1992.
R.D. Robarts and M.L. Bothwell (Eds.). N.H.R.I. Symposium Series 7, Environment Canada, Saskatoon.
AQUATIC ECOSYSTEMS IN SEMI-ARID REGIONS: IMPLICATIONS FOR RESOURCE MANAGEMENT.
OVER-SUMMERING STRATEGIES OF MACROINVERTEBRATES IN
INTERMITTENT STREAMS IN AUSTRALIA AND ARIZONA
A. J. Boulton,
Department of Zoology, University of Adelaide, Adelaide, 5001 South Australia
E. H. Stanley, S. G. Fisher,
Department of Zoology, Arizona State University, Tempe, 85287 AZ U.S.A
and P. S. Lake
Department of Botany and Zoology, Monash University, Clayton, 3168 Victoria, Australia
ABSTRACT
Aquatic macroinvertebrates must adopt physiological and/or behavioural strategies
to avoid desiccation during waterless periods in intermittent streams. Differential survival
determines which biota are present when flow resumes and may influence the direction
of subsequent succession. In two Australian intermittent streams, 23 taxa sought refuge
below the stream bed in the hyporheic zone (8 taxa), under dried litter and rocks (11), in
crayfish burrows (6) or in dry sediments in riffles (18). Most of these taxa were common
in epigean samples collected soon after flow resumed. In a Sonoran desert stream, fewer
major taxa (13) occupied these refuges and there was little overlap with the fauna collected
when flow began, or with the fauna using similar refuges in Australia. These results imply
that in the Australian intermittent streams, some stability of the epigean ecosystem to
drying disturbances is apparently due to the presence of a variety of over-summering
refuges whereas in-stream refuges in a Sonoran desert stream contribute little to the
resistance of the aquatic benthos to drying. However, conclusions from inter-continental
comparisons of the ecology of arid stream ecosystems must be tempered with recognition
of the extreme variability inherent in these systems, even at the broad scale used here.
Results from a larger number of intermittent streams subject to similar drying regimes in
both continents are needed to fully test our hypothesis.
INTRODUCTION
In intermittent streams, the loss of surface water during the dry season ("drying") is
probably the most significant abiotic disturbance affecting the epigean (surface) aquatic
biota, although flash floods are major disturbances in desert streams whose beds are
unstable (Fisher et al. 1982; Grimm and Fisher 1989). Most published work on drying has
been descriptive, detailing physiological and behavioral responses by particular
invertebrates (e. g., Clifford 1966; Smith and Pearson 1985, Williams 1987) or listing taxa
collected from various over-summering refuges (e. g., Williams and Hynes 1977; Boulton
1989). To date, no one has considered the implications of their results to ecosystem
stability through resistance and resilience to drying.
Ecosystem response to disturbance is judged in terms of ecosystem stability (sensu
Webster et al. 1975); stability persists if state variables are not deflected by disturbance
(resistant) or recover swiftly (resilient) (cf. Webster et al. 1975; Connell and Sousa 1983).
Recently, there has been much interest in the role of disturbance in lotic ecosystem
structure (reviewed by Resh et al. 1988), mainly concentrating on recovery after spates
(e. g., Fisher et al. 1982; Power and Stewart 1987; Grimm and Fisher 1989). These studies,
coupled with small-scale experimental manipulations (e. g., Boulton et al. 1988; Lake et al.
1989) imply survival of algal and invertebrate communities through resilience rather than
Present address
227
resistance, especially when disturbances shift substrata. In streams where spates are
frequent but less severe, mechanisms of resistance may be more important (Fisher and
Grimm 1988).
While much attention has been directed toward the role of floods in determining
community and ecosystem structure, little is known about changes caused by drying,
despite its widespread occurrence and destructive effects on aquatic organisms. As with
flooding, responses to drying can be defined in terms of resistance and resilience (Stanley
and Fisher 1992). In this paper, we present data on invertebrates over-summering in
several refuges (viz. hyporheic zone, crayfish burrow water, below rocks and dried litter,
dry sediments) in two intermittent streams in south-eastern Australia and a desert stream
in central Arizona, comparing them at a broad taxonomic level to try and minimize
differences due to endemicity. We also examine the degree of overlap between epigean
taxa found soon after flow resumed with those found in the refuges, interpreting the
results in terms of stability of the epigean ecosystem. Comparing taxa in the refuges with
taxa present in the surface stream when flow resumes allows us to assess circumstantially
the contribution of these refuges to resistance of the benthic fauna to drying (Resistance
II sensu Stanley and Fisher 1992).
