GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - H2UZQ2C4IECBB675K

Marine ecosystems certainly have less permanence than terrestrial ecosystems. Ashore, 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. It is only on the millennial scale that such boundaries migrate significantly, or that characteristic regional ecosystems disappear. Urban 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. Although 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. We can accomplish this only indirectly by atmospheric modification, resulting in a changed global climate and a shifted ocean circulation. Indeed, if we are agreed that the regional characteristics of marine ecosystems are consequent on the characteristics of the physical environment, then we must assume that ecological conditions are as impermanent as the physical conditions themselves. And these, it is now well understood, are in continual flux and state of change at all scales of variability.