In this post Juan P. González-Varo, Rafael G. Albaladejo, Marcelo A. Aizen, Juan Arroyo and Abelardo Aparicio discuss their recent paper ‘Extinction debt of a common shrub in a fragmented landscape’.

A key question with direct implications for biodiversity conservation and restoration in fragmented areas is whether the persistence of those species we currently observe in habitat remnants is ensured in the long-term. Habitat-specialist species, especially long-lived species such as trees and shrubs, may be initially retained within remnant patches after habitat loss and fragmentation. Yet, the disruption of demographic processes along with adult mortality can cause delayed extinctions. The extinction debt represents the number of populations expected to eventually become extinct after habitat destruction. Extinction debts can be assumed if current species occurrences are better predicted by past than present landscape features. The rationale is that some species occurrences may be mirroring past landscapes that sheltered more habitat cover than nowadays. In other words, that ‘living dead’ populations still persist in the current unfavourable landscapes thanks to the longevity of their individuals. The importance of identifying extinction debts is that there are still opportunities to counteract future biodiversity loss if we know which are the causal mechanisms that lead to recruitment collapse and adult mortality. The latter is essential for developing effective management plans for conservation and restoration.

Example of contemporary woodland loss in the Guadalquivir Valley. Unviable populations of long-lived plant species may occur in the new habitat remnant just because they still mirror the larger woodland patch of the past. Photo credit: Google Earth © 2015 DigitalGlobe.
Example of contemporary woodland loss in the Guadalquivir Valley. Unviable populations of long-lived plant species may occur in the new habitat remnant just because they still mirror the larger woodland patch of the past. Photo credit: Google Earth © 2015 DigitalGlobe.
Woodland remnants in the Guadalquivir Valley are typically small and highly disturbed, and surrounded by an agricultural matrix. Photo credit: Juan P. González-Varo.
Woodland remnants in the Guadalquivir Valley are typically small and highly disturbed, and surrounded by an agricultural matrix. Photo credit: Juan P. González-Varo.

In our paper, we prove there is an extinction debt for Mediterranean myrtle (Myrtus communis) populations in the Guadalquivir Valley, south-west Spain. The myrtle is a long-lived sclerophyllous shrub that inhabits warm areas of the Mediterranean Basin and it is a key component of well-preserved woodlands. The Guadalquivir Valley is a lowland region that has lost nearly half of the remnant woodland cover during the last fifty years as a result of intensive agriculture, urbanization and infrastructure development. Nowadays, the expected past ubiquity of this common shrub across the valley is mostly represented by small populations composed of adult plants in small and highly disturbed woodland patches where regeneration is negligible owing to reduced habitat quality (edge effects and disturbance). We show that myrtle occurrence is associated with past (1950s) – rather than present (2000s) – woodland cover in the surrounding landscape – a pattern that arises because the current distribution of the species still mirrors the more forested landscapes found in the region fifty years ago. But paying the debt is just a matter of time considering the elevated susceptibility of small and disturbed populations to environmental stochasticity and the impacts that currently threaten the remnant woodlands of the region.

The Mediterranean myrtle is currently a common species that provides flowers and fruits to several flower-visiting insects and frugivorous birds. Photo credit: Juan P. González-Varo.
The Mediterranean myrtle is currently a common species that provides flowers and fruits to several flower-visiting insects and frugivorous birds. Photo credit: Juan P. González-Varo.

We encourage authorities to devote conservation efforts on preserving those ‘yet’ common species that, as myrtle, serve as indicators of ecosystem functionality. Because myrtle provides food sources through its flowers and fruits to several flower-visiting insects and frugivorous birds, preserving myrtles is expected to have multiple effects at the community level. Specifically, we encourage authorities to preserve certain woodland patches as micro-reserves in order to avoid disturbances such as grazing, trampling, clearings, or fires, and promote succession; thus, improving habitat quality. This is not in disagreement with certain uses such as small-game hunting, which currently happens in nearly half of patches.

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