How can seals affect horses?
In nature seals and horses are hardly seeing each other and are not connected by common food chains. Nevertheless, on Sable Island (Nova Scotia, Canada) researchers observed clear connections between the gray seals and feral horses.
The reason for these connections is the fact that seals transfer significant amounts of nitrogen from the seas to the land. Researchers studied this transfer by measuring d15N-values in marram grass, an important food plant for feral horses on Sable Island. d15N-values correlated with the density of seal pups on the island and with protein content, i.e. nutritive value, of the grass. Given this higher nutritive value, there was also a positive correlation between d15N and horse foraging and scientists speculated that the presence of seals improves living conditions for the horses on Sable island.
Source: McLoughlin et al., Ecology 9, 1929 – 1937.