ICES Journal of Marine Science, 54: 593–599. 1997
The relationships of seabird assemblages to physical habitat
features in Pacific equatorial waters during spring 1984–1991
Christine A. Ribic and D. G. Ainley
Ribic, C. A. and Ainley, D. G. 1997. The relationships of seabird assemblages to
physical habitat features in Pacific equatorial waters during spring 1984–1991. – ICES
Journal of Marine Science, 54: 593–599.
The association of seabird species groups with physical habitat was investigated in the
eastern tropical Pacific Ocean, far from any breeding colonies. This avoided birds that
commute between colony and feeding habitat, behaviour that confuses associations
with specific water types and current systems. Seabirds were counted on duplicate
tracks in the eastern tropical Pacific each spring from 1984–1991. On each cruise,
seabird habitat was measured on the basis of six factors and focused on three species
groups: (A) black-winged petrel and white-winged petrel, (B) Juan Fernandez petrel,
wedge-tailed shearwater, and sooty tern, and (C) Leach’s storm-petrel and wedgerumped storm-petrel. Group A was associated with the South Equatorial Current,
particularly in cooler waters (median of 26.4)C); both petrel species followed this
assemblage association with current. Group B was associated with areas characterized
by deep thermoclines (median of 60 m) and low salinities (median of 34.33). Within
Group B, two of the three species’ responses were consistent with the group pattern;
Juan Fernandez petrel differed by occurring more often where thermocline slopes were
steep (median of 9.8 deg C m "1). Group C was not associated with any physical
habitat variable. This was due to species in the group being associated with different
habitats: Leach’s storm-petrel with the tropical and equatorial surface water masses
and wedge-rumped storm-petrel with waters having shallower thermocline depths
(median of 22 m). Overall, two of the three assemblages appeared to be associated with
physical habitat during spring with consistency among the species in the group. An
association with thermocline depth may indicate that productivity was an important
predictor of assemblage presence.
? 1997 US Government
Key words: assemblages, habitat choice, physical features, seabirds.
C. A. Ribic: USGS BRD, Department of Wildlife Ecology, University of Wisconsin, 226
Russell Labs, 1630 Linden Drive, Madison, WI 53706-1598, USA. D. G. Ainley: H. T.
Harvey & Associates, 906 Elizabeth St., PO Box 1180, Alviso, CA 95002, USA.
Correspondence to C. A. Ribic: tel: +6082636556; fax: +6082626099; email:
[email protected]
Introduction
Seabird assemblages, unrelated to breeding activities,
have been identified in different areas of the world’s
oceans, such as the South Atlantic (Veit, 1995), the
Benguela Current (Abrams and Griffiths, 1981; Berruti
et al., 1989), the Antarctic (Ribic and Ainley, 1988/89;
Ryan and Cooper, 1989; Veit and Hunt, 1991; Ainley
et al., 1984, 1994; Hunt et al., 1994), the Indian Ocean
(Pocklington, 1979), the Peru Current (Ribic and
Ainley, 1988/89), and the California Current (Briggs
et al., 1987). In the Equatorial Pacific, work on seabird
habitat relationships, thus far, has focused mainly on the
structure of feeding flocks in association with tuna
(Ainley and Boekelheide, 1983; Au and Pitman, 1986;
1054–3139/97/040593+07 $25.00/0/jm970244
Au, 1991; Ballance, 1993; Ballance et al., in press),
but other research is beginning to identify species
associations with physical habitat features (Ainley and
Boekelheide, 1983; Ribic and Ainley, 1988/89; Ribic
et al., in press). In regard to the latter, much is known
about associations of individual species with environmental features (King, 1970; Pitman, 1986; Spear et al.,
1995), but work on identifying species assemblages
(Ribic et al., 1997) and the relationship of species’
assemblages to environmental features has been minimal
(except Ribic and Ainley, 1988/89).
In an analysis of species-habitat relationships (Ribic
et al., in press), we noted two patterns. The first was that
species appeared to be found in groups along an
environmental continuum and the second was that
? 1997 US Government
594
C. A. Ribic and D. G. Ainley
20°N
Hawaii
Pacific Ocean
10°
2
2
7
2
3
0°
6
8
3
2
2
3
2
Galapagos
Islands
6
3
10°
Tahiti
20°S
160°W
150°
140°
130°
120°
110°
100°
90°
80°
Figure 1. Cruise tracks for study carried out in the eastern tropical Pacific in spring, 1984–1991. Numbers next to the tracks denote
the number of cruises over a given track line. Tracks with no adjacent number were travelled on a single cruise.
