Climate change may drive cave spiders to extinction

Submitted by editor on 7 December 2017. Get the paper!

By: Marco Isaia, Stefano Mammola and Sara Goodacre

Despite the impact of global climate change being documented on a broad
range of biotas, the effects of altered temperatures on biological
communities in subterranean ecosystems have seldom been explored. This
is rather surprising, considering that caves are spectacular model
in which to study ecological processes, in particular, the
effects of climate change on biological communities.  In fact, most
caves have an almost constant temperature over the year, with an annual
variability usually varying by only a few tenths of a degree. Direct
field observations as well as theoretical models, have already pointed
out that anthropogenic climate change may significantly influence and
modify the underground climate. As a consequence, fragile underground
ecosystems and their peculiar conenoses are expected to be affected by
the ongoing human-driven climatic shifts because most subterranean
species have lost the ability to withstand temperature variations over
their evolutionary history.

We have been studying the underground spider fauna of the Western Alps
since 2007, when we described our first species of subterranean spider.
Besides the enjoyable part of working in caves (compared with laboratory
work, caving can be extremely entertaining!), we sensed that caves could
be used as natural laboratories in which we could develop different
kinds of ecological studies: communities are relatively simple (low
abundance and diversity), food chains are shorter and species
interactions are simpler than outside, and there are no pronounced
seasonal effects interfering with the interpretation of the data.

Since our first cave survey, we have accumulated a substantial quantity
of information, which has mostly resulted in taxonomical and faunistic
papers. We described several species new to science and assembled all
available data on cave-dwelling Arachnids in a monographic work,
published in 2011. The result was an impressive collection of occurrence
data, scattered on a coherent biogeographic area of the Alps: a
remarkable baseline to be used for developing species distribution
models aimed at unravelling macroecological patterns.

The work to link subterranean species and cave ecosystems with climate
change was the focal idea of a research project proposal in 2011. The
experience gained in our previous cave-based researches, the involvement
of several stake holders and the collaboration with experts from other
disciplines, such as physics, geomorphology and climatology were
important keys to the success of the CAVELAB project. The project
started in February 2012 with a monitoring program of 33 caves and other
subterranean habitats. Our first aim was to shed light on the thermal
stability of the caves and on the relationship between the temperature
measured in the cave and the mean annual temperature recorded outside.
Once this pattern was confirmed, we started to investigate the
relationship between cave temperature and the occurrence of certain
specialized subterranean species (Troglohyphantes) that inhabit cool
caves scattered through the alpine chain.

We modelled the distribution of Troglohyphantes spiders in relation to
cave microclimate, inferred past glacial dynamics and other cave
features in order to assess their sensitivity to climatic variation
induced by climate change. In particular, we hypothesized that
Troglohyphantes spiders were significantly correlated with different
climatic conditions as a function of their adaptation to narrow and
specific ranges of temperature, and that climate affected their
distribution in the past and will affect it in the future, through a
decline in habitat suitability.

What we found was quite remarkable. Our
indicate a clear-cut link between the distribution of these
spiders and both past and current climatic conditions. More importantly,
a clear decline in the suitable subterranean habitat is predicted,
although with a certain time-lag due to the cave thermal inertia. Whilst
the results need further confirmation from an ecophysiological point of
view, our findings raise concerns for the long-term survival of these
subterranean predators
, which play a crucial role in the fragile food
web of these extreme ecosystems.

A current project focusing on the thermal tolerance of these
subterranean organisms is currently ongoing, providing further support
to our inferred trends. Our findings emphasized the importance of
considering subterranean organisms as model species for ecological
studies dealing with climatic changes, and to extend such investigations
to other subterranean systems worldwide. We hope that more scientists
will find inspiration in caves for their ecological studies as we did
with this work!