line ran in a different direction than they had initially thought. Armed
with the location of both bolts, they meticulously constructed a map of
squares in between, precisely repeating what Hewatt had done. In all,
they counted 125,590 animals, representing 136 different species.
Once the counting began, Gilman and Sagarin quickly saw dra-
matic changes in the makeup of the intertidal animal and plant com-
munity. What they found was a substantial species shift in the inter-
vening 60 years that was best explained as a northward migration of
species away from warming water—exactly the kind of response pre-
dicted by models of human-caused climate change.
When Hewatt first looked at the tidepools, there were no tube
snails or sunburst anemones at all, although both were prevalent fur-
ther south. When Gilman and Sagarin did the same, they found hun-
dreds of the southern anemones, and as many as 229 tube snails
crowded into 1 square yard. In all, ten of eleven species previously iden-
tified as southerners increased significantly. Six of eight northern
species decreased significantly. Those changes showed up regardless
of what the animals ate, how they reproduced, or where they sat in the
taxonomic hierarchy. Meanwhile, daily temperature records showed
waters there had warmed about 1.8°F since Hewatt squatted in the
surf.
They looked at all the masses of data in front of them, and consid-
ered different reasons for the changes. The best they found was the
increase in temperature. The combination of counting so many species
and having such a rich set of temperature data going so far back means
that their findings withstood substantial statistical scrutiny. In other
words, it was almost impossible that what they found could be simply
chance. “If you had a table with a diagonal line and some marbles,”
Sagarin says, “and you just dropped them randomly, there’s no way
you’d ever get the pattern we saw.”
Gilman did some separate work looking at one seaweed species
growing on the granite. Using a study from the 1960s by Peter Glynn,
now a researcher at the University of Florida, Gilman did a similar
rock-by-rock comparison of turfy red algae called Endocladia muricata.
“The areas where Glynn worked just became deserts, in a sense,
because there was no algae there,” she explains. “But if you looked a foot
116 Orna Izakson