How large is the extinction debt, and when will it be paid?

Measure how many species are committed to delayed extinction by past habitat loss and fragmentation, and how long until they are lost.

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1	# How large is the extinction debt, and when will it be paid?
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3	After a forest is cleared or a grassland fragmented, many species hang on for years or decades before disappearing. These delayed losses, the extinction debt, mean that today's species counts can overstate what a landscape will keep, but while the species persist there is still time to act.
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5	## Question
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7	What is the magnitude of the extinction debt, the species committed to extinction by habitat loss and fragmentation that has already happened, and when will it be paid? It applies to any taxon and landscape.
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9	## Background
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11	The question is one of 100 fundamental ecological questions drawn up for the British Ecological Society's centenary from 754 questions submitted by 388 participants, in the theme on human impacts and global change [1]. Conservation targets based on present-day species richness may look met while losses are still pending.
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13	## What is known
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15	- A model of competing species in patchy habitat predicted that even moderate habitat destruction causes time-delayed but deterministic extinction of the dominant competitors; because these extinctions come generations after fragmentation, the authors called them a debt [2].
16	- Debts have been detected across many ecosystems and taxa, estimated at 9–90% of current species richness and sustained for 5–570 years so far, with payment projected to take up to 1,000 years [3].
17	- In 2,880 US breeding-bird communities, current diversity is still largely explained by past landscapes, implying extinction debts across 52% of the country, notably in recently urbanised areas, and colonisation credits elsewhere [4].
18	- Measurement is contested. Debts inferred by extrapolating species–area relationships may be largely a sampling artefact, overestimating extinctions by more than 160% [5]; yet of three species–area methods, only the one needing past species richness, rarely recorded, detected debts in all habitats studied, so debts may also be under-reported [6].
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20	## What would count as progress
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22	No single result settles a question like this; contributions like these would move it forward:
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24	- Open resurvey datasets that record species at the same sites before and long after dated habitat loss, so that debts are measured directly rather than inferred.
25	- A model that predicts which species and sites carry a debt from traits and landscape history, validated against later resurveys.
26	- Restoration experiments that test whether enlarging or reconnecting habitat prevents predicted extinctions.
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28	## References
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30	1. Sutherland, W. J. et al. (2013). [Identification of 100 fundamental ecological questions](https://doi.org/10.1111/1365-2745.12025). Journal of Ecology 101(1), 58–67.
31	2. Tilman, D., May, R. M., Lehman, C. L. & Nowak, M. A. (1994). [Habitat destruction and the extinction debt](https://doi.org/10.1038/371065a0). Nature 371(6492), 65–66.
32	3. Figueiredo, L. et al. (2019). [Understanding extinction debts: spatio–temporal scales, mechanisms and a roadmap for future research](https://doi.org/10.1111/ecog.04740). Ecography 42(12), 1973–1990.
33	4. Haddou, Y. et al. (2022). [Widespread extinction debts and colonization credits in United States breeding bird communities](https://doi.org/10.1038/s41559-021-01653-3). Nature Ecology & Evolution 6(3), 324–331.
34	5. He, F. & Hubbell, S. P. (2011). [Species–area relationships always overestimate extinction rates from habitat loss](https://doi.org/10.1038/nature09985). Nature 473(7347), 368–371.
35	6. Ridding, L. E. et al. (2021). [Inconsistent detection of extinction debts using different methods](https://doi.org/10.1111/ecog.05344). Ecography 44(1), 33–43.
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37	## Source
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39	Imported from [Identification of 100 fundamental ecological questions, question 73](https://doi.org/10.1111/1365-2745.12025), curated by Sutherland et al. Listed as open in the source (2013) and still an active research question as of 2026-10-20. The statement above is written in our own words; see the source for the original wording, history and discussion.