Marine species diversity in the recent past, present and future
by Chhaya Chaudhary | Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung Am Handelshafen 12, 27570 Bremerhaven, Germany
Abstract ID: 58
Event: The 3rd AsiaEvo Conference
Topic: Paleo- and macro- ecology in tropical Asia
Presenter Name: Chhaya Chaudhary

The literature has widely discussed latitudinal patterns in marine species richness and their origins, i.e., the richness peaks at low latitudes and declines with increasing latitude. The tropics have always accommodated the highest biodiversity and harbours maximum unique bioregions at present. The deviation from unimodality to bimodality in richness, with a dip at the equator and peaks in sub-tropics, under climate warming has been recently reported.

This talk discusses the change in the bimodal pattern resulting from the widening of the equatorial dip and northward shift (richness) in the northern hemisphere since the 1980s. The shift is in line with the increasing temperature over time, suggesting that the tropics are too warm for species to survive, and species are tracing their habitat in cooler waters. Species richness across latitudinal bands was sensitive to temperature, reached a plateau or declined above a mean annual sea surface temperature of 20 °C for most taxa. The paleo data also observed the equatorial dip during mass extinction under extremely high temperatures.

Using species-specific global climate suitability models (of corals, molluscs, fish, crustaceans, and polychaetes- representing coral reef ecosystem),  the potential coherence and differences and their cumulative impact on the ecosystem under present and future climate scenarios (RCP 4.5 and 8.5) were assessed. Under a warming future, nearly 90% of 30 warm-water species would coherently lose their suitability in the parts of the Indo-west Pacific, the Coast of Northern Australia, the South China Sea, the Caribbean Sea, and the Gulf of Mexico. Irrespective of their taxonomic group, the species with wider distribution ranges (thermal and geographic) will gain higher suitability than their stenothermal counterparts, suggesting an increase of generalist species and a decline of specialist (endemic) species of the ecosystem. This indicates local mass extinction risk in native habitats and a high species turnover across the ecosystem. Such changes may destabilise predator–prey dynamics in the ecosystem, especially if foraging specialists dominate coral food webs and adversely affect the associated countries. As the prediction models plays a crucial part in the strategic planning of marine biodiversity resources, they must be improved to assess the risk more accurately.  One important aspect of the improvement is the inclusion of life stage bottlenecks.

Tropical fish spawners, comparably vulnerable life stage than adults, are already facing temperatures closer to their thermal maxima, coinciding with the equatorial dip. Another bottleneck is the younger life stages, which serve as a primary diet component in marine ecosystems, are more sensitive to extreme temperatures than adults. Such life stage bottlenecks would restrict  species' overall thermal and geographical range.  One crucial part that is currently missing in the prediction models is the inclusion of life-stage bottlenecks, including this information in the models is expected to improve their robustness.