STUDY SITES
The Australian study sites are situated in the upper reaches of the Werribee and
Lerderderg Rivers, approximately 100 km north-west of Melbourne, Victoria. The Werribee
River ceases flow annually for about nine weeks. The Lerderderg flows throughout the
summer one year in three. Except in drought years, pools persist during summer in both
rivers. Mean annual rainfall is approximately 950 mm; maximum summer temperatures
seldom exceed 40°C (Boulton 1988). Both rivers drain catchments vegetated by tall open
eucalypt woodland and the study sites are heavily shaded. Full details of vegetation,
hydrology and physico-chemistry are given by Boulton and Smith (1985) and Boulton and
Lake (1990).
Sycamore Creek is a spatially-intermittent Sonoran Desert stream that drains 505 km'
of mountainous terrain vegetated by desert scrub. Unlike the Australian sites, the flow
regime is unpredictable, the stream is prone to flash floods capable of moving the unstable
substrata, and the study sites are unshaded. Average annual rainfall is 450 mm and the
mean maximum summer temperature is 41°C. Grimm and Fisher (1989) present further
details on vegetation and environmental features.
To increase comparability between the sites in Australia and Arizona, all study reaches
were located in riffles where the substrata are heterogeneous mixtures of gravel, pebbles
and cobbles, interspersed by occasional boulders (phi = -6 to -1). However, drying is far
more rapid and continues for longer periods of time at the unshaded sites at Sycamore
Creek than at the Australian sites. Furthermore, crayfish burrows are absent in Sycamore
Creek and large permanent pools seldom persist near the study reaches.
MATERIALS AND METHODS
Sampling techniques
Three to six weeks after surface flow ceased, hyporheic water was sampled using the
Karaman-Chappuis technique (Williams 1984). Pits were dug in the dry streambed until
water was reached. Samples of approximately 4 L were sieved through a 50 jim mesh
hand-net and live-sorted under 12-63X magnification. Animals living in the water of
crayfish burrows ("pholeteros" sensu Lake 1977) were collected by excavating the opening
228
of the burrow and sucking the water out through a plastic tube. Samples were sieved
through 50 pm mesh and live-sorted as above. Dried mats of filamentous algae and leaf
litter lying on the stream-bed were overturned and all aquatic invertebrates removed with
forceps. Samples of litter and dry algae were returned to the laboratory and scanned
under 12-63X magnification.
Plastic bags were filled with surface sediments (upper 10 cm) and organic matter and
returned to the laboratory. The sample was then emptied into an aquarium and flooded
with dechlorinated tap water or distilled water. Within 30 minutes of inundation, a 50 pm
mesh hand-net was swept vigorously through the tank and the sample was live-sorted
before returning it to the aquarium. The process was repeated until no further taxa were
collected. Subsequently, samples were taken at irregular intervals over the next 3-14 days,
always returning specimens to the tank.
As soon as flow resumed at the study sites, benthic invertebrates were sampled using
a suction sampler (Boulton 1985) at the Australian sites and a core sampler (Fisher et al.
1982) in Sycamore Creek. Further samples from Sycamore Creek were obtained using a
50 pm mesh hand-net. Invertebrates were identified to the lowest possible taxon and their
abundance ranked as "present" (1-2 individuals), "common" (3-10) or "abundant" (> 10).
Given the variety of sampling methods and unequal numbers of samples, more precise
quantification was unwarranted.
