Because of fences, differing policies, or different cultures, national borders also mark habitat boundaries for animals and plants. More at Edible Geography.
For example, the antlion surplus in Israel can be traced back to the fact that the Dorcas gazelle is a protected species there, while across the border in Jordan, it can legally be hunted. Jordanian antlions are thus disadvantaged, with fewer gazelles available to serve "as 'environmental engineers' of a sort" and to "break the earth's dry surface," enabling antlions to dig their funnels.
Meanwhile, the more industrial form of agriculture practised on the Israeli side has encouraged the growth of a red fox population, which makes local gerbils nervous; across the border, Jordan's nomadic shepherding and traditional farming techniques mean that the red fox is far less common, "so that Jordanian gerbils can allow themselves to be more carefree."
Update: As this satellite view shows, the US-Canada border quite literally forms a line that cuts through the landscape. I had no idea this fenceless border was so visible. (thx, jonathan)
