Marine ecosystems certainly have less permanence than terrestrial ecosystems. Ashore, ecologists are not confronted with shifting ecological discontinuities, or with changes in thecharacteristic conditions of individual ecosystems, because, unless man intervenes, the tree lineon a mountain or the passage between grassland and savannah remains approximately static overa human lifetime. It is only on the millennial scale that such boundaries migrate significantly, or thatcharacteristic regional ecosystems disappear. Urban sprawl, deforestation, overgrazing, andintensive agriculture are accomplishing in a few decades what nature cannily do in centuries, butthat sad fact does not alter the argument. Although the human population explosion can producepressures that rapidly shift ecological boundaries and modify ecosystems ashore, it isparadoxically more difficult directly to modify the average locations of the ephemeral and shiftingecological boundaries of the sea. We can accomplish this only indirectly by atmosphericmodification, 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 onthe characteristics of the physical environment, then we must assume that ecological conditionsare 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.

The primary purpose of the passage is to:

a. examine differences in the threats faced by marine and terrestrial ecosystems.
b. explain how human intervention has affected marine and terrestrial ecosystems.
c. discuss a distinguishing feature of marine ecosystems
d. present a debate regarding the causes of marine ecosystem impermanence
e. discuss certain consequences of marine ecosystem impermanence