Data analysis
For the broad comparisons presented here, taxonomic resolution was only as far as the
family level in most cases. Although the operator and the techniques used to sample
potential refuges were identical among streams, different numbers of samples were
collected in several cases. To compare the relative species richness in the refuges in both
regions given unequal sample sizes, two approaches were used. Cumulative species plots
of numbers of taxa with increasing sample number were drawn to ascertain the species
richness where the curve levelled off (Southwood 1984) and to determine whether
sufficient numbers of samples had been collected from each refuge. Secondly, mean
species richness per refuge was computed by taking the average number of species in all
the samples collected from the refuge.
There were sufficient samples collected from each refuge for the cumulative curve of
species richness with sample number to level off (see Results). Therefore, we compared
the assemblage composition among the refuges and the surface samples using multivariate
analyses, incorporating species identity and rank abundance into the comparisons among
refuges between continents. The samples were classified using two-way indicator species
analysis (TVVINSPAN, Hill et al. 1975; Hill 1979), a polythetic divisive technique
recommended for hierarchical classification because it is effective and robust (Gauch and
Whittaker, 1981). Each measure of rank abundance was set to a "pseudospecies cut level"
(sensu Hill 1979) and the analysis continued until the group size was two elements or less.
The data were also ordinated to provide an alternative means of comparing
assemblage composition as recommended by Gauch (1982). We used hybrid non-metric
multidimensional scaling (HMDS, Faith et al. 1987) computed using ECOPAK (Minchin
1986). HMDS has several advantages over more commonly used ordination methods
(Faith et al. 1987; Minchin 1987). The secondary matrix was calculated using the
Kulczynski coefficient (advocated by Faith et al. 1987) and the relationships among refuges
based on assemblage composition were plotted on the first two axes.
229
Initial multivariate analyses emphasized the endemicity of the fauna, even at such a
broad taxonomic level, so it was necessary to derive a second data matrix comprising
major taxa common to both continents and amalgamating taxa that were "ecological
equivalents" (i. e., sharing similar morphology and habitat requirements). This second
matrix was analyzed in the same way described above.
Table 1. Mean numbers of major taxa per sample (± SE) in three refuges sampled when flow ceased
in the Werribee and Lerderderg Rivers (Australia) and Sycamore Creek (Arizona).
Below dried
litter
Dry substrata flooded
in laboratory
Hyporheic
Australia
6.7 ± 1.6
5.9 ± 0.2
3.4 ± 0.6
Arizona
4.6 ± 0.8
3.4 ± 0.6
5.8 ± 0.5
Location
zone
RESULTS
Species richness
12
All cumulative plots of species richness against number of samples approach
an asymptote (Figure 1), indicating that
sufficient numbers of samples were collected to capture most of the species in
each refuge in the streams on both continents. Most taxa were collected from reinundated dry sediments from the Australian rivers; 16 versus 13 for an equivalent
number of Sycamore Creek samples (Figure 1). There were no inter-continental
differences in the number of taxa recorded
from below dry litter or from the hyporheos (Figure 1).
Assemblage composition
10
8
6
Cumulative number of major taxa
More taxa per sample were found in
dry substrata inundated in the laboratory
and below dried litter from the Australian
streams than from Sycamore Creek (Table
1). However, the hyporheic zone of Sycamore Creek harboured more taxa than
that of the Australian streams (Table 1).
Within two weeks of resumption of flow,
20 major taxa were collected from the
Werribee and Lerderderg Rivers whereas
only 9 major taxa occurred in samples
from Sycamore Creek.
Below dried litter
2
0
2
o
3
4
6
5
7
10
86
4
Hyporheic zone
20
o
2
6
4
10
Inundated dry sediments
10
20
30
ao
50
Sample number
Figure 1. Cumulative species curves of numbers of
major taxa plotted against sample number for the
Australian streams (dashed line) and Sycamore Creek,
Arizona (solid line) for three refuges: below dry litter, in
the hyporheic zone, and recovered from inundated dry
sediments.
Twenty-three major taxa, primarily arthropods, were recovered from four oversummering refuges in the Werribee and Lerderderg Rivers (Table 2). Amphipods
(Austrochiltonia australis) were abundant under dry litter. Substrata flooded in the
230
laboratory yielded large numbers of taxa with desiccation resistant stages (e. g., nematodes,
leptophlebiid mayfly nymphs) or those capable of surviving in moist, aerated sediments
(e. g., oligochaetes, elmid beetle larvae). Only nematodes and oligochaetes were common
in the hyporheic zone whose faunal composition overlapped considerably with that of the
pholeteros (Table 2).
