0:00:03.409,0:00:05.014
A pedra angular original

0:00:05.014,0:00:08.460
refere-se a uma laje em forma de cunha[br]no topo de um arco de pedra.

0:00:08.460,0:00:12.030
Se retirar a pedra angular,[br]corre o risco de tudo cair.

0:00:12.030,0:00:17.081
Alguns ecossistemas funcionam assim,[br]dependendo de uma espécie-chave.

0:00:17.080,0:00:18.906
Em biologia, as espécies-chave

0:00:18.906,0:00:22.127
têm um impacto[br]desproporcionalmente grande no seu habitat

0:00:22.127,0:00:23.700
tendo em conta a dimensão ou número.

0:00:23.700,0:00:26.840
As suas atividades diárias afetam[br]todos os tipos de outras espécies

0:00:26.840,0:00:28.430
direta ou indiretamente.

0:00:28.430,0:00:32.155
Uma alteração no seu número[br]pode causar uma cascata trófica

0:00:32.155,0:00:34.407
em que os efeitos[br]se propagam pelo ecossistema

0:00:34.407,0:00:35.790
de forma surpreendente.

0:00:35.790,0:00:38.650
Veja os lobos-cinzentos[br]do Parque de Yellowstone, no Wyoming.

0:00:38.650,0:00:41.605
O seu número caiu para zero[br]no início da década de 1930

0:00:41.605,0:00:42.860
sobretudo devido à caça.

0:00:42.860,0:00:46.580
Mas em 1995, os lobos[br]foram reintroduzidos no parque

0:00:46.580,0:00:49.138
e o seu impacto[br]tem sido monitorizado desde então.

0:00:49.138,0:00:52.140
Afinal, o impacto é muito importante.

0:00:52.140,0:00:54.510
A ausência e a reintrodução dos lobos

0:00:54.510,0:00:57.800
alteraram Yellowstone[br]de forma significativa e surpreendente

0:00:58.130,0:01:00.160
até a forma como os rios correm.

0:01:00.160,0:01:03.309
Por um lado, os lobos alimentam-se[br]dos alces, e os alces sabem-no.

0:01:03.309,0:01:06.220
Assim, o seu comportamento muda[br]quando os lobos estão na zona.

0:01:06.220,0:01:09.880
Os alces são nómadas, mantendo-se[br]em movimento enquanto comem plantas.

0:01:09.880,0:01:13.825
Mas sem lobos por perto, os alces[br]tornaram-se muito menos cautelosos,

0:01:13.825,0:01:16.860
preferindo instalar-se e comer muito[br]num só lugar de cada vez.

0:01:16.860,0:01:19.270
Os alces até comeram até à margem do rio

0:01:19.270,0:01:22.280
onde não se atreveriam a ficar[br]se houvesse lobos por perto.

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