Both sentences convey a similar meaning, but there is a slight difference in emphasis due to the use of "that" versus "which." "Other animals are disappearing that at first do not seem to be essential." In this sentence, "that" suggests a restrictive clause, implying that the animals disappearing are specifically those that don't seem essential. It implies a stronger connection between the disappearing animals and their lack of perceived essentiality. "Other animals are disappearing which at first do not seem to be essential." In this sentence, "which" introduces a non-restrictive clause, providing additional information about the disappearing animals. It suggests that while some animals are disappearing, it's not necessarily limited to those that don't seem essential. This construction places a bit less emphasis on the direct correlation between disappearing animals and their perceived essentiality. In essence, the first sentence (using "that") emphasizes a tighter connection between the disappearing animals and their lack of apparent essentiality, while the second sentence (using "which") presents this as one of the characteristics of the disappearing animals without implying as strong a direct relationship.

they both mean the same, they are disappearing in the beginning nobody noticed or thought of as not important but know realise that the disappearance is significant

Does the first one have the connection between other disappearing animals and lacks of essentiality while the second one has the connection between disappearance in animals and lacks of essentiality?