Download Habitat Use and Community Structure in an Assemblage of Cottid

Habitat Use and Community Structure in an Assemblage of Cottid Fishes
Author(s): Stephen F. Norton
Source: Ecology, Vol. 72, No. 6 (Dec., 1991), pp. 2181-2192
Published by: Ecological Society of America
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Ecology, 72(6), 1991, pp. 2181-2192
© 1991 by the Ecological Society of America
HABITAT
USE AND COMMUNITY STRUCTURE
ASSEMBLAGE OF COTTID FISHES'
IN AN
STEPHENF. NORTON
Fisheries Research Institute and Friday Harbor Laboratories, University of Washington,
620 University Road, Friday Harbor, Washington 98250 USA
Abstract. I applied an observational approach, using strip transects, to document the
abundance and the spatial distributions of members of an assemblage of subtidal fishes
(Scorpaeniformes: Cottidae) in the San Juan Islands, Washington, USA. I then assessed
the extent to which species-specific distributions were correlated with habitat availability
and the potential for interspecific competition to modify patterns of abundance and resource
use. The six common cottid species were Artedius fenestralis, A. harringtoni, Asemichthys
taylori, Chitonotus pugetensis, Icelinus borealis, and Jordania zonope. A comparison of
their observed distributions with the distributions of microhabitats along the transects
revealed that all species demonstrated significant affinities for or avoidance of many of the
predominant substrates: clean rock, silt rock, shell sand, gravel, mud, and algae. Using a
stepwise regression technique, I found that the densities of individual cottid species were
significantly correlated with the percentage cover of these substrate types along the transects
and with transect depth.
Several lines of evidence suggest that interspecific competition does not play a strong
role in this assemblage. Species pairs that co-occurred on at least some transects did not
show disjunct distributions. Microhabitat overlap of these species pairs was higher in
sympatry (comparison of transects with both species present) than in allopatry (comparisons
of transects with only one species present), counter to the expectation of niche divergence.
Correlations of species densities after correcting for habitat heterogeneity with the SchoenerCrowell-Pimm technique revealed only one significant negative density correlation out of
thirty potential correlations. Cottid diversity was not correlated with structural diversity
along transects in spite of strong associations of each species with habitat structure. This
suggests that the transect areas were undersaturated in the number of individuals and/or
species of cottid fishes. The apparent lack of competition among subtidal cottids contrasts
with studies on intertidal fish communities dominated by cottids for which there is observational evidence of interspecific competition.
Key words: communitystructure;Cottidae;habitatselection;interspecificcompetition;microhabitat overlap;observationalapproach;resourcepartitioning;speciesdensity;temperatefish communities;
unsaturatedhabitats.
INTRODUCTION
A central goal of ecology is the identification of the
roles that abiotic and biotic factors play in determining
the distribution and abundance of species. In particular, attention has been directed to the roles of these
factors in determining patterns of resource use among
closely related species. This interest stems from one of
the central paradigms of ecology, the principle of competitive exclusion (Gause 1934). A major focus of ecological research has been to identify those conditions
that allow coexistence of species using similar resources, e.g., predation (Paine 1974), non-equilibrial
physical processes (Sale 1977, Wiens 1977, Sousa 1979)
and non-linear competitive hierarchies (Buss and Jackson 1979) or that reduce overlap in resource use, e.g.,
niche partitioning (Werner 1977, Hixon 1980, Pacala
and Roughgarden 1982), character displacement (Grant
' Manuscriptreceived 12 March 1990; revised 12 January
1991; accepted 14 January1991.
and Schluter 1984), and disjunct distributions (Hairston 1980, Case 1983).
The term "resource partitioning" has been used in
a general sense to describe any differences in resource
use that exist between species and in a strict sense to
describe only those changes in resource use due to competitors (Ebeling and Laur 1986). That species differ
in resource use is a common ecological observation
(see reviews in Schoener 1974a, Toft 1985, Ross 1986).
More pertinent to the understanding of community
dynamics is assessing to what extent these differences
in resource use reflect inherent interspecific inequalities
in the abilities to use and/or the preferences for using
resources (autecological interactions) and to what extent biotic processes, including predation and competition (synecological interactions) modify autecological differences (Strong 1983).
Both experimental and observational approaches
have been used to investigate the factors influencing
the distribution and abundance of guild members. The
experimental approach, manipulation of a single factor
2182
STEPHEN F. NORTON
Turn Is.
while holding others constant, is the stronger method,
but is not without drawbacks (Cody 1974, Schoener
1974a, Connell 1975, Underwood and Denley 1984,
Diamond 1986). In the observational approach, correlations are made between the density or distribution
of a species and the abundance or intensity of a biotic
or abiotic factor as both vary spatially or temporally.
The weakness of the observational approach is the leap
from correlation to causation. Several studies have provided examples in which experiments have refuted the
importance of processes inferred by observed patterns
or found that the effects were even reversed (see
McGuinness 1988). Yet, in multispecies guilds, observational studies can be valuable in generating hypotheses, especially in identifying subsets of factors or species that then may be subjected to experimental analysis
(e.g., Brown et al. 1986).
