Freshwater Biology (1997) 38, 247–261
Structure and diversity of stream invertebrate
assemblages: the influence of temperature with
altitude and latitude
D E A N J A C O B S E N , * R I K K E S C H U LT Z * A N D A N D R E A E N C A L A D A †
*Freshwater Biological Laboratory, University of Copenhagen, 51 Helsingørsgade, DK 3400 Hillerød, Denmark
†Departamento de Ciencias Biológicas, Pontificia Universidad Católica del Ecuador, Apartado 17–01–2184, Quito, Ecuador
S U M M A RY
1. Structure and diversity of the macroinvertebrate fauna were studied in relation to
altitude and latitude among three groups of streams from Ecuador (lowland: 100–600 m,
Central Valley: 2600–3100 m, páramo: 3500–4000 m), and one group from the temperate
lowland region of Denmark. The streams in the four regions were comparable with
regard to physical characteristics such as size, current and substratum.
2. In terms of faunal composition the Ecuadorian highland streams bore more
resemblance to the Danish lowland streams than the Ecuadorian lowland streams. The
greater similarity between the Ecuadorian highland and the Danish streams, however,
was due to the large number of insect families in the Ecuadorian lowlands, many of
which were not found in the other regions. Of ten physico-chemical parameters
measured, maximum stream temperature explained by far the most variability in faunal
composition.
3. The number of insect orders and families increased linearly with maximum stream
temperature and therefore decreased with altitude and latitude. A compilation of
literature data on insect richness and maximum water temperature from streams around
the world confirmed this pattern, yielding a common linear relation for both temperate
and tropical streams. This pattern may arise due to a direct temperature effect on
speciation but is probably also related to geological history and the influence of climatic
changes on stream ecosystems. We estimate that small, tropical, lowland streams have,
on average, a two- to fourfold higher species richness than temperate lowland streams.
Introduction
Species richness among most groups of organisms in
the sea and on the continents increases markedly
from high latitudes towards the tropics (Fischer, 1960;
Rosenzweig, 1995). This pattern is much less clear
in the freshwater biota. Species richness of aquatic
angiosperms is lower in tropical than in temperate
regions (Crow, 1993), and richness of molluscs is about
the same (Hubendick, 1962). Apparently only the fish
have reached a far greater species richness in tropical
freshwaters (Lowe-McConnell, 1987). The latitudinal
pattern of aquatic insect diversity is unclear, as some
authors have found higher diversity in the tropics
(Bishop, 1973; Stout & Vandermeer, 1975; Pearson,
© 1997 Blackwell Science Ltd
Benson & Smith, 1986), while others have found
the same diversity in tropical and temperate regions
(Patrick, 1964; Arthington, 1990; Flowers, 1991). Our
ability to evaluate this question is limited, however,
because few studies are available from the tropics, all
differing in sampling technique, and because knowledge on the systematics of tropical freshwater insects
is incomplete.
Temperature affects stream invertebrates in a number of ways (Ward & Stanford, 1982; Allan, 1995) and
is the most apparent abiotic variable related to latitude
as well as to altitude. Studies on altitudinal succession
from tropical (Illies, 1964; Hynes, 1971; Williams &
247
248 D. Jacobsen, R. Schultz and A. Encalada
Hynes, 1971) and temperate regions (Kownacka &
Kownacki, 1972; Allan, 1975; Ward, 1986) demonstrate
clear changes in faunal structure and a decrease in
diversity with altitude. Illies (1964) suggested that an
analogous succession in life-types to that from high
to low altitudes should also exist from high to low
latitudes, and temperature was suggested to be the
main variable governing this pattern. However, longitudinal zonation studies following the same stream
from mountain brook to lowland river obscure the
effect of temperature itself because small streams and
large rivers, even at the same altitude, are two different
habitat types with different faunal assemblages
(Vannote et al., 1980), and stream size itself has an effect
on species richness (Brönmark et al., 1984; Minshall,
Petersen & Nimz, 1985; Jacobsen & Friberg, 1997).
Comparative studies on the overall faunal structure
and diversity of similar stream types located at different altitudes (Ormerod et al., 1994) and latitudes (Stout
& Vandermeer, 1975) are few. Hence, the specific
influence of temperature on faunal composition along
altitudinal and latitudinal gradients has remained
elusive.
This study compares quantitative composition and
diversity of the macroinvertebrate fauna from four
groups of streams. Three groups were from different
altitudes in tropical Ecuador, and one group was from
the temperate, lowland region of Denmark. All streams
were sampled in the same way, and a consistent level
of identification to family was applied to all samples.
Furthermore, the streams were comparable with
regard to physical parameters such as size, current
and substratum. Temperature was the main physical
parameter differing systematically among the stream
groups. The purpose of the study was to examine the
general patterns in structure and diversity of the
invertebrate fauna across altitudes and latitudes and
subsequently relate the patterns to stream temperature.
Materials and methods
Localities
The study included three groups of streams in Ecuador
and one group in Denmark. Each group consisted of
eight streams. The first group in Ecuador included
streams located in the coastal lowlands at altitudes of
100–600 m above sea level (m a.s.l.). These streams
drain disturbed, tropical, lowland forest, pasture and
extensively cultivated areas. The maximum distance
between any two of the streams was 25 km. The
lowland streams were sampled in June 1995. The
second Ecuadorian group included streams at altitudes
between 2600 and 3100 m in the Central Valley of the
Andes around the capital city of Quito. The streams
drain secondary scrub vegetation and extensively cultivated areas. These streams were up to 85 km apart
and were sampled in September and October 1994.
