NZ’s unique fossil record of marine molluscs helps scientists predict extinction risk – before it’s too late

James
S. Crampton, Te
Herenga Waka — Victoria University of
Wellington

Scientists are
increasingly worried we may be witnessing the start of the
“sixth
mass extinction” – the first to be caused by human
activities.

Five earlier mass extinctions, caused
by dramatic but natural events, have devastated life on
Earth during the past half billion years.

The most
famous, though not the largest, was triggered by an asteroid
impact 66 million years ago and caused the demise of the
dinosaurs – with the exception of a feathered, meat-eating
group that continued to evolve
into modern birds.

Today, the rapidly
increasing rate of human-caused extinctions is one of
our greatest challenges.

To slow these accelerating
rates, we need to understand and minimise the factors that
raise extinction risk. But therein lies a problem. Although
predictable to some extent, extinction is subject to fickle
outcomes and can only be truly understood in hindsight, when
it is too late.

For example, the
spectacularly abundant passenger pigeon in North America –
likely the most abundant bird on the planet at the time –
was driven
to extinction within a few decades during the 19th
century.

In contrast, the tuatara persisted for 80
million years in New Zealand as the last vestige of a group
of reptiles that were formerly globally
widespread and abundant.

Extinction risks are
relatively easy to predict in highly visible and
well-studied species such as the blue
whale and New Zealand’s flightless night parrot kākāpō.
They are very hard to predict in myriad marine invertebrate
species that keep ocean ecosystems functioning.

But
that is where New Zealand is uniquely placed to quantify and
predict marine extinction risks, using the fossil record. We
can quantify a growing “extinction debt” – a measure
of the number of species committed to future extinction
because of human actions today – and act on that before
the debt is locked in.

Why do we care?

Extinction and
loss of biodiversity threaten the systems that sustain life
on our planet – the complex interactions between physical,
chemical and biological processes that maintain Earth in a
habitable state.

But most of the species we are
losing are never listed on the International Union for
Conservation of Nature’s red list. This list
is biased towards birds and mammals because only a small
fraction of invertebrates, particularly those in the sea,
have been evaluated against conservation criteria.

To fill this gap and improve our understanding of
extinction in the marine realm, we can turn to the countless
“natural experiments” recorded by long-extinct fossil
species.

Using fossils, we can identify which
particular subsets of marine invertebrates have been most
vulnerable to extinction in the past and, therefore, might
be at particular risk of future extinction. In this way, we
don’t need to wait until species have gone extinct to
figure out which are most vulnerable.

The importance of New Zealand’s
data

New Zealand’s geographic isolation and
highly endemic marine fauna mean we are studying a
“closed” system. Species have been “captive” and
unable simply to move elsewhere when the going got tough.

Both New Zealand’s living
and fossil marine faunas are among the best-known
on the planet. This gives us rich
and large datasets to analyse.

This captive and
well-documented marine fauna gives New Zealand scientists
the potential to explore the drivers of extinction risk in
ways that are relevant globally.

The shellfish
(molluscs) are the most abundant and important (by biomass)
group of fossil marine invertebrates available for study.
They provide a valuable proxy for bottom-dwelling marine
animals in general.

Our previous
work on New Zealand fossil molluscs suggested their
extinction risk was related to their geographic range, body
size and position on or within the seafloor sediment, among
other factors.

Our current research extends this
work to produce extinction predictions for different types
of molluscs into the future, using climate models to
simulate the next few centuries. This is work in progress
and there is so much more to learn about marine extinction
risk from New Zealand’s wonderful fossil
record.

Are we entering a sixth mass
extinction?

Debate
about this question is perhaps a distraction.

It is methodologically difficult to place current
and future extinctions into the context of past
Earth-changing mass extinctions. This is because modern
extinction concerns focus on land species and those that are
already rare, whereas the fossil record is dominated by the
histories of common marine organisms.

The modern
extinction rate is also unconstrained. We have no idea how
quickly future extinctions will happen.

Will a
threatened species become extinct in a few decades (like the
passenger pigeon) or will it persist in low abundance for
thousands of years, or longer, like the tuatara?

Leaving these questions to one side, recent
research by myself and colleagues has shown that at
least some past mass extinctions have been defined not by a
very high rate of extinction, but by the duration of
elevated extinction rate.

This unexpected finding
gives us another argument, should we need it, to reduce
modern human-driven extinctions as quickly as possible, and
to avoid going too far into extinction debt.

James
S. Crampton, Professor of Paleontology and Stratigraphy,
Te
Herenga Waka — Victoria University of
Wellington

This article is
republished from The
Conversation under a Creative Commons license. Read the
original
article.