Norm - Some types of invertebrates are much better at colonizing new waters than others. Some of the early colonization is due to the proximity of a nearby water body. Many invertebrates to name a few, clams and snails, are sometimes very slow at colonizing new waters. Some invertebrates get introduced but do not establish themselves either short or long term for one reason or another. Sometimes in new water bodies the early colonists have low diversity compared to old established waters. Inflowing stream water is responsible for higher diversities in a lot of new waters. Isolated relatively new water often has high abundances but low diversity.

I am not sure why terrestrial colonization is slower, if it truly is. Is there proof of this? Maybe I misread your last question. Are you talking about, why is it more diffcicult for aquatic species to colonize land habitats vs aquatic habitats? or colonization in general?

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