Bird Predation On Spiders: Ecological Mechanisms And Evolutionary Consequences

2007·24.2 MB·English

by  Bengt Gunnarsson

2007 (2008). The Journal ofArachnology 35:509-529
REVIEW ARTICLE
BIRD PREDATION ON SPIDERS: ECOLOGICAL MECHANISMS AND
EVOLUTIONARY CONSEQUENCES
Bengt Gunnarsson: Department ofPlant and Environmental Sciences, Goteborg University,
Box 461, SE 405 30 Goteborg, Sweden. E-mail: bengt.gunnarsson(gdpes.gu.se
ABSTRACT. Birds are common predators ofarthropods in many ecosystems but their impact on spiders
has not been assessed. Therefore, the experimental evidence for bird predation effects on spider populations
wasexamined. Inparticular, thepresent review focuses on thequestions: what aretheecologicalmechanisms
and what are the evolutionary consequences? Data from 17 field experiments, mainly in forest ecosystems,
showed that spider communities were often significantly affected by bird predation. Comparisons of
experimental effects were based on the ratio ofmean density on experimentally enclosed vegetation and on
controls. In 27 tests, a significant effect was detected (mean ratio 3.03) but in 9 tests the effect was non-
significant (mean ratio 1.03). Furthermore, field experimental studies on bird predation effects on certain
spiderspeciesorcertaingenerawerereviewed. Inthreeinvestigations, significantpredationeffectswerefound
on agelenid, linyphiid and theridiid spiders but there were no significant effects on lycosids. Selective bird
predation on large individuals has been shown in studies on spider communities and single species. Data on
bird predation effects on species richness were lacking although impact on large species was expected to be
important. Three field experiments showed that different spider families may experience differences in bird
predation pressure. An aviary experiment showed that frequently moving spiders had a higher risk of
predation than sedentary individuals, but the evidence from field experiments supporting the hypothesis of
high predation pressure on moving spiders was limited. This included sex-specific differences in size and
movement, althoughatleastoneexperimentshowedthatmaleshadhigherwintermortalitythanfemales. One
experiment showed thatbird predation can affect anti-predatorbehavior. Inconclusion, thepresentevidence
showed that bird predation on spiders in several contrasting forest ecosystems is strong. However, there are
many hypotheses regarding bird predation on spider populations that should be examined in future field
experiments.
Keywords: Avian predation, field experiments, natural selection, spider community
Birds in terrestrial habitats consume insects Arachnologists have long acknowledged
and other arthropods in large quantities. This birds as a potential threat against spider
fact has recently led a number of authors to populations but they have had different opin-
emphasize the importance ofbird predation as ions about their importance. In the first edition
part of the “ecological services” that human of “Biology of Spiders” Foelix (1982) stated
societies rely upon (e.g., Sekercioglu et al. 2004; that “The influence of birds as a factor
Fayt et al. 2005; Sekercioglu 2006). Many controlling spider populations is generally
studies have focused on the economic impor- overestimated.” He argued that spiders often
tance because of the large values that can be hide, sit motionless, are cryptic, or are active
gained by reduction of insect herbivore popu- during the night and, therefore, spiders would
lations in agricultural areas or managed forests bepoorlyperceived bybirds. Moreover, he said
(Takekawa & Garton 1984; Mols & Visser that adult birds rarely eat spiders but certain
2002). However, insectivorous birds are often species may feed spiders to their young. There
generalists and may feed on many trophic were no references to support these claims. In
levels. This means that birds are not only the second edition, Foelix (1996) added that
eating herbivores that are damaging human “spiders are amajorpreyformanybirdsduring
food or other resources, but also include in winter” and referred to the works by Askenmo
their diet arthropods (e.g,, spiders) that other- et al (1977) and Hogstad (1984). Thus, results
wise compete with birds for insect prey. from experimental studies have gradually been
509
510 THE JOURNAL OF ARACHNOLOGY
accepted in textbooks but still much ofthe data selective agents on spider populations is sum-
referred to are from old, mainly descriptive marized. Finally, bird predation on spiders is
studies. Wise (1993) in his book on spider evaluated in an ecological and evolutionary
ecology discussed birds as important natural context and this also leads to new hypotheses.
