I would add that the word 'purge' implies a deliberate effort to remove something- and it is usually an idea of purification. So you might try to purge an illness from your body- it is quite an aggressive and deliberate term. This means your sentence "Before we purge the marsh of all wildlife, we should change the way we use the land" implies that we are deliberately killing the wildlife, rather than doing so out of carelessness. Otherwise, your understanding is correct.