Pine martens were virtually extinct in England and Wales by the 20th century. Reintroduction programmes are establishing new populations, with the added benefit of suppressing invasive grey squirrels.
Pine marten welfare post-reintroduction is generally good - animals translocated using best-practice soft release techniques show good survival and rapid establishment of territories. Individual welfare costs of translocation (capture, transport, release into unfamiliar environment) are real but short-term compared to the lifetime welfare benefit of access to suitable habitat. The grey squirrel suppression benefit creates an interesting indirect welfare outcome - fewer grey squirrels mean fewer animals experiencing the suffering of displaced from habitats through competition, and potentially better conditions for red squirrel populations. The long-term welfare question is whether sufficient mature woodland habitat is available to support expanding populations without conflict with human land use.