Of course they exist. Even animals recognise them in order to mate themselves, to recognise what to hunt and what to avoid. It's a weird opinion that cats learn to avoid german shepard, bulterier, canis lupus etc and ...
They recognize species {in-group) and {out-group}. That's all that is required to exist, survive and breed. They would also learn to be wary of lynx and bobcat and mink.
Cats (and other animals) learn by experience. Unfamiliar type animals are treated with wary skepticism regardless of taxon group they come from. Parents (when around) pass experience to offspring. Learning works both ways: cats raised with dogs are not wary of dogs. That cats can peacefully coexist with dogs disproves your assertion.
They also certainly do not recognize {mammal} as a group - or any other taxon division - because it is an arbitrary group based on one of many common ancestors.
There is no classification beyond species that is NOT arbitrary on where the lines are drawn for {part of group} vs {not part of group}.
Message 89It depends what exactly you remove. If you remove 75% animals from each species I would say nothing happens - remaining individuals will geometrically fill niches.
Seeing as the fossil evidence is that it was 75% of the species this does not address the real issue but is a non sequitur.
It also neglects the issue of those remaining 25% being isolated in two or more pockets of their former range, now in reproductive isolation. This would allow each sub-population to evolve differently as there would no longer be gene flow between the groups. Thus 75% reduction of each species could lead to massive opportunity for change that would not exist before.
Message 89That's the question. Why crocodiles and sharks didn't occupy emptied niches?
Opportunity. Crocs and sharks did diversify as well, they just didn't compete sufficiently with whale ancestors to prevent their diversification. There was sufficient opportunity for whale ancestors to survive, breed and diversify.
Why after Ambulocetus left estuaries no other mammals entered it again?
Because they did not leave it all at once but in stages. As each stage was taken there was only PART of the "niche" to fill and the opportunities of all the competing species to take bits and pieces along the way means that no complete "niche" was left.
There are also lots of mammals that live in estuary environments, so this "niche" is not left empty nor vacant nor bereft of mammals.
You keep thinking that the niche defines the species. It's the other way around. As the species changes the niche changes with it because it is defined by the species.
Enjoy.
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