enemies of spiders and referred to several
SPIDERS AS BIRD PREY
experimental works. He concluded ‘'birds take
substantial numbers of overwintering spiders Bristowe (1941) referred to several field
from spruce branches, and birds have been observations of birds eating spiders, especially
implicated as important predators upon large in investigations on the effect ofbirds on pests,
orb weavers in tropical forests.” Thus, in this in the early 20th century. However, the
book birds are rated among the most promi- importance of spiders as prey of birds has
nent predators on spiders. However, in more mainly been supported by analyses ofstomach
recent papers, authors have questioned whether contents and fecal samples. In his review of
birds really are important as mortality agents spider enemies, Bristowe (1941) evaluated the
(e.g., Blackledge et al. 2003). importance of bird predation as a mortality
The present review is not intended to be an agent of spiders mainly by using data from
exhaustive evaluation of all papers discussing stomach analyses. The stomach data were used
the importance ofbirds as predators on spiders to estimate averagenumbers ofspiderseatenby
or their egg-sacs. There have been many various bird species suggesting, e.g., that a
conclusions about the effects ofbird predation blackbird would eat 106 spiders per year. In
on spider populations based on rather poor further calculations, various assumptions and
data or interpretations that go beyond the data rough estimates about population sizes ofbirds
at hand. Instead, the focus in the present paper and spiders led to the conclusion that birds in
will be on the experimental evidence for the England and Wales did not pose a serious
impact of birds on spiders in natural popula- “threat” to spider populations (Bristowe 1941).
tions. Experiments in field ecology have be- In terms of population dynamics this means
come increasingly popular as a way to examine that Bristowe suggested that birds did not
hypotheses about predation in different habi- control (regulate) spider populations.
tats (Hairston 1989). Many of the conclusions Stomach analysis has often been used to
in Wise’s (1993) book on spider ecology were identify the number of individuals of various
based on experimental evidence. The experi- prey items. For instance, in a quantitative
mental approach has certain advantages. Ex- examination of prey of goldcrests {Regulus
periments provide good insights into cause and regulus) in winter, Hogstad (1984) suggested
effect and are a means to test hypotheses. This that, on average, 60% of the individual prey
approach may also help to disentangle complex were spiders. In mid-winter (January and
interactions in natural habitats. It was in the February) the percentage increased to about
1970s that field experiments of bird predation 80%. On the other hand, spiders can be quite
on spider populations became well established rare in other birds’ diets. In an analysis of the
in the ecological literature (e.g., Askenmo et al. stomachs of the meadowlark {Sturneiia magna
1977; Holmes et al. 1979). These pioneering argutulla) in Florida, Genung & Green (1974)
experiments and their successors will be the found a few remains oflycosids in 12.7% of63
focus ofthe present review. birds examined.
Two questions of importance will be high- Another possible approach was to examine
lighted: what are the ecological mechanisms of fecal samples. The feces were easier to sample
bird predation on spiders, and what are the and the sample size could often increase
evolutionary consequences? In a previous considerably in comparison to stomach sam-
review of bird predation on arthropods. ples. It was also convenient because birds were
Holmes (1990) emphasized these two functions not harmed by sampling. For instance, in a
of bird predation. First, I will present some study of the threatened California gnatcatcher
work on birds’ diets. Second, experimental (Polioptiia c. californica), fecal samples showed
studies on bird predation effects on spider that the adult gnatcatchers selected more and
assemblages will be scrutinized. Third, investi- larger prey, including spiders, to feed their
gations of bird predation on single species are chicks than adult birds ate themselves (Burger
reviewed. Fourth, the importance of birds as et al. 1999). However, there was constantly a
GUNNARSSON—BIRD PREDATION ON SPIDERS 511
risk that estimations ofprey numbers would be which means that even small effects on
biased. In a study ofthe diets ofthe willow tit individual species are combined into the
{Pams montanus) and crested tit {P. cristatus), observed grand mortality. There were several
Jansson (1982) showed that numbers ofspiders experimental field studies ofbird predation on
and beetles were overestimated in fecal samples arthropod populations in contrasting habitats
in comparison with estimates of prey brought between 1977 and 2007 (Table 1). In certain of
to the nest by using color photographs from these experiments the focus was on spider
cameras at the nest entrance. populations and in others, in which many
Both stomach analysis and fecal samples arthropod taxa were studied, the results for
have a number of important limitations when spiders were reported separately.
