Download Temporal and Spatial Variation in Species Diversity of Wandering

Temporal and Spatial Variation in Species Diversity of Wandering Spiders
(Araneae) in Deciduous Forest Litter1•2•3
GEORGE W. UETZ
Dept. of Ecology, Ethology, and Evolution, University of Illinois, Champaign 61820
ABSTRACT
A guild of wandering spiders was studied in an oak-tuliptree-maple forest in northern
Delaware. Specimens were collected by pitfall trapping and weather data recorded at
weekly intervals over the summer season (3 months). A seasonal peak in species diversity (R') and species richness in midsummer was significantly correlated with prey
abundance but not with seasonal temperature, humidity, or rainfall. Annual patterns
of detritivore productivity in temperate forests and their influence on niche partitioning
and seasonal abundance of species are discussed as a possible explanation. Spatial differences in species diversity were significantly correlated with the amount of litter and a
measure of habitat space, but not with microclimatic moisture and temperature, vegetative diversity, or prey abundance. Physical aspects of the litter habitat, either as structural microhabitats or refuges from predation, are suggested as being important in regulating within-habitat species diversity. Interaction of diversity-regulating environmental
factors in space and time are discussed.
Well-known latitudinal and successional gradients
in species diversity have received much attention
from ecologists (MacArthur
1965, Pianka 1966,
Uetz 1974a), and current research seeks to explain
why some communities have more species than
others. Increased species diversity in tropical areas
and climax communities has been attributed to climatic stability, long evolutionary history, high productivity, and spatial heterogeneity.
Since factors affecting the structure of communities are varied and often interacting, a better under"
standing of species diversity might be gained by
studies of groups with extremely similar ecological
requirements, i.e., guilds (Root 1973). (A guild is
a group of organisms exploiting a single resource or
type of resource in a similar manner.) Spiders can
easily be divided into several guilds based on methods used to capture prey, and provide interesting subjects for such investigation.
The wandering spider guild (Breymeyer 1966) is
composed of the families Clubionidae, Gnaphosidae,
Lycosidae, Pisauridae, Thomisidae, and some representatives of the Agelenidae and Hahniidae. These
families are similar in size, general body form, and
mode of prey capture. The Thomisidae, or crab
spiders, are an exception with regard to body form
(legs I and II are laterograde rather than prograde)
but have been observed to forage in the same manner
as other spiders-running
down or pouncing on
prey. Wandering spiders are common in most com-
in community energy flow (Moulder and Reichle
1972).
As spiders are small animals, differences in species
diversity may be found within a single habitat. In
this study, spatial and temporal distribution of foraging of wandering spiders was sampled by pitfall
trapping and the data used to calculate indices of
diversity. Patterns of species diversity in space and
time were compared with several aspects of the litter
environment in an attempt to determine which factors regulate species diversity in this guild.
munities, inhabiting the ground and lower vegetation.
placed in the bottom as a preservative.
They are abundant and diverse in forest litter microcommunities (Norton 1973), constituting more than
43% of ground-dwelling spider species (Drew 1967).
Feeding at several trophic levels in the detritus food
webs of forest litter, wandering spiders are important
located at random within 50x50-m grids. They were
emptied at weekly intervals over the summer season,
from June to September 1970.
There has been considerable discussion about the
interpretation of ecological information obtained by
pitfall trapping (Breymeyer 1966, Turnbull 1973,
1 This study was supported
by a USDA For. Servo McIntireStennis Grant awarded to the Univ. Delaware Agric. Exp. Stn.
• This paper represents a portion of a thesis submitted in
partial fulfillment of the requirements for the degree of Doctor
of Philosophy at the Univ. Illinois.
"Received for publication Feb. 24, 1975.
719
Methods and Study Area
The University of Delaware Experimental Woodlot, located in the Agricultural Experiment Station
Farm in Newark, Del., was the site of this investigation. The woodlot is a 35-acre tract of mature secondary growth forest over a silt loam soil on relatively
level topography. Oaks, tuliptree, and maples are
common canopy trees. Vegetation, soil, and microclimatic factors in this woodlot have been described
in more detail by Preiss (unpubl.)'. The woodlot provides a mosaic of environments for forest floor
arthropods, through variable microtopography, moisture, herbaceous cover, litter depth, and litter structure.
