Past exam of the year 2 of the earth sciences (geology) course of the University of Oxford / 2023 / Paper 1 / 12

A mass extinction is a geologically brief, geographically widespread loss of a large fraction of species across multiple major groups, substantially exceeding background extinction rates. Severity, global extent and rapidity distinguish it from the disappearance of a local population or the ordinary loss of individual lineages. A threshold near $75\%$ of species is often used operationally, but the qualitative definition does not depend on one universal numerical cutoff. The duration is short on geological timescales, and need not mean that every species disappeared simultaneously in a single day.

A mass extinction is driven by changes that overwhelm organisms' physiological tolerances or destroy their food and habitat systems. Distinguish the initiating forcing from the linked environmental mechanisms: one event can generate several contributors.

These examples supply more than four drivers or contributors. The historical effects of volcanism, warming, oxygen loss and elevated carbon dioxide can act together rather than compete as mutually exclusive explanations.^[ref]

Extinction selectivity means that the probability of extinction depends systematically on biological or ecological traits, rather than being equal across all species. An empirical association needs a specified event, group and taxonomic level; a trait protective in ordinary background turnover may not protect during every mass extinction.

Geographic range: at the end-Cretaceous crisis, geographically widespread marine bivalve genera had lower extinction intensity than narrowly distributed genera. This makes range breadth a demonstrated selective trait for that dataset. The same analysis found no clear selectivity by body size, so that result should not be generalized to every trait or event.^[ref]

Body size: large-bodied mammals suffered disproportionately during late-Pleistocene and later Quaternary losses, including the loss of many giant herbivores and carnivores. A global analysis over the last $125000$ years found strong size-selective losses across continents and trophic modes. Large size can correlate with slow reproduction, high resource demand and exposure to human hunting, but the observed size pattern and its possible mechanisms should be distinguished.^[ref]

Sexual dimorphism and reproductive investment: a study of fossil cytheroid ostracods found much higher estimated extinction rates in species with stronger male–female carapace dimorphism, interpreted as greater male investment in reproduction. This supplies a third empirical trait association; the observed relationship is not a universal claim that sexual selection is always harmful.^[ref]

The examples concern different time intervals and levels, including genera in the first example and species in the third. They demonstrate extinction selectivity without assuming one set of trait effects is universal across all crises.

Extinction debt is future extinction committed by past environmental change but not yet realized because populations persist temporarily. After habitat loss or fragmentation, surviving adults and small isolated populations can remain visible even when successful recruitment and recolonization are too low for long-term persistence. Long generation times can delay the observed species loss.^[ref]

Thus current species richness can overstate the diversity sustainable under present habitat conditions. Restoration or renewed connectivity before the delayed losses occur may avert part of the debt; the concept is a lagged consequence of past change, rather than a prediction that every threatened population is irreversibly doomed.