The series portrays a world where there are occasional winters lasting several years. This world seems to be inhabited by plants and animals very similar to those on Earth: We often see mention of horses, cows, goats, chickens (also wild animals like deer, eagles and wolves, and aquatic animals such as various kinds of fish, octopus and squid) and many others, as well as oaks, cedars, pines and so on.
A Song of Ice and Fire's biosphere is the same as Earth's, with minor differences.
I think it is generally assumed that many of the species found on Earth are also found on Westeros, and vice versa. One could of course claim that what the Westerosi call, say, a goat, is actually an alien beast with purple tentacles, since we never actually see a goat described in detail (a few animals have been described, and often match closely with Earth fauna), but I think that's a bit silly - for most animals, we can probably assume that if it's called a goat it's probably just the same old goat we know and love.
There are select exotic animals that definitely do not exist on Earth. There are of course the dragons; but also manticores, shadowcats, krakens and dire wolves seem like creatures genuinely unique to the world of SOIAF (and mammoths, while not unique, are uniquely not-extinct). There are also a few that seem like "cleverly disguised" mundane animals: Striped horses (apparently zebras), lizard-lions (apparently just crocodiles), snow bears (polar bears) and so on. Likewise with plants, there are a few obviously unique ones like weirwood and ghost grass (which glows in the dark). There are also a few magical creatures like ice spiders, in which case it is unclear whether these are bonafide species of a wild organism, or exclusively artificial.
We could also extend this to microorganisms: Clearly there are some infectious diseases like Greyscale that have no analog in our world (and must be caused by novel microorganisms), but otherwise many diseases are described as having symptoms identical to common Earth ones (colds, flus, poxes, wound infections, cholera).
At any rate, the exceptions aside, the world of SOIAF appears to have flora and fauna that closely resembles Earth. The exceptions do not seem to have caused major distortions in the ecosystem that would undermine this resemblance (for instance, Earth flora and fauna would probably be very different if dragons existed, but this does not seem to apply to SOIAF).
The wildly different climate would have significant consequences on the biosphere
However, the flora and fauna of Earth would be utterly incompatible with SOIAF's climate. Many animals and plants have life cycles dependent on annual winter/summer cycles. Without it, they would be unable to effectively mate and proliferate. Moreover, I don't believe any species on Earth (outside of the Arctic, which is more of an exception that proves the rule) would be capable of surviving multi-year winters - almost all would outright starve, and the plants would perish. Even ocean ecosystems would be devastated as many key species (such as algae and krill) drastically drop in numbers. In short, if Earth started having 7-year winters tomorrow, it would probably cause a mass extinction and our biosphere would take millions of years to recover.
On the other hand, if Earth had 7-year winters from the start (eg. a much bigger orbit and stronger sun), then it seems inevitable that our planet's natural history would be significantly impacted. Many animals and plants would evolve in very different ways. This is not just in the sense of chance events that could have come out differently (eg. our hearts could have ended up on the right) but also the fact that an animal that can weather a 5 month winter must have a very different anatomy and behavior than a similar animal that can survive a 5 year winter: Imagine what it would take for a Westerosi bear to hibernate for the whole long winter.
I don't recall seeing anything in the books that addresses this.
- If the seasonal situation is ancient (millions of years or more), this would imply dramatic adaptations in wildlife. Are such adaptations ever mentioned? Or is it explained how the wildlife survives without them?
- If the seasonal situation is recent (eg. thousands of years), the biosphere should be massively disrupted (since many plants and animals would be unable to survive the long winters). Is there any evidence of such disruption?