Table 2. Aquatic taxa collected from four refuges sampled when flow had ceased in the Werribee and Lerderderg
Rivers, Victoria. Abundance categories are semi-quantitative 1+ = present (1-2 individuals), C = common (310), A = abundant (>10) because of differing numbers of samples (n) and methods (see text).
Refuge
TURBELLARIA
Neorhabdocoela
NEMATODA
Nematoda
GASTROPODA
Hydrobiidae
Ancylidae
OLIGOCHAETA
Oligochaeta
CRUSTACEA
Isopoda
Janiridae
Amphipoda
Ceinidae
Decapoda
Parastacidae
HYDRACARINA
Hydracarina
INSECTA
Ephemeroptera
Leptophlebiidae
Plecoptera
Gripopterygidae
Coleoptera
Dytiscidae
Helodidae
Psephenidae
Elmidae
Diptera
Tipulidae
Psychodidae
Chironomidae
Ceratopogonidae
Stratiomyidae
Empididae
Muscidae
Trichoptera
Calocidae
Leptoceridae
Below dried
litter (7)
+
Crayfish
(Engaeus sp.)
burrows (12)
C
Dry substrata flooded
in laboratory (47)
Hyporheic
zone (5)
C
+
A
C
C
+
C
C
A
C
C
+
A
C
C
A
+
A
+
C
+
+
+
A
+
+
+
+
+
+
+
C
+
C
+
+
+
+
+
C
Total number of
taxa (23)
11
6
18
8
Percent of total
48
26
78
35
231
In Sycamore Creek, thirteen major taxa were collected from three refuges (Table 3).
Nematodes were abundant in all refuges. Large numbers of ceratopogonid larvae, oligochaetes and planorbid snails (Helisoma sp.) were also found in the hyporheic zone (Table
3). Peracarids, mayfly and stonefly nymphs, and caddisfly larvae were not recorded from
refuges in Sycamore Creek (Table 3), accounting for much of the difference in total taxa
recorded. Peracarids are absent from the surface water of Sycamore Creek and stonefly
nymphs are rare and only seasonally common (Stanley unpub. data, Gray 1981).
Table 3. Aquatic taxa collected from three refuges sampled when flow had ceased in Sycamore Creek, Arizona.
Details as in Table 2.
Refuge
TURBELLARIA
"Microturbellaria"
NEMATODA
Nematoda
GASTROPODA
Physidae
Planorbidae
OLIGOCHAETA
Oligochaeta
HYDRACARINA
Hydracarina
INSECTA
Coleoptera
Hydrophilidae
Diptera
Tipulidae
Chironomidae
Ceratopogonidae
Stratiomyidae
Muscidae
Tabanidae
Below dried litter
(7)
Dry substrata
flooded in laboratory
(22)
Hyporheic
zone
(10)
C
C
A
A
A
+
C
+
+
+
A
C
C
A
+
+
C
+
C
C
+
C
C
A
C
+
+
C
+
C
+
+
C
Total number
of taxa (13)
11
13
9
Percent of total
85
100
69
There were inter-continental differences in the fauna found in these refuges (cf. Tables
2 and 3). In Sycamore Creek, the hyporheic zone was favoured by two species of gastropods (Table 3) whose Australian equivalents were usually found after flooding dry substrata (Table 2). Several families of Diptera were also common in the hyporheic zone of Sycamore Creek (Table 3). Dytiscid beetles were abundant in the surface waters of all the
study streams but were never found under litter in Sycamore Creek - a refuge they often
used in the Australian streams. However, hydrophilid larvae and adults were common
in moist stream sediments in Sycamore Creek but were not found in refuges in the
Werribee and Lerderderg Rivers where they are frequently found in the surface waters
(Boulton unpubl. data).