—
assessing the importance of bird predation. Methods. All the studies employed experi-
First, the digestion ofprey makes it difficult to mental methods using different types of ex-
correctly assess the numbers of various prey closures to prevent birds from foraging. Typ-
taxa. It is true that many arthropods have ically, the exclosures covered some part the
distinctive sclerotized body parts but still it vegetation in which spiders and other arthro-
could be hard to provide counts ofindividuals pods were dwelling. Nearly all studies utilized
based on different numbers ofcheliceral fangs, nets with coarse mesh size (10-58 mm) so that
tarsal claws, leg parts, etc. Second, biased data arthropods were free to leave and enter the
will lead to incorrect estimates ifit is magnified exclosures without restrictions. Possibly some
several times. Samples including a few bird large arthropods such as adult butterflies may
individuals will not be sufficient for an unbi- have been hindered in their movements by the
ased estimate of the predation pressure on the netting, but in general the authors considered
arthropod prey. Third, the relevance of bird such effects as negligible. In a few cases, fine
predation on spider populations is dependent mesh size (1 X 1 mm) was used but those
on local abundance and this is usually un- experiments were performed during the winter
known in studies based on ingested food. Ifthe when movements of spiders and their prey are
population size ofthe spidersis rathersmallbut usually minimal.
increasing, then relatively moderate predation A factor that usually is not controlled for in
pressure can cause a significant decline of most of the experiments concerns the intro-
spiders locally. On the other hand, even high duction ofan additional structural component
predationpressurebybirdswouldnotinfluence to the habitat in the form of the net. This is a
anextremelylargepopulationto any significant classical problem in exclosure experiments in
extent. These types of interactions between other habitats and taxa (Hairston 1989). The
birds and their arthropod prey have been primary disturbance caused by addition of the
observed in several study systems in forests extra net-structure may be to support more
(e.g., Crawford & Jennings 1989; Holmes web building spiders (Rypstra 1983; Green-
1990). Different spider populations probably stone 1984). This could increase the carrying
show a similar response to varying bird capacity of spider populations if there is
predation pressures. In order to evaluate the shortage of sites for attaching webs in the
importance of bird predation on local spider study habitat (Robinson 1981; Greenstone
populations, some estimate of the effect of the 1984; Wise 1993; Rypstra et al. 1999). How-
predation must be made, preferably by com- ever, there are reasons to believe that the bias
paring data on spiders exposed to birds vs. due to the extra net-structure is rather weak.
spiders protected from bird predation. First, nearly all experiments have been per-
BIRD PREDATION ON SPIDER formed in trees or bushes with abundant
foliage (including leaves, needles, branches,
ASSEMBLAGES COMMUNITIES
/ twigs etc.) for attaching webs. There is,
Studies on spider assemblages, or communi- therefore, no reason to expect that structure
ties, offer good opportunities to evaluate the in itself is limiting in the studied habitats, but
impact of bird predation. The advantages of it would be highly welcome if appropriate
such studies are that it is usually easier to study controls using sheets of netting (without
the effects of manipulated bird predation enclosing the foliage) were included in future
pressures on entire communities than single experiments. However, in at least one experi-
species. The response is the sum of all spiders mental study the authors used procedural
512 THE JOURNAL OF ARACHNOLOGY
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514 THE JOURNAL OF ARACHNOLOGY
controls (Van Bael & Brawn 2005). These the ground. In these studies, spider densities
dummy exclosures had open sides that allowed were related to number of leaves, leaf area, or
birds to forage in the foliage. There was no leafmass.