This study uses an observational approach (multiple
strip transects at several subtidal sites) to document
habitat use and to identify possible mechanisms underlying habitat partitioning among members of a guild
of subtidal fishes in the family Cottidae. Ninety cottid
species can be found between Baja California and the
Aleutian Islands (Howe and Richardson 1978). Within
this geographic range cottids are often the dominant
fishes in tidepool and nearshore subtidal habitats (Simenstad et al. 1977, Yoshiyama et al. 1986). Many
studies have documented the importance of physical
and biological factors, including competition, in determining the distribution of intertidal species (Morris
1961, Graham 1970, Green 1971, Nakamura
1976a, b, Cross 1981, Richkus 1981, Yoshiyama 1981,
Grossman 1982, 1986b, Wells 1986). Subtidal cottids
are also quite abundant, but the factors determining
f - Eagle Point
g - FHL Mud
h - Turn Rock
the distributions and abundances of these species are
poorly known. In part, this reflects the small size, sedentary behavior, and cryptic coloration of many spe-
Vancouver
''_
(V
San Juan Is.
S e a t t Ie
North
0Or) c a s Is.
D<:Jt)
^^
(4^ ',9
S anXvJua^
.
n
'
J
tl9h"
\
/7
\
\
L0IsLopez
v^
°
CO^-)
i:,@1960,
a
b
c
d
-
East Sound
1
- East Sound 2
- Bell Is.
- Point George
Ecology, Vol. 72, No. 6
e
-
FIG. 1. Diagram of the San Juan Islands, Washington, cies, which make them much less visible during unshowing the six major study sites (a-f) and the two minor derwater investigations (e.g., Burge and Schultz 1973,
study sites (g and h).
Ebeling et al. 1980, Larson and DeMartini 1984).
The goal of this study was to document patterns of
abundance and habitat use among members of the assemblage of subtidal cottids, to correlate species-speDistribution of transects and average cottid density on each transect by site. Habitat abbreviations: CR = clean
rock, SR = silt rock, G = gravel, SS = shell sand, M = mud, A = algae. Species abbreviations: Ahar - Artedius harringtoni;
'ABLE 1.
Number of
transects
Total
area
(m2)
CR
SR
G
SS
M
A
Turn Island
Bell Island
East Sound 1
East Sound 2
Point George
Eagle Point
Friday Harbor ILaboratories
Turn Rock
6
8
8
5
12
4
1
1
85
106.5
120
67.5
170
57.5
15
15
52
59
0
0
75
34
0
98
13
0
10
40
6
0
0
0
15
0
20
0
1
23
2
1
8
40
39
57
12
35
0
0
5
0
28
0
3
0
66
0
7
1
3
2
3
8
31
1
Totals
45
636.5
42
10
8
28
8
4
Site
Cover of substrates (%)
2183
COTTID COMMUNITY STRUCTURE
December 1991
cific patterns of habitat use with patterns of habitat
availability, and to assess the degree to which interspecific competition appears to modify patterns of species abundance and resource use. I examined three
patterns that would be expected if intraguild competition is a strong force influencing the distribution and
abundance ofcottid species: disjunct distributions, niche
divergence, and inverse density relationships. Although other mechanisms besides competition may also
be consistent with these patterns, invalidating these
patterns would decrease the likelihood that intraguild
competition was a strong mechanism influencing the
distributions and abundances of these species.
MATERIALSAND METHODS
Study area
I conducted this study in the waters around the San
Juan Islands of Washington state, USA. The San Juan
Islands lie at the junction of Puget Sound, the Strait of
Juan de Fuca, and Barkely Sound (Fig. 1). The complex
topography of these islands (Mayers and Bennett 1973)
and the mixed semidiurnal tides (Herlinveaux 1954)
interact to create a variety of physical habitats (e.g.,
deep mud-bottomed fjords, current-swept rock reefs,
sand flats) within a small area. These highly productive
waters are well known for their rich marine life
(MacPhee and Clemens 1962, Ellis 1971, Kozloff 1973,
Foreman and Root 1975). Cottids are no exception; I
have found in the San Juan Islands 31 of the 90 species
described from the northeast Pacific (S. F. Norton,
personal observation).
Fish and habitat sampling
Fish transects were conducted at six primary sites
(4-12 transects/site) and two secondary study sites (one
transect/site) (Fig. 1) in the summer of 1985. These
sites are mosaics of several common habitat types in
the nearshore subtidal in the San Juan Islands (Table
1). Most of the 45 transects were 30 m long, but several
of the deeper transects were shorter due to time constraints imposed by scuba-diving at these depths. Each
transect was placed parallel to shore within a narrow
depth range, and an effort was made to maximize the
diversity of substrates sampled by each transect. The
influence of depth on cottid abundances was evaluated
by running transects at several depths (between 7 and
30 m) at each primary site.
The initial pass along each transect sampled habitat
use and density of cottids; a second sampled the availability of different substrates along the transect. During
the first pass I recorded the species and the substrate
on which it was lying for each cottid individual encountered within 0.5 m of the right side of the transect
line. I carried a 0.5-m rod as a reference width for the
transect. To accurately sample these cryptic sedentary
fishes I swam slowly along the bottom at - 1.5 m/min.
