Last update: August 2002
Todd A. Blackledge
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Lecturer 3136 Comstock Hall Cornell University Ithaca, NY 14853 voice
(607) 255-7153 fax (607) 255- Email tab42@cornell.edu Ph.D. 2000 Entomology,
Ohio State University |
Research Interests:
Causes and consequences of adaptive radiations: orb-weaving spiders in Hawaii
In
an adaptive radiation prolific speciation is often accompanied by expansion in the
ecologies of members. Yet, the community context and consequences of adaptive
radiation are not well understood. I am using a radiation of orb-weaving
spiders (Tetragnatha) across the Hawaiian archipelago to test whether
adaptive radiation results in predictable evolutionary patterns of community
structure. The predatory lifestyle of these spiders is intimately tied to their
building of orb webs, which provides spiders with the means to capture prey,
sense their environment, defend against predators, and select mates. Thus,
variation between species in web architecture can reflect differences in the
life histories of spiders. I have found that sympatric species of Tetragnatha
exhibit little overlap in the shapes of their webs while species on different islands
have evolved surprising similarities in web architecture, independently of one
another. These patterns suggest that adaptation to different selective optima
within habitats may have played a role in the extraordinary diversification of
Hawaiian spiders.
Cylcosa
(Araneidae) is the only other group of orb-weaving spiders, besides Tetragnatha
(Tetragnathidae), to have dispersed to and speciated across the isolated
Hawaiian Islands. In contrast to Tetragnatha, Cyclosa comprises a
relatively species poor radiation. These two groups of spiders provide a unique
opportunity to study why speciation is higher in one lineage than in another.
Both genera are sympatric, found within the same habitats, but are segregated
temporally because Cyclosa is diurnal while Tetragnatha is
nocturnal. Thus, competition does not explain this difference in
diversification. Preliminary data also suggest that the Hawaiian Cyclosa
are not especially younger than Hawaiian Tetragnatha. Instead,
comparison of resource use between these two radiations may give insight into
the causes of adaptive radiation. By quantifying variation in the web
architectures and resource use of endemic Hawaiian spiders and reconstructing
their phylogenetic relationships I can answer two hypotheses about diversification.
Do species diversify in adaptive radiations because they exploit greater
ecological opportunity? Or, do species divide resources more narrowly, or
tolerate greater niche overlap, so that adaptive radiations can occur
regardless of ecological opportunity?
Click here to see images of Hawaiian spiders and
webs.
Predator-prey conflict and sensory drive: behavioral ecology and evolution of stabilimenta in spider webs
My research focuses on the conflict that
arises, between signaling presence of webs to predators and to prey, when
spiders include stabilimenta in their webs. These conspicuous silk lines,
crosses and spirals may have several defensive functions including camouflage
of spiders, startling predators, and acting as aposematic warnings for the
presence of webs (Blackledge & Wenzel 1999, 2001). However, my research
indicates that insect prey can also use stabilimenta as a signal in avoidance
of webs, indicating that there should be selection against the use of
stabilimenta in web avoidance by insects (Blackledge & Wenzel 1999). The
reflectance spectrum of the silk used to build stabilimenta suggests that the
silk is cryptic to insects, unlike more primitive spider silks (Blackledge
1998b). This is supported by my experiments demonstrating that honey bees can
learn to forage at targets made from primitive spider silks but not targets
made from stabilimentum silk (Blackledge & Wenzel 2000). I suggest that the
evolution of silk coloration has occurred through a process termed sensory
drive, where innate biases in the color vision of insects has selected for the
cryptic properties of stabilimenta. This system is unusual because most
examples of sensory drive involve sexually selected signals but spiders’ silks
have evolved under natural selection from predators and prey.
Evolution of defensive adaptations of spiders
Spider webs
result from complex behaviors that have evolved under many selective pressures.
Webs are primarily considered to be foraging adaptations, leaving the potential
role of predation risk in the evolution of web architecture neglected. Yet,
spiders are confronted with their own suite of predators – transforming the
hunter to the hunted. I am interested in the role defensive adaptations may
have played in the evolution of spider web architecture. Wasps in the Sphecidae
and Pompilidae are ubiquitous predators of most web-building spiders and likely
have an important role in regulating spider densities. This allows wasps to act
as selective agents in the evolution of spider defensive behaviors. One such
adaptation may have been the evolution of three-dimensional web architectures
from ancestral orb webs. Such cob webs and sheet webs surround spiders with matrices
of silk that may defend them against predatory wasps, which capture many fewer
of these spiders than expected from their abundance in the environment.
Foraging and web-building in Dictyinidae
Webs provide behavioral
manifestations of the foraging investments of spiders, providing a model system
with which to examine foraging decisions by sit-and-wait predators. While orb
weaving spiders have received a great deal of attention in recent years, little
is known about how tangle web spiders manipulate their webs in response to
foraging success. Yet, tangle web spiders likely represent the ancestral
condition to the better studied orb-weavers. And, unlike orb weavers, tangle webs
spiders do not rebuild their webs on a daily basis. Instead they add silk to
their webs over many days. This difference can have major consequences for how
spiders should manipulate construction of webs.
I am examining how a common dictynid spider (Dictyna
volucripes) responds to temporal variation in prey capture. My research
suggest that dictynids use information from both previous success and previous
effort when making decisions about current foraging effort. My current focus is
on producing a dynamic model to explain this integration of information in
foraging decisions.