Even at the broad taxonomic resolution employed in the study, endemicity between
the continents was obvious when the data were classified (Figure 2a) and ordinated (Figure
2b). In the Australian streams, there was greatest overlap between the assemblage
232
composition of samples collected
soon after flow resumed and that
harboured in the dry sediments
whereas the second division of
TWINSPAN separated the Sycamore Creek surface samples from
those in the three refuges sampled, implying that postwetting
recolonization from these refuges
contributes little.
HydracarHa, Physkta•
Planorbidep
CormapageolOse
SS
AS
SD
AD
AP
AH
2
(5)
Re-analyzing a smaller data
set composed of taxa common to
both continents and assigning
•
some taxa to "ecological equivalents" produced different
SL
0
results. The assignment to "eco•
logical equivalents" emphasized
functional differences and overcame taxonomic distinctions
•
•
SH
(endemicity), and merely entailed
grouping similar taxa (e. g., pulss
0
monate snails in different fam2
Axis I
ilies in Australia and Arizona).
The first TWINSPAN division Figure 2. TWINSPAN dendrogram (a) and two-dimensional
split samples from all the Syca- HMDS ordination plot (b) of the similarities among the refuges in
more Creek refuges and the re- total faunal composition of the Australian and Arizonan streams.
inundated Australian sediments Australian refuges are denoted: AS = stream fauna when flow
from the remaining Australian resumed, AL = fauna found below litter, AP = pholeteros, AH =
refuges and the surface samples hyporheos, and AD = fauna recovered from inundated dry sedifrom all three streams (Figure ments. The fauna in refuges in Sycamore Creek. are prefixed by S;
no pholeteros existed in Sycamore Creek. Indicator species of the
3a). This emphasizes the lack of
major taxa listed on the dendrogram.
overlap between fauna in the
refuges in Sycamore Creek and that found when flow resumes. The fauna found in the
Australian refuges that held free water (hyporheic zone and crayfish burrow water)
grouped together but was distinct from the hyporheos of Sycamore Creek (Figure 3a).
Both multivariate analyses indicated considerable overlap between the fauna found when
flow resumes in the three streams (Figure 3) when endemic taxa are removed. These
common "early flow" taxa included turbellarians, oligochaetes, simuliids, chironomids,
tipulids, ceratopogonids, stonefly and mayfly nymphs, and nematodes. However, even
when endemic taxa were omitted, there was little overlap in the use of refuges between
the continents.
AL
AP
So
AD
AS
DISCUSSION
Preliminary comparisons of the aquatic invertebrate fauna resistant to drying disturbances in intermittent streams suggest differences in the relative importance of refuges
within and between continents. Furthermore, there are differences in the degree of overlap
between fauna using the refuges and those collected when flow resumes. Interpreting
these results in terms of ecosystem stability conferred by resistance to drying, we conclude
that in the Australian rivers, many of the taxa found shortly after flow resumed
successfully resisted drying by surviving in the hyporheic zone, crayfish burrows, below
dried litter and filamentous algae, and in the surface sediments. Many others survived in
233
permanent pools along the river bed
(Boulton 1989), a refuge not considered in
the present study.
Physklat, Plano.kW,
TIpullela• NemMods
leptoc.d.
HydrAcarkto
Hydroblitlwr
AS
AD
'furl.Ilene
Stretlernyklm
SL
SD
AL
AP
SL
0
AP
•
SD
0
S.
•
SS
Conversely, there is little overlap between the fauna collected from three
refuges in Sycamore Creek and those
found soon after flow started. This implies that there are few resistant stages in
these refuges and that the stability of the
epigean aquatic fauna probably is maintained by recolonization from aerial
sources, as is the case with post-flood
recolonization of Sycamore Creek (Gray
and Fisher 1981). Major taxa found within
two weeks of resumption of flow in Sycamore Creek included simuliids, baetid
mayflies and capniid stoneflies, all capable
of aerial recolonization (e. g., Harrison
1966; Hynes 1975; Abell 1984).
