The Integration and Application Network at the University of Maryland Center for Environmental Science has developed a set of free Illustrator symbol and swatch libraries of environmental and life sciences topics. There are over 1500 symbols of agriculture, plants, fish, reptiles, birds, animals, insects, boats, buildings, mapping, weather and more, as well as 100 custom swatches. These high-quality color vector symbols and swatches can be used in Macintosh or Windows Illustrator 10 or newer.
If you work in a life sciences-related field, you definitely need these. If not, you still will probably find a few things you can use here. More information about working with symbols in Illustrator can be found here.