De l'échelle locale à l'échelle globale : l'importance d'une approche intégrée pour la conservation d'une espèce de reptile en déclin, la vipère péliade (Vipera berus, L. 1758)

Today, evidence points to a global decline in reptiles. However, while the main
causes of this decline are now well known, the mechanisms by which ecosystem
disturbances affect reptiles remain poorly investigated. Indeed, ecosystem
disturbances can act on multiple spatial scales and have cascading consequences on
numerous parameters affecting reptiles. In such a context, a comprehensive analysis
of the complex mechanisms affecting reptile populations cannot be confined to a
single spatial scale. The aim of this thesis is therefore to highlight the multi-scale
aspect of the mechanisms (micro-meso-macro influence spatial scale) governing the
dynamics of reptile populations in Western Europe. To meet this objective, we
decided to build our research strategy around a sentinel species, the common adder
(Vipera berus).
First, our research strategy focused on the role played by the local context of
vegetation structure (micro-influence scale) in the modulation of the predation
pressure exerted on the adder. Using 2400 artificial adders, we were able to
demonstrate that the local context and especially the increase in the local vegetation
complexity had a positive impact by creating microhabitats in which
thermoregulatory behaviour presents a lower risk of predation. In addition, our
results highlighted the increased risk of mammalian predation in the vicinity of linear
elements structuring habitats (hedgerows and forest edges). Our results showed the
modulating role played by the local vegetation structure and microhabitats
availability on predation risks. This study has significant management implications
for snake conservation leading to the formulation of practical recommendations.
We then focused our research on the influence of the local vegetation context
(micro-influence scale) on the abiotic conditions available within microhabitats
(microclimates) and on the abundance of reptiles found within forest edges. Our
results showed that high local canopy cover induces a general cooling of
microhabitats, resulting in a decrease of microhabitat quality within interior forest
edges. Our analyses based on repeated count data of common lizards and adders
present in our network of 55 edges also highlighted the importance of the local
structure of the vegetation within forest edges. Indeed, reptile abundance was
positively related to the amount of light reaching microhabitats at ground level.
Consequently, internal forest edges with an intermediate vegetation stratum along
the exploitation driveway harbour higher estimated abundances than narrow or
steep forest edges. Taken together, these results demonstrate that abiotic
conditions defined by local vegetation structure are of paramount importance for reptiles within habitats with a high degree of canopy closure such as forests. Based
on these results, several practical recommendations have been formulated to
combine forestry activities and reptile conservation.
Finally, we have left the micro-scale to focus on the influence of the global
context (meso- and macro-influence scale) in which populations evolve, and the
impact of global context on the local dynamics of the populations of common adder
found in Western Europe.
To achieve this goal, we have compiled the largest database of standardised
monitoring data on adder population in western Europe. This large database,
comprising 251 populations monitored for at least 7 years, was then used to propose
a multi-site analysis in which we examined, among others, the role played by habitat
parameters (mesp-influence scale) and macroclimatic parameters (macro-influence
scale). Our analyses have confirmed the significant decline of the adder on a sub-continental scale, with an estimated annual loss in population size of 2.05%. Our
analyses also highlighted the positive influence of a high availability of suitable
habitats and of a high diversity of habitats encountered on meso scale (3 km radius
around populations). Finally, our results demonstrated the important role played by
the anthropic management of semi-natural habitats to maintain open environments
suitable for the species. However, this positive effect of anthropic pressure on
natural areas seems to reach a breaking point when anthropic pressure becomes too
high and no longer allows the development of microhabitats favourable to the
species. However, our data did not allow us to demonstrate a significant effect of
measured climatic variables with a macro-scale influence on population dynamics.
Taken as a whole, the results of this thesis underline the importance of
adopting a more holistic and integrative multi-scale approach to better understand
the mechanisms impacting reptile populations. Our results clearly indicate that
habitat characteristics, both on a micro scale (microhabitats-microclimates) and on
a meso scale (landscapes), play a crucial role in explaining the dynamics observed.
However, our analyses were unable to significantly determine the role of climatic
variables (macro-influence scale) on population dynamics. However, the local
context in which individuals evolve is proving to be of considerable importance in
modulating the exposure of reptiles to constraints on a larger scale. Considering the
significant decline in reptile populations, it is essential to develop a conservation
strategy adapted to the 21st century modern landscapes. To this end, we suggest a
reassessment of the traditional dichotomous choice between the concepts of land-sharing and land-sparing. Our proposal is to develop conservation strategies that
combine the concepts of land-sharing and land-sparing within unique agro-ecological landscapes. Consequently, within agricultural landscapes, maintaining and
protecting lenses of favourable habitats is essential to ensure the role of core areas
for reptiles. In parallel, a major effort needs to be made to improve the landscape
matrix permeability to make it more reptile-friendly by extensification of farming and
forestry practices. In these landscapes combining elements of land-sharing and land-sparing, movement between core areas (including migration) is possible, enabling
the establishment of metapopulations. These metapopulations of reptiles are then
more resistant to disturbances acting at various spatial scales, as well as to changes
in the local and global context.