Changing species

Have you noticed that with most (natural) waters, the status quo is rarely maintained for very long? I'm talking about species, which species dominate, size etc. Very rarely does a water fish the same say 10 years down the line.

What are the reasons for this?

I know that some fish notably Perch, are cyclical. My understanding is that is mainly because of the Perch disease coming and going. I also understand how stocked fish i.e. in most cases Carp, can dominate to the detriment of species such as Tench. I'm really talking about natural waters though, where only nature does the stocking. I understand how Cormorants can decimate a certain size of fish, but as far as I'm aware they are not selective of species.

a good example of this is my local river. It used to be full of small/ medium Chub, alongside a few Trout. Nowadays Chub are extremely rare, i haven't caught one for 3 years. Trout are steady. There has been an explosion of Grayling, they are now the dominant species. Why should Grayling replace Chub? There are lots of Crayfish in the river.

How about your own experiences, and any thoughts why this happens?

its actually best described in ecological terms as an arms race, the physical, animal and vegatable environment is in a constant state of flux, it is dynamic in its quest for survival and niche enhancement and adaptation

on some of the local waters in somerset,tench seem to be on the decline,at the same time there has been an explosion in the roach population.there has also been a significant rise in the numbers of grayling on one river too.i suppose that it`s natural for all species to have their "peaks and troughs" of populations.

Dumb luck plays a part, too - Darwin might just as well have talkesd of the survival of the fortunate. Most populations in mature environments struggle along, but every now and then, conditions favour one group - in your case, it looks as thought the grayling had a very successful spawning a few years back - and they seem to take over for a while. It won't last; enjoy it while you may!
It may also be that a boom has occurred in some food item which nourishes grayling better than chub- did you have really low levels in the spring a couple of years back? That could have increase the area of "riffles", which would allow very shallow-water species - net-spinning caddis for example - to "boom".
If grayling fry feed in much shallower water than chub fry - as I would suspect - then they, too would be set to "explode".