The third group of Ecuadorian streams was located in
the Eastern Cordillera of the high Andes at 3500–
4000 m a.s.l. These streams drain páramo, a humid
grassland vegetation characteristic of the Northern
Andes. The páramo streams were up to 110 km apart
and they were sampled from February to May 1995.
Finally, a group of Danish lowland streams at 0–
100 m a.s.l. located in mid-Jutland, was included. The
streams drain secondary deciduous forest, pasture and
cultivated areas and were maximally 75 km apart. The
streams in Denmark were sampled from May to July
1993. All streams in Ecuador and Denmark included
in the study were first-, second- or third-order streams,
rather fast-flowing with coarse substratum and with
no or very little aquatic macrophyte growth. None of
the streams had significant sources of pollution.
Physicochemical measurements
In Ecuador, physicochemical parameters were measured on three occasions during the dry season in each
of the lowland and Central Valley streams, but only
once in each of the páramo streams. The highest
daytime temperature recorded and mean values of all
other parameters are used in this study. Measurements
in the Danish streams were performed once in March.
The maximum temperatures, however, originate from
several years of measurements in each stream. Conductivity, normalized to 20 °C, was measured with a
WTW LF96 meter, pH was measured with a WTW
pH96 meter, and alkalinity was measured by means
of gran titration with 0.1 N HCl (Mackereth, Heron &
Talling, 1978). Mean current velocity (in 0.4 3 water
depth) was measured with a Höntzch digital anemometer with a vane wheel sensor at four to eight points
along three cross-sections. The composition of the
substratum was measured by registering the type of
substratum in 58–113 points along six to eight randomly chosen transects across the stream. The mineral
substratum types were assigned to the following six
© 1997 Blackwell Science Ltd, Freshwater Biology, 38, 247–261
Structure and diversity of stream invertebrates 249
categories: silt, sand, gravel, pebble, cobble and
boulder (Hynes, 1970).
were identified using Roldán (1992) and Merritt &
Cummins (1996).
Invertebrate sampling
Data treatment
Benthos samples from the Ecuadorian streams were
collected in the dry season, when taxon richness and
density of invertebrates are highest (D. Jacobsen &
A. Encalada, unpublished data), while the Danish
streams were sampled in summer, from May to July.
Winter samples of benthos were available from six
of the eight Danish streams. The mean number of
individuals was 20% higher, and, on average, one to
two more families were found in the winter samples
than in summer samples.
In each stream, a 10- to 20-m reach was chosen for
the study. Invertebrate samples were collected by
standardized ‘kick-sampling’ using a 25 3 25-cm
nylon handnet (mesh size: 0.5 mm) placed on the
stream bottom. A sample was obtained in the following
way: three transects across the stream were chosen,
and at each transect the stream bottom in front of the
net was disturbed by kicking twice in the substratum
at four positions: next to bank, at 25%, at 50% and at
75% of the stream width. Although this method is
not strictly quantitative, it allows comparison among
samples. The benthos samples were preserved in
70% ethanol. Samples were sorted without use of
magnification.
To achieve the same level of identification of invertebrates from Ecuador and Denmark, all insects were
identified to family level and non-insects to class.
At present we must either work with operational
taxonomical units (OTU) as applied by Stout &
Vandermeer (1975), use taxonomically well known
groups as indicators of biodiversity, or keep identification to a consistent taxonomic level such as family
(Williams & Gaston, 1994). We recognize that the
family level is not absolutely consistent among insect
orders, but is influenced by the maturity and tradition
of the taxonomy of each order. However, keeping
identification at the family level is probably more
consistent than using a mixture of species, genera and
families, as so often seen in the literature. Family
richness of insects at individual stream sites is highly
correlated to species richness (Bournaud et al., 1996;
Wright, Moss & Furse, 1997) so, when identification
to species is not consistently possible, family richness
will reflect species richness. Insects from Ecuador
Similarity in overall faunal structure among the thirtytwo streams was analysed using the computer software package PRIMER (Clarke & Warwick, 1994) to
construct a dendrogram. PRIMER ’s multivariate techniques are based on non-parametric calculations of
similarity between samples. PRIMER was also used
to perform multivariate non-parametric correlations
(weighted Spearman harmonic) of faunal composition
to environmental parameters. In this way the parameters, or combination of parameters, best explaining
the faunal variability could be found.
‘Species’ accumulation curves were made for the
four regions to estimate regional insect family richness,
regarding each of the eight streams in a group as a
locality. The order of plotting the eight streams was
randomized 1000 times using jack-knifing and the
mean value of family number was plotted. A
Michaelis–Menten regression was fitted to the data
points because this is an asymptotic function that
allows estimation of total richness within a confined
region through extrapolation (Colwell & Coddington,
1995). This procedure gives three parameters of interest: (α) the point diversity, or mean number of taxa
found at one locality, by some authors called alpha
diversity, (γ) the maximum richness of a region or taxon
pool, called gamma diversity (Vmax in the original
Michaelis–Menten equation), and (β) the number of
localities needed to represent half of the region’s taxon
pool (Km in the original equation). The β-value is here
regarded as a measure of the so-called beta diversity,
which is a measure of the rate of species accumulation
as one moves from locality to locality.
