Numbers of species in the equator started declining since the last age and before industrialisation, but more species will be lost due to climate warming
The graph shows the number of species at different latitudes during the ice-age (blue), pre-industrial centuries (green) and predicted for 2090s (red).
Previously we found that instead of marine biodiversity being highest at the equator as widely assumed (because it is higher in the tropics generally), that it actually dips there (Chaudhary et al. 2016, 2017, Saeedi et al. 2017). We thought that this may be due to global warming because this is the region with most solar radiation. Now, in collaboration with an international team, we see that this dip occurred before the industrial revolution and thus may not be entirely due to global warming (Yasuhara et al. 2020).
The study used data on microfossils, shells of planktonic Foraminifera from deep-sea sediment cores. By counting the numbers of shells of different species in sediment cores from around the oceans, the number of species occurring in the distant past can be plotted, like using tree-rings to age trees. The dating is not as accurate as tree rings but enabled us to compare how many species existed at different latitudes during and since the last glaciation ice-age the high latitudes were covered in ice.
We found that the number of species was similarly high across the equator during the ice age, but has dipped there since and before industrial times. However, we predict a further decrease due to global warming. Whether this is already underway is the subject of current research.
In contrast to the loss of species in low latitudes, we have predicted that species richness of razor clams and deep-sea shrimps will increase in high altitudes (Basher and Costello 2016, Saeedi et al. 2016).
Basher Z, Costello MJ. 2016. The past, present and future distribution of a deep-sea shrimp in the Southern Ocean. PeerJ 4, e1713. DOI 10.7717/peerj.1713
Chaudhary C., Saeedi H., Costello 2016. Bimodality of latitudinal gradients in marine species richness. Trends in Ecology and Evolution, 31 (9), 670-676. http://dx.doi.org/10.1016/j.tree.2016.06.001
Chaudhary C., Saeedi H., Costello 2017. Marine species richness is bimodal with latitude: a reply to Fernandez and Marques. Trends in Ecology and Evolution 32 (4), 234–237. http://dx.doi.org/10.1016/j.tree.2017.02.007:
Saeedi, H, Basher Z, Costello 2016. Modelling present and future global distributions of razor clams (Bivalvia: Solenidae). Helgoland Marine Research 70 (23). DOI 10.1186/s10152-016-0477-4
Saeedi, H, Dennis TE, Costello 2017. Bimodal latitudinal species richness and high endemicity in razor clams (Mollusca). Journal of Biogeography 44 (3), 592–604. DOI: 10.1111/jbi.12903
Yasuhara M, Wei C-L, Kucera M, Costello MJ., Tittensor D, Kiessling W, Bonebrake TC, Tabor C, Feng R, Baselga A, Kretschmer K, Kusumoto B, Kubota Y. 2020. Past and future decline of tropical pelagic biodiversity. Proceedings of the National Academy of Sciences.
A planktonic foraminifera, Globigerinoides sacculifer. Photo credit: Katsunori Kimoto.
Scanning electron microscopy image of a planktonic foraminifera species, Globigerinella adamsi. Photo credit: Briony Mamo.