Age specific impacts of vegetation functional traits on gastro-intestinal nematode parasite burdens in a large herbivore

Gastro-intestinal nematode (GIN) parasites play an important role in the
ecological dynamics of many animal populations. Recent studies suggest
fine-scale spatial variation in GIN infection dynamics is important in
wildlife systems, but the environmental drivers underlying this variation
remain poorly understood. We used data from over two decades of GIN
parasite egg counts, host space use, and spatial vegetation data from a
long-term study of Soay sheep on St Kilda to test how spatial
autocorrelation and vegetation in an individual’s home range predict
parasite burden across three age groups. We developed a novel approach to
quantify the plant functional traits present in a home range to describe
the quality of vegetation present.  Effects of space and
vegetation varied between age classes. In immature lambs, strongyle
parasite faecal egg counts (FEC) were spatially structured, being highest
in the north and south of our study area. Independent of host body weight
and spatial autocorrelation, plant functional traits predicted parasite
egg counts. Higher egg counts were associated with more digestible and
preferred plant functional traits, suggesting the association could be
driven by host density and habitat preference. In contrast, we found no
evidence that parasite FEC were related to plant functional traits in the
host home range in yearlings or adult sheep. Adult FEC were spatially
structured, with highest burdens in the north-east of our study area,
while yearling FEC showed no evidence of spatial structuring. Our findings
support the importance of fine-scale environmental variation for wildlife
disease ecology and provide new evidence that such effects may vary across
demographic groups within a population. Parasite burdens in immature
individuals appear more readily influenced by fine-scale spatial variation
in the environment, highlighting the importance of such heterogeneity for
our understanding of wildlife epidemiology and health.