Demographic response to patch destruction in an endangered amphibian

Is rehabilitation always a good thing? Hugo Cayuela suggests alternative approaches for forest managers following the recently published article, Demographic response to patch destruction in a spatially structured amphibian population.

Economic activities such as logging and mineral extraction can result in the creation of new anthropogenic habitats (e.g. temporary aquatic habitats) that may host specific biodiversity, including protected species. However, legislation in many Western European countries requires the rehabilitation of ‘damaged’ areas after logging and mining operations. This can eliminate those early successional habitats. Conservation managers face a dilemma in these situations and often lack knowledge about the impacts of environmental rehabilitation on the population dynamics of pioneer species. Therefore, they  are unable to take this into account in their actions.

In this study, we examined how breeding patch destruction caused by environmental rehabilitation might affect the demographic processes and long-term viability of a spatially structured population of a pond-breeding amphibian, the yellow-bellied toad Bombina variegata. This endangered species has recently become extinct in several regions of Western Europe. It is protected by national legislation over much of its range and is registered in Appendix II of the Bern Convention and Appendices II and IV of the EU Habitat Directive. In harvested forests,  the yellow-bellied toad breeds in artificial waterbodies (e.g. ruts and residual puddles) resulting from logging operations. We analyzed the effects of patch destruction on adult survival and the long-term population growth rate in an SSP consisting of 28 breeding patches (i.e. discrete groups of waterbodies). Over the nine years of the study (2000–2008), environmental rehabilitation policy led to frequent events of patch destruction (N=13), which occurred during either the overwintering or the breeding period. Our analysis revealed that dispersal not resulting from patch loss was relatively high and was sex-biased. They also showed that patch destruction had a negative impact on adult survival. Moreover, simulations showed that the frequency of patch destruction had a strong negative influence on the population growth rate. This impact was intensified if female fecundity was also affected.

Bombina variegata in its reproductive habitat (i.e. ruts) in harvested woodland (A) and a breeding patch used by Bombina variegata in 2006 (B), which was destroyed during the toad overwintering period in winter 2006 by logging rehabilitation operations (C). Image A. by Matthieu Berroneau.

In light of these findings, we considered how conservation measures might mitigate the effects of patch destruction on B. variegata spatially structured population dynamics. In Western Europe, forest management policies seek to promote environmental rehabilitation at the end of logging operations to improve future forestry activities and transport access. This confronts forest managers with a dilemma: on the one hand, rehabilitating the area damaged by repeated skidder operations, resulting in the destruction of the species’ breeding habitat and provoking adult over-mortality; on the other hand, maintaining breeding patches at the cost of possible economic losses related to silviculture and track usability for future wood exploitation. To solve this dilemma, our primary recommendation is to maintain a relatively stable number of patches between years to reduce the risk of extinction of the whole SSP. Today, the SSP considered in this study is virtually extinct due to the high frequency of patch destruction and the dramatic diminution of the amount of breeding patches (from 28 in 2008 to 3 in 2015–2016). To avoid this situation in other harvested woodlands, we recommend the following measures:

(1)        Avoid the systematic rehabilitation of the breeding patches at the end of logging operations. Our study revealed that the extinction risk to SSPs can be mitigated by reducing the frequency of patch destruction to one event every 10 years or less and by limiting the effect of patch destruction on female fecundity.

(2)        Compensate for patch loss resulting from rehabilitation by creating substitute breeding patches near the destroyed patches, in areas that do not substantially affect forest regeneration and vehicle use. The substitute patches should be created before patch destruction to allow individuals to reach them directly after the rehabilitation operations.

(3)        Maintain a patch network that remains unaffected by rehabilitation. Future studies should investigate how the proportion of unaffected patches and the destruction frequency in unprotected patches affect long-term population viability to propose detailed management recommendations.

Read the full article, Demographic response to patch destruction in a spatially structured amphibian population in Journal of Applied Ecology.

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