Large herbivore grazing modulates the effect of inundation frequency on macrodetritivorous invertebrates

Macrodetritivores play an important role in the functioning of salt marshes through the breakdown of organic matter and creation of soil structure. However, macrodetritivores may be negatively affected by large herbivore grazing, implemented as a salt marsh conservation strategy to promote plant biodiversity. This study investigated how low intensity large herbivore grazing affects macrodetritivore and plant communities along a salt marsh elevational, i.e. inundation frequency, gradient; an important driver of salt marsh communities. In both grazed and ungrazed areas, we assessed macrodetritivore communities, plant communities, and various plant and soil parameters. Macrodetritivore abundances were similar in the grazed and ungrazed low marsh, but decreased more steeply with increasing elevation on the grazed marsh. Concurring with this trend, halobiotic species were more abundant on higher elevations in ungrazed compared to grazed areas. Inundation-sensitive macrodetritivores were generally most abundant on the ungrazed high marsh and in tall vegetation patches on the grazed marsh. Canonical Correspondence Analysis indicated that macrodetritivore communities were associated with salt marsh elevation, combined with indirect associations with grazing through vegetation height, litter mass, and increase in soil bulk density. In contrast to negative trends for macrodetritivore communities, plant species richness increased with elevation on the grazed marsh while declining on the ungrazed marsh. Given the contrasting findings for plants and macrodetritivores, large herbivore grazing could have unintended consequences for salt marsh functioning through reduced macrodetritivore abundances.