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Why don’t invasive species introduced in a single event die out due to inbreeding

Today I read a BBC Report about how Pablo Escobar had once imported 4 hippos (1 male, 3 female) into his estate in Colombia for his provate zoo. After his downfall, while other species were shipped out, hippos were considered too big to move and expected to not survive.

However, to the surprise of all the hippos are thriving and are so numerous that there have been calls to cull them. From the report:

Numbers are projected to only get bigger. Ms Castelblanco and her
peers say the population will reach over 1,400 specimens as early as
2034 without a cull – all of them descended from the original group of
a male and three females. In the study, they envisaged an ideal
scenario in which 30 animals need to be culled or castrated every year
to stop that happening.

My understanding is that since there was only 1 male, the gene pool would be limited and lead to lot of inbreeding in the descendants. This would cause population to not explode because some individuals would be unfit to survive.

Why has this not happened in case of hippos? Is it because there are 3 females (probably unrelated to each other) which keeps gene pool large enough? Or can mutations explain this phenomenon? Would resuls have been different if originally 2 females had been moved and only 1 retained?

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