Table 1. Definitions of surface water masses found in the study area and relationship with
major current systems (based on Wyrtki, 1967).
Surface
water mass
Tropical
Equatorial
Subtropical
Latitudinal boundaries
Physical boundaries
North
South
sst
sal
15)N&5)
24)N
25–28)C
33–34
24)N
20)
20)
10)S
20–28)C
19–28)C
34–35
35–36.5
the placements of species and groups along the
continuum was similar from year to year, regardless
of perturbations such as El Niño–Southern Oscillation
(ENSO) events. We investigated the first pattern in Ribic
et al. (1997). The purpose of the present study was to
investigate the second pattern, the relationship of seabird species’ assemblages with oceanic characteristics,
including water masses and currents. We also investigated whether the individual species comprising the
assemblages had similar relationships with the environmental variables, rather than assuming that this would
be true. The work presented here, then, is an extension
of the results of Ribic et al. (1997). We concentrated on
the spring season because more than one assemblage
was found, allowing comparison between assemblages.
Our work, though still observational in nature, differs
from that of Ribic and Ainley (1988/89) by focusing on
a specific season and area of the eastern tropical Pacific
(ETP) using cruise tracks repeated in space and time,
features not present in the previous work.
Current
North Equatorial,
Equatorial Counter
Current
South Equatorial
South Equatorial
Methods
Single cruises were conducted each boreal spring for
eight years, 1984–1991, in the ETP between 10)N and
10)S latitude and between 140) and 90)W longitude (Fig.
1). Oceanographic features of the area included three
water masses and three current systems, although most
of the sampling occurred in the Equatorial Countercurrent and the South Equatorial Current (Table 1). Spring
cruises occurred between April and June (the 1988 cruise
extended into the first week of July) and were within the
study area, on average, 25 days (range 17–35 d). The
data were collected as part of the larger EPOCS-TOGA
project, and we did not have control over the exact cruise
track. Nevertheless, the census of seabirds along a consistent cruise track over a long time period, covering two
to three water masses and two current systems, provided
an uncommon research opportunity.
Strip transects of 500–600 m width (depending on
height of observation platform) were conducted using
Seabird assemblages and physical habitat
the methodology described in Spear et al. (1995). Counts
were adjusted for species’ flight speeds and directions
(Spear et al., 1992; Spear and Ainley, 1997). Continuous
transects, partitioned into one half hour segments, were
conducted during daylight hours whenever the ship was
underway, weather permitting.
Various physical factors were considered in the
assemblage/habitat relationship analysis. Sea-surface
temperature ()C), sea-surface salinity (ppt), windspeed
(km h "1), and swell height (to the nearest 0.3 m) were
measured at the start of each transect. Thermocline
depth (m) and gradient ()C m "1 depth) were interpolated from XBTs deployed four to six times daily,
approximately 85 km apart, and CTDs. Using information from Longhurst and Pauly (1987), we categorized transects as falling into one of three water
masses on the basis of sea-surface temperature and
salinity characteristics (Table 1) and, separately, into
one of two current systems on the basis of thermocline
depth and gradient. Cruises were coded as to whether
they occurred during El Niño–Southern Oscillation conditions (1987 and 1991), during La Niña or cold-water
conditions (1988), or neither (1984–1986, 1990).
An assemblage was defined to be any group of two or
more species that consistently co-occurred during spring
1984–1991. Co-occurrence was defined based on cluster
analysis and recurrent group analysis (using an alpha of
0.05). Consistency was defined as those groups of species
that were found in over 50% of the years (details in Ribic
et al., 1997). This definition of assemblage differed from
that used previously (Ribic et al., 1992); in this paper, no
assumption was made regarding constrainment to water
masses.
