单选题 .  ①Marine ecosystems certainly have less permanence than terrestrial ecosystems. ②Ashore, ecologists are not confronted with shifting ecological discontinuities, or with changes in the characteristic conditions of individual ecosystems, because, unless man intervenes, the tree line on a mountain or the passage between grassland and savannah remains approximately static over a human lifetime. ③It is only on the millennial scale that such boundaries migrate significantly, or that characteristic regional ecosystems disappear. ④Urban sprawl, deforestation, overgrazing, and intensive agriculture are accomplishing in a few decades what nature cannily do in centuries, but that sad fact does not alter the argument. ⑤Although the human population explosion can produce pressures that rapidly shift ecological boundaries and modify ecosystems ashore, it is paradoxically more difficult directly to modify the average locations of the ephemeral and shifting ecological boundaries of the sea. ⑥We can accomplish this only indirectly by atmospheric modification, resulting in a changed global climate and a shifted ocean circulation.40.  The primary purpose of the passage is to ______