'Horses for courses', 'one man's meat is another's poison' comes to mind. No doubt if we we had to eat the diet tribes in the rain forest eat, we'd soon become pretty ill.
It could be those yew eating sheep (Ewe? bit of serendipity going on here ) had become tolerant of the yew's toxicity.
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It could be those yew eating sheep (Ewe? bit of serendipity going on here
Cambridgeshire/Norfolk border.