Move the mouse cursor to any position on the edge of the target species and then press and hold down the Mouse-Left-Button. Whilst holding down the Mouse-Left-Button trace around the species whose perimeter you wish to measure. Remember that releasing the Mouse-Left-Button indicates that you have finished tracing and the perimeter is "closed" by completing the line from the position where the mouse button is released to the point where the mouse button was first pressed. The process of holding down the Mouse-Left-Button, tracing the outline, and releasing the button can be repeated as often as you like. If the picture starts to look messy with all your tracing remember you can Redraw the image to give you a fresh canvas to continue with.
Note It is relatively easy, at least after a bit of practice, to trace quite accurately. In the examples shown here the traces were intentionally made outside the intended target in order that the trace be easily visible, for instructional purposes.