Age specific impacts of vegetation functional traits on gastro-intestinal nematode parasite burdens in a large herbivore
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Jun 22, 2023 version files 799.09 KB
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data_manuscript_Wiersma2023.csv
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Abstract
- 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.
Methods
This dataset consists of long-term field data and samples collected by St Kilda research teams between 1988 and 2017, and calculated vegetation functional traits based on vegetation surveys of the island between 2008 and 2012. We used census data from the full year to calculate home ranges and strongyle faecal egg counts (FEC) and weight from the August catch each year.
Ten study area censuses of the Soay sheep population are conducted during spring, summer, and autumn, and the location of each sheep is noted (resulting in 30 censuses a year). A home range calculation was run on these sightings, resulting in a home range for each individual per year. Vegetation trait values for each species were obtained from the LEDA TraitBase and Ellenberg values and a sheep's average value for vegetation traits was determined by community-weighted means within its home range.
In August, sheep are captured and morphometric measurements/faecal samples are collected. Gastrointestinal nematode eggs were counted using a modified McMaster technique. Due to difficulty distinguishing species, the number of eggs of five main nematode species was included as one measurement (FEC). Individual faecal samples were associated with an individual's averaged census location for INLA spatiotemporal modelling.