indication that the dummy exclosures attracted In five studies, moderately high trees were
arthropods, or reduced the light. Second, birds used for experimental purposes. Saplings of
could always reach spiders sitting in their Eucalyptus spp. up to a height of 3 m were
retreats or on their webs attached to the sampled in Australia (Evelegh et al. 2001
outside or on the inside of the cage or net. Recher & Majer 2006) and in these cases the;
This ability ofbirds to reach spidersjust inside number ofarthropods was counted per sapling
the exclosure, ofcourse, depends on mesh size without compensation for size differences.
but nearly all studies have used a coarse mesh Other investigations were done in coffee
size which should be penetrable by bills of plantations, either in shrubs of Coffea arabica
insectivorous birds. If spiders prefer to place (Greenberg et al. 2000) or in branches of Inga
their webs close to the net this could lead to an trees 3^ m above the ground (Philpott et al.
underestimate of bird predation pressure. 2004). In Hawaii, spider populations in 2-3 m
Third, in some of the studies the effects of tall trees of Metrosideros were examined for
extra net-structures were possibly minimized bird predation effects (Gruner 2004). The
by a long experimental period. For instance, if densities of spiders in these three studies were
the study was performed in temperate or related to the mass offoliage.
boreal zone with cold winters, interaction One study was done in several rain forests in
between the net-structure and spiders is Panama (Van Bael & Brawn 2005). The height
probably minimal during cold periods.
— ofthe tree top canopies varied between 30 and
The database on bird predation effects. The 40 m above the ground and in some ofthe sites
studied habitats are mainly forests in different
the predominant tree species were Anacardium
successional stages with shrubs and trees as
excelsum, Brosimum utile and Manilkara biden-
experimental units. Coniferous trees as well as
tata. The tree tops were reached by using
broadleaf species have been examined. Three
canopy cranes with a 50-m tower that had a
studies were from coniferous forests in south-
gondola attached to a boom. In this way, the
ern Scandinavia with Picea abies as the
predominant tree (Askenmo et al. 1977; Gun- investigators could reach tree branches on all
heights and each crane covered between 0.70
narsson 1983, 1996). The investigations focused
and 0.88 ha of the forest canopy. The arthro-
on branches in the lowerpart ofthecanopy (up
m pod densities were estimated as numbers per
to 4 above the ground) that were reached by
leaf area.
using a transportable platform. Spider densities
were related to the branch-mass. Three studies One of the studies was performed in open
in Colorado were also in coniferous forest. In land in Oregon. In this experiment, plants of
one case, the samples were all from rather small Artemisia tridentata were manipulated to ex-
Pinus trees, not taller than 3 m (Mooney & amine the effects by bird predation and plant
Linhart 2006). In the other experiments, pines chemistry (Wiens et al. 1991). Finally, Rypstra
were between 4.5 and 13.5 m tall (Mooney (1984) conducted an experiment with a stan-
2006, 2007). The spiderdensitiesweremeasured dardized number of spiders placed in the
as number of animals per tree (Mooney & vegetation of a broadleaf forest in temperate,
Linhart 2006), or biomass per branch (Mooney subtropical, and tropical environments. The
2006, 2007). disappearance rates of spiders were used for
One early study on bird predation on comparison between treatments.
arthropods was done in a broadleaf forest in The studies ranged over five geographic
New Hampshire (Holmes et al. 1979). Howev- regions. There were five experiments performed
er, the experiment was performed in understory in Northern Europe (Sweden and Finland), five
shrubs {Acer pensylvanicum). In Northern in North America (New Hampshire, Oregon,
Europe, Sipura (1999) investigated bird preda- and Colorado), three in Central America
tion effects in Salix shrubs and Gunnarsson & (Mexico, Guatemala, and Panama), one in
Hake (1999) examined arthropods on branches Hawaii and two in Australia. In addition, one
in canopies of Quercus and Betula trees. A experiment was performed on three continents
“sky-lift” was used to sample up to 6 m above (USA, Peru, Gabon).
