Grazing management in saltmarsh ecosystems drives invertebrate diversity, abundance and functional group structure

1. Saltmarsh conservation management often involves livestock grazing
to maximise plant diversity and provide suitable breeding habitat for over-wintering
coastal birds. The effect of grazing on invertebrates is rarely quantified, but results
from limited studies of terrestrial and coastal grasslands demonstrate greater abundance
and species richness in un-grazed grassland.
2. The impact of short sward (<8 cm) cattle grazing on the ground dwelling invertebrate
community was assessed on an English inter-tidal upper salt marsh using pitfall
traps. Abundance, species richness, functional group structure, abundance of
coastal specialists, environmental factors that influence invertebrate habitat choice
and food web composition were compared for grazed and un-grazed marsh.
3. In total, 90 000 invertebrates were sampled. Predatory, zoophagus and detritivorous
Coleoptera were significantly more abundant on the un-grazed marsh. In contrast,
predatory Hemiptera and Araneae were significantly more abundant on the
grazed marsh. Sheet weaver spiders were significantly more abundant on the grazed
marsh, foliage running hunters and space web builders more abundant on the ungrazed
marsh. Most inter-tidal coastal specialist species exhibited clear habitat preference
for the grazed marsh. Total species richness was not significantly different
between grazing treatments.
4. Linear direct gradient analysis showed that two environmental variables influenced
by grazing intensity, soil temperature and vegetation height, significantly
explained the composition of invertebrate functional groups. Larger bodied invertebrates
dominated the un-grazed food web.
5. We conclude that both short sward cattle grazed and un-grazed saltmarsh habitat
should be maintained to maximise invertebrate abundance and diversity and provide
suitable habitat for coastal specialists.