These data were recorded in 5-m blocks. During the
second pass I sampled the availability of different substrates (microhabitats) by recording the substrate under
the transect line every 10 cm. Observations from preliminary dives were used to identify six dominant substrates: (1) clean rock (CR), characteristic of high-current sites; (2) silt rock (SR), rock covered with a fine
layer of silt, characteristic of low-current sites; (3) shell
sand (SS), a complex of shell fragments and sand that
collect below rockwalls; (4) gravel (Gr); (5) mud (M);
and (6) foliose algae (A).
Data analysis
I examined the influence of biotic and abiotic factors
on the distribution and abundance of cottids in two
ways. The microhabitat analysis compared the distribution of individuals of each cottid species to the distribution of substrates available along the transects.
This analysis also examined the distributional patterns
of species pairs for disjunct distributions and niche
shifts. I defined a microhabitat as a physically uniform
area of substrate (e.g., shell sand, mud, etc.) within
which a cottid might spend part of the day. In the
macrohabitat analysis I applied the regression technique of Schoener (1974b) and Crowell and Pimm
(1976) to examine the role that substrate abundance
and the densities of other cottid species have on the
density of each species along the transects. Each transect was viewed as a macrohabitat containing a unique
distribution of microhabitats.
Afen = A. fenestralis; Asem = Asemichthys taylori; Chit = Chitonotus pugetensis; Ice = Icelinus borealis; and Jord
zonope.
(Indsm
Ahar
Afen
0.81 + 0.17
0.68 ±+ 0.29
0
0
0.31 ± 0.42
0.37 + 0.42
0
2.60
0.41 ± 0.53
0
0
0.24 + 0.23
1.35 ± 0.98
0
0
0.13
0
0.20 + 0.52
Jordania
Cottid density
transect-', X + 1 SD)
-2
Asem
Chit
Ice
Jord
± 0.13
± 0.52
+ 0.18
+ 0.47
+ 0.11
+ 0.20
0
0
0.31 + 0.42
0
0
0.29 ± 0.26
0.72 + 0.45
0
0
0
0
0.13 ± 0.29
0.38 ± 0.39
0.08 ±_0.11
0
0
0.73 + 0.56
1.63 ± 1.34
0.47
0
0.42 ± 0.67
0.51 ± 0.26
0.80 + 0.39
0
0.07 + 0.05
0.34 ± 0.22
0.75 + 0.57
0.13
0.53
0.39 + 0.39
0.07
0.63
0.13
0.96
0.09
0.40
STEPHENF. NORTON
2184
Microhabitat analysis
Intersite consistency in the microhabitat distribution
of a species was measured using Kendall's coefficient
of concordance and was tested for significance by
Friedman's rank test (Kendall 1962). Because no cottid
species was found at all sites, the rank tests for each
species only included those sites where the species was
observed. For both Artedius fenestralis and Chitonotus
pugetensis, encountered only at the two East Sound
sites, the coefficient of rank concordance and Friedman's test were evaluated among transects.
A view of the overall distribution of each species
among subtidal microhabitats in the San Juan Islands
was constructed by combining the data on the microhabitat association of individuals over all transects.
The hypothesis that individual species were distributed
among microhabitats in proportion to the availabilities
of these microhabitats was evaluated with G tests. Because all species demonstrated non-random habitat use,
additional tests were conducted to determine which
microhabitats were significantly "preferred" or
"avoided" by each species (i.e., over- and under-represented). In these tests, the observed microhabitat distributions on a single substrate and on the sum of the
other substrates were compard to expected distribution
of individuals on these two substrate classes if cottid
distributions were based on microhabitat availability.
Each microhabitat was singled out in turn (e.g., comparison of the observed distribution of Artedius harringtoni on shell sand and all other substrates with the
distribution expected if A. harringtoni used shell sand
and the other substrates in the frequencies sampled on
the transects). The critical G values were adjusted to
reflect the a posteriori nature of these comparisons
(Sokal and Rohlf 1981).
If competition among cottid species is influencing
the microhabitat distribution, one might predict that
species pairs would show checkerboard or disjunct distributions, where one species excludes another from
an area. This hypothesis was evaluated by totaling the
number of 5-m transect sections (2.5 m2) where (1)
both species of a pair were present, (2) both species
were absent, and (3) only one species was present. Only
transects where at least one member of a species pair
was present were included, reducing the interdependence of the comparisons (Toft et al. 1982). Independence in the distribution of species pairs among 5-m
transect sections was evaluated using G tests.
Even if species pairs do not show checkerboard distributions, competition by one member of a species
pair may influence the microhabitat distribution of the
other. If this occurs, one would expect the species pairs
to show higher overlap in microhabitat use when in
allopatry than when in sympatry (i.e., niche divergence). The potential of one species to alter the distribution of another was evaluated using G tests that compared the microhabitat distribution of one species in
Ecology, Vol. 72, No. 6
those 5-m transect sections that also contained the potential competitor (sympatry, i.e., transects containing
both members of a species pair) with the microhabitat
distribution in 5-m transect sections that lacked the
potential competitor (allopatry, i.e., transects containing only one member of a species pair). An overlap
index (Schoener 1970) was used to calculate microhabitat overlap between members of species pairs in
sympatry and in allopatry.