Fifteen pitfall traps were used to collect wandering spiders. A trap consisted of a plastic cup (9 cm
diam, 12 em depth) buried in the soil, the open end
flush with the soil surface. Ethylene glycol was
Traps were
• Preiss, F. J. 1967. Nest site selection, microenvironment,
and predation of yellow jacket wasps. Vespula ",aculifrolls (Busson), (Hyrnenoptera:Vespidae)
in a deciduous Delaware woodlot. M.S. thesis, Univ. Delaware, Newark.
720
ENVIRONMENTAL
Uetz and Unzicker 1975). An activity-based trapping method actually samples a quantity that is the
product of activity and density, referred to as "penetration" (Breymeyer 1966) or "active density" (Uetz
and Unzicker 1975). These workers have found pitfall trapping to be an adequate sampling method for
studies of species diversity but recommend limiting
its use to cursorial groups. Despite drawbacks it
remains the best available means of sampling wandering spiders.
Data obtained from pitfall traps, representing
frequency of foraging activity or active density, was
used to, calculate species diversity indices in time (as
weekly samples from the whole woodlot) or in space
(seasonal totals of 15 trap ]ocalities). Traps were
operative for 12 wk, providing 15 trap-wk of information each week and a total of 180 trap-wk for the
summer.
Species diversity has 2 components: (1) species
richness-the
number of species, and (2) equitability-or
the relative equality of species abundance.
Diversity is high when there are many species and!
or when the sample is composed of several equally
abundant species. Diversity in a time period or ]ocation was measured by a count of the number of
species (species richness) or by calculation of a commonly used "information" index that is sensitive to
differences in equitabi]ity. The Shannon-Weiner information formula (Shannon 1948) :
s
=-
H'
~
Pi ]Og2 Pi
i=1
(Pi = proportion of total individuals in species i;
s = number of species) was used to calculate diver-
sity. Evenness (Pie]ou 1966) was also calculated as
an estimate of the equitability component of diversity:
J
Vol. 4, no. 5
ENTOMOLOGY
Gnaphosidae
Hahniidae
Lycosidae
Lycosidae
Pisauridae
Thomisidae
Phrurotimpus alarius (Hentz)
Phrurotimpus borealis (Emerton)
Phrurotimpus minutus (Banks)
Drassodes neglectus (Keyserling)
Drassyllus virginianus Chamberlin
Gnaphosa fontinalis Keyser]ing
Litophyllus rupicolens Chamberlin
Orodrassus sp.
Sergiolus capulatus (Walckenaer)
Sergiolus variegatus (Walckenaer)
Antistea brunnea (Emerton)
Arctosa virgo (Chamberlin)
Lycosa avara (Keyserling)
Lycosa gulosa Walckenaer
Lycosa helluo Walchenaer
Pardosa saxatalis (Hentz)
Pirata arenicola Emerton
Pirata insu/arius Emerton
Pirata marxi Stone
Pirata sedentarius (Comstock)
Schizocosa crassipes (Walckenaer)
Schizocosa ocreata Emerton
Pisaurina mira (Walckenaer)
Dolomedes tenebrosus Hentz
Oxypti/a americana Banks
Xysticus elegans Keyserling
Xysticus fraternus Banks
Species richness, species diversity (H'), and evenness (1) rose to a peak in midseason (July), then
dropped to lower levels (Fig. 1). The seasonal curve
of wandering spider diversity (H') is due mainly to
differences in the number of species collected each
week rather than to differences in equitability among
species. About 63-65% of the variation in species
= H'obs! H' max
Environmental data from concurrent studies was
made available by several other biologists. Microclimate data (temperature, rainfall, humidity) was
availab]e from weather stations maintained by the
University within the woodlot. Soil data and vegetation analysis data were available from previous
studies and from 1970 census data (Robert E. Jones,
pers. comm.). Litter depth and structure data were
taken by the author as described in Uetz (1974).
~
·~2.0
'Ii
.'"(S
~ 1.0
.~
~
Results
A total of 5200 specimens was collected, representing 8 families, 22 genera, and 34 species of wandering spiders, as follows:
Family
Agelenidae
Anyphaenidae
Clubionidae
1.0
I"'J
~
Species
Cicurina robusta Simon
Aysha gracilis (Hentz)
Agroeca minuta Banks
Agroeca pratensis Emerton
Castaneira cingulata (Koch)
Castaneira longipalpus (Hentz)
Clubiona kastoni Gertsch
.5
5
:::
<II
L
June
July
August
1.-Seasonal patterns in spider species diversity
species richness, and evenness (1) compared with
the abundance of prey.