© 1997 Blackwell Science Ltd, Freshwater Biology, 38, 247–261
Results
Physicochemical characteristics
Of the measured physical characteristics, significant
differences among the stream groups were only found
regarding temperature and substratum composition,
the Danish streams generally having a less coarse
substratum (Table 1). However, the four groups of
streams were dominated by relatively coarse substratum and good current, and all thirty-two streams
40a
(18–67)
192b
(101–317)
56a
(19–119)
331c
(165–450)
24.8a
(22.4–26.3)
14.6b
(10.7–17.8)
9.8c
(6.4–14.8)
15.3b
(12.0–18.0)
Coastal lowlands
Central valley
Páramo
Denmark
1.55b
(0.23–2.53)
0.63a
(0.14–1.50)
1.51b
(0.55–2.79)
0.37a
(0.17–0.62)
Conductivity Alkalinity
(mEq l–1)
(µS cm–1)
Max. temp.
(°C)
7.64b
(6.35–8.31)
7.39ab
(6.18–8.25)
7.88b
(6.82–8.90)
6.89a
(6.30–7.49)
pH
183a
(50–272)
213a
(123–350)
227a
(108–443)
323a
(140–643)
Width
(cm)
57a
(1–120)
106a
(5–289)
148a
(9–689)
122a
(6–394)
Discharge
(l s–1)
24a
(8–39)
23a
(5–53)
24a
(7–43)
20a
(4–42)
Current
(cm s–1)
36bc
(18–47)
10a
(0–24)
25ac
(0–69)
8a
(0–29)
silt 1 sand
54a
(17–69)
35b
(10–56)
37b
(18–66)
63a
(46–72)
gravel 1 pebble
Inorganic substratum (%)
9c
(0–20)
55b
(39–81)
37ab
(7–62)
30a
(0–51)
cobble 1 boulder
Table 1 Mean values and ranges (in parentheses) of some chemical and physical chacacteristics of the four groups of eight streams in the coastal lowlands, Central valley and
páramo in Ecuador and from the lowlands of Denmark. Significant differences among values are shown by different index letters (P , 0.05, Mann–Whitney U-test, one-tailed)
250 D. Jacobsen, R. Schultz and A. Encalada
© 1997 Blackwell Science Ltd, Freshwater Biology, 38, 247–261
Structure and diversity of stream invertebrates 251
Table 2 Number of individuals and taxa (insect families and non-insect classes), Margalef’s richness index and Pielou’s evenness
index from ‘kick-samples’ from Ecuadorian and Danish streams. Mean values with ranges in parentheses are given for each group
of eight streams. Significant differences among values are shown by different index letters (P , 0.05, Mann–Whitney U test,
one-tailed)
Individuals
Taxa
Richness
Evenness
436a
(114–1040)
29.3a
(24–33)
3.38a
(2.89–3.81)
0.756a
(0.622–0.849)
Valley
1382b
(587–4100)
20.9b
(15–26)
1.97b
(1.52–2.27)
0.516b
(0.337–0.751)
Páramo
653ac
(335–1310)
13.5c
(10–20)
1.35c
(1.02–1.84)
0.634b
(0.520–0.799)
Denmark
750bc
(318–1070)
20.0b
(17–24)
2.01b
(1.59–2.48)
0.556b
(0.386–0.644)
Ecuador
Lowland
had from four to six of the applied inorganic types of
substrate. Temperature clearly divided the streams
into three groups: the Ecuadorian lowland streams
had a mean maximum temperature of 24.8 °C, the
páramo streams of 9.8 °C, while the Central Valley
and Danish streams had similar mean maximum temperatures of 14.6 and 15.3 °C. All chemical parameters
differed clearly among the stream groups, reflecting
the nutrient-poor waters of the Ecuadorian lowland
and páramo streams relative to the Central Valley and
Danish streams.
Structure of fauna
The density of invertebrates was significantly lower
in the Ecuadorian lowland streams compared to the
Central Valley and the Danish streams while the
number of higher taxa (insect families 1 non-insect
classes) was higher in the lowland streams (Table 2).
Two indices were calculated to evaluate the two basic
components of diversity, richness and equitability: the
Margalef richness index as a measure of the number
of taxa taking the sample size into account, and the
Pielou evenness index as a measure of equitability
irrespective of the number of taxa. Both the Margalef
and the Pielou indices were significantly higher in the
lowland streams than in any other of the three stream
groups (Table 2). Thus, the fauna of the lowland
streams were less dominated by few taxa.
The fauna of the lowland and the Central Valley
streams of Ecuador was completely dominated by
insects (95% and 93%, respectively), while the proportion of insects in the páramo and in the Danish
© 1997 Blackwell Science Ltd, Freshwater Biology, 38, 247–261
streams was lower (70%) (Table 3). The Crustacea
(Gammaridae) showed a clear increase with altitude,
from absence in the Ecuadorian lowland, 3% in the
Central Valley, to 14% on the páramo. Gammarus was
a very prominent part of the Danish stream fauna.
Oligochaeta also increased in proportion from the
Ecuadorian lowland to the páramo. Turbellaria
(Planaridae) and Bivalvia (Pisidiidae) were other
important groups in the páramo streams. The number
of insect orders decreased with altitude. In the lowland
streams nine orders were found. Odonata comprised
8%, Hemiptera 7%, Megaloptera 0.8% and Lepidoptera
0.5% of the lowland fauna, but, except for a few
Odonata in the Central Valley, these four orders were
absent in the Central Valley and the páramo streams.
