Global Warming is affecting the Equatorial Marine species

We all know that global warming is impacting our planet's biology in a very serious manner. Not only terrestrial species, but marine species are also badly affected. Researchers have seen a lot of changes in behaviour, food habits and the population of marine species across the globe. One such recent study by a group of researchers consisting of David Schoeman, Chhaya Choudhary, Anthony Richardson and Mark Costello concluded that there has been a decrease in the diversity of marine species in the equatorial region. 

They analyzed the data since 1955 covering about 48,660 species to verify whether climate change influenced the change in the richness of marine species. The study clearly shows that there has been a decrease in the variety of species at the equator. These changes were sensitive to the temperature. Since the 1970s, with an increase in surface sea temperature, these species (especially pelagic species) have shifted towards the north in the northern hemisphere. Pelagic species include fishes like mullet, billfish, cutlassfish and tuna. This consistent pattern through many decades clearly indicates that climate change is impacting latitudinal gradient in marine biodiversity on a global scale. Shifting of a few species might be ignored, but a large scale shifting rings an alarm for the humans and the whole ecosystem. 

With the review of 27 data models and around 65000 species, Chhaya Choudhary concluded that the distribution of marine diversity is bimodal. In statistics, bimodal is data having two maxima. The image below shows the distribution graphs of some species across the globe. The image is a snapshot from the research paper authored by the above four researchers.

This clearly shows that almost all marine species show almost similar bimodal distribution. The two edges of the graph show polar regions. This suggests that there is a sharp decline in distribution towards the pole. The dip at the centre shows the equatorial regions. Over the past few decades, there has been a clear decrement in species distribution. The distribution of both, pelagic and benthic species have decreased in the water with temperatures greater than 20° C. Species like bivalves, gastropods, bathydemersal showed a clear dip at the temperature greater than 20° C.  Now the real question is, how the increase in temperature is related to the dip?

To answer this, we need to take a quick look at SST (Sea Surface Temperature) data from the past few decades. From 1955 to 1974, there has been increase in water temperature by 0.06° C. The temperature further increased by 0.1° C from 1975 to 1995 and by 0.8° C from 1996 to 2015. Until 1974, the species distribution was weakly bimodal. Some species even showed no bimodality. However, after 1974 the pattern started to change. With an increase in temperature, the subtropic regions gradually witnessed an increase in the variety of species, creating a dip in the equatorial region. Along with intensity, the width of the dip also gets increasing. It is concluded that these marine species cannot bear the changes in water temperature in equatorial regions and they eventually shift towards the north. The image below shows the distribution graph throughout the different time gaps.

The intensity of shifting goes on increasing more and more northwards since 1974. The species in the southern end of the equator gradually shifted to the southern hemisphere. However, it is not significant in the southern hemisphere. The diversity graph is perhaps one of the best biogeographic patterns ever known. Despite less warming in oceans than on land, marine species are showing rapid shifting activities than those on land. It also shows the need for research on possible evidence of the shifting of land species. 

The sharp, non-linear relationship between sea temperature and species richness clearly explains that equatorial dip over time increases only due to ocean warming. Pelagic species respond more quickly to climate change than benthic species. A possible explanation is benthic species have access to thermal refugia in the deeper water. This is not the first record of an equatorial dip in marine biodiversity. Earlier in the late Quaternary and Triassic periods the ocean warming led to an equatorial dip. 

The only conclusion of this research is that global warming is harming equatorial biodiversity at an alarming rate. Also, the gradual increase in marine population in northern regions creates another series of problems. The equator has already become too warm for many fishes to live. Apart from shifting, a sudden decline in their population can also occur if the ocean water temperature goes on increasing. Despite having different food habits and biological compositions, almost all species showed the same pattern of shifting, which is a very rare event.