FIG.
(H'),
October 1975
UETZ:
VARIATION
IN SPIDER
SPECIES
721
DIVERSITY
Table I.-Correlation coefficients for indices of spider
diversity and environmental factors in space or time.
H'
r = .850
~<.005
No.
species
J
o
o
Seasonal factors: (10 df)
x weekly temperature
x weekly RH%
x weekly rainfall
x prey numbers
.075
.069
.041
.808*
.069
.071
.069
.600*
.089
.053
.045
.794**
.042
.219
.368
.405
.371
.093
.112
.331
.365
.372
.891**
.850**
.125
.335
.374
.391
.850**
.872**
.108
.224
.263
.281
.739**
.694**
o
Spatial factors: (13 df)
soil moisture %
soil organic matter %
soil ten1perature
(weekly range)
herbaceous cover
tree species H'
herb species H'
litter depth
litter habitat space
• .oJ < p <
< .01.
•• P
~
-..::.
1.0
r
=
.872
~<.005
I/)
~
c;:
.5
~
Lt
.05 .
richness and species diversity was explained by the
abundance of prey arthropods.
Correlations between weekly prey numbers and diversity (H', and
o
r = .694
P2< .005
o
~
o
o
~ 3.0
tl
~
0
0
~
0
r = .891
~2.0
Habitat
~< .005
.~
)
FIG. 3.-Relationship between species diversity measures and litter habitat space.
.~
III
C5
2
Space (cm3/cm
1.0
I/)
.lll
~
species richness) were significant, while weather
factors were not significantly correlated with any of
the diversity measures (Table 1).
Species diversity of trap localities was related to
the quantity of litter habitat available at each site.
Diversity, evenness, and species richness were significantly correlated with litter depth (Fig. 2) and
habitat space (S') (Uetz 1974b), a measure of interstitial volume (Fig. 3). Data on moisture, temperature, soil organic matter, herbaceous cover, vegetative diversity, and prey abundance were not significantly correlated with measures of spider diversity
(Table 1). Spatial differences in H' were due mainly
to variation in equitability rather than variation in
species diversity.
III
~
•.•...• 1.0
~
':::
III
c;:
c;:
r = .850
~<.005
.5
~
~
0
I/)
10
r = .757
.lll
~
~<.OO5
III
~
.•..tl
Discussion
5
0
~
2
3
Litter Depth (em)
4
5
FIG. 2.-Correlations between species diversity (H'),
evenness (1), species richness, and litter depth.
The temporal patterns of species diversity observed in this study (Fig. 1) are consistent with other
studies of temperate arthropod communities (Evans
and Murdoch ] 968, Lewis ]969, Pimentel and
Wheeler 1973, Root 1973), where the greatest number of species was found in midsummer. The strong
correlation between spider species diversity and prey
abundance supports the hypothesis of Pianka (1974),
722
ENVIRONMENT
Arthropod
/
,'''
biomoss
;-,/
,
\
I
Litter
energy and
nutrients
.........f
".
f
f
."
/
/
"/.
/
"
"
I
'.
'.
...................
..
'
JFMAMJJASOND
FIG. 4.-Seasonal
patterns in arthropod biomass and
numbers in temperate deciduous forest litter (drawn
from Moulder et al. 1970). Hypothetical annual pattern
of litter decomposition constructed from information in
Bocock (1964) and Ovington (1965).
who has suggested that more productive habitats (in
this case, time periods) can support more species of
foraging animals. With an increase in the standing
crop of insect prey, each species may utilize less of
the total range of available food and so more species
can coexist. Since wandering spiders may partition
prey resources by temporal stratification (Luczak
1959, Breymeyer 1966), we might expect that the
maximum tolerable niche overlap could be greatest
at times when food is most abundant. The 34 species
collected in this study are evenly divided into "early"
and "late" occurring species, as in Berry (1971), although there are varying degrees of overlap in temporal occurrence (discussed in a separate paper in
prep.) . Coexistence of more species during times
of increased prey abundance is apparent in the seasonal pattern of the species richness component of
diversity. The equitability component of diversity,
though significantly correlated with prey numbers,
contributes less to the seasonal pattern in H'.
It is interesting that no significant correlations were
found between diversity and seasonal weather factors.
