A couple of options come to mind.
1. Convert the legend to graphics, ungroup and edit the parts. But once you convert it to graphics it's no longer live and won't update if the data changes.
2. Add a couple of "dummy" layers to the map, one to represent the species and another to represent the sizes. Turn them off so they don't show in the map, but turn them on in the legend in place of the legend item for your actual layer. More work to set up, but the legend will remain live.