Quotations are EXTRAORDINARILY rare in scientific writing--perhaps one in every 50
    manuscripts (just a rough guess here but you should get the point!).  Your job as the author is to
    relate the work of others to your project.  As a result, the preferred tact is to paraphrase the
    work of others and more directly relate this citation to your point.  One much less wordy way
    of using the ideas of Aldo Leopold would be:

           A knowledge of social systems has long been considered of great importance to the
            conservation of a species (Leopold 1948)