In order to analyse the species assemblage/
environmental relationships, we considered the simple
presence/absence of the species assemblages. Sample
sizes for the analyses ranged between 1514 and 2210
transects, depending on the species and species assemblage being analysed. We were interested in looking at
the relationships combined over all years. As the ranges
of the sampled environmental variables were different
between years (e.g. absolute sea-surface temperatures
were colder during the 1988 cruise and warmer during
the 1987 cruise), we standardized each variable to have
zero mean and unit variance within each year. This gave
relative measures of the environmental variables (e.g.
warmer sea-surface temperatures would have positive
values while colder temperatures would be negative) for
use in the species assemblage/environmental relationship
analysis. We are considering, then, the response of the
assemblages to a relative pattern in the environmental
variables (e.g. an assemblage would tend to be found in
warmer areas of the study site regardless of the absolute
values of the variables).
We defined species assemblage/environmental variable
relationships using classification tree methodology
595
(Breiman et al., 1984). All calculations were carried out
using S-plus (Statistical Sciences, 1995). Trees were
selected using minimum-risk complexity and a moderate
complexity penalty of 0.01 (Breiman et al., 1984; Miller,
1994). The output of the technique is a dendrogram,
which we present with the relative frequencies of seeing
the assemblage at each terminal node. The standardized
variables were transformed back to the original scale
and averaged over all the years for ease of interpretation. We also report the misclassification rate for the
final tree. Classification and Regression Trees (CART)
has been used in different areas of ecology (Verbyla,
1987; Grubb and King, 1991; Lynn et al., 1995; Walker,
1990; Bell, 1996) and is useful in situations where
complicated interactions between variables are suspected
to be present.
Results
The three spring assemblages considered were those
identified by Ribic et al. (1997) to be recurring from year
to year: (A) black-winged petrel (Pterodroma nigripennis) and white-winged petrel (P. leucoptera); (B) Juan
Fernandez petrel (P. externa), sooty tern (Sterna fuscata), and wedge-tailed shearwater (Puffinus pacificus);
and (C) Leach’s storm-petrel (Oceanodroma leucorhoa)
and wedge-rumped storm-petrel (O. tethys).
Group A, a petrel assemblage, recorded on 22% of all
transects, had the highest probability of occurrence
in the South Equatorial Current, particularly in cooler
waters (median=26.4 )C, misclassification rate=0.22;
Fig. 2a). Note that during ENSO, the assemblage was
seen in the South Equatorial Current in areas of warmer
temperatures (median=27.5)C) and higher salinities
(median=35.2; relative frequency of occurrence=0.55),
but that during La Niña, the assemblage had a higher
probability of being seen in the Equatorial Counter
Current (relative frequency of occurrence=0.39; Fig.
2a).
The two species making up the assemblage, when
considered individually, were consistent with the assemblage pattern. White-winged petrel, observed on 9% of
all transects, was more likely to be found in cooler
waters, particularly during La Niña (median=22.6)C,
relative frequency of occurrence=0.29, misclassification
rate=0.09; Fig. 2b). In the other years, it was seen in
waters associated with the South Equatorial Current
with a median temperature of 27.1)C (relative frequency
of occurrence=0.11; Fig. 2b). Black-winged petrel,
recorded on 14% of the transects, was also more likely to
occur in the South Equatorial Current, particularly in
areas of higher salinity (median=35.26, relative frequency of occurrence=0.42, misclassification rate=0.13;
Fig. 2c). It was also seen in areas within the South
Equatorial Current that were of lower salinity
596
C. A. Ribic and D. G. Ainley
(a)
Current
ECC
SEC
sst (°C)
<=26.6
>26.6
Year
Normal,
ENSO
LNSO
0.05
0.39
0.42
Year
Normal,
LNSO
ENSO
0.37
Salinity
<=35.09 >35.09
0.11
(b)
0.55
sst (°C)
<=27.7
>27.7
0
Year
Normal,
ENSO
LNSO
0.29
Current
SEC
ECC
0.02
(c)
0.11
(median=35.03) and cooler (median=26.3)C, relative
frequency of occurrence=0.27; Fig. 2c). However, these
salinity and temperature differences did not lead to a
specific water mass being an important discriminator of
species presence.
Group B, the Juan Fernandez/sooty tern/wedge-tailed
shearwater assemblage, was seen on 35% of the
transects, and was most likely to be found in areas with
deeper thermoclines (median=60 m) and lower salinities
(median=34.33, relative frequency of occurrence=0.55,
misclassification rate=0.33; Fig. 3a).
