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Back home, five million years ago, the warm, wet climate of the Miocene sloped downwards into the cooler, drier Pliocene before descending even further into the more so Pleistocene. The slope was so gradual that to my knowledge, no extinction events happened.

But not in this alternate Earth. Five million years ago, the Miocene skipped the Pliocene and took a sharp drop to the Pleistocene. This drop in temperature happened so dramatically and so quickly that half of all plant and animal species became extinct, and the once-widespread rainforests had been cut down to their equatorial margins. (Like the refugia of back home.)

But considering that rainforest species have always been resilient against ice ages, one must ask, in this ice age mass extinction, which latitude would actually be hit harder--the lower tropics or the middle temperate?

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Tropical species would be more affected

There were actually massive extinctions due to the Miocene cooling. A lot of animal groups were wiped out in the late Miocene-Pliocene due to climatic changes, including the chalicotheres, bone-crusher dogs, three-toed horses, oreodonts, sparassodonts, megatooth sharks, protoceratids, pseudotooth birds, small baleen whales, a lot of rhino groups, and more. The oceans were hit particularly hard and still haven't recovered by the present day (https://www.nature.com/articles/s41559-017-0223-6). It's just the rate of extinction wasn't high enough to qualify as a mass extinction.

In real life, it was the tropical species that got screwed over the hardest. Most species adapted by moving further south, and for many species they got pushed so far south that there wasn't a lot of habitat left for them anymore (or even moving further south where conditions were nicer wasn't an option like in North America before the Panamanian landbridge formed), and they died.

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  • $\begingroup$ The kalliks died out 1.8ma, the bone-crushers 2.6. And the sparassodonts died out of competition with invading placental saberteeth. So it wasn't the cooling climate that killed those groups. $\endgroup$ – JohnWDailey Mar 19 at 11:14
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    $\begingroup$ @JohnWDailey The sparassodonts didn't die out from competition. There's been a lot of research on the subject and cooling climate is the currently implicated factor. Checks refs here (en.m.wikipedia.org/wiki/Great_American_Interchange). The last species of Borophagus is 1.8 Ma. And that's kind of the point. The die off was pronouncded but gradual from about 5.3-1.8 Ma, so the extinction rate wasn't a single measurable event that would normally be registered as a mass extinction. It was a gradual die off with elevated extinction rates. $\endgroup$ – user2352714 Mar 19 at 19:30
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That would depend on region... and what you mean by hard.

Rain forests such as those found in India, Northern Australia, Soath East Asia would likely be very heavily hit. The rapid cooling will likely be playing havok with their monsoons which are critical for replenishing the water they loose.

Rain forests that can sustain their own rainfall, such as the Amazon Rain Forest, and rainforest built over great untapped aquifiers, such as those around the K2 impact crater could sustain themselves in the face of interrupted rainfall for some time.

All of them will be disrupted by the rapidly changing temperature. Expect the die off of tightly coupled species particularly if those species have some sort of mechanic around temperature, such as migration, seeding, nesting, etc...

Temperate regions rely less on rainfall for maintaining the forest. Their chief issue will be how cold adapted the trees are.

  • Regions heavily populated by conifers will probably handle well. Conifers have been through many periods of extended cold.
  • Broadleaf evergreens will be hit hard as their leaves are destroyed by frost.
  • Broadleaf deciduous will survive, though only growing during the "summer" presuming summer months warm sufficiently.

However this is if you take the view that a forest is the expected biome.

What will likely happen is that many forests will transition in grasslands/tundra. Producing a resurgence in C4 plants. This will naturally provide a field day for every C4 herbivorous animal, and the resulting food chain that they bring with them.

Naturally this isn't a terribly nice world for Nut/Fruit eaters...

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  • $\begingroup$ "Broadleaf annuals", as in deciduous trees, like oak and maple? Because I've been hearing the term "annual" applied only to herbs. $\endgroup$ – JohnWDailey Mar 19 at 1:24
  • $\begingroup$ True, i'll update. $\endgroup$ – Kain0_0 Mar 19 at 2:22
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I don't think this would really be a matter of latitude per se.

Since the equaliser between all the rainforests of the world is a general resiliency against ice ages, I'd argue that the answer will then be:

the rainforests of the latitude that has the fewest finicky species will be the less hard hit in your scenario. We tend to think of equatorial rainforests as being home to all kinds of species that rely on specific diets and environments. Those would probably be the first species to go extinct during a worldwide rapid cooling event, and thus it would make sense that the tropical latitudes would be harder hit.

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