Why there are so many species at the equator and so few at the poles?

Why there are so many species at the equator—and so few at the poles?
A Spondylus americanus (left) from the Florida Keys, a tropical species with long spines that act as defense against predation; and Arctica islandica, sometimes known as the Methuselah clam for its long life, which is found only in cold waters like the Gulf of Maine. Credit: Katie Collins

Earth is teeming with strange life forms—crabs with 12-foot-long legs scuttling off the Japanese coast, mushrooms that glow at night in eastern North America, butterflies that drink the tears of Amazonian turtles.

Among all of the world's natural kingdoms, however, one rule reigns supreme: There are lots of different at the tropics, but their numbers drop off sharply as you move toward the poles. "This holds true across virtually all kinds of life and in all kinds of environments, but the reasons why are still hotly contested," said Prof. David Jablonski, a leading University of Chicago scientist of extinction and biodiversity. "This is a fundamental question that goes back before Darwin."

Jablonski's goal is to understand biodiversity, and the stakes are big—including how species will adapt to climate change.

Scientists like Jablonski—whose research on mollusks has shaped the field—have traditionally researched either a species' form (the shape of an organism's body) or its function (the way it makes a living). For example, a clam's shell can be spiny or smooth, and it can make a living eating sunken logs off a coast or by filtering plankton in tidal flats. Each way of looking at the animal tells you something different about evolution, niches and the patterns of biodiversity; but each is so complex on its own that they're rarely studied in unison. Jablonski believes integrating the two could yield important insights.

Two recent studies from his lab take this approach, combining forces with other specialists to investigate the diversity shift from topics to poles in an innovative way.

In the first study, they worked with bird biologist Prof. Trevor Price to compare Jablonski's mollusk data with Price's bird insights on how species across the world live in .

In , some ways to make a living are packed with species—lots that eat insects on tree branches, for example—and others support just a few. This pattern persists midway into the latitudes, but then there's a distinct tipping point and the number of ways to make a living, and the uneven distribution among them, drop off.

The exact same is true for mollusks. "That result knocked our collective socks off," said Jablonski, who is the William R. Kenan Jr. Distinguished Service Professor of Geophysical Sciences.

"For animals, you don't get much different than a bird and a bivalve, but you see this strikingly similar pattern," said Stewart Edie, a postdoctoral researcher and the joint first author of both papers. "That usually suggests we're looking at a higher-order control that's operating on a large scale around the planet, both on land and sea."

The theory is that in the tropics, there's more room for fine-scale specialization among species—not just birds that eat seeds, but birds that eat only one kind of seed, in one part of the environment (branches high in the forest canopy, for example). But with increasing latitude, the climate becomes more seasonal and more difficult to survive in, and it's less viable to be a specialist. "A lot of the ways of life are still present; you just have to eat every kind of seed, or live anywhere in the canopy," Jablonski said.

Why there are so many species at the equator—and so few at the poles?
A map of the researchers' study area for the Florida Keys and Gulf of Maine. On the top row are the shells at higher latitudes, which tend to be smoother and plainer than their spiky, colorful counterparts closer to the equator, on bottom row. Credit: Katie Collins

That's something new about how biodiversity works, he said, and it may have implications for how things will play out as climate change progresses. "For example, what's going to happen to parasites that attack crops or plants we care about—species will arrive that can focus on specific hosts as it gets warmer," he said.

The second paper, headed by postdoctoral researcher Katie Collins, turned to technology to analyze how the forms of mollusks change from tropics to poles.

They used a micro CT scanner to scan samples of 95 percent of all the species found in the Florida Keys and the Gulf of Maine, yielding a treasure trove of 3-D images. Most studies just work with shell length and height, "but that's not the whole picture," Collins said. "A third dimension adds a new layer of understanding." "Now we can put hard numbers on shell form where we just had general impressions before," added co-author Rüdiger Bieler, curator of invertebrate zoology at Chicago's Field Museum and member of UChicago's Committee on Evolutionary Biology.

When you go to a beach in the Florida Keys, your eye is drawn to large, brightly colored shells with spines and ridges and knobs. But north of Cape Cod, you don't find those. The shells are all smooth, plain and often small, which at first blush seems to indicate a massive shift in species form.

That's not what scientists found, though. "It turns out there are plain shells everywhere," Collins said."The tropical shells are actually a mix of plain and fancy species; the fancy shells just drop out of the mix as you go north. So the individual species aren't transforming to adapt to new conditions; whole lineages of fancy things drop out."

"It looks like the high-latitude climate narrows down the range of viable shell forms, and some lineages are just frozen out, so to speak," said Jablonski.

This too has implications for humans as they continue to alter the climate and environment.

"What this suggests is that most animals are more likely to drop out than to adapt to the tougher conditions at high latitude," Collins said. "This could affect humans in a very real way. Bivalves serve a huge role in fisheries, and we don't know if the ones we like to eat most, like oysters or scallops or mussels, are going to disappear, or move out of their accessible fishing grounds, as the climate changes around them."