The importance of thermocline depth and salinity
carried over to two of the three individual species. Sooty
terns, seen on 15% of the transects, were most likely to
occur in waters of higher salinities (median=34.99) and
deeper thermocline depths (median=60 m) (misclassification rate=0.14); relative frequency of occurrence was
0.22 (Fig. 3b). Wedge-tailed shearwaters, recorded on
16% of the transects, were associated with areas of
deeper thermoclines (median=60 m) and higher salinities (median=35.13) (relative frequency of occurrence=0.37, misclassification rate=0.14), regardless of
year (Fig. 3c). In contrast, Juan-Fernandez petrels, seen
on 19% of the transects, were seen in waters with the
steepest thermoclines (median=9.8 deg C m "1) (relative
frequency of occurrence=0.34, misclassification rate=
0.19, Fig. 3d).
Group C, a storm-petrel assemblage, was not associated with any of the environmental variables. There was
no assemblage response because, individually, each
storm-petrel was found in association with a different
variable. Overall, Leach’s storm-petrel, seen on 34% of
the transects, had the highest probabilities of being seen
in the tropical and equatorial surface water masses
(misclassification rate=0.31), particularly during years
with no major climatic perturbation (i.e. ‘‘normal’’
years; relative frequency of occurrence=0.44; Fig. 4a).
In contrast, wedge-rumped storm-petrels, observed on
17% of the transects, had the highest probability of
being seen in areas with shallower thermocline depths
Current
ECC
0.06
SEC
Salinity
<=35.16
>35.16
sst (°C)
<=26.4
>26.4
0.27
0.12
0.42
Figure 2. Classification trees for: (a) the black-winged and
white-winged petrels assemblage; (b) white-winged petrel; and
(c) black-winged petrel. The numbers in the boxes are the
relative frequencies of seeing the species as predicted by
the standardized variables in the tree. The average values of the
environmental variables that split the data into more homogeneous units are at the top of each split. ECC = Equatorial
Counter Current; SEC = South Equatorial Current; ENSO =
El Niño-Southern Oscillation; LNSO = La Niña or cold-water
event; normal = no major climatic perturbations; sst = seasurface temperature. The combination of environmental variables that result in the frequencies of occurrence in the boxes
are found by following the splits through the tree. For example,
in (a), the assemblage was seen with a relative frequency of 0.37
in the South Equatorial Current in areas where waters were
warmer than 26.6)C in normal and LNSO years.
Seabird assemblages and physical habitat
597
(median=22 m), regardless of year (relative frequency of
occurrence=0.29, misclassification rate=0.17; Fig. 4b).
(a)
Thermocline depth (m)
<=41.7
>41.7
0.24
Discussion
Salinity
<=34.76
>34.76
0.55
0.32
(b)
Salinity
<=34.29
>34.29
0.01
Thermocline depth (m)
<=39.6
>39.6
0.04
0.22
(c)
Thermocline depth (m)
<=41.4
>41.4
0.04
Salinity
<=34.81
>34.81
0.36
0.12
(d)
–1
Thermocline slope (°C m )
<=8.4
>8.4
0.16
0.34
Ballance (1993), focusing on feeding flocks of seabirds in
the eastern tropical Pacific, defined two groups: flocks
forming at convergences and those of the subsurface
predator-dependent guild. In our study, these two
types of flocks corresponded to the storm-petrel assemblage (Group C) and the Juan-Fernandez petrel/sooty
tern/wedge-tailed shearwater assemblage (Group B),
respectively. Ballance (1993) found that sooty tern
flocks were associated with the deepest thermocline
depths and that Juan-Fernandez petrel/wedge-tailed
shearwater flocks were associated with medium deep
thermoclines. In our study, which considered species
regardless of feeding flocks, deeper thermoclines, along
with medium salinities, defined the areas of greatest
presence of Group B. Individually, this held for sooty
tern and wedge-tailed shearwater, with some specific
differentiation by salinity. The petrel assemblage (Group
A) also had a consistent response to a common environmental feature, in this case the South Equatorial Current. In contrast, the storm-petrel assemblage did not
show any consistency in response to the physical habitat.
