An influential early view held that ecosystems contain niches for a limited number of species and that competition for resources among species – whether native or nonnative invading ones – determines ecosystems' species composition. However, factors other than competition often help explain invading species' success. For example, the American grey squirrel, often cited as a classic example of competitively superior invading species, was introduced in England in 1876 and now thrives, while the native red squirrel population has declined. Although scientists have found gray squirrels to be more efficient foragers than red ones, they also note that even before the gray squirrel's arrival, Britain's red squirrel populations had a periodic tendency to die out, only to be subsequently reintroduced. Furthermore, many gray squirrels are silent carriers of a disease fatal to red squirrels.