Under different scenarios of competition, resource
overlap may vary directly or inversely with the intensity of competition (Lawlor 1980). One difficulty with
traditional overlap measures as approximate measures
of competition is that overlap measures based on proportional utilization confound the electivity of the consumer with the availability of resources in the environment (Lawlor 1980). Lawlor proposed that similarity
of electivities between two species may be a better
measure of potential competition. I used the asymmetric MacArthur-Levins equation to calculate both
interspecific microhabitat overlap, based on proportional utilization, and microhabitat similarity, based
on electivities determined using Manly's (1974) formula. The former measure is an estimate of overlap
of the realized niches of the species pairs, while the
latter should be a better indicator of overlap of the
fundamental niche of these species.
Macrohabitat analysis
The method of Schoener (1974b) and Crowell and
Pimm (1976), as modified by Rosenzweig et al. (1984),
was used to examine the influence of substrate abundance and the density of potential competitors on cottid densities. In the Schoener-Crowell-Pimm (SCP)
method, an initial multiple regression uses the density
of a single species measured at several sites as the dependent variable and several habitat variables (e.g.,
rainfall, percentage cover of different substrates, temperature, etc.) as the independent variables. The regression coefficients provide estimates of the influence
of the habitat variables on species abundance. The residuals of this regression should be estimates of species
densities at each site that are largely free of habitat
heterogeneity. A second set of multiple regressions uses
the matrix of these residuals for all the species in the
guild to assess the strength of interspecific interactions.
Each species serves as the dependent variable and is
regressed against the residuals of the other species. Negative interspecific regression coefficients derived from
the SCP technqiue have been invoked as evidence for
competition (e.g., Crowell and Pimm 1976, Hallett and
Pimm 1979, Dueser and Hallett 1980, Rosenzweig et
al. 1984, Dueser and Porter 1986, but see Rosenzweig
et al. 1985 and Abramsky et al. 1986). Simulation
studies indicate that competition coefficients calculated
by the SCP technique are robust to deviations of populations away from equilibrium (Hallett and Pimm
December 1991
TABLE2. Rank correlation of microhabitats. Friedman's test,
a non-parametric multisample rank-correlation statistic, was
used to test for consistency of habitat use among sites or
among transects (species designated with *). Kendall's W
is indicative of the sample correlation coefficient.
Species
2185
COTTID COMMUNITY STRUCTURE
No. of
sites
No. of
substrates
W
P
Artediusharringtoni
Asemichthystaylori
Icelinusborealis
5
6
4
6
6
6
0.57
0.60
0.43
<.025
Jordania zonope
Artedius fenestralis*
Chitonotus pugetensis*
6
12
12
6
5
5
0.51
0.33
0.48
<.01
<.01
<.001
<.01
<.25
1979) and for problems with the independence of the
variables (Carnes and Slade 1988).
In this study the seven independent variables for the
habitat-species regressions were the percentage cover
estimates for the six substrate types on each transect
and the depth of each transect. In the species-species
regressions the residual densities of a single species
from the habitat regressions served as the dependent
variable and the residual densities of the other species
were the independent variables. The distribution of
percentage cover data was not significantly different
from normal after arcsine square-root transformation.
Similarly, the distribution of the density data was normally distributed after square-root transformation.
In both stepwise regressions (habitat-species and
species-species) a minimum significance level of 0.15
was required at each step before entry of any variable
into the overall model. Other regression approaches
(full regression models or models using the original
cottid densities as independent variables in the speciesspecies regressions) were explored as suggested by Rosenzweig et al. (1984), but were poorer descriptors than
the stepwise method; i.e., they accounted for less of the
variation in cottid density. There were strong qualitative similarities in the results regardless of the type
of regression model. Examination of plots of the residuals vs. the independent variables of the habitatspecies and species-species regressions revealed no obvious curvilinear relationships (i.e., no evidence for
concave zero-isoclines; Schoener 1974b).
RESULTS
Cottids are abundant in the nearshore subtidal of the
San Juan Islands; eight species totaling 1172 individuals were encountered during 45 transects. Cottid density was 1.8 ± 0.16 individuals per square metre per
transect (mean + 1 SE). The six common cottids were
Artedius fenestralis, A. harringtoni, Asemichthys taylori, Chitonotus pugetensis, Icelinus borealis, and Jordania zonope (Table 1). Two other cottid species,
Hemilepidotus hemilepidotus (N = 1) and Rhamphocottus richardsoni (N = 8) were rare and are excluded
in the analyses. Fishes sampled on the transects included both newly settled juveniles and adults of all
species. Other fishes encountered occasionally during
the transects included Pholis clemensi, Ronquilus jotdani, the rockfishes Sebastes auriculatus, S. caurinus,
S. emphaeus, and S. flavidus, and the hexagrammids
Ophiodon elongatus and Hexagrammos decagrammus.
Selection of microhabitats by cottid fishes
The microhabitat distributions of cottid species were
generally consistent among sites or transects (Table 2).
