In some situations ON and ONTO are interchangeable. In others they are not. The child fell ONTO the floor (from the sofa) and then got back onto/on the sofa. In the first use of 'onto' it makes it clear what action is happening. The child fell TO the floor. The child ended up ON the floor. If you said 'The child fell ON the floor' it would not be clear if the child fell from the sofa or fell whilst on the floor. The context might make it clear, of course. So if you said the walruses were forced to move ON the beach - that would mean they were already on the beach and just had to move a bit. You could say they were forced TO the beach. That would make sense. But the best is ON and TO together. They were forced TO move and they went ON to the beach (or this particular bit of Alaskan beach). Presumably, before, they were on nice solid floating sea-ice. But now they have had to move ONTO the beach.

Forced to move on a skinny patch of Alaskan shoreline

The relationship between 'on' and 'onto' is the same as that between 'in' and 'into'. In + to = into On + to = onto The 'to' indicates a movement. If I've picked up a book and put it into my bag it is now in my bag. If I've picked up a book and put it onto the table it is now on the table. The walruses have been forced out of their natural habitat onto a skinny patch of Alaskan shoreline, and they are now on that shoreline. NB Some people insist that 'onto' should be two separate words.

With some verbs, and in some cases, it will not make much difference whether you use "on" or "onto." However, in cases such as your sentence, "onto" is necessary. Think of it this way: you need the aspect of "on" because walrus are now living "on" this patch of shoreline - they do NOT "live the shoreline." They live ON the shoreline. The aspect of "to" is required because of the movement that took place: They were forced to move FROM one place TO this shoreline.

Hello Gio: We can: Jump onto a stage to sing a song.