as disturbed

systems, with equilibrium conditions rarely being estab-

lished (e.g. Reice, 1994). McCreadie et al. (1997) provide

strong evidence for non-equilibrium communities of black-

flies in small streams (� 2 m wide) in Yellowstone National

Park, U.S.A. The first impact is to remove organisms,

creating opportunities for new individuals to colonise

(Reice, 1994). After a disturbance, local species richness

returns to pre-disturbance levels over a period of time

(Lake, 2000). Hence, the number of species at a local site

might depend on where on the recovery curve the sample is

taken.

In this study, mean species richness was highest in the

Sandhills ecoregion. However, the total number of species

in this region during the spring was lower (11 species) than

in either the Piedmont ecoregion (16 species) or the Moun-

tain ecoregion (16 species). The Sandhills in the spring was

the only data set in which a significant association occurred

between stream-site conditions and richness, suggesting

that, at least for this area and time, local conditions played

a more important role than regional richness in determining

local species richness.

Some of the potential problems that might have influ-

enced results in previous studies of regional effects on local

diversity were addressed in this study. First, an implicit

assumption in many studies is that members of the same

taxon are responding in the same manner. Caley and

Schluter (1997), for example, assumed that all Lepidoptera

(butterflies) or all Odonata (dragonflies) could be treated as

Table 2. Results of PCA and correlation analysis between stream variables and derived principal components (PCs) for spring collections in

the Sandhills ecoregion of South Carolina.

Stream sites Principal components

Variables Min. Max. Mean (� SE) PC1 PC2 PC3 PC4 PC5

Temperature (�C) 7 15.6 9.9� 0.51 �0.494 0.164 �0.107 �0.197 �0.397

pH 5.6 7 6.9� 0.09 0.732 * �0.150 �0.288 �0.651* 0.049

% Dissolved oxygen 93.8 104.3 99.2� 0.66 0.270 0.338 �0.343 �0.341 �0.650*

Conductivity (mS cm�1, 25�C) 18 51 30.1� 1.84 0.634 * 0.014 0.138 �368 0.384

Depth (m) 0.13 1.73 0.50� 0.08 0.643 * 0.414 �0.343 0.054 �0.082

Velocity (m s�1) 0.17 0.44 0.26� 0.02 0.213 �0.225 �0.812 * 0.130 0.327

Width (m) 0.6 15.0 6.34� 1.01 0.663 * 0.411 0.044 0.177 �0.116

Discharge (m3 s�1) 0.01 3.69 0.87� 0.21 0.678 * 0.519 �0.154 0.263 �0.101

Seston (mg l�1) 2 10 4.7� 0.38 0.451 �0.435 0.184 0.418 �0.192

Streambed-particle sizea mud silt �0.355 0.302 �0.533 �0.017 0.388

Riparian vegetationa open forest �0.416 0.818 * 0.409 �0.003 0.259

Canopy covera open complete �0.384 0.793 * 0.409 �0.030 0.346

% variance explained in PCA

Proportion 25.1 22.2 14.6 10.9 9.9

Cumulative 25.1 47.3 62.0 72.9 82.8

aRanked variables: streambed-particle size 1–6; riparian vegetation 1–3; canopy 1–3; rankings follow those of McCreadie and Colbo (1991).

*P< 0.01.

Table 3. One-way analysis of variance of mean species richness

(response variable) of immature blackflies among three ecoregions

(main effect) in South Carolina.