This study was done on modern species; the next step will be to add fossils. (Scientists love mollusks because their shells are easily fossilized.) "For example, we want to know whether the fancy lineages turn over more rapidly over time are more extinction-prone or if they're stable," Collins said.

"Taken together, these new studies show that the different dimensions of biodiversity don't all change together on a global scale, and the mismatches tell us something new about the forces shaping life on earth," said Jablonski.


Explore further

Scientists use technology to examine questions around climate, biodiversity

More information: M. Schumm et al. Common latitudinal gradients in functional richness and functional evenness across marine and terrestrial systems, Proceedings of the Royal Society B: Biological Sciences (2019). DOI: 10.1098/rspb.2019.0745

K. S. Collins et al. Spatial filters of function and phylogeny determine morphological disparity with latitude, PLOS ONE (2019). DOI: 10.1371/journal.pone.0221490

Citation: Why there are so many species at the equator and so few at the poles? (2019, August 30) retrieved 22 October 2019 from https://phys.org/news/2019-08-species-equator-poles.html
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User comments

Aug 30, 2019
"The theory is that in the tropics, there's more room for fine-scale specialization among species"

-Well sure. No 2 species can inhabit the same niche, but in the tropics there are lots more niches to inhabit.

Aug 31, 2019
Um...how about the obvious? Year-round warmth is better than seasonal warmth and cold.

Sep 01, 2019
DuH....Lamest most obvious outcome ever.

Sep 01, 2019
So why do you three think there are more species in the tropics?
The tropics don't sustain more biomass than higher latitudes but MUCH more species.
https://upload.wi...here.jpg
Because more niches? Why are there more niches? Because more species?
Which caused which?

Did you read the article?
This research indicated that it is because less seasonal variation in the tropics allows for more specialization. Species don't need different seasonal strategies to survive. Specialized species inhabiting a narrower niche allows for more species.

Sep 01, 2019
There is a major flaw in the conclusions.
1. It does not account for physical and magnetic pole shifts. It is the major cause of migration in animals annually, biannually etc.
2. It does not take into account the Earth's magnetosphere.
3. The changing of ocean and brackish rivers and estuaries salinity.
4. The loss of the ice caps. the article discounted fossils at both pole areas showing no such stratification, with the exception of Australia. It is clear Australia separated first from the unicontinent.
JMHO

Sep 01, 2019
So why do you three think there are more species in the tropics?
The tropics don't sustain more biomass than higher latitudes but MUCH more species.


I don't care what your wiki says; I look at Yukon tundra and Congo river basin and there are orders of magnitude more biomass in the Congo River basin than the Yukon Tundra or Greenland, in toto.

Sep 01, 2019
Everything in the tropics is scared sh!tless of penguins and polar bears. Simples.

Sep 01, 2019
So why do you three think there are more species in the tropics? Because more niches? Why are there more niches? Because more species?
More niches dude.

"Tropical rainforests vary because of differences in latitude, altitude, soils, or water supply. Because of their immense diversity, they provide many different niches in which organisms can live... The important point here is not to categorize all possible niches and habitats within forests, but to recognize their immense number and diversity. This is an opportunity for an explosion of species, and so rainforests are generally the most species-rich ecosystems on earth"

and

"K. Rohde, who has suggested that a vacant niche can be defined as the possibility that in ecosystems or habitats more species could exist than are present at a particular point in time, because many possibilities are not used by potentially existing species..."

-You dont need an inhabitant to define a niche.

Sep 01, 2019
Did you read the article?
Did you search the internet?
I don't care what your wiki says; I look at Yukon tundra and Congo river basin and there are orders of magnitude more biomass in the Congo River basin than the Yukon Tundra or Greenland, in toto
-What makes you think this means there are more niches to inhabit?

Sep 01, 2019
So, in summary --
Tropics == Warmth == Life flourishes.
Poles == Cold == Life sucks.

Sep 01, 2019
It's about the basic starting nutrient source(s) which initiate/support life that goes on to start/expand the forms in the local food-chain. Then it's about cycling/re-cycling options/processes which can naturally occur/establish within the local temperature ranges that determine how variedly and how quickly chemical/physical processes responsible for nutrient retention/cycling can work within the local conditions/seasons etc. Obviously the lower temps at the polar regions limit both diversity/recycling of initial nutrient sources; and that also limits the further rate of recycling of said nutrients for local retention/wildlife. Hence why the limited animal species that can take advantage of the polar region nutrient/food sources and also survive the extreme cold/seasonal effects which allow only certain adapted 'specialist' species (like whales, penguins, krill etc) to range freely there. Whereas equatorial regions have more 'generalist' species which can survive/thrive there. :)

Sep 18, 2019
So, nice and interesting!

Though some disagree:

DuH....Lamest most obvious outcome ever.


Duh, someone does not know how science (or technology) progresses: by checking everything.

There is a major flaw in the conclusions.
1. It does not account for physical and magnetic pole shifts.


That is by design, it was not yet looking for geological time and fossils, see the article.

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