Ephemeroptera dominated the insect fauna in all
three regions in Ecuador, but were less numerous in
Denmark (Table 3). Plecopterans, in contrast, were rare
in all three regions in Ecuador but were numerous in
the Danish streams. Coleoptera and Trichoptera were
more important and Diptera less dominant in the
Ecuadorian lowland streams than in the other
stream groups.
A total of fifty-four families of insects were found
in Ecuador and forty-four in the lowlands (Table 4).
Twenty-three insect families were restricted to the
lowlands, mainly families of Odonata and Hemiptera,
whereas eleven families were restricted to the
Central Valley and the páramo. Only one family
(Limnephilidae: Trichoptera) was restricted to the
páramo. Fifteen families were found at all altitudes
and many of these were common or abundant at all
altitudes (Baetidae, Elmidae, Glossosomatidae,
252 D. Jacobsen, R. Schultz and A. Encalada
Table 3 Composition of the macroinvertebrate fauna given as a total of kick-samples from eight streams in three Ecuadorian and
one Danish region
Ecuador
Turbellaria
Oligochaeta
Hirudinea
Bivalvia
Gastropoda
Arachnida
Crustacea
Insecta
Plecoptera
Ephemeroptera
Odonata
Hemiptera
Coleoptera
Megaloptera
Trichoptera
Lepidoptera
Diptera
Lowland 100–600 m
Valley 2600–3100 m
Páramo 3500–4000 m
Denmark 0–100 m
n
%
n
%
n
%
n
%
106
26
5
5
31
12
–
3300
7
1034
265
213
834
25
434
16
453
3.0
0.7
0.1
0.1
0.9
0.3
–
94.7
0.2
29.6
7.6
6.2
24.0
0.8
12.5
0.5
13.0
18
183
81
12
1
66
334
10 316
19
4145
3
–
591
–
1110
–
4448
0.2
1.7
0.7
0.1
0.01
0.6
3.0
93.3
0.2
37.5
0.03
–
5.3
–
10.1
–
40.3
217
473
3
167
11
–
719
3629
7
1372
–
–
540
–
259
–
1450
4.2
9.1
0.1
3.2
0.2
–
13.8
69.5
0.1
26.3
–
–
10.4
–
4.9
–
27.8
63
84
3
129
21
34
1487
4197
887
534
–
5
274
7
140
–
2350
1.0
1.4
0.05
2.1
0.3
0.6
24.8
69.8
14.7
8.9
–
0.1
4.5
0.1
2.3
–
39.1
Hydroptilidae,
Leptoceridae,
Ceratopogonidae,
Chironomidae, Empididae, Simuliidae and Tipulidae).
Overall, the Ecuadorian lowland and highland fauna
(Central Valley and páramo combined) shared twenty
families. The Danish stream fauna (thirty insect families collected) shared sixteen families with the Ecuadorian lowland streams, sixteen with the Ecuadorian
highland streams and nineteen families, overall, with
the Ecuadorian fauna.
A dendrogram based on fourth-root-transformed
data to downweigh very abundant taxa showed that
the Ecuadorian lowland and the Danish streams composed two clearly distinct faunas, while there was
greater resemblance between the Central Valley and
páramo streams (Fig. 1). More interesting though, the
Ecuadorian highland streams showed more similarity
to Danish lowland streams (42%) than to Ecuadorian
lowland streams (33%).
Faunal composition in relation to environmental
parameters was also analysed by PRIMER ’s multivariate
technique. Maximum temperature had the highest
correlation coefficient (rs 5 0.49), and was therefore the
single parameter best explaining the faunal variation.
None of the other parameters showed nearly as good
correlations, the second best correlated parameter
being pH (rs 5 0.17), and conductivity the third
(rs 5 0.16). The best combination of parameters to
account for the variation in fauna was temperature
and conductivity (rs 5 0.58).
Diversity of fauna
Alpha and gamma diversities were highest in the
Ecuadorian lowland region and lowest on the páramo
(Table 5). The streams in the Central Valley and in
Denmark had intermediate and similar α and γ values.
Between 85 and 92% of the theoretically estimated
regional insect family pool (γ diversity) was reached
from the eight sampled streams. The β-diversity was
significantly higher on the páramo than in any other
region (P , 0.05, t-test) and lowest in the Ecuadorian
lowlands. The γ diversity of the four groups was
linearly and very closely related to mean maximum
temperature of each group (r2 5 0.98).
Regression analyses between family richness and
the physicochemical parameters on the thirty-two
stream sites showed that family richness was highly
related to maximum stream temperature (r2 5 0.74)
(Fig. 2). Family richness was also, but to a much lesser
extent, related to stream width and the percentage of
gravel and pebble in the substratum. None of the
chemical parameters showed significant correlations.
