1.Censuses and surveys can be used to estimate population numbers, but do not provide information on causes of death.
可通过普查和调查来估算人口数量，但这无法提供死亡原因信息。
2.The first step was to use some very vigorous sampling-based methods to estimate population densities of herbivores .
第一步就是采用一些有效的取样方法估算食草动物的种群密度。