King (1970) noted that sooty tern and black-winged
petrel were most abundant in warmer waters and that
Leach’s storm-petrel was most abundant at low air and
water temperatures. However, King (1970) did not find
any relationships with surface salinity, in contrast to our
findings, and did not measure the other variables we
considered.
Species assemblages related to distinct physical
habitats have been identified by many researchers
(Pocklington, 1979; Ainley and Boekelheide, 1983; Ribic
and Ainley, 1988/89; Wahl et al., 1989; Hunt et al., 1990;
Veit, 1995). Hunt et al. (1990) suggested that seabird
assemblages had a common response to the physical
characteristics of the water masses in their study area,
but few researchers have addressed this question in
detail (Wahl et al., 1989; Ainley et al., 1994; Veit, 1995).
Figure 3. Classification trees for: (a) the Juan-Fernandez petrel/
sooty tern/wedge-tailed shearwater assemblage; (b) sooty tern;
(c) wedge-rumped shearwater; and (d) Juan-Fernandez petrel.
The numbers in the boxes are the relative frequencies of seeing
the species as predicted by the standardized variables in the
tree. The average values of the environmental variables that
split the data into more homogeneous units are at the top of
each split. The combination of environmental variables that
result in the frequencies of occurrence in the boxes are found by
following the splits through the tree. For example, in (a), the
assemblage was seen with a relative frequency of 0.55 in areas
with thermoclines deeper than 41.7 m and salinities less than or
equal to 34.76.
598
C. A. Ribic and D. G. Ainley
(a)
Water mass
TSW,
ESW
SSW
0.22
Year
Normal,
ENSO
LNSO
0.44
0.16
(b)
Thermocline depth
<=32.0 m
0.29
>32.0 m
Thermocline depth
<=58.0 m
>58.0 m
considered competitors for food. In the case of the
storm-petrel assemblage in our study, shared foraging
strategies may be the important link for the
co-occurrence of these species (Spear and Ainley,
unpublished). In addition, Ballance et al. (in press) used
thermocline depth as an indicator of productivity and
related the occurrence of the different types of feeding
flocks to a productivity gradient. If thermocline depth
can be used as an indicator of productivity, then we
found that productivity was an important predictor of
the presence of the Juan-Fernandez petrel/sooty tern/
wedge-tailed shearwater assemblage, as well as the
wedge-rumped storm-petrel. Our analyses on prey
species are in progress and, therefore, we cannot discuss
prey availability as a possible influence on seabird
associations here. It is clear that much work remains to
be done in understanding the processes that determine
seabird associations in the eastern tropical Pacific and
elsewhere. Our results thus far do not rule out the
possibility that birds within an assemblage are responding to a common resource base.
Acknowledgements
0.16
0.07
Figure 4. Classification trees for: (a) Leach’s storm-petrel; and
(b) wedge-rumped storm-petrel. The numbers in the boxes are
the relative frequencies of seeing the species as predicted by the
standardized variables in the tree. The average values of the
environmental variables that split the data into more homogeneous units are at the top of each split. TSW=Tropical
Surface Water Mass, ESW=Equatorial Surface Water Mass,
SSW=Subtropical Surface Water Mass, ENSO=El NiñoSouthern Oscillation, LNSO=La Niña or cold-water event, and
normal=no major climatic perturbations. The combination of
environmental variables that result in the frequencies of occurrence in the boxes are found by following the splits through the
tree. For example, in (a), Leach’s storm-petrel was seen with a
relative frequency of 0.44 in Tropical and Equatorial Surface
Water Masses in normal and ENSO years.
We thank the officers and crews of the NOAA ships
‘‘Malcom Baldridge’’, ‘‘Discoverer’’, and ‘‘Oceanographer’’, and personnel of the Pacific and Atlantic Marine
Environmental Laboratories who allowed our participation on their NOAA cruises and facilitated our efforts.
We thank all observers who helped collect the data,
particularly L. B. Spear. We thank T. W. Miller and
L. B. Spear for reviewing an earlier draft of the paper.
We thank two anonymous reviewers for their comments.
Our project was funded by the National Science Foundation, Division of Biological Oceanography (grants
OCE-8515637, -8911125) and National Geographic
Society (3321-86, -89).
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