To maintain plant gene flow, don’t ditch the ditches

The intensification of agriculture is usually associated with increased habitat fragmentation and loss of diversity.  However, agricultural lands bring with them an unexpected but extensive corridor network in the form of ditches.  Although their intention is to divert water flow around agricultural landscapes, ditch networks also act as linear landscape elements that aid in the dispersal of seeds and pollen, particularly for wetland species.  Ditches can be complex corridors, since they often have varying hydrologic conditions and can be interrupted by dispersal barriers such as culverts.  How this may impact gene flow, and consequently remnant plant diversity in agricultural landscapes, is the focus of recent research by Favre-Bac et al. in their new paper in Heredity.

In comparing the genetics of two hydrochorous wetland species found in ditch networks in France – one rare (Oenanthe aquatica), the other more commonplace (Lycopus europaeus) – they found that both showed substantial genetic differentiation and a significant spatial genetic structure. They also observed high rates of inbreeding due to self-fertilization in L. europaeus.  Ditches played a key corridor role in shaping and facilitating gene flow patterns, with water flow in particular affecting the extent of the local genetic neighborhood for each species.  Genetic diversity was impacted in a species-specific way, although for both species, diversity increased with the number of connected ditches within medium and/or large catchment areas, and decreased with a higher proportion of culverts.

So what makes a ditch a good corridor?  The answer is somewhat species-specific, and management decisions should take into account present abundance and habitat requirements of target species.  For rare species that live in stagnant ditches, such as O. aquatica, altering mowing practices to prevent cutting flowering individuals as frequently could enhance reproduction for individuals in road verge ditches.  If managed correctly, ditches have the potential to act not as marginal habitats, but as key corridors in agricultural landscapes that support rare or declining species and contribute to overall plant biodiversity.