Posted 02 February 2013 - 05:04 AM
Yes it mainly depends on species, but a general trend may be that generalist species with no special needs in terms of habitat, diet, climate etc (also called ubiquitous, euryoecious or eurytopic, considering the different factors) have stable populations or increase even (examples are rats, mice, raccoons, sparrows, pigeons etc), whereas specialist species with quite narrow niches and specialist needs concerning the mentioned factors have often declining populations (stenotopic, stenoecious species). Examples are the koala, many amphibian species, wildcats, polar bears, bats).
I think nowadays more or less all factors influencing this are influenced finally by human beings. The most important factor here is the loss of habitats and landscapes where such species can live, such as rainforest or wetlands (the discussion about this should be known, agriculture is the keyword here). Other factors are pollution (see condor), poaching (especially large animals) or climate change (displacing animals with special climate needs).
I think the best data are available for large animals, birds and also amphibians, as here public interest and therefore good research networks and money exists. But it also depends on the areas/continents. Places such as US or Europe are more or less well surveyed but others such as southern America, Africa or as an extreme example the deep sea are less or not at all investigated.
Domestic animals and cattle are a different topic, as they are out of the natural cycles and depend on human support almost only. I'd say their numbers depend on economic factors, and because today the meat consumption increases, the prices are falling, their numbers increase too (as efficient production as possible of high numbers for a low price similar to other mass-produced goods).
About fish I've not much idea but I agree that the economically used species are massively overfished. They will not die out but are finally so rare (and then stable at low level) that it's useless to try to catch them and then other species are more used. Anyway some really endangered species exist (some tuna and shark species).
One must presume that long and short arguments contribute to the same end. - Epicurus
...except casandra's that belong to the funniest, most interesting and imaginative (or over-imaginative?) ones, I suppose.