Equatorial decline of thousands of marine species is gain for higher latitudes

A new paper from Chhaya Chaudhary’s PhD shows that the latitudinal gradient of species richness has been changing in concert with climate change since at least the 1950s (Chaudhary et al. 2021). Thousands of species have left the equator for higher latitudes as the oceans warm.

While previous studies predicted such changes in the distribution of marine life, including on marine species in the North Atlantic and Pacific (Burrows et al. 2019), razor clams (Saeedi et al. 2016) and Antarctic shrimps (Basher et al. 2016), this paper shows it has been happening across all kinds of species for decades. The study included 48,661 animal species obtained from the Ocean Biodiversity Information System (OBIS).

The pattern applied to species living on the seabed (benthic) and in open water (pelagic), fish, molluscs and crustaceans. Pelagic species had declined more at the equator and shifted poleward in the northern hemisphere more than benthic. The lack of a similar shift in the southern hemisphere was because ocean warming has been greater in the northern than the southern hemisphere.

Previously, the tropics were considered stable and an ideal temperature for life because so many species occur there. Now, we realise that the tropics are not so stable and are increasingly too hot for many species.

Last year, a study led by Moriaki Yasuhara showed that while marine biodiversity peaked at the equator during the last ice age, 20,000 years ago, it had already flattened before industrial global warming. That study used fossil records of marine plankton buried in deep-sea sediments to track the change in diversity over thousands of years (Yasuhara et al. 2020).

Chhaya’s work began following a previous PhD student, Hanieh Saeedi, noticing that the richness of razor clams declined at the equator and peaked in the subtropics, so the latitudinal gradient was bimodal (Saeedi et al. 2017). Together, we published two reviews showing that this was the case for all marine taxa, even when sampling biases were accounted for (Chaudhary et al. 2016, 2017). Surprisingly, all previous studies either did not comment on this or plotted their data in ways that concealed the pattern. The equatorial dip also seems present in amphipods, polychaetes and biogenic habitat-forming taxa (Arfianti and Costello 2020, Pamungas et al. 2021, Pages et al. in preparation). However, not all taxa may follow this gradient. The results of another study suggested that benthic invertebrates may be richer in species and abundant in colder latitudes due to release from fish predation (Edgar et al. 2017, Pages et al., in preparation).

The graphs below show the average species richness for each 5-degree latitude band (dots) and modelled average richness (lines) that accounts for sampling effort. They suggest a decline of about 40% of species at the equator in the past 60 years, comprised of 17% benthic and 67% of pelagic species.

Equatorial declines of benthic species are about 3,000 to 2,500 (17%), of pelagic 900 to 300 (67%) and overall species 4,000 to 2,500 (37%).

References

Arfianti T., Costello 2020. Global biogeography of marine amphipod crustaceans: latitude, regionalization, and beta diversity. Marine Ecology Progress Series 638: 83–94.

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

Burrows MT, Bates AE, Costello MJ, Edwards M, Edgar GJ, Fox CJ, Halpern BS, Hiddink JG, Pinsky ML, Batt RD, Molinos JC, Payne B, Schoeman D, Stuart-Smith RD, Poloczanska ES. 2019. Ocean community warming responses explained by thermal affinities and temperature gradients. Nature Climate Change 9, 959–963.

Chaudhary C., Saeedi H., Costello MJ. 2016. Bimodality of latitudinal gradients in marine species richness. Trends in Ecology and Evolution, 31 (9), 670-676.

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

Chaudhary C, Richardson AJ, Schoeman DS, Costello 2021. Global warming is causing a pronounced dip in marine species richness at the equator. Proceedings of the National Academy of Sciences, online. DOI 10.1073/pnas.2015094118.

Edgar GJ, Alexander TJ, Lefcheck JS, Bates AE, Kininmonth SJ, Thomson RJ, Duffy JE, Costello MJ, Stuart-Smith RD. 2017. Abundance and local-scale processes contribute to multi-phyla gradients in global marine diversity. Science Advances, e1700419.

Pamungkas J, Glasby CJ, Costello 2021. Biogeography of polychaete worms (Annelida) of the world. Marine Ecology Progress Series 657, 147-159. https://doi.org/10.3354/meps13531

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, 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 117 (23), 12891-12896. www.pnas.org/cgi/doi/10.1073/pnas.1916923117