Give preference to mixed flocks of sheep to limit the spread of exotic plants in their fleece - DISPERSAL

The Loire carries sediments laden with seeds from native and exotic plants, which are deposited during flood and recedeing waters cycles on the river floodplains. At the interface between aquatic and terrestrial environments, DispersaL has investigated the impact of sheep grazing (as part of the Pasto’Loire initiative) on areas managed by the CEN (Conservatoire des Espaces Naturels) Centre Val de Loire. The focus was put on the ecological processes associated with the presence of domestic herbivores on the spatial and temporal dynamics of the flora in grazed areas, in particular herbivory and zoochorous dispersal. Certain benefits of sheep grazing are well established, notably the maintenance of open habitats and the risk reduction of log jams, but the potential risks associated with the spread of exotic plants within areas renowned for their biodiversity, in landscapes designated as UNESCO World Heritage sites, are much less so.

Approaches

DISPERSAL assessed riverine inputs across various components of the flora: the soil seed bank, the local expressed flora, the grazed flora and the flora dispersed by sheep. In parallel, and on the same sampling plots, the entomofauna associated with pastoralism and involved in the recycling of organic matter was studied over two annual cycles.

In addition to in situ observations, an experiment was conducted under controlled conditions with Romane breed ewes and ewe lambs reared at the INRAE P3R experimental unit in Osmoy. It tested the effect of animal sociality on the epizoochorous dispersal of Xanthium strumarium, an invasive alien species found along the banks of the Loire river.  We assessed the effect of the ewe-to-lamb ratio within the group on the retention time and dispersal distance of this plant.

Results

Cocklebur Xanthium strumarium, an invasive alien species found along the banks of the Loire river, produces fruits with hooks that facilitate their epizoochorous dispersal (though they can also float and be carried downstream). Its fruits were found in the fleeces of the majority of the sheep studied (18 out of 24 fleeces). Experiments with ewes and ewe lambs have also shown that these fruits can be dispersed over significant distances (several kilometres). The conditions are such that sheep grazing on sites where this plant is established act as vectors for the species’ spread, and that, particularly as they move along grazing routes, this plant can invade the landscape.

The more ewe lambs there are in the flock, the shorter the retention times and the shorter the dispersal distances of the fruits. The social behaviour of ewe lambs (more contact with other animals and more individual grooming) causes the fruits to detach more quickly from the fleece or be transferred to other sheep with poorer fruit adhesion. From a practical perspective, managing mixed flocks comprising both adults and young tends to reduce the dispersal distances of this plant’s fruits.

In the 24 fleeces studied, 35 different species were identified (between 3 and 13 different species and between 8 and 377 diaspores of a single species per fleece). These results confirm the high epizoochorous potential of sheep.

The floristic study conducted shows that sheep exhibit little food selectivity, with grazing having a significant effect, leading to a reduction in biomass (coverage by species), maximum plant height, species density and diversity. The stocking density present in each of the enclosures over a period of around ten days exerts grazing pressure that very rapidly reduces food availability and therefore requires regular movement from grazed areas to ungrazed areas.

The study of the soil seed bank (carried out by UMR Citeres) identified 117 taxa capable of germination. Five species account for more than half of the germinations (Portulaca oleracea, Urtica dioica, Lindernia dubia, Digitaria sanguinalis and Cyperus fuscus) and very few exotic species were detected.

The expressed local flora differs from the soil seed bank, but seed fall helps to explain part of the expressed flora.

The presence of sheep grazing along the banks of the Loire maintains rich communities of dung beetles (39 species identified, some of which are subject to protection status) involved in the recycling of organic matter.

Participants

INRAE units involved

• UR EFNO - Unité de recherche Écosystèmes Forestiers
• UE P3R - Pôle de Phénotypage des Petits Ruminants

Partners 

• Conservatoire d'espaces naturels Centre-Val de Loire
• UMR CITERES - INSA Centre-Val de Loire
• Biodiv'AG - University of Angers
• Laboratoire d'éco-entomologie (environmental consultancy services)