Let’s face it: at some point or
another, you’ve had to deal with a spider in your space – crawling up your bedroom wall or deep in a corner of
your basement. Whether you live in an urban area or a rural one, arachnids and
other “creepy crawlies” seem suited to life alongside human habitation. Increasingly,
they must, to be able to persist in the face of habitat loss and fragmentation.
As our many-legged neighbors continue to make space for themselves in our homes
and gardens, ecologists have been forced to reconsider whether their discipline
is most relevant in increasingly rare pristine habitat or amid the brick and
mortar of the urban jungle.
In spite of this
this urban takeover (or reclamation), ecologists still do not have a good grasp
on how many species, including spiders, select their habitat. For spiders, the
choice of where to bunker down is crucial to success or failure: web creation
and maintenance takes a lot of energy, and relocation takes even more. Because
spiders can only determine prey availability through experience – and
experience takes silk – it is thought that web selection is based on abiotic (aka
environmental) factors, such as temperature, light exposure, and humidity. Those
spiders you try to avoid cozying up in your bathroom corner are there because
finding other suitable habitat may not be worth the energy cost.
What constitutes “suitable habitat?” Voss
et al. (2007) set out to answer this question for one urban arachnid
species, a common wall spider known as Oecobius
navus.
Two < 4mm Oecobius navus on a stretch of wall habitat – one of which appears
to be in a web. Image from SpiderBytes.org
For
their study, Voss et al. visited 28 walls around an Australian university known
to harbor the diminutive arachnid. They chose two sections of each wall to
sample from, noting physical characteristics of the site such as substrate type,
substrate temperature, amount of solar radiation, air temperature, wind speed,
and relative humidity. They counted the number of spiders which set up
residence within the transects over a period of six weeks. Using regression
analysis, Voss et al. found that Oecobius
navus abundance changed most strongly with humidity, ambient temperature,
and sunlight exposure: more spiders were found as humidity increased, and fewer
spiders were found with higher temperatures and more sunlight exposure. These
results suggest that O. navus are
vulnerable to desiccation, or drying out, and must choose their habitat
accordingly.
Voss
et al. focused primarily on what uncontrollable environmental factors
influenced the distribution of spiders throughout their environment. However,
it was not within the scope of the study to look at how the presence of Oecobius navus affected others of their kind. Did they maintain
a safe distance from potentially aggressive competitors? Were the spiders
randomly spaced, or was there an underlying pattern to their distribution? How Oecobius navus distribute themselves in
the face of limited prime habitat could have had real impacts on the study
results: for example, spiders who would have preferred to set up shop on shaded
walls may have been pushed out by more aggressive spiders.
Questions brought
up through close reading of the Voss et al. paper set the stage for our own spider
distributional experiment, here at the Grinnell College campus in Iowa. Nestled
in the midst of prairie-turned-cornfield, the Grinnell campus is a unique
pocket of suburban wilderness, and its lack of intense landscaping management
may make it more suitable habitat for insects (and their predators) than the vast,
pesticide-cultivated monocultures surrounding it. We aim to study the
distribution of the common grass spider, Agelenopsis
spp., in one such “suburban wilderness,” a cluster of boxwood bushes in a
quiet area of campus on which their webs are abundant.
Agelenopsis
on a web among the boxwood branches at our study site. Does this spider
compete with others in the area? Photo by author.
Like Voss et al.,
our study focuses on how (and potentially why?) spiders arrange themselves the
way they do in space. However, by noting physical characteristics of these
spider webs in relation to each other, we hope to gain insight to how these
spiders share their “habitat.” As suitable wild habitat becomes harder to come
by and our many-legged friends move in, how they make use of limited space – how
they distribute – may determine success or failure in an increasingly urban
world.
Voss, S. C.,
Main, B. Y., & Dadour, I. R. (2007). Habitat preferences of the urban wall
spider Oecobius
navus(Araneae, Oecobiidae). Australian Journal of Entomology, 46(4), 261-268
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