Agricultural landscapes provide a home for numerous plant and animal species. But by reducing the diversity and quality of habitats and their spatial connections, the intensification and simplification of farming practices constitute a threat to biodiversity. In order to clarify the mechanisms underlying the relationships between agricultural landscapes and the maintenance of biodiversity, a research project has been ongoing since 1994 at the CNRS Centre d’Etudes Biologiques in Chizé (CEBC), in partnership with INRA, focused on a cereal-growing plain to the south of Niort. This 450 km2 site is situated in a region traditionally dedicated to mixed farming and livestock.
As an indicator of the biodiversity present at Chizé, the common vole, Microtus arvalis, is the most abundant mammal in European agrosystems. In western France, this rodent is subject to marked demographic cycles, with a regular, three-year alternation of high densities followed by very low densities. These cycles govern the population dynamics of their mammalian and avian predators, which include Montagu's harrier, Circus pygargus. The persistence of this bird of prey, whose populations are declining throughout Europe, is thus to a great extent dependent upon that of the common vole.
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Some of the research carried out by CNRS and INRA aimed to understand the landscape factors that govern the functioning of common vole populations in agrosystems, and more specifically at Chizé. One important aspect regarding maintenance of these populations is the ability of common voles to exploit both perennial environments (such as grasslands) and ephemeral environments (habitats that are only available for part of a year, such as annual crops). Their ability to circulate between these habitats is crucial to their persistence in agrosystems, because agricultural practices, and particularly tillage, destroy colonies and eliminate a large proportion of the population.
In order to characterise the principal aspects of common vole dispersal at this study site, the researchers studied the genetic profiles of different colonies at different spatial scales and as a function of the gender of animals. Through statistical analysis, they showed that the dispersal rates of males (68%) were double those of females (28%). However, more detailed analysis of this dispersal at the spatial scale provided some additional information. In particular, dispersal at a very local scale appeared to be achieved practically only by males, with estimated dispersal rates lower than 1% for females but reaching 44% for males in colonies on the same agricultural plot. By contrast, dispersal rates were more equilibrated at a larger spatial scale (between distant plots): 27% for females and 24% for males.
These differences suggest that dispersal is dependent on different mechanisms in males and females. Thus the dispersal of females may be governed by competition for resources, which probably leads to the colonisation of vacant areas rather than transfers between colonies. On the other hand, an avoidance of consanguinity and competition for reproduction may be the principal factors governing dispersal in males.
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These different modes of dispersal in males and females have important consequences in terms of the functioning of vole populations, and enable a clearer understanding of how important gene flows are maintained at very large spatial scales. Dispersal movements by females allow them to colonise vacant habitats (notably ephemeral habitats in the agrosystem), particularly when densities grow too high in the event of an irruption. By favouring the (temporary) installation of colonies in ephemeral environments, the dispersal of females encourages spatial cohesion of the population. This ability to colonise ephemeral habitats is particularly important in agricultural matrices where perennial habitats are relatively rare (nearly 15% of arable land on the study site), dispersed and only available for part of the year. In particular, it enables the establishment of important gene flows between different habitats, thanks to the intense dispersal of males for reproduction. Thus, although perennial habitats are generally more distant that the mean dispersal distance estimated for the common vole, the colonisation capacities of females, coupled with the intense dispersal of males, make it possible to maintain flows between these environments (see schematic representation above. Philopatry, by contrast with dispersal, characterises individuals who remain where they were born in order to reproduce).
A marked reduction in the proportion of perennial habitats would certainly result in a global decline of common vole populations and flows of individuals – and hence genes – through an agricultural matrix made up of ephemeral environments. This reduction could even cause the disappearance of vole populations if perennial habitats became too rare or too isolated. Maintaining or inserting grasslands into intensive agricultural landscapes could therefore constitute an important means of preserving biodiversity.
Reference:
Sex-biased dispersal patterns depend on the spatial scale in a social rodent. Proceedings of the Royal Society (2009) ; 276, 3487-3494. B. Gauffre1, E. Petit2,3, S. Brodier4, V.Bretagnolle4 and J.F. Cosson1. 1INRA-EFPA, UMR Centre de Biologie et de Gestion des Populations (INRA/IRD/Cirad/Montpellier SupAgro), Campus International de Baillarguet, CS 30016, 34988 Montferrier-sur-Lez cedex, France 2INRA UMR BIO3P (INRA/Agrocampus Ouest/Univ. Rennes 1), Domaine de la MOTTE, 35653 Le Rheu Cedex, France 3UMR ECOBIO (Univ. Rennes 1/CNRS), Campus de Beaulieu, 35042 Rennes cedex, France 4CNRS, Centre d’Etudes Biologiques de Chizé, 79360 Beauvoir sur Niort, France
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