None of the cottid species were found to use microhabitats in proportion to their availability along the
transects (Fig. 2; G tests, P < .001); each species showed
significant microhabitat preferences and avoidances.
Both J. zonope and Artedius harringtoni were found
predominately on clean rock surfaces and avoided nonsolid substrates. C. pugetensis and Asemichthys taylori
were found almost exclusively on shell sand and avoided clean rock. Encounters with A rtediusfenestralis were
divided between shell sand and silt-covered rock. I.
borealis was common on gravel substrates and rare on
silt rock and shell sand.
Interspecific microhabitat overlap (based on realized
overlap of resource use) and interspecific microhabitat
similarity (based on overlap of resource electivities)
varied greatly among the species pairs (Table 3; overlap: mean ± 1 SD = 0.46 ± 0.42; similarity: mean +
1 SD = 0.40 ± 0.25). These measures of the overlap of
actual and potential resource use were highly correlated
(r2 = 0.58, F = 38.7, P < .001). Nine of 30 overlap
coefficients and 4 of 30 similarity coefficients were in
the "biologically significant" range, >0.60 (see references in Grossman 1986a). High overlap values were
found for J. zonope and A. harringtoni, Asemichthys
taylori and C. pugetensis, Artedius fenestralis and C.
pugetensis (all reciprocal), and for J. zonope and A.
TABLE3. Two measuresof interspecificoverlapin the spatial
distributions of six common cottid species in the San Juan
Islands. The table shows the overlap or similarity of the
column species vs. the row species. Species abbreviations
as in Table 1.
Ahar
Afen
Asem
Chit
Ice
Jord
A. Asymmetrical interspecific microhabitat overlap (MacArthur-Levins index).
Ahar
0.091
1.076
0.123
0.081
0.557
Afen
0.156
1.068
0.943
0.290
0.116
Asem
0.102
0.518
0.843
0.219
0.058
Chit
0.090
0.609
0.236
0.043
1.122
Ice
1.238
0.376
0.585
0.475
1.296
0.058
0.925
0.060
0.034
0.501
Jord
B. Asymmetrical interspecific electivity similarity (MacArthur-Levins index).
Ahar
0.428
0.238
0.169
1.134
0.499
Afen
0.507
0.404
0.367
0.462
0.336
Asem
0.284
0.081
0.134
0.333
0.696
0.115
Chit
0.131
0.420
0.965
0.349
0.502
0.507
Ice
0.394
0.450
0.452
0.101
0.793
0.329
0.103
0.315
Jord
STEPHENF. NORTON
2186
1(00
Ecology, Vol. 72, No. 6
100
A. fenestralis
80
80 -
60
60 -
40
40
I-
20
20
*-
126 individuals
CR SR G
100
**
i(O
8
80( -
°
60-
D
40O
20-
60
°
I
80 -
ni
247 individuals
Jor~dania
c
|
183 individuals
-
60
°CR
SR G
I. borealis
A
M SS
Chitonotus
80 -
260 individuals
60
60-
40
40
20-
20
CRSRG
**
60 -
20
AMSS
M SS
.Asemichthys
80 -
.
CRSRG
A
AMSS
0R
87 individuals
SR G
A M
SS
100
80
60
40
0
CR SR G A M SS
Substrate Type
FIG.2. Microhabitatassociationsof six cottid speciesand the frequencyof occurrenceofmicrohabitatsalongthe transects.
or negative substrate associations are indicated: * P .05, ** P
Significant positive or
rock, SR = silt rock, G = gravel, A = algae, M = mud, SS = shell sand.
harringtoni on I. borealis, and Asemichythys taylori on
Artediusfenestralis. High microhabitat similarities were
found for J. zonope and A. harringtoni, and for Asemichthys taylori and C. pugetensis (each reciprocal). Both
indices indicate that if substrate availabilities were limited, competition was likely to occur among these cottid species pairs.
In contrast to the expectation of negative co-occurrence patterns under the competition paradigm, six
cottid species pairs showed neutral co-occurrence pat-
<
.01. Habitat abbreviations: CR = clean
terns and three had positive co-occurrence patterns;
none were negative (Fig. 3). Another possibility consistent with competition is that a species may alter its
microhabitat use in the presence of a competitor (niche
divergence). Although several species pairs did show
significant differences in patterns of microhabitat use
in the presence of potential competitors, interspecific
overlap in microhabitat use was higher in sympatry
than in allopatry, opposite to predictions of niche divergence under competition (Fig. 4).
COTTID COMMUNITYSTRUCTURE
December 1991
2187
Species Co-occurrence
-------------------
A. fenestralis
Chitonotus
.
Asemichthys
|
Icelinus-
A. harringtoni
Jordania
A- --------FIG.
positive
neutral
3. Co-occurrenceof speciespairsin 5-m transectsections.Speciespairsconnectedby arrowsdemonstratedsignificant
positive co-occurrence patterns. Species pairs connected by dotted lines demonstrated independent patterns of occurrence.
