请讨论比较级补出还是不补出

1.      rain forests contain more trees, and in higher density, than any other ecological system.

than 下面划线，这是最简单的一个sc了，只有一个单词划线。第一眼没仔细想，差点选错，答案里有than, than that of, than those of, than does, 还有一个不记得了，应该是错的，最后选了than does.

我觉的是 THAN. 因为比较的是主语.

举例:Although the pace of exploitation of wetland resources perhaps did not parallel that of the felling of virgin timber or the breaking of the prairies, they now constitute one of the last frontiers of unutilized land and, for this reason, are dwindling in acreage faster than any other ecological system.