Trace mineral supplies for populations of little and large herbivores

Abstract

Copper (Cu), iron (Fe), and zinc (Zn) are essential trace minerals for the reproduction, growth, and immunity of mammalian herbivore populations. We examined the relationships between Cu, Fe, and Zn in soils, common plants, and hepatic stores of two wild herbivores to assess the effects of weather, sex, and population density on the transfer of trace minerals from soils to mammals during the growing season. Soils, grasses, woody browse, hispid cotton rats (Sigmodon hispidus), and white-tailed deer (Odocoileus virginianus) were sampled across 19 sites. Concentrations of Cu, Fe, and Zn in grasses and browse species were not correlated with concentrations of those minerals in soils sampled from the same areas. Leaves of woody browse were higher in Cu, lower in Fe, and similar in Zn when compared with grasses. Available concentrations of soils were positively related to liver Cu and Zn in hispid cotton rats, which was consistent with the short lives and high productivity of these small mammals that rely on grass seed heads. Interactions between soil concentrations and weather also affected liver Cu and Fe in deer, which reflected the greater complexity of trophic transfers in large, long-lived, browsing herbivores. Population density was correlated with liver concentrations of Cu, Fe, and Zn in hispid cotton rats, and concentrations of Cu and Fe in deer. Liver Cu was < 5 mg/kg wet weight in at least 5% of animals at two of eight sites for hispid cotton rats and < 3.8 mg/kg wet weight in at least 5% of animals at three of 12 sites for deer, which could indicate regional limitation of Cu for populations of mammalian herbivores. Our data indicate that supplies of trace minerals may contribute to density dependence of herbivore populations. Local population density may therefore influence the prevalence of deficiency states and disease outbreak that exacerbate population cycles in wild mammals.

Fig 1. Locations of grassland study sites (n = 19) in relation to long-term environmental conditions in Texas.

Precipitation and temperature gradients are 30-year annual averages from 1981–2010 adapted from open access data through PRISM Climate Group [27], across Texas. Ecoregion classification, latitude, and longitude for each site are listed in S1 Table.

Fig 2. Dry-weight concentrations of trace minerals across trophic levels.

Box-plot of copper (Cu), iron (Fe), and zinc (Zn) concentrations for soils, grasses, woody browse, hispid cotton rat livers (HCR; Sigmodon hispidus), and white-tailed deer livers (WTD; Odocoileus virginianus) across Texas grasslands (S4 Table).

Fig 3. Stable isotope values of grasses, browse, hispid cotton rats, and white-tailed deer.

Values for δ¹⁵N and δ¹³C in two species of grass [i.e., little bluestem (Schizachyrium scoparium; n = 23) & silver bluestem (Bothriochloa saccharoides; n = 23) (S5 Table)], two species of woody browse [i.e., eastern red cedar (Juniperus virginiana; n = 18) & honey mesquite (Prosopis glandulosa; n = 20) (S5 Table)], and two herbivores [i.e., hispid cotton rats (HCR; Sigmodon hispidus; n = 73) (S5 Table) and white-tailed deer (WTD; Odocoileus virginianus; n = 283) (S5 Table)] across grasslands in Texas (S1 Table). Symbols are average values with standard deviation bars.

Fig 4. Deer density relationship with deer diet.

Relationship between density of deer (#/km²; Odocoileus virginianus) and δ¹³C values of heart tissue (± 95% CI) across 11 grassland sites in Texas (S1 Table). Symbols are values for males (solid) and females (hollow) at each site predicted by mixed model regression.