Ecoregion Mean (� SE)* n F d.f. P

Spring

Mountains 3.18� 0.25a 22 7.97 2, 54 0.001

Piedmont 3.19� 0.34a 16

Sandhills 4.74� 0.37b 19

Summer

Mountains 3.28� 0.25 18 1.73 2, 66 0.185

Piedmont 3.68� 0.24 31

Sandhills 4.00� 0.23 20

*Means with different letters are significantly different at a family

(group) error rate of P< 0.05, based on the Tukey multiple

comparison test.



a single unit. A few studies have shown, however, that

factors associated with richness vary among taxa or func-

tional feeding groups (e.g. Hawkins et al., 1982), invalidat-

ing this assumption for lotic systems. This study therefore

was restricted to an ecologically (lotic) and functionally

(mostly filter-feeding) homogeneous group. Establishing

empirical relationships of insect communities could be com-

promised by a paucity of species-level descriptions

(McCreadie & Adler, 1998). But given their taxonomic

status, blackflies are an ideal group for examining biodiver-

sity in lotic systems (Adler & McCreadie, 1997). Finally, the

current study used 12 or more local sites to estimate local

richness for each region. Studies based on single samples of

local richness (e.g. Caley & Schluter, 1997) run the risk of

producing a curvilinear relationship resulting from sample

bias rather than from saturated communities. A single sam-

ple, for example, might not reflect the heterogeneity of

habitats across a region. Because habitat heterogeneity is

believed to promote species richness of lotic insects, a num-

ber of single local samples in a species-poor homogeneous

area, collected from heterogeneous species-rich regions,

could produce curvature that mimics a saturated community.

Most studies conducted to date have suggested that local

species richness increases in a simple linear manner as

regional species richness increases; that is, in most cases,

communities are not saturated (Cornell, 1985, 1993; Caley

& Schluter, 1997). In contradistinction, results of the regres-

sion analyses performed here showed a significant curvilinear

relationship between local and regional species richness

of blackflies. This relationship did not change whether

the complete data set or only samples from Nearctic

streams were used. This result was interpreted to indicate

that simuliid communities are saturated, meaning there is

an upper limit to the number of species in a local simuliid

assemblage. Several recent studies have suggested that

species richness for lotic insects is remarkably constant (e.g.

Death & Winterbourn, 1994) and that local regulation of

species richness is strong (e.g. Downes et al., 2000). Both

findings would be expected to occur in saturated communities

and therefore strongly support the conclusion that simuliid

communities are best described as saturated.

Finally, it is emphasised here that species richness of

blackflies in temperate streams is greater than in tropical

streams at both local and regional scales. This result con-

trasts markedly with the well-known phenomenon of hyper-

richness of terrestrial insects in the Neotropics vs. the

Nearctic Region. Whether the results from blackflies can

be generalised to other aquatic insects must await until the

taxonomy of Neotropical aquatic insects is better under-

stood. However, evidence has been accumulating for some

time that lotic insects richness may not be greater in tropical

habitats (e.g. Illies, 1969; Patrick, 1975).

Acknowledgements

We thank C. R. L. Adler, L. Aquino, R. I. Barbosa,

J. E. Binda, C.E. Beard, D.S. Bidlack, J. Bosco, J.F. Burger,

D.C. Currie, C. Delgado, F.F. Xavier Filho, R.D. Gray,

P.G. Mason, and C. Steiner for providing some collections.

We also thank two anonymous reviewers for helpful

comments. This work was partially funded by a National

Geographic Grant (5731-96) to J.W.M., an NSF grant

(DEB 0075269) to J.W.M. and P.H.A., an NSF grant

(DEB-9629456) to P.H.A., and INPA/PPI 1-3400 and a

CNPq fellowship (201165/93-7) to N.H.

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Data

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model

Regression

coefficients t-value P R2

Nearctic and Neotropical Linear 0.158X 11.70 <0.001 85.6%Quadratic 0.306X 14.91 <0.001 96.1%

0.006X2 7.68

Nearctic Linear 0.150X 10.58 <0.001 86.8%Quadratic 0.295X 12.26 <0.001 96.3%

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Regional richness5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0

Mea

n lo

cal r

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nes

s

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5.0

Fig. 1. Scattergram of mean local and regional richness for

samples from 20 sites in the Nearctic and Neotropical Regions.

Several sites were sampled on two occasions.



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Accepted 21 June 2004



Documents

Patterns of species richness for blackflies (Diptera: Simuliidae) in the Nearctic and Neotropical regions