when speaking of abundant biodevirsity and number of different species on islands - why is it happening? Is it because the islands are stranded and natural selection is more intensive there or is it just the opposite - is the natural selection weak there?

Not sure I completely follow your question. There is no particular expectation that selection will be stronger or weaker per se on islands than on mainlands, although isolation can be a key factor in speciation (selection typically - but not always - tends to drive divergence among local populations but this can be swamped by homogenising gene flow if the populations are well connected by individuals moving from one to another to breed).

There is however, a large body of theory called "island biogeography" that gives some good pointers about levels of diversity (e.g. number of species) we should find. Have a look here for the basics

https://en.wikibooks.org/wiki/Ecology/I … ogeography