Influence of habitat structure and heterospecifics
on cottid densities
and Asemichthys taylori were higher on transects with
a high percentage cover of silt rock and shell sand. The
density of Artedius harringtoni decreased with increasing depth and increased in areas dominated by clean
rock surfaces. Abundance of C. pugetensis showed negative relationships with clean rock, gravel, and algal
abundances. The density of J. zonope was positively
correlated with the percentage cover of clean rock and
algae. Abundance of I. borealis increased with increasing depth and in areas dominated by gravel and algae,
but this species avoided silt rock and shell sand.
Densities of each species were significantly influenced by the relative abundances of particular habitat
types in an area (Table 4). Stepwise regressions using
the macrohabitat variables account for between 40 and
60% of the variance in species density (Table 4: R2
Habitat). As with microhabitat associations, the density of each species demonstrated significant positive
and negative correlations with areas dominated by particular habitat types. Densities of Artedius fenestralis
Allopatric - Sympatric Overlap
A. fenestralis
<
0.36 -0.67
Chitonotus
>
Asemichthys
|
Icelinus
|
1 0.57
I
+
0.63
Jordania
|!
A. harringtoni
I
I
FIG. 4. Changes in microhabitat use between sympatry and allopatry and microhabitat overlap in allopatry (left value)
and sympatry (right value) for cottid species pairs. Overlap measures were calculated using the index described in Schoener
(1970). An arrow connecting two species indicates that the species at the head of the arrow demonstrates significant differences
in its distribution among substrates when in sympatry vs. in allopatry on 5-m transect sections with the species at the tail of
the arrow.
2188
Ecology, Vol. 72, No. 6
STEPHEN F. NORTON
TABLE4. Stepwise regression coefficients of cottid densities vs. habitat variables and of species vs. species residuals after
removal of habitat effects. Species and habitat abbreviations as in Table 1.
Independent
Independent
variables
t Significant at P
Afen
Ahar
9.133**
CR
SR
G
SS
M
A
Depth
Ahar
Afen
Asem
Chit
Ice
Jord
R2 Habitat
R2 Density
R2 Total
12.43**
Dependent variables
Asem
Chit
-8.37**
3.84t
-4.28*
11.00**
4.72*
-14.16**
-0.07t
Jord
Ice
8.25**
-6.98*
6.60t
-4.43t
29.34**
G.19**
10.40t
0.32*
0.05**
-0.40*
0.38*
-0.29t
0.24t
1.02**
-0.21t
0.30t
0.400
0.214
0.614
<
0.35*
0.440
0.258
0.698
.15; t significant at P < .10; * significant at P
From the results of the second series of regressions,
the subtidal cottid community in the San Juan Islands
shows little evidence of being structured by competition for space. Of 30 possible interspecific coefficients,
only 10 passed the initial significance cutoff (P < .15)
in the stepwise regression models; 6 of these are significant at the .05 level or better (Table 4). Only one
of these six coefficients is negative (A. fenestralis on A.
harringtoni). The interspecific regression models account for between 3 and 26% of the total variation in
cottid densities (Table 4: R2 Density). Together the
habitat and interaction regressions account for between
48 and 79% of the variation
in the initial densities
0.608
0.180
0.788
0.448
0.072
0.520
of
the cottid species (Table 4: R2 Total).
Diffuse competition (the aggregate influence on one
species by the other members of a guild, MacArthur
1972) does not appear to be important in this guild. I
conducted a regression analysis for each species in which
the dependent variable was the residual density of the
single species and the independent variable was the
sum of the residual densities of all the other species
after habitat effects had been removed. None of the
regressions indicated a negative relationship as predicted under a diffuse competition model (Table 5).
On the contrary, abundances of both Asemichthys taylori and J. zonope were significantly higher in areas
with high densities of other cottids.
DISCUSSION
The results from this study indicate that subtidal
cottids in the San Juan Islands are distributed differentially among six common structural habitats. The
patterns of distributions and the abundances of these
species are more consistent with mechanisms based on
autecological responses of species to spatial heterogeneity in the distribution of habitats (the individualistic
concept, Gleason 1926) than they are with synecolog-
<
0.407
0.150
0.557
0.450
0.031
0.481
.05; ** significant at P < .01.
ical mechanisms. Under the individualistic concept,
differences in resource use among guild members (resource partitioning sensu lato, e.g., Schoener 1974a,
Ross 1986) reflect the unique adaptations and evolutionary histories of each species, and not coevolutionary interactions among guild member (resource partitioning sensu stricto, Roughgarden 1976). The
individualistic concept has been invoked to explain
niche differences in a variety of systems (Strong 1983),
e.g., terrestrial plant communities (Gleason 1926),
aquatic snails (Aho et al. 1981), shrub-steppe birds
(Wiens and Rotenberry 1981), and temperate fishes
(Grossman et al. 1982, Ebeling and Laur 1986).
Influence of habitat factors
The influence of habitat on species distributions may
change when examined over different spatial scales (e.g.,
Wiens and Rotenberry 1981, Morris 1987, Snyder and
Best 1988). Within the San Juan Islands, most cottid
species demonstrated significant rank consistency in
the use of substrates at different sites or among transects. On a broader scale, the habitat distributions of
subtidal cottids in the San Juan Islands are similar to
qualitative observations of their distributions in other
TABLE5. Diffuse competition: influence of the aggregate
density of other cottid species on the density of single species after correction for habitat effects.
