What's observable? That some toads have longer legs than others? It's always been that way, where is the change?
Uh, no. That's the point of the whole article, that it hasn't "always been that way". Not to the present degree. Did you even read the article?
So for years Shine and his colleagues have been tracking cane toads, and as a matter of course they weigh the toads and measure them. Those records came in handy when they discovered that some cane toads at the invasion front were covering an unprecedented mile-and-a-quarter (two kilometers