We've talked repeatedly about the large impacts humans have on the global environment and about the consequences those impacts have on contemporary rates of extinction. In particular, I argued that the impact of projected extinctions would be distributed unevenly among taxa, that we would tend to lose narrowly distributed endemic taxa and that they would be replaced with widespread cosmopolitan species, making a world that not only has fewer species but one that has fewer biotic differences among regions. What we haven't talked about are the cumulative effect of the individual losses on ecological communities. It probably won't come as a surprise that just as extinction affects some types of organisms more than others, some biomes will be more affected by changes forecast over the next century than others.1