—
GUNNARSSON—BIRD PREDATION ON SPIDERS 515
Five of the studies were long-term with The data taken together could be used for a
an experimental period over 12 months and rough estimation of the percentage of experi-
another study continued for 11 months. These mental tests that resulted in significant effects
investigations covered at least an annual cycle of bird predation on spider densities. A study
and thismeans that the data were not the result that reported the most dramatic effect (25-80-
of a temporary predation effect. The seasonal fold increase) was excluded because it focused
variation in bird predation pressure could on a single, invasive species (Gruner 2005).
potentially be large because ofvariation in bird Overall, 63% ofthe tests indicated a significant
density (e.g., seasonal migration) orintensity of reduction in densities of spider communities.
foraging (e.g., raising hatchlings). It has also This suggests that bird predation could be
been proposed (Askenmo et al. 1977) that bird regarded as a potentially important mortality
predation could be strong during the non- agent.
breeding season because of non-favorable It should benotedthatnearly allexperiments
climate in winter. This means that non-migra- examined report on bird predation effects in
tory birds have to increase their efforts of forest ecosystems or in habitats with dense
finding food in winter to meet their energy vegetation of bushes, often including at least a
demands (e.g., Norberg 1978). Some of the few trees. Only Wiens et al. (1991) reports on
studies focus on the winter situation, or the dry bird predation in an open habitat with sage-
season, and the effects on prey populations. brush {Artemisia tridentata). This could be
Other studies were short-term experiments explained in two ways: either there have been
focusing on the direct effects on arthropods few experiments performed in open habitats, or
during summer, or the breeding season. the experiments have mostly produced non-
ExperimentalresultsatthecommunityleveL significant results that have been difficult to
Summarizing the data in Table 1, it can be palusbolisphossiinblreefetrheaetdajounrunmalbse.rItofis,stoudfiecsourpseer,-
concluded that bird predation often significant-
ly reduces spider densities. Comparisons were formed in forest habitats have remained un-
published because they did not show any
based on the ratio of mean density on
significant results. However, several studies
experimentally enclosed vegetation and con-
have focused on the effects on the arthropod
trols without such restrictions. Data showed
populations in the study habitat. This means
that spider densities on plants protected from
that a large number oftaxa are often analyzed,
bird foraging significantly exceeded controls in
some groups showing a significant response to
ratios between 1.25 and 8.93. The mean ratio
bird predation others showing no significant
for experimental tests with a significant effect
was 3.03 (SD 1.93, n = 27). This estimate rsetusdpyo,nsew.hicPohssiinbcllyu,detshissevteyrpael oafrtehxrpoeproidmetnatxaal,
includes more than one test in each paper
reduces the risk of “hiding” non-significant
because many ofthe experiments reported were
results, because a significant effect by birds on
divided into separate investigations on different
at least one arthropod group often justifies
tree species and sites, etc. The studies included publication. In published studies, spiders are
could be regarded as independent tests, al- often among the taxa that are numerically
though in some cases data were analyzed in an affected by birds. In some experiments, bird
overall ANOVA. The mean ratio in tests with a predation effects were detected in spiders but
non-significantresultwas 1.03 (SD0.35, n = 9). not in other arthropods, or in few other taxa.
Six studies, however, could not be included in Forinstance, Eveleghetal. (2001) examinedthe
the density ratios. Two studies (Rypstra 1984; effects of bird predation on 19 different
Wiens et al. 1991) were not included because of arthropod orders of Eucalyptus saplings but
the experimental design (see below), and in two they only found evidence for a significantly
cases it was not possible to calculate any ratio higher density of spiders in net-protected
because no data were presented (Sipura 1999; canopies. On the other hand, Gunnarsson &
Mooney & Linhart 2006). Finally, in two Hake (1999) found that bird predation nega-
studies, there were no density estimates provid- tively affected arthropod abundances in six out
ed; instead the biomass was estimated from of nine orders (including spiders) examined in
spider length distributions (Mooney 2006, birch {Betulapendula) and oak {Quercus robur)
2007). canopies.