Species
Artedius harringtoni
Artedius fenestralis
Asemichthys taylori
Chitonotus pugetensis
Icelinus borealis
Jordania zonope
Regression
coefficient
0.025
-0.061
0.245
0.004
-0.069
0.118
F
P
0.14
1.31
19.73
0.02
1.34
7.13
>.70
>.25
<.001
>.90
>.25
<.05
December 1991
COTTID COMMUNITY STRUCTURE
parts of their ranges (MacPhee and Clemens 1962,
Wilkie 1963, Burge and Schultz 1973, Peden and Wilson 1976, Moulton 1977, Leaman 1980, Yoshiyama
et al. 1986).
In this study, habitat structure had similar effects in
the micro- and macrohabitat distributions of cottids.
The densities of a cottid species were enhanced in those
habitats dominated by substrates for which it had high
microhabitat electivities. Of the 21 significant microhabitat associations and 10 significant macrohabitat
correlations (i.e., P < .05), seven pairs of habitat effects
were shared across the two scales of analysis, e.g., positive microhabitat association and positive macrohabitat correlation of Artedius harringtoni and clean rock.
Conflicts existed for only one species: on a microhabitat scale Asemichthys taylori was rarely observed on
silt rock, but on a macrohabitat scale its overall density
was enhanced on transects with a high percentage cover
of this substrate. This likely reflects the positive association of these substrates in East Sound transects.
At this low-current site, the source of shell fragments
is from barnacle tests from the silt-covered rockwalls.
Studies of intertidal cottid communities have documented a similar strong role of habitat structure on
their distributions. Important environmental features
correlating with distributional patterns of intertidal
cottids include the predominant substrates in the tidepool and the degree of wave exposure, tidal height, and
pool size (Green 1971, Nakamura 1976a, b, Cross 1981,
Yoshiyama 1981, Yoshiyama et al. 1986). As with
subtidal cottids, the associations of intertidal cottids
with particular substrates are relatively consistent
throughout their geographic ranges (up to 1000 km) in
spite of changes in the composition of the intertidal
fish community (e.g., Yoshiyama et al. 1986). Tests of
microhabitat choice have shown that the associations
of several intertidal species with particular environmental features in the field can be replicated in the
laboratory and do not change in the absence of potential competitors (Nakamura 1976a, Richkus 1981).
Marliave (1977) demonstrated that the substrate preferences of several intertidal fishes, including two cottids, appear to be set at the time of larval settlement.
Influence of interspecific competition
Three lines of evidence indicate that interspecific
competition does not influence the distribution of subtidal cottids. First, there is no evidence that some species excluded others from an area (disjunct distributions). Second, although the presence of other species
was associated with changes in the microhabitat associations of several species, the microhabitat overlap
between all species pairs was higher in sympatry (transects containing both species) than in allopatry (transects with one species absent), opposite to the expectation under typical competitive models. This reflects
some convergence of species onto the predominant
substrate type in sympatric areas. Finally, using the
2189
Schoener-Crowell-Pimm (SCP) method, only 1 of 30
interspecific interaction coefficients was negative, indicating a lack of strong negative correlation of species'
densities.
Recently, the SCP method has come under intense
scrutiny regarding the mechanics of the technique and
the experimental corroboration of its results (Rosenzweig et al. 1984, Pimm 1985, Schoener 1985, Abramsky et al. 1986, Cames and Slade 1988). The SCP method has been applied repeatedly to examine the influence
of competition in small mammal communities (Crowell and Pimm 1976, Dueser and Hallett 1980, Hallett
1982, Porter and Dueser 1982, Rosenzweig et al. 1984);
a consistent result of these studies has been that an
overwhelming predominance of significant and negative interspecific interactions remain after removal of
habitat effects. This pattern has been interpreted to be
the result of strong competition among members of
these communities, but experimental verification of
these putative competitive interactions has yet to appear.
In the one experimental test of the SCP method, it
failed to detect consistently competition between two
bee species that were shown experimentally to be strong
competitors (Abramsky et al. 1986). The predictions
of the SCP method were upheld when the experimental
manipulations of bee populations were conducted over
a range of densities similar to those used in the regressions, but failed when the experiments were conducted at densities far beyond the range used in the
regression analysis. Pimm (1985) has argued that the
"failure" of this second test may less reflect inadequacies of the SCP method than inherent difficulties with
linear extrapolations beyond the range of observed values, especially if competitive effects are non-linear as
predicted in several theoretical treatments (Rosenzweig 1981, Schoener 1985).
The SCP method for calculating interspecific interaction coefficients may be viewed as a conservative
test of competition. It assumes that habitat effects on
species distributions have priority over interspecific
effects. As originally formulated, this arrangement was
necessary to adjust species densities at multiple sites
that differed in the equilibrium densities of potential
competitors due to differences in the distribution of
habitats (one of the major drawbacks of the observational approach) before comparing species densities
(Schoener 1974b). One result of this arrangement is
that some of the variation in cottid densities attributed
to habitat in the initial habitat-species stepwise regression may actually be niche partitioning caused by
competitive interactions among species. This would
reduce the ability of the SCP technique to detect significant negative species interactions.
