Patterns of avian malaria in tropical and temperate environments: testing the "The enemy release hypothesis"

According to the enemy release hypothesis (ERH) the spread of invasive species will be facilitated by release from their enemies as they occupy new areas. However, the ERH has rarely been tested by comparing populations of native (non-invasive, long established) species with expanding or shifting ranges, to the same species as invasive in another area. We tested the ERH with respect to blood parasite levels (prevalence and intensity of Plasmodium spp. and Haemoproteus spp.) of (a) two closely related, widely distributed species of thrush (Turdus leucomelas and T. merula), and (b) an invasive sparrow (Passer domesticus) whose range has expanded from the Old World to the New World since the 18th century. A total of 158 birds were sampled in Portugal and 99 in Brazil. All bird species were parasitized, and 55% of the individuals collected were parasitized, and the mean intensity of infection was of 28 parasites per 10,000 erythrocytes. We assessed whether differences in levels of infection (prevalence and intensity) were due to site (tropical/New World and temperate/Old World) or host species. The ERH was supported: Passer domesticus and Turdus merula had higher levels of parasitism in the Old World than in the New World. Thus, P. domesticus seems to be benefitting from its "recent" range expansion, compared to T. leucomelas, through ecological release from its native parasites and because the parasites of the recently invaded area seem to be infesting native species instead. Resumo: De acordo com a hipótese da liberação do inimigo (HLI), a disseminação de espécies invasoras será facilitada pela liberação de seus inimigos ao ocuparem novas áreas. No entanto, a HLI raramente é testada comparando-se as populações de espécies nativas (não invasivas, estabelecidas há muito tempo) que apresentam expansão ou alteração de habitats, com populações das mesmas espécies em habitats que foram invadidos. Testamos a HLI com relação aos níveis de parasitas no sangue (prevalência e intensidade de Plasmodium spp. e Haemoproteus spp.). De (a) duas espécies estreitamente relacionadas e amplamente distribuídas de Turdus (Turdus leucomelas e T. merula), e (b) um pardal invasor (Passer domesticus) cujo alcance se expandiu do Velho Mundo para o Novo Mundo desde o século 18. Um total de 158 aves foram amostradas em Portugal e 99 no Brasil. Todas as espécies foram parasitadas e 55% dos indivíduos foram parasitados, sendo que a intensidade média da infecção foi de 28 parasitas por 10.