516 THE JOURNAL OF ARACHNOLOGY
The experimental studies by Rypstra (1984) reveal how the predation affects life-history
on predation (predominantly by birds) on web- traits. There are few studies focussing on the
building spiders in three continents, and by effects of bird predation on a specific spider
Wiens et ah (1991) of bird predation on species orgenus. Here, Iwill reviewfourstudies
arthropods in sagebrush {Artemisia tridentata) that employ experimental methods. The first
were not included in the mean value of study concerns the funnel-web spider Agele-
predation effects reported above. The reason nopsis aperta (Gertsch 1934) in two distinct
for excluding them is that the comparison was habitats in Arizona (Riechert & Hedrick 1990).
not based on control vs. experimental density. The funnel functions as a retreat that protects
In Rypstra’s study (1984), a fixed number of the spider from predatory attacks.
spiders (240 individuals in four groups in each The two study sites included a desert
ofthe study sites) were placed in forest sites and grassland and a riparian habitat in woodland,
their survival during 6-h periods was measured. respectively. In their study, Riechert & Hedrick
Thus, there was no estimation of the natural (1990) estimatedpredation rates onA. apertain
density, or the effect on the spider community. two ways. First, in each study site an area of
However, Rypstra (1984) could show that the 900 m^ was enclosed with sheet metal flashing.
relative disappearance rate of spiders was All spiders in webs were marked and pitfall
higher during the daytime than at night at the traps at regular intervals inside and outside the
Peru (subtropical forest) and Gabon (tropical metal flashing made it possible to detect
forest) sites but not at the site with temperate migrating individuals that were marked and
forest in the USA. She concluded that preda- released on the opposite side of the metal
tion by vertebrates (most likely birds) was flashing. Second, a field experiment was per-
significant on web-spiders during the daytime formed by protecting certain webs from bird
in subtropical and tropical sites. predation by means ofnetting. The survival of
Wiens et al. (1991) used an approach that individuals at protected webs was compared
differs from all other experiments reported on with those at unprotected webs in 1-week
here; i.e. arthropods were first removed from periods.
shrubs and the recolonization ofdifferent taxa In the population monitoring, the losses of
on caged or exposed shrubs was observed. No individuals differed significantly between the
exact calculations for arachnids were given in habitats. In the riparian site, spiders disap-
the text. However, raw data provided in the peared at a rate of 63% to 72%. However, in
paper’s Appendix 1 could be used to assess the the grassland area, the disappearance rate
importance ofbird predators on the abundance varied between 4% and 10%. The experimental
of spiders on sagebrush. The relevant compar- exclusion of bird predation resulted in large
ison is the one between shrubs that were caged differencesindisappearance rate(30% and 51%
and open shrubs because both types of shrub difference per year) between protected and
were defaunated at the same time and the only unprotected webs in the riparian habitat, with
difference was caging. Wiens et al. (1991) used higher survival ofspiders in protected webs. In
several sub-experiments in their design and the the grassland habitat, there were no significant
longest experimental period was 56 weeks. The differences in spider survival at protected and
mean value forcaged shrubs after 56 weekswas exposed webs. Consequently, bird predation on
1.89 spiders and for open shrubs the mean was A. aperta seemed to be important in the
1.15 spiders. This difference was significant riparian habitat but no measurable effect could
{P < 0.001) using a /-test. Consequently, the be found in the grassland. Riechert & Hedrick
experimental result supports the hypothesis (1990) also referred to a study by Greene (1989)
that birds affect the abundance of spiders in that reported significant bird predation effects
sagebrush. on canopy-living spiders in a pine habitat
adjacent to their riparian habitat.
BIRD PREDATION ON SINGLE SPECIES
Vertebrate predation on two species of
Studies of ecological mechanisms can be lycosids, Schizocosa ocreata (Hentz 1844) and
difficult to perform on communities because S. stridulans Stratton 1984 was studied in an
the various species respond in very different exclosure experiment in secondary forest in
ways. Therefore, experimentalinvestigations on Kentucky (Wise & Chen 1999) over two years.
bird predation effects on single species may These wolf spiders were very common on
GUNNARSSON—BIRD PREDATION ON SPIDERS 517
the floor of forests dominated by oak and 1993). In samples collected in the fall, the
maple and > 90% of lycosids in the study site mean densities in net-enclosed branches were
belonged to the genus Schizocosa. Birds and 2.3-2.6 times the densities of controls. The
other potential vertebrate predators, such as predation effects on males and females were
shrews, were common in the experimental similar in three out of four samples (Gun-
plots. In late spring, five locations were narsson 1998). In the fall samples, the mean
established. In each location, two 50 m^ areas number of individuals for each sex on net-
were selected at random as a “removal-exclu- enclosed branches was 2.1-2.3 times the
sion” treatment, or as “open reference.” controls. A more pronounced difference be-
Vertebrates inside each exclosure were trapped tween the two branch categories was found
and removed. The density estimates of Schizo- after the winter. The mean number of males
cosa included all stages and the analysis was on protected branches was 10.3 times the
based on eight samples during two years. controls, and for females the effect was 10.6
The removal ofvertebrate predators, includ- times. However, in the second spring sample
ing birds, did not affect lycosid densities. The there was a sex-related difference in the bird
treatment factor (i.e., the exclosure) ap- predation effect. In males, there were 5.5
proached P = 0.10 in an ANOVA but times more individuals on net-enclosed
Schizocosa densities tended to be higher in the branches compared with control branches. In
“open reference” plot rather than the protected females, the predation effect was only 2.8
one. Wise & Chen (1999) suggested that the times more individuals on protected branches.
removal ofvertebrate predators might cause an These results suggest that bird predation can
increase in the density of another, unknown be sex-specific under certain circumstances
major arthropod enemy of Schizocosa that and this will be discussed further below (see
depressed the lycosid density. However, they “Selective predation”).
emphasized that this was a speculative sugges- Another experimental study of bird preda-
tion. tion was carried out on a theridiid spider,
In a study during two years, the effects of Achaearanea cf. riparia (Blackwall 1834) on the
bird predation on the linyphiid Pityohyphantes island of Hawai’i. This species is an exotic to
phrygianus (C.L. Koch 1836) was examined by the study site and it is presently expanding its
manipulation of predation pressure (Gunnars- distribution on several locations on the island
son 1993, 1998). This sheet-web spider lives on of Hawai’i (Gruner 2005). The experimental
branches ofconiferous trees and itisexposed to site was a high altitude, wet forest in early
predation bypasserinebirdsduring all stages of succession and the vegetation was dominated
the biennial life cycle. Many ofthe bird species by the tree Metrosideros poiymorpha. Birds
are non-migrating and they form multi-species were common insectivores in the study site.
flocks that patrol the same area during The results on A. riparia were part of a
overwintering. larger experiment that examined the relative
The results were part of an experiment on influences of bottom-up and top-down effects
bird predation effects on the spider community in a Hawaiian food-web (Gruner 2004). Fifteen
in spruce (Gunnarsson 1996). Randomly se- individuals ofA. riparia were collected in total
lected spruce branches between 1.5 and 4 m from 28% ofthe plots at the start ofthe study.
above the ground were enclosed in coarse- However, at the end of the experiment, 939
meshed nets (mesh size 10 X 10 mm) and other individuals were collected in caged plots but
branches were left as controls. Four sampling only 22 individuals in control plots where birds
periods, fall and spring in each of two years, could continue their foraging without any
were used and in all cases removal of bird restrictions. If the abundance is expressed as
predation significantly increased the density of spider density (No. per 100 g foliage), the mean
P. phrygianus. density on caged trees was approximately 28
In the spring samples, i.e., after the winter times the mean density on control trees. A
periodwithacombinationofbirdpredationand closer look at the size distribution ofA. riparia
other, temperaturerelated, mortalitycauses, the in the treatment groups suggests that large
spider mean density of experimental, net- individuals benefited even more from exclusion
enclosed, branches was 4.1-10.5 times the ofbird predation. The possible size selection by
mean of control branches (Gunnarsson birds on spiders will be discussed now.
518 THE JOURNAL OF ARACHNOLOGY
SELECTIVE PREDATION is common in many spiders (Elgar 1998), the
Bird predation pressure on spider popula- males reaching the adult stage early will have a
tions can have consequences that are much mating advantage. This could be achieved by
more far-reaching than just a reduction of the enhanced growth rates and large size, which ji
lporceaylwaibtuhnsdaonmcee.sorItfobfirddispcrreidmaitnoartsionsetlehcetrethweiillr lsuugngdest&s thEaatgesresxturalomsel1e9c7t7i;on Gisunancatirnsgso(nWik&- '
be unequal risks of being eaten depending on Johnsson 1990), but early maturation could ;
the phenotype of the prey. This will lead to a also be associated with small size and fewer
selective advantage to certain individuals and molts (Vollrath 1987). In addition, it should be
over time the frequencies of different pheno- noted that in many species there is obviously
selection for small male size because of
tpyrpeedsatiinont.heThpoepuprlearteiqouniswiitlelscfhoarnsguechduaeptroocbeisrsd predation risk (e.g., Vollrath & Parker 1992).
In female spiders, there is ageneral relationship
are (i) variation in the studied trait (size, color, between large size and fecundity (Vollrath
etc.), (ii) a genetic basis for the trait, and (hi) &
1987; Marshall Gittleman 1994). This has
fitness differences between individuals display-
ing the trait. When these conditions are been suggested as amajor selective forceforthe
evolution of size dimorphism exhibited in
fulfilled, natural selection occurs (Endler 1986).
several spidergenera, e.g., Nephila(Coddington
There are several studies on birds acting as
et al. 1997). However, predation by visually
selective agents on their prey. Especially birds
in forest ecosystem may have a large evolu- hunting predators is potentially an important
selective force acting against large size.
tionary impact on their arthropod prey
Experiments on bird predation effects on the
(Holmes 1990). There are a number of studies
entire spider population in a forest ecosystem
ofpredation effects on spider populations that
have often revealed that there are differences in
can be used to assess the importance ofbirds as predation pressure on various size categories.
selective agents. In reviewing these studies, I Eor instance, in a coniferous forest in Sweden
put special emphasis on the experimental there was a significant decrease in population
evidence but in certain cases I also include size by large spiders (body length > 2.5 mm)
investigations in which there are good reasons but this was not found for small spiders during
to assume bird predation as an important winter (Askenmo et al. 1977). In the same
mortality factor. — system, Gunnarsson (1983) found a significant
Selection on size, sex and color. One of the decrease in the density of both small
most obvious traits for selection is size. (< 2.5 mm) and large (> 2.5 mm) spiders
Theoretical models suggest, for instance, that during winter but bird predation pressure
size-specific predation pressure can affect the varied. A comparison of spring densities
evolution oflife history ifthe coupling between expressed as the ratio between experimental
predator and prey is strong (Day et al. 2002). (branches protected from predation) to control
Size is an extremely important trait that affects mean density showed that for small specimens
many aspects of the life history of spiders and there was a 2.0-fold increase in density on
other arthropods. Data suggest that females of branches protected from bird predation. The
the agelenid Agelenopsis aperta can benefit in increase in the number of large specimens on
terms of fitness by mating with a large male protected branches was 6.1. This suggests that
(Riechert & Johns 2003). The offspring of the predation pressure on large spiders was
females mated to large males were “bolder” higher than on small ones.
and more aggressive than the offspring sired by In another experiment in a spruce {Picea
small males. There are numerous studies abies) forest ofsouthern Sweden, themean (and
showing that large males have a higher median) size was significantly larger on branch-
probability of winning contests with smaller es protected from predation via exclosures
opponents over females (see Elgar 1998 for (Gunnarsson 1996). The experiment started in
examples). Moreover, in some species large spring and the first sampling was done in fall,
males mature early and this is favored ifthere is five months later, and the second sampling was
protandry, i.e., males on average maturing performed after the winter, six months after the
before the females (e.g., Gunnarsson & Johns- first one. Another two samplings were executed
son 1990). Ifthere is first-sperm priority, which in fall and spring with a six-month interval