© 1997 Blackwell Science Ltd, Freshwater Biology, 38, 247–261
Structure and diversity of stream invertebrates 253
Table 4 Insect families collected from kick-samples from eight streams in three Ecuadorian and one Danish region
Ecuador
Lowland 100–600 m
Valley 2600–3100 m
Páramo 3500–4000 m
Denmark 0–100 m
Plecoptera
Perlidae
Gripopterygidae
Perlidae
Gripopterygidae
Leuctridae
Nemouridae
Perlidae
Ephemeroptera
Baetidae
Caenidae
Euthyplocidae
Leptohyphidae
Leptophlebidae
Calopterygidae
Coenagrionidae
Gomphidae
Libellulidae
Megapodagrionidae
Gerridae
Hebridae
Naucoridae
Veliidae
Hemiptera indet
Dryopidae
Elmidae
Hydrophilidae
Lampyridae
Psephenidae
Ptilodactylidae
Corydalidae
Calamoceratidae
Glossosomatidae
Helicopsychidae
Hydrobiosidae
Hydropsychidae
Hydroptilidae
Leptoceridae
Philopotamidae
Polycentropodidae
Xiphocentronidae
Pyralidae
Ceratopogonidae
Chironomidae
Culicidae
Dixidae
Empididae
Muscidae
Psychodidae
Sciomyzidae
Simuliidae
Tipulidae
Baetidae
Leptohyphidae
Leptophlebidae
Oligoneuridae
Baetidae
Leptophlebidae
Baetidae
Ephemeridae
Leptophlebidae
Aeshnidae
–
–
–
–
–
Dytiscidae
Elmidae
Gyrinidae
Ptilodactylidae
Scirtidae
Elmidae
Scirtidae
–
Anomalopsychidae
Glossosomatidae
Helicopsychidae
Hydrobiosidae
Hydropsychidae
Hydroptilidae
Leptoceridae
Polycentropodidae
–
Anomalopsychidae
Glossosomatidae
Helicopsychidae
Hydrobiosidae
Hydropsychidae
Hydroptilidae
Leptoceridae
Limnephilidae
Dytiscidae
Elmidae
Helophoridae
Hydraenidae
Hydrophilidae
Scirtidae
Sialidae
Goeridae
Hydropsychidae
Leptoceridae
Limnephilidae
Polycentropodidae
Rhyacophilidae
Sericostomatidae
–
Blepharoceridae
Ceratopogonidae
Chironomidae
Empididae
Ephydridae
Muscidae
Psychodidae
Simuliidae
Tabanidae
Tipulidae
Diptera indet
–
Blepharoceridae
Ceratopogonidae
Chironomidae
Empididae
Muscidae
Simuliidae
Tabanidae
Tipulidae
–
Ceratopogonidae
Chironomidae
Dixidae
Empididae
Lonchopteridae
Psychodidae
Sciomyzidae
Simuliidae
Thaumaleidae
Tipulidae
31
21
30
Odonata
Hemiptera
Coleoptera
Megaloptera
Tricoptera
Lepidoptera
Diptera
Total number
of families
44
To test whether the relation between stream temperature and insect family richness was generally valid,
we compiled data available from the literature on
© 1997 Blackwell Science Ltd, Freshwater Biology, 38, 247–261
temperate-arctic and tropical streams at different altitudes, and thereby provided a latitudinal as well as
altitudinal gradient. In the plot we used only single
254 D. Jacobsen, R. Schultz and A. Encalada
Table 5 Data on diversity of aquatic insect families from
Ecuadorian and Danish streams showing α-diversity,
β-diversity, γ-diversity and proportion of the regions family
pool occurring in one stream (α/γ)
Fig. 1 Dendrogram showing the similarity between faunal
assemblages in streams in three Ecuadorian regions and in
Denmark. The data were fourth-root-transformed before
calculating similarities.
locality data and did not include large rivers
(Fig. 3). The richness of aquatic insects (y) increased
linearly with maximum stream temperature (x) for
both temperate-arctic (y 5 1.93x – 2.96, r2 5 0.84,
P , 0.001) and tropical streams (y 5 1.72x – 2.80, r2 5
0.48, P , 0.01). The regressions were similar for temperate-arctic and tropical streams (P . . 0.05, t-test).
Discussion
Overall patterns of invertebrate community structure
In the present study faunal similarity among widely
different regions was analysed using family level
identification because the aim of the study was to
identify overall patterns in faunal composition. It
would have been difficult to find patterns had the
analysis been performed at the species level, because
none or only very few species are probably shared
by the regions. The study showed that the faunal
composition of Ecuadorian mountain streams was
more similar to Danish lowland streams than to
Ecuadorian lowland streams. This result seems sur-
α
γ
β
α/γ
r2
Ecuador
Lowland
Valley
Páramo
26.1 (22–30)
16.9 (13–21)
10.0 (6–16)
47.7
35.2
24.6
0.89
1.13
1.48
0.55
0.48
0.41
0.985
0.998
0.998
Denmark
15.0 (11–18)
34.8
1.36
0.43
0.998
prising at first, considering that Ecuador is located in
the tropics and Denmark in the north-temperate region
on another continent 11 000 km away, while the highland and lowland streams of Ecuador are only 100 km
apart. However, the finding supports the hypothesis
by Illies (1964) that a rhithron biocenosis adapted to
cold conditions is found in both temperate and tropical
latitudes, but at progressively higher altitudes the
closer to the equator the streams are located. Illies
based his concept primarily on ‘life-types’ adapted to
similar niches in temperate lowland and high altitude
tropical streams. Here we demonstrate that the concept
is also valid for the composition of higher taxa and
the structure of the invertebrate fauna comparing
Neotropical and European faunas, although the neotropical rhithron fauna is derived from early Paleoantarctic elements and later Nearctic immigrants (Illies,
1969).
The number of families shared by the Ecuadorian
highland and Danish streams (sixteen) was, however,
exactly the same as between the Ecuadorian lowland
and Danish streams. The higher similarity between
the Ecuadorian highlands and Denmark is, therefore,
primarily due to the high number of insect families
present in the Ecuadorian lowlands, many of which
were not found in the other regions. The majority of
the families from Ecuador, which were not collected
in the group of Danish streams, do in fact occur in
Denmark (e.g. Veliidae, Libellulidae, Hydroptilidae)
but usually not in the type of stream investigated.