On the other hand, some of the variation in cottid
densities attributed to species interactions in the species-species regressions may be due to covariance between environmental factors not included in the initial
2190
STEPHENF. NORTON
habitat regressions (e.g., food supply, substrate relief,
etc.) and species densities. The effects of these "hidden"
habitat factors on species densities would not be "corrected" when calculating the residual densities of some
species. The positive associations between several pairs
ofcottids (Table 4) are probably due to such covariance
and not to mutualistic interactions between species.
Although mindful of the drawbacks of the SCP approach, it has fulfilled one of the advantages accrued
when using an observational approach to examine resource partitioning within complex assemblages, i.e.,
identification of specific physical factors that are most
likely to influence the distribution and abundance of a
cottid species and identification of the species pairs
that show inverse density relationships consistent with
interspecific competition. These results predict that experimental manipulations of most cottid species are
not likely to reveal strong competitive interactions.
The one negative interaction coefficient, Artedius fenestralis on A. harringtoni, may be an artifact due to
covariance with an environmental variable not included in the original analysis (e.g., current speed), or may
be due to strong competitive interactions between
closely related species that have similar diets (Norton
1989). Experimental manipulations of these two species are necessary to distinguish between these possibilities.
Another line of evidence consistent with a general
lack of interspecific competition among subtidal cottids is the lack of a correlation between cottid diversity
and habitat diversity (Shannon-Weiner index, H') along
the subtidal transects, despite strong association of individual cottid species with habitat variables (r2= 0.047,
F = 3.784, P > .05). Other studies have documented
a positive relationship between complexity of the physical environment and species diversity (see references
in Gorman and Karr 1978, Boecklen 1986). This observation implies that the subtidal cottid community
may be undersaturated; some transects lack either the
full complement of species and/or individuals that one
would expect given the available habitats. Undersaturation may be due to mortality caused by predators
(e.g., Paine 1974) or by physical processes (Sousa 1979,
Grossman et al. 1982), or to recruitment limitations
(Shulman 1985, Victor 1986).
Ecology,Vol. 72, No. 6
that manipulated the densities of putative competitors
(Yoshiyama 1981) or the abundance of spatial resources (Cross 1981) have been inconclusive. However, Grossman (1986a) has argued that strong competition does exist among intertidal fishes, but that the
limiting resource is food, not space.
In light of the apparent importance of competition
among members of the intertidal assemblage, it is surprising that the subtidal cottid community of the San
Juan Islands shows little evidence of competitive regulation, especially in the absence of other structuring
forces (e.g., predation or physical disturbance). Examination of the diets of other potential predators and
competitors (e.g., rockfishes and hexagrammids) indicates that these fishes rely primarily on prey sizes
and types that are not utilized by cottid fishes, and
cottids are not normally encountered in the diets of
these species (Moulton 1977; F. S. McEuen, personal
communication, S. F. Norton, personal observation).
Also, physical disturbance is much less pronounced in
the subtidal than in the intertidal (Cross 1981, Levings
et al. 1983).
The densities of subtidal cottids may be kept below
levels necessary to saturate the available resources by
recruitment processes, especially when compared to
the densities of intertidal species. The transport mechanisms (the strong currents in the San Juan Islands and
a larval period of several weeks or months for cottids)
seem sufficient to ensure access of potential recruits to
all intertidal and subtidal habitats. However, the supply of recruits of subtidal species may be limited by
heavy predation on benthic eggs in the subtidal by
invertebrates and by other fishes (DeMartini 1978).
In conclusion, habitat partitioning by subtidal cottid
species is more likely the result of inherent micro- and
macrohabitat preferences of individual species for particular substrates rather than responses to the presence
ofheterospecifics. The lack of any relationship between
species diversity and habitat structural diversity despite strong habitat preferences implies that subtidal
cottid populations are below environmental carrying
capacity. In the absence of great physical disturbance
and predation pressure, population regulation may be
due to recruitment limitation, especially through egg
predation.
ACKNOWLEDGMENTS
Comparison of intertidal and subtidal
cottid communities
Several authors have concluded that the distributions of rocky intertidal fishes are strongly structured
by competition for space (Yoshiyama 1981, Cross 1981,
Yoshiyama et al. 1986). These conclusions rest on observations of apparent resource partitioning via niche
complementarity (Cross 1981, Yoshiyama 1981), of
reciprocal patterns of abundance (Cross 1981), of increased microhabitat breadth in areas of decreased species richness (Cross 1981), and of high microhabitat
overlap (Yoshiyama 1981). Results from experiments
I would like to thank my dive partners,T. Miller, D. Hollister, and S. Herbert, for their assistance in collecting the
transectdata. Lab space and diving facilities were provided
by FridayHarborLaboratories.P. Raimondi,A. Ebeling,and
D. Reed provided helpful comments on the analysis. The
comments of A. Ebeling,S. Sweet, R. Warner,M. Carr,and
G. Grossmangreatlyimprovedthe manuscript.This research
was supportedby grantsfrom Sigma Xi, the TheodoreRoosevelt Fund, the LernerFund, the Raney Fund, and from the
GraduateDivision of UCSB.
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