Science - USA (2022-01-07)

GRAPHIC: KELLIE HOLOSKI/

SCIENCE

SCIENCE science.org

By Moriaki Yasuhara^1 and Curtis A. Deutsch^2

I

n the ocean, the most powerful forces

of climate change may not be as visu-

ally striking as the storms and wildfires

on land. Yet small changes in seawater

temperature, acidity, or oxygen content

can have substantial impacts on the

marine biota ( 1 ). Some species will relo-

cate, whereas those with limited mobility

will either adapt to the changing condi-

tions or perish ( 2 ). The difference between

these responses across thousands of spe-

cies will determine the structure of eco-

logical communities for centuries to come.

However, the outcomes are notoriously dif-

ficult to detect in sparse and patchy his-

torical data, and even harder to attribute,

without a time machine. On page 101 of

this issue, Salvatteci et al. ( 3 ) document

changes in the abundance of fish verte-

brae, alongside indicators of the physical

and chemical environments, by using sedi-

ments off the west coast of South America

deposited over the past 130,000 years.

The new data provide a window into a

crucial time: the last interglacial period,

when Earth’s climate was warmer than it

is today. The data reveal a propitious place:

a warm tropical region with low oxygen—

properties that synergistically shape spe-

cies physiology and biogeography ( 4 ). The

region’s Humboldt Current also lies at the

heart of the El Niño phenomenon (the pe-

riodic warming of the ocean surface in the

central and eastern tropical Pacific Ocean)

whose effect on fish populations was known

to indigenous fishing communities long be-

fore modern scientists formally identified

the phenomenon ( 5 ).

The fossil records obtained by Salvatteci

et al. reveal that the Humboldt Current

during the last interglacial period was not

dominated by the anchovy species that

drive the extraordinary fishery productiv-

ity today. Instead, the most abundant fossils

were of smaller, goby-like fishes. This shift

to smaller species was not accompanied

by changes in the productivity of the area,

so the change was unlikely to be driven by

food supply. Instead, the shift coincides

with indicators of a more extreme oxygen

minimum zone, suggesting that regional O 2

content was lower. Warmer water and lower

O 2 concentration are both factors that can

produce smaller fish sizes, either within a

population or among different taxa as water

properties change ( 6 ). The shift to smaller

fish in a warmer Humboldt Current was

not achieved by an existing species’ adapta-

tion to reach smaller body sizes, but by an

ecological replacement of anchovy with a

dominant species of smaller fishes.

If organismal growth is limited by O 2

concentration in seawater, one may expect

species to adapt by having a smaller size

when fully grown ( 7 ). However, physiolo-

gists have argued that tolerance to lower

environmental O 2 concentration is indepen-

dent of body size both within and among

species and is an evolutionary necessity

for growth in water with limited O 2 avail-

ability ( 8 ). Salvatecci et al. also found that

the size of fossil vertebrae within species

showed no signs of shrinking, which goes

against the prediction for smaller orga-

nism sizes as an intraspecific adaptation

to maintain aerobic balance in a warmer

and lower O 2 environment. However, the

evidence may also be interpreted as a re-

flection of the migratory strategy of more-

mobile species, such as the anchovy ( 9 ).

Among less-mobile species, fossil records

show that body-size reductions within the

taxa often accompany climate warming

( 10 ). Whether the small size of the goby-like

fishes was essential or coincidental, their

interglacial ascendance is crucial to the

interpretation and generalization of the re-

sults. Answering that question will require

more explorations in paleobiology, loaded

with more data on the modern distribution

and physiological traits of fossilized species.

Whether body sizes played a causal or

incidental role in species turnover in the

Humboldt Current, the fossil records add

to the evidence that the community struc-

ture of tropical marine ecosystems responds

strongly to climate across a wide range of

magnitudes and time scales. Species dis-

placements may have a domino effect, with

migrations out of the tropics, because of in-

creasing ocean temperature and/or declining

oxygen concentration, affecting the competi-

tive fitness of extratropical species, which

then propagate toward higher latitudes near

the poles. These changes may increase diver-

CLIMATE CHANGE

Paleobiology provides glimpses of future ocean

Fossil records from tropical oceans predict biodiversity loss in a warmer world

(^1) School of Biological Sciences, Area of Ecology and
Biodiversity, Swire Institute of Marine Science, and State
Key Laboratory of Marine Pollution, The University of Hong
Kong, Kadoorie Biological Sciences Building, Pokfulam Road,
Hong Kong SAR, China.^2 Department of Geosciences and
High Meadows Environmental Institute, Princeton University,
Princeton, NJ 08544, USA. Email: [email protected]
Polar
Cold water
Warm water
Hot water
Coral reef
development
Sea ice
Water
temperature
Oxygen
concentrations
Temperate
Tropical
Past (preindustrial)
Species
Present Future (next centuries)
7 JANUARY 2022 • VOL 375 ISSUE 6576 25
The future of ecosystems in tropical, temperate, and polar oceans
A warming and deoxygenating ocean will make species smaller and push them from the tropical zone to the
temperate zone, from the temperate zone to the polar zone, and from the polar zone to extinction, resulting in a
loss of biodiversity in the tropics and higher biodiversity in higher latitudes. Overexploitation and other direct
human impacts are not considered in this conceptual model.