October 25, 2021

I, Science

The science magazine of Imperial College

Benno Simmons discusses extinction debt and why biodiversity might be in an even worse state than we realise

Black rhino dawn Ray Morris1 6149870420_ae31345456_bGlobal biodiversity is declining at an alarming rate as extinction drivers, such as habitat loss, invasive species and climate change, cause huge and immediate biodiversity loss. These are the statistics we see reported in newspapers, magazines and the journal articles every day and they don’t paint a pretty picture. However, things might be worse than we realise. While many species become extinct soon after an environmental perturbation, others take a significant amount of time to disappear: a habitat could be destroyed, but a species living within it not become extinct for another fifty years. This is the worrying phenomenon known as extinction debt, the time lag between an environmental disturbance affecting a species, and its final disappearance. If it’s widespread, there could be large numbers of extant species, committed to extinction, ‘living dead’ species simply waiting to disappear unless something is done. This may sound worrying but it’s important to remember that our understanding and knowledge of extinction debt is actually quite limited. The number of national or regional scale studies is small, so we don’t really know how widespread the problem is, though preliminary evidence is concerning. For example, a recent study from Dr Rob Ewers’ group at Imperial found that, in the Brazilian Amazon, while local extinctions of forest-dependent vertebrate species have been minimal, more than 80% of extinctions are still to come, incurred from historical habitat loss. Another study found that, in Africa, most countries have a debt equal to 30% of forest primate fauna. extinct Dallas Krentzel 3825133154_fc4ac819b4_bWe also don’t know much about how different species might be affected. So far, studies have generally focused on vascular and forest cryptograms (an assembly of organisms like lichen and algae), with studies of animals, other than birds, being in the minority. Theoretically, however, species with low turnover rates (such as perennial plants or mammals) and those with habitat specialisms are the most likely to incur a debt. There’s also more likely to be a debt in landscapes where habitat loss has occurred quickly. Ultimately though, we don’t know much, and there is an urgent need for extinction debt research concentrating on greater range of organisms and conducted at larger scales. The potential implications of many species being doomed to regional or local extinction, even with no further environmental pressures, are understandably wide-ranging. Most obviously, if a debt exists, there will be a window of conservation opportunity to try and prevent it being paid. There has not been much research into the best ways to do this, as empirical examples of how and where predicted extinctions can be prevented by concentrated conservation action are rare, although there are a few examples where such action has been successful. It is also essential for conservation managers to accurately determine whether any extinction debt exists in protected areas: simply designating an area as protected can shield species within it against future threats but we now know there may be unpaid debt from historical pressures. Ultimately, then, extinction debt is a frontier for biodiversity conservation. It has the potential to be incredibly important, yet we know very little about it. The fact that many species in protected areas could be declining to extinction, based on past threats that are no longer acting is definitely worrying, but our lack of knowledge means we don’t really know if this is the case. Extinction debt needs continued focus if we are to save species before those windows of conservation opportunity close for good. Benno Simmons is studying for an MRes in Ecology, Evolution and Conservation Research Images: Black rhino dawn by Ray Morris; Teleoceras proterum and Barbourofelis loveorum by Dallas Krentzel