We are accustomed to thinking of the evolution and distribution of life in terms of its interaction with the environment. The view that polar bears live in cold areas (and thus are white and have large amounts of fat and fur) spreads to the microbial world. Certain organisms are adapted to living under temperatures as high as 121℃ and some thrive in the mild (from an anthropocentric point of view) range of 15-30℃. Of course, the biogeography of these organisms is highly influenced by their habitat, since the latter are cosmopolitan and the former are restricted to the [relatively] few hotspots on and beneath the surface of the Earth. By obtaining information on an environment we are able to already start determining the type, distribution, past and future of the inhabitant life forms. This, as you might already deduce, is prone to bias.
There is another factor in deciding the fate of life forms which is at least as important as the environment.
It perhaps is best observed in the largest ecosystem on the planet – the oceans. In comparison to the large heterogeneity of terrestrial ecosystems, some layers of the oceans can be quite homogenous. The environmental conditions of the surface layer remain mostly constant – salinity, pH, temperature and even nutrients can (and potentially do) remain stable over extended periods of time.
In such places there is no need for organisms to compete for space, nutrients, exposure to light and other environmental factors that are essential to determining the evolutionary success of the species on land (take tropical jungle vegetation as a great example). Still we can observe that some species are more abundant than others, and success can be often seen in a very fast and dramatic manner – algal blooms for example. What gives an advantage of one species against another, if not ability to survive and thrive in the environment?
The answer is grazing and predation. Not only the higher levels of the food chain, but some of the smallest organisms known to man, so small (and admittedly, different) that some do not consider them a part of life at all – viruses. Altogether the success of a species in such an environment is determined largely by its ability to defend against the aforementioned factors, or their ability to destroy (or utilize) our ecological “winners”.
From this discussion it can be deduced that a small change in the food chain can cause massive repercussions across all of its levels. Nowhere it is more evident than in the oceans. And don’t forget that half of the oxygen we breathe comes from there. We better be careful.