An annual increase in spider species diversity to a
peak in. midseason and a decline as autumn approaches gives the outward appearance of being
related to annual patterns of temperature. Because
of the trophic position of spiders, it is possible that
diversity is related to seasonal patterns in energy
and nutritent release in the detritus food web. Using
data from several studies of temperate forest litter
community energetics and nutrieut cycling (Bocock
1964, Ovington 1965, Moulder et al. 1970), I have
constructed a hypothetical diagram of these patterns
(Fig. 4). The seasonal peak of litter arthropod numbers occurs in July in the diagram, which is similar
to the data found in this study. It seems probable
that seasonal patterns of forest productivity, governed
AL ENTOMOLOGY
Vol. 4, no. 5
by weather, could regulate the seasonal species diversity of predatory arthropods like spiders through
annual fluctuations in prey abundance. Such an explanation is consistent with current ideas about growing season length, productivity, and latitudinal gradients in species diversity (Pianka 1967, Janzen 1967,
Willson 1973). In lower latitudes where the growing
season is longer and forest productivity is spread
more evenly over the seasons, coexistence of a greater
number of species may be possible due to temporal
resource partitioning .
The species diversity of wandering spiders in different areas within the woodlot is correlated with the
depth of litter (Fig. 2) and the amount of interstitial space within the litter (Fig. 3). Several previous studies have found increased spider abundance
with increased litter depth (Lowrie 1948, Hagstrum
1970, Berry unpubl."), but species diversity has not
been mentioned. The litter habitat may affect spiders
by providing prey, reducing microclimatic temperature fluctuations, maintaining moisture, providing
structural retreats, and introducing a heterogeneous
substrate for refuging from predation. Since no correlations were found between spider diversity and
moisture, temperature, or prey abundance, the physical structure of the litter might be important.
Physical complexity of environments has been correlated with diversity in birds (MacArthur and MacArthur 1961, Cody 1968), lizards (Pianka 1967),
molluscs (Kohn 1967), crustaceans (Abele 1974),
and insects (Murdoch et al. 1972, Turner and Broadhead 1974). Species diversity was found to be
greater in habitats with more vertical strata or more
complex substrates. As litter depth and complexity
increase, a greater variety of structural microhabitats
may become available, enabling more species to coexist. Also, it is possible that species could partition
the habitat vertically (Huhta 1971). Thus, microhabitat diversity and vertical stratification would tend
to increase species richness. However, differences in
species diversity (H') between trap localities appear
to be largely due to variation in the equitability component. Evenness (1) correlates strongly with the
habitat space measure, suggesting that the litter environment regulates spider species diversity by equalizing the relative abundance of species, as well as by
adding species. Spatial heterogeneity in the litter
environment might influence the relative abundance
of spider populations by affecting predatory interactions within and between species. Predation on
young spiders by adults of the same or other species
is common in wandering spiders (Breymeyer 1966).
Edgar (1969) found experimentally that a 3-dimensionallitter substrate reduced cannibalism in Pardosa
/ugubris (Walckenaer).
Increased survivorship of
immatures could account for greater equitability in
areas with greater litter depth, thus contributing to
increased diversity.
The diversity of species found in a single area of
5 Berry, J. W.
1967. The distributional ecology of spiders in
the old-field succession of the Piedmont region of North Carolina. Ph.D. thesis, Duke Univ., Durham, N.C.
October
1975
UETZ; VARIATION IN SPIDER SPECIES DIVERSITY
5.0
10
FIG. 5.-Hypothetical
sity in time and space.
gradient of spider species diver-
a larger habitat has often been referred to as "point"
diversity (Slobodkin and Fishelson 1974), or "alpha"
diversity (Whittaker
1969).
In this study, samples
of spider diversity were taken at points in space and
in time. Although 34 species were collected from the
entire area, no more than 12 were present at any
point. We might predict that factors influencing species diversity in either space or time might interact
to create gradients of species diversity within a single
forest (Fig. 5) as observed in this study. More information
is needed to assess the role of any of the
factors
discussed,
as correlation
does not imply
causation.
Future studies, with manipulation
of one
or more of the variables, will give a better understanding of the factors regulating species diversity in
spiders.
Acknowledgment
I wish to thank the Department
of Entomology
and Applied Ecology, University
of Delaware,
for
the opportunity
to do this research,
and especially
R. E. Jones, E. Catts, R. Roth, and J. Steinhauer for
providing
me with environmental
data from their
own research.
I also wish to thank P. W. Price, L.
L. Getz, J. D. Unzicker,
G. C. Kulesza, and A. L.
Noble for their criticisms of the manuscript.
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