Ecosystems on Earth are diverse, and the distribution of life is not even, with life near the equator being the most diverse.
Species diversity increases as we move across the tropics toward the equator, and this pattern exists both on land and in the ocean, from mammals and birds to ants and even trees, recorded in a variety of flora and fauna.
A representative example of the latitudinal gradient of biodiversity: changes in species diversity of terrestrial vertebrates. Red represents the highest and blue represents the lowest. | Image credit: Mannion, P. D., Upchurch, P., Benson, R.B. J. & Goswami, A., based on work by Clinton Jenkins via Wikimedia Commnons under CC BY
Although this pattern is so obvious today, the distribution of biodiversity at different latitudes, that is, the latitude gradient of biodiversity, is not always the same. Studies have shown that at certain times in Earth's history, species diversity may have been highest at latitudes far from the equator.
Academia has proposed several different hypotheses to explain why high biodiversity aggregation occurs at certain latitudes. But both today, and in earth's history as shown in the geological record, climate is often seen as a key driver. Climate affects organisms in many ways, including where they live, when they reproduce, and even how they control their own internal processes, such as body temperature regulation.
Modern biodiversity peaks in the equatorial regions of low latitudes, such as the rainforests of the Amazon and Central Africa. This pattern is more likely to be found in the historical record of the "Ice Chamber" period, when ice sheets appeared at the same time at both poles, much like today.
However, during warmer periods known as the "greenhouse" state of the Earth, patterns of bimodal distribution may appear in the records. That is, the two bands have the highest biodiversity, and they occur in the mid-latitudes of the Earth, that is, south of the equator and around 25° and 65° north.
Changes in biodiversity with latitude are often closely linked to mass extinction events. Today's planet faces the problems of climate change, habitat loss and global biodiversity loss, and understanding the causes of the latitudinal changes in biodiversity over hundreds of millions of years is critical to us.
Looking back at geological times, we find a shocking picture of what we would lose if we failed to solve the problem of rising global temperatures.
The fossil record provides us with the best window into Earth's ancient biodiversity. But estimating patterns of biodiversity from the fossil record is not easy because it is full of missing and incomplete information that limits our understanding.
But over the past two decades, new analytical techniques have allowed paleontologists to estimate patterns of prehistoric biodiversity from sporadic data. In recent years, these advanced technologies have revealed what the latitude of diversity looked like 200 million years ago, after the most devastating mass extinction event on record.
The mass extinction event of the late Permian occurred about 251 million years ago and caused the extinction of more than 80% of species on Earth. Extinction events are caused by an unstable climate following widespread volcanic eruptions. At that time, and for the next 50 million years of the Triassic period, the global continent was "spelled" into a single continent, that is, the Pancontinent.
The climate during this period was hotter and drier than it is now, and there was a vast desert around the equator. The polar regions did not have ice caps, but instead exhibited a temperate climate, as we do in the mid-latitudes today. At the same time, marine life not only has to withstand temperatures of up to 40 degrees Celsius at sea surface temperatures near the equator, but also faces the effects of declining oxygen levels and ocean acidification.
A period of recovery followed by the mass extinction at the end of the Permian. A recent study found that for most of the Triassic period (201-251 million years ago), the oceans had a latitude gradient of diversity very similar to today.2 million years ago.
Marine biodiversity. | Image credit: Carljohnthegreat via Wikimedia Commons under CC BY-SA
However, shortly after the mass extinction event, a gentle pattern of biodiversity gradients emerged. At any latitude, there is no peak in species diversity, which is due to extreme warming and ocean hypoxia (oxygen depletion in seawater), and a high rate of species extinction near the equator.
On land, vertebrates that survived the mass extinction soon formed a bimodal pattern, with the highest peak occurring in the lower latitudes of the Northern Hemisphere and the subculture occurring in the mid-latitudes of the Southern Hemisphere. The formation of this pattern is likely to be driven by extreme climatic conditions across the Pan continent, including high temperatures and seasonal heavy rainfall, associated with the formation of "giant monsoons".
In the late Triassic period, as another mass extinction event approached, most land vertebrates, including early mammals and early dinosaurs, showed a high degree of diversity in the mid-latitudes north and south of the equator. This pattern is similar to the records of terrestrial vertebrates in the Permian period prior to the mass extinction.
Pseudo-crocodiles. | Image credit: Dallas Krentzel via Flickr under CC BY
But one exception is the pseudosuchian, a group of crocodiles and their fossil relatives. Interestingly, although the latitudinality of the biodiversity of other species changed over the next 200 million years to reach today's equator, their biodiversity has remained at its highest level at lower latitudes throughout their evolutionary history.
This may be due to their physiological characteristics, especially their resistance to high temperatures. Reptiles are thermotropes, commonly known as "cold-blooded" creatures, that rely on the external environment to regulate body temperature. Today, crocodiles and other reptiles are confined to warmer, more stable regions of the world, and their fossil relatives may be the same.
These insights into past mass extinction events are critical to understanding how biodiversity regions on Earth are currently changing. As global temperatures continue to rise, some studies predict that species will spread from the equatorial region to the poles. However, if the rate of change is too fast, they are at risk of extinction.
Others argue that global warming may cause climates at different latitudes to become more similar, potentially creating peaks in biodiversity in the mid-latitudes. There is already evidence that over the past 50 years, the latitude of marine biodiversity has become increasingly closer to the bimodal model.
Scientists believe that the so-called "sixth mass extinction" is imminent, and we need a long-term perspective on how to maintain the earth's biodiversity in the future.
#创作团队:
Original author: Emma Dunne (Postdoctoral Fellow, School of Geography, Earth and Environmental Sciences, University of Birmingham)
Bethany Allen (PhD candidate, School of Earth and Environment, University of Leeds)
Compilation: Hairy tip
#参考来源:
https://theconversation.com/prehistoric-creatures-flocked-to-different-latitudes-to-survive-climate-change-the-same-is-taking-place-today-163309
#图片来源:
封面：J.T. Csotonyi via Wikimedia Commons under CC BY-SA