".....role do you feel the invertebrates play in the grand scheme of marine keeping?"
I'm assuming "invertebrates" to mean other than corals but given that the reefs have an unbelievably huge diversity of inverts, and if one is doing their best to recreate a marine ecosystem, such inverts have to be considered and included which usualy involves having to provide additional habitats. The typical coral display systems that I'm familiar with usualy only include such inverts as part of a clean up crew to wage war on any and all algae or as sessile show pieces (clams, feather dusters...) Thankfully though, a good many hitch hiking inverts (pods and such) are introduced unintentionaly and still manage to survive and multiply regardless of how anti-reef a system is set up as. I'm obviously a big fan of diversity and biased towards inverts as they are extremely abundant and play very important roles within the reef's ecology. BUT... being enclosed systems, there has to be some regulations as its a simple fact that if you introduce a predator/parasite with a few individual prey species that do not themselves rapidly multiply, the prey species is not going to be around long nor is the predator as most have highly specific diets. As such, most inverts that do remain and thrive are those that the systems provides for in way of food (algae/detritus) and are capable of reproducing. Which still leaves a lot of diverse choices, that is, if their habitat is also provided and not restricting that diversity to that which uses rock substrates.
"The reefs ( as a land mass ) are in decline, the fish we keep are in decline....but....what are your thoughts about a decline of invertebrates? Is there one? if not, why not? "
If there is a decline in reefs, being habitat would of course also see a decline in all species within that habitat, inverts included. Such decline almost always involves a shift to algae dominance which then alters the entire community structure, pretty much extending the algae reef flats and its primary production yet the corals are not around to benefit from it anymore and much habitat and diversity is lost. Its not a pretty sight.
Chuck