gs
The major reason for this intercontinental difference is probably the severity of
•
the drying rather than the time since
inundation as this period was similar
between studies. In unshaded Sycamore
Figure 3. TWINSPAN dendrogram (a) and two-dimen- Creek, drying occurs far more swiftly than
sional HMDS ordination plot (b) of the similarities in the Australian streams and often lasts
among the refuges in faunal composition of the Austra- for several months instead of several
lian and Arizonan streams, after removing endemic weeks. Sediment surface temperatures
major taxa and amalgamating ecological equivalents (see frequently exceed 60°C and litter mats are
text). Refuges are denoted as in Figure 2. Indicator not as thick or well-developed as in the
species of the major taxa are listed on the dendrogram. Lerderderg or Werribee Rivers. Many of
the taxa persisting in the Australian streams were found in moist sediments; in Sycamore
Creek, such sediments dried within days, rendering this refuge less habitable for many
taxa.
AS
Although the hyporheic zone of Sycamore Creek contains over 50 taxa (Boulton et al.
1992), most of these are found below the wetted stream at depths exceeding 30 cm.
Furthermore, few of these taxa ever enter the surface stream and many of them are blind
and unpigmented (e. g., amphipods, isopods, bathynellaceans), comprising obligate
members of the hyporheos. As such, they do not contribute directly to the stability of the
epigean biota of Sycamore Creek. Few obligate inhabitants of the hyporheos were
collected in the Australian streams although the survey was not exhaustive. However,
these refuges and crayfish burrows held free water, allowing macroinvertebrates such as
oligochaetes, decapod crayfish, and janirid isopods to resist drying.
At a broad taxonomic level and after removing endemic taxa, the fauna collected when
flow resumed in the Australian streams was similar to that found in Sycamore Creek and
to that found in intermittent streams in Africa (Harrison 1966; Chutter 1968; Hynes 1975),
Canada (Williams and Hynes 1976; 1977), California (Abell 1984) and other parts of
Australia (Towns 1985). At this broad level, the composition of epigean communities in
intermittent streams where flow has just begun is relatively predictable, satisfying one of
the tenets of "equilibrial" communities summarized by Lake and Barmuta (1986). This is
234
essential if equilibrial concepts of stability are to be applied because assessment of
resistance or resilience entails measuring displacement from some equilibrium point or
limit cycle (Connell and Sousa 1983). Thus, the equilibrial concept of stability is potentially
useful at this juncture because it allows an assessment of the extent of resistance and/or
resilience displayed by the fauna to drying.
The application of equilibrial concepts to the community ecology of temporary streams
is, however, limited (Boulton and Suter 1986). The predictable composition of the
assemblage found soon after flow resumes allowed us to apply this concept to the
disturbance wrought by drying upon the epigean aquatic benthos of intermittent streams.
With time after flow begins, the composition of the aquatic benthos in the Australian
streams becomes less predictable, and variations among the sites become pronounced.
These assemblages possess many properties of nonequilibrial communities (Boulton 1988),
potentially confounding efforts to apply equilibrial concepts of stability to describe
resistance or resilience to disturbances imposed on later assemblages (e. g., floods).
In conclusion, this preliminary inter-continental comparison illustrates the insights
gained when the concept of stability to a disturbance exemplified by drying in intermittent
streams is applied to community composition on a broad scale. When methods and
operators are consistent, differences can be ascribed to variations in aspects of the
disturbance (cf. Pickett and White 1985) - in this case, intensity and magnitude. If the
assemblages are broadly similar, as they are shortly after flow resumes, the contribution
of each refuge to resistance to drying can be assessed. Of the refuges examined here, most
invertebrates in the Australian streams recolonized from the sediments whereas in-stream
refuges played little role in resistance to drying in Sycamore Creek. However, it must be
emphasized that such ecosystem comparisons should be restricted to equivalent streams.
The magnitude of the differences reported here could easily result from differences among
streams with different intensities of drying disturbance on the same continent (cf. Fisher
and Grimm 1991).
ACKNOWLEDGEMENTS
We thank John Whitney of Dos S Ranch for allowing us access to Sycamore Creek.
In Australia, A. J. B. was supported by a Commonwealth Postgraduate Research Award;
research in Arizona was funded by NSF grant # BSR 88-18612. Constructive comments by
Dr. N. B. Grimm and a reviewer improved drafts of this manuscript.
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