Slightly more families and individuals would have
been obtained in the Danish streams had the collection
taken place during winter, but the overall faunal
composition at the family level would not have been
markedly different.
Illies (1964) proposed a faunal transition from
rhithron to potamon biocenosis at about 2500 m in a
© 1997 Blackwell Science Ltd, Freshwater Biology, 38, 247–261
Structure and diversity of stream invertebrates 255
Peruvian Andes stream, and Statzner (1975) found the
transition at 1700 m in a stream in Zaı̈re. These findings
agree with the findings in the present study, as the
fauna of the Central Valley (2600–3100 m) was more
closely related to the fauna of the páramo than to the
lowlands. Hynes (1971), however, studied longitudinal
succession in a stream in Trinidad and found a major
shift in faunal composition at an altitude of only 30 m.
Hynes argued that the transition from rhithron to
potamon fauna occurs where the hill stream meets the
plain and therefore depends on the nature of the
stream bed, and that temperature (which did not vary
much in his study) is yet another factor superimposed
on the stream system. This disagreement reflects the
difficulty of interpreting studies of longitudinal faunal
succession in relation to altitude and suggests that
it may be more appropriate to regard altitudinal
succession in streams as a continuous process rather
than dividing streams into distinct zones. Ward (1985)
has a more thorough discussion of stream zonation in
relation to temperature.
Surprisingly few studies have adressed directly the
importance of temperature as a structuring parameter
for the stream invertebrate communities under normal
conditions in comparable streams. The streams
included in this study did not differ greatly regarding
size, current, substratum or aquatic macrophyte
growth, and the chemical parameters did not explain
much of the faunal variability or family richness.
We suggest therefore that temperature is a major
parameter structuring the stream fauna.
The global relationship between stream temperature
and insect richness
We acknowledge the problems involved in discussing
patterns in diversity of a complete fauna composed
of such different insect orders as Plecoptera (which
primarily is a temperate order; Illies, 1969), Odonata
(which primarily is a tropical group; Corbet, 1980)
and Trichoptera (which seems to have diversified at
temperate as well as at tropical latitudes; Ross, 1967).
Nevertheless, Fig. 3 indicates that overall family richness of stream insects is linearly related to maximum
Fig. 2 Significant regressions of insect family richness of
stream insects and mean stream width, percentage of gravel
and pebble in the substratum and maximum stream
temperature for three regions in Ecuador and one in Denmark.
s 5 streams from the Ecuadorian lowland, u 5 streams from
the Ecuadorian Central valley, n 5 streams from the
Ecuadorian páramo, and d 5 the Danish lowland streams.
Regression coefficients are provided.
© 1997 Blackwell Science Ltd, Freshwater Biology, 38, 247–261
256 D. Jacobsen, R. Schultz and A. Encalada
Fig. 3 Regression of family richness of stream insects and
maximum temperature from single locality studies found in
the literature (y 5 1.60x – 0.18, r2 5 0.71, P 5 3.65*10–9). s 5
temperate-arctic streams and d 5 tropical streams. Temperate
and arctic data-points from: Andersen et al., 1989 (Denmark,
one); Elgmork & Sæther, 1970 (Sweden, one); Glenstrup, 1981
(Denmark, one); Heitkamp, Lessmann & Piehl, 1985 (Germany,
two); Jones, 1948 (Wales, one); Mackay, 1969 (Canada, one);
Matthias, 1983 (Germany, one); Petts & Bickerton, 1994
(Switzerland, two); Slack, Nauman & Tilley, 1979 (Alaska,
three); Stoneburner, 1977 (U.S.A., two); Ulfstrand, 1968
(Sweden, two); Ward, 1986 (U.S.A., one); Winterbourn, 1978
(New Zealand, one). Tropical data from: this study (Ecuador,
three); Arthington, 1990 (Australia, one); Bishop, 1973
(Malaysia, one); Dudgeon, 1989 (Hong Kong, one); Henry
et al., 1994 (Brazil, one); Illies, 1964 (Peru, one); Machado &
Roldán, 1981 (Colombia, two); Matthias & Moreno, 1983
(Colombia, one); Patrick, 1966 (Peru, two); Turcotte & Harper,
1982 (Ecuador, one). In Dudgeon (1989) only a fauna list
combining four streams is given. The number of families in
one stream is estimated from the species-accumulation curve
established for the Ecuadorian lowland streams. In Henry et al.
(1994) maximum stream temperature is not given, but is
estimated by the authors of the present study.
stream temperature and that this pattern represents a
global trend. However, species richness, rather than
family richness, is what primarily concerns us when
patterns of biodiversity are examined. The number of
species per family may not be constant on a global
scale; we might suspect that families become more
species-rich closer to the equator.
We made an attempt to compare species : family
ratios for aquatic insect orders in South America,
North America and Europe. Such an analysis should
be interpreted with caution because of the incomplete
knowledge of the South American fauna. Many more
species are to be described from South America,
but probably also several more families, so future
species : family ratios are difficult to predict. At present
the data suggests that, overall, South American aquatic
insect families do contain more species than families
in North America and Europe (Table 6). It is also quite
clear that there are more species of aquatic insects in
the neotropics than at higher latitudes. More species
of, especially, Odonata, Heteroptera, aquatic
Coleoptera and perhaps Trichoptera are responsible
for this difference. This does not mean, however,
that Fig. 3 cannot be regarded as representing species
richness as well as family richness in relation to
stream temperature. The data in Table 6 concern whole
continents, while Fig. 3 concerns local diversity found
at single stream sites, and individual streams in the
tropics do not necessarily house more species of each
family occurring, although more species taxonomically
belong to each family.
If individual stream sites do, in fact, have more
species per family closer to the equator, the relationship
between species richness and maximum stream temperature will be exponential instead of linear, but a
global relationship should still persist. Fig. 3 suggests
that tropical lowland streams have about one and a
half to two times more insect families than temperate
lowland streams. In addition, Table 6 suggests that
aquatic insect families in South America have, on
average, one and a half to two times more species per
family. This leads us to suggest that, overall, tropical
lowland streams should have a two- to fourfold higher
species richness than temperate lowland streams.
The critical part on the graph is where tropical highaltitude streams and temperate streams overlap (10–
20 °C). Illies (1969) suggested that at the species level
the diversity of the neotropical rhithron fauna is very
low compared to the European fauna because the
coherent nature and the north–south orientation of
the Andes has given little opportunity for isolation and
speciation during the interglacial periods compared to
the many individual mountain regions in Europe.
Illies used plecopterans as an example, which at that
time had five families and sixty-five species in South
America, while Europe had seven families and 340
species. Later the number of plecopteran families in
South America has been reduced to two. Therefore, at
least
regarding
rhithron
plecopterans
the
species : family ratio is now the same on the two
continents, but little information is available for other
insect orders.
© 1997 Blackwell Science Ltd, Freshwater Biology, 38, 247–261
Structure and diversity of stream invertebrates 257
Table 6 Number of families, species and the species : family (S : F) ratio for aquatic insect orders on three continents. Data for
South America from Hurlbert, Rodriguez & Dias dos Santos (1981), data for Europe from Illies (1978) and data for North America
from Merritt & Cummins (1996). The asterix denotes that the number of species in tropical South America is estimated.
Megaloptera includes Neuroptera. Diptera is not included because of the very incomplete knowledge of the order
South America
Europe
North America
Families
Species
S:F
Families
Ephemeroptera
Plecoptera
Odonata
Heteroptera
Megaloptera
Coleoptera
Trichoptera
10
2
19
14
4
18
14
184
100*
1491
715
42
1913
1500
18.4
50.0
78.5
51.1
11.5
106.3
107.1
17
7
10
12
4
23
22
217
387
127
129
16
967
895
12.8
55.3
12.7
10.8
4.0
42.0
40.7
Total
81
5945
73.4
95
2738
28.8
Speciation in relation to temperature and climatic
history
As part of the voluminous debate on the possible
causes for latitudinal gradients in biodiversity Rohde
(1992) concluded that temperature itself may be a main
factor governing diversity because high temperature
leads to higher mutation rates and shorter generation
times, thereby speeding up evolution and speciation.
Therefore, more families (and species) of stream insects
have probably evolved in tropical lowland streams
than in tropical mountain or higher latitude streams.
It is difficult to say whether temperature during
evolution works on a regional or a local scale. Regional
(γ) species richness affects, and is tightly related to,
local richness (Cornell & Lawton, 1992), obviously by
setting an upper limit to local richness. However,
temperature was more closely related to γ diversity
than to local richness, probably because of the confounding effect of local abiotic and biotic conditions
at the individual stream sites. Furthermore, local richness is probably affected by the partly stochastic
process of dispersal. Dispersal between localities is
related to the β diversity. The higher β diversity of the
páramo streams relative to the Ecuadorian lowlands
may simply result from the fact that the páramo
streams covered a larger land area than the Ecuadorian
lowland streams. However, β diversity may also have
been affected by low dispersal of flying adult insects
among the younger mountain streams.
Temperature itself may not be the sole cause of the
relation between richness and temperature.
Quaternary climatic history is related to temperature
© 1997 Blackwell Science Ltd, Freshwater Biology, 38, 247–261
Species
S:F
Families
Species
S:F
21
9
10
18
4
18
23
599
577
422
421
70
1214
1385
28.5
64.1
42.2
23.4
17.5
67.4
60.2
103
4688
45.5
and should also be considered to explain regional
differences in taxa richness. During the last ice age,
some 20 000 years ago, large lowland areas at temperate latitudes, including most of Denmark, were
covered by glaciers. In the tropical high Andes the
mean air temperature was about 6 °C lower than
today, with permanent snow cover probably extending
down to 3500–4000 m, and glaciers to an altitude of
3000 m (Colinvaux, 1987; Schubert, 1988). Hence, the
highland stream fauna was probably driven down to
lower altitudes (and the northern fauna to lower
latitudes) and many species became extinct. Consequently, most stream faunas of the high Andes and
of northern Europe (or North America) are relatively
young and still under colonization from lower altitudes or more southerly streams.
The neotropical lowlands may also have undergone
climatic fluctuations in historic or geological time
leading to extinctions, while still reaching higher
species richness (Fischer, 1960). In particular, lowland
streams may have been affected by lower temperatures
and extensive droughts during the Pleistocene
(Covich, 1988). However, such climatic fluctuations
may also have contributed to speciation through isolation in refuges. The northern part of western Ecuador
is actually part of a postulated pleistocene refuge
(Colinvaux, 1987).
The influence of predation and food sources on insect
diversity
Species richness in tropical vs. temperate streams has
been thought to be affected by several abiotic and
258 D. Jacobsen, R. Schultz and A. Encalada
biotic factors such as available food sources, habitat
diversity (Covich, 1988) and predation (Stout &
Vandermeer, 1975; Fox, 1977). No indications were
seen in the present study of a higher habitat diversity
or lower food diversity in neotropical streams compared to temperate streams as suggested by Covich
(1988). Flowers (1991) also proposed that food sources
are basically the same in tropical and temperate streams, namely detritus and algae.
Fox (1977) suggested that higher proportions of
predators in tropical streams may lower invertebrate
densities and increase diversity by maintaining competitively superior prey species at lower densities and
thereby allowing the existence of less competitive
species. It is noteworthy that the two insect orders
that primarily contribute to the higher species richness
of aquatic insects in the neotropics (Odonata and
Heteroptera) (Table 6) both are predatory, and these
groups mainly occur in the lowland streams. The
proportion of invertebrate predators (by numbers) is
higher in Ecuadorian lowland streams (24%) than in
Danish streams (6%) (Schultz, 1997), and also higher
than that found in the Ecuadorian Central Valley
streams (9%) (D. Jacobsen & A. Encalada, unpublished
data). A study on the Ecuadorian páramo streams
showed that predator abundances were generally low
(3%), but local dominance of Planarians may increase
this figure to 65% (Encalada, 1997). Hence, high densities and diversities of invertebrate predators may
influence invertebrate community structure and
diversity in tropical streams.
Neotropical lowland streams often have an
abundant and diverse fish fauna (Lowe-McConnell,
1975). Some experimental studies in temperate as well
as in tropical streams (Flecker, 1992; Dudgeon, 1991,
1993) have found effects of fish predation on invertebrate community structure, but it is unclear just how
important and general the impact from predatory
fish is. Flecker (1992) found that herbivorous and
detritivorous fish, which are much more abundant in
neotropical streams than in temperate streams, had a
stronger indirect effect than predatory fish on the
dominant invertebrate taxa by depleting food
resources that were shared between the fish and
the invertebrates. Both predatory and herbivorous/
detritivorous fish may thus be an important controlling
factor for invertebrate community structure and
diversity in neotropical streams.
The effect of environmental stability on insect diversity
Environmental variability and disturbance is regarded
as a key factor for biotic diversity in stream communities (Stout & Vandermeer, 1975; Stanford & Ward,
1983; Arthington, 1990; Reice, Wissmar & Naiman,
1990). Although some temperate regions are characterized by highly variable stream flow, tropical lowland
streams in general seem more prone to spates than
temperate or high altitude streams, and the global
maximum stream temperature gradient may run somewhat parallel to a ‘discharge unpredictability gradient’.
Ward & Stanford (1983) proposed that the general
‘intermediate-disturbance hypothesis’ (Connell, 1978)
should be applicable to stream invertebrate communities, so that maximum richness is reached at intermediate levels of disturbance. However, we do not
know the exact optimum level of disturbance. Therefore, whether the influence of increasing disturbance
has either a positive or a negative impact on species
richness may depend on the location of the streams
along the ‘disturbance-gradient’. Although intuitively
appealing, the review of Reice et al. (1990) found little
evidence for the hypothesis. ‘Rock-tumbling experiments’ teach us little about patterns in diversity among
streams and very few studies have examined species
richness and quantified overall stability in a set of
streams. However, in such a study, Death &
Winterbourn (1994) found that unstable streams had
fewer species than more stable streams. Hence, there
may be a negative impact of the suggested latitudinal
disturbance-gradient on diversity of stream insects
closer to the equator. Still, we know little about how
variability in flow regime affects regional richness and
speciation of stream insects.
Tropical streams are, on the other hand, more constant than temperate streams in terms of temperature
regime. It has been suggested that a wide annual
temperature range (as found at temperate latitudes)
may enhance species diversity by allowing the coexistence of differently synchronized life cycles, thereby
ensuring temporal separation of major periods of
resource use and reducing active competition (Vannote
et al., 1980). Likewise, wider daily variation in temperature increases species packing by providing a wider
range of temperature optima, even though suboptimal
conditions consequently will occur for each species as
well (see references in Ward & Stanford, 1982). It
seems, therefore, that both temperature variability and
© 1997 Blackwell Science Ltd, Freshwater Biology, 38, 247–261
Structure and diversity of stream invertebrates 259
discharge variability should favour higher diversity
in temperate streams than in tropical streams.
In conclusion, the global relationship between family
richness and maximum stream temperature is suggested to be related to geological history and climatic
changes in addition to the direct effect of temperature.
We propose that more families and species have
evolved in warm than in cold regions because of a
higher speciation rate for a longer time period (high
geological age), and that cold streams at high altitudes
in the tropics (the páramo) or at high latitudes
(Denmark) have young biotas because of former glaciations and therefore are still under the process of
colonization.
Acknowledgements
We thank Nikolai Friberg for the Danish invertebrate
data, Klaus P. Brodersen for giving us access to the
PRIMER software, Peter C. Dall for making the jackknife computer program and Kaj Sand-Jensen, Niels
Peder Kristensen and two anonymous referees for
valuable comments on the manuscript. The
Departamento de Ciencias Biológicas at the Pontificia
Universidad Católica del Ecuador in Quito kindly
provided laboratory facilities. This work was funded
by grant 104.Dan8/630 from Danida, Danish Ministry
of Foreign Affairs.
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