This is a topic rarely commented on, but the surrounding biome is adjusting to the urban environment and often surprisingly well. Birds in particular have often taken to the urban lifestyle. The big surprise to myself was how well raptors have adjusted. Yet the prey birds have certainly blossomed and thev raptors have simply followed.
This integration will only become deeper and more successful as time
progresses.
The unexpected here is the swift improvement in size for spiders even
if it is completely understood.
BY NICK
STOCKTON
08.20.14
Something about city
life appears to be causing spiders to grow larger than their rural
counterparts. And if that’s not enough to give you nightmares,
these bigger urban spiders are also multiplying faster.
A new study
published today in PLOS One shows that golden orb weaver
spidersliving near heavily urbanized areas in Sydney, Australia
tend to be bigger, better fed, and have more babies than those living
in places less touched by human hands.
The study’s
authors collected 222 of the creatures from parks and bushland
throughout Sydney, and correlated their sizes to features of
the built and natural landscape.
They dissected each
specimen back at the lab, and determined its size, health, and
fecundity by measuring four attributes: the length of the
spider’s longest leg segment, the ratio of that leg segment to
overall body weight, the amount of fat on the spider, and its ovary
size.
To
measure urbanization, the authors looked primarily at
ground cover throughout the city, at several scales, where they
collected each spider: Are surfaces mostly paved? Is there a lack of
natural vegetation? Lawns as opposed to leaf litter?
“The landscape
characteristics most associated with larger size of spiders were hard
surfaces (concrete, roads etc) and lack of vegetation,” said
Elizabeth Lowe, a Ph.D student studying arachnids at the University
of Sydney.
Humped golden orb
weavers are a common arachnid along Australia’s east
coast. They get their name from their large, bulging thorax, and
the gold silk they use to spin their spherical webs. They
typically spend their lives in one place, constantly fixing the
same web (which can be a meter in diameter). Each web is
dominated by a single female, though 4 or 5 much smaller males
usually hang around the edges of the web, waiting for an opportunity
to mate (only occasionally does the female eat them afterwards).
Paved surfaces and
lack of vegetation mean cities are typically warmer than the
surrounding countryside. Orb weavers are adapted to warm weather,
and tend to grow bigger in hotter temperatures. The
correlation between size and urban-ness manifested at every scale.
Citywide, larger spiders were found closer to the central business
district. And, their immediate surroundings were more likely to be
heavily paved and less shady.
More food also leads
to bigger spiders, and the scientists believe that human
activity attracts a smorgasbord of orb weavers’ favorite prey.
Although the study wasn’t designed to determine exactly how
the spiders were getting bigger, the researchers speculate that
things like street lights, garbage, and fragmented clumps of plant
life might attract insects. They also believe that the heat island
effect might let urban spiders mate earlier in the year, and might
even give them time to hatch multiple broods.
The orb weavers
could also be keeping more of what they catch. Because they are
such prolific hunters, orb weavers’ webs are usually home to
several other species of spiders that steal food. The researchers
found that these little kleptos were less common in
webs surrounded by pavement and little vegetation.
Lowe says quite a few
species of spider are successful in urban areas, and she wouldn’t
be surprised if some of these other species were also getting bigger.
Despite how terrifying this sounds, she assures me that this is
actually a good thing. “They control fly and pest species
populations and are food for birds,” she said.
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