Grazing management creates environments that are more or
less conducive towards the development, survival and availability
of free-living stages of certain parasitic nematode species.
High stocking densities can influence levels of pasture contamination
that may upset the natural sustainable evolutionary balance
between gastrointestinal nematode parasites and their sheep hosts,
with reference to the principle that the parasites do not compromise
their hosts to an extent that will threaten the survival of their
future generations, especially when climatic and environmental
conditions are optimal for the development of contaminating eggs.
It is intriguing that biodiverse environments such as that grazed by
the study flock of Soay sheep may provide a variety of microhabitats
that favour the build-up of infective larval challenge of some
parasitic nematode species, while restricting the development or
contamination with others as a consequence of differential optimal
temperature and moisture requirements, or susceptibilities to
plant secondary metabolites (Githiori et al., 2006; Rios de Alvarez
et al., 2012). Selective grazing or browsing of flora with bioactive
anthelmintic properties, enabled by extensive management, but
prevented by intensification, may also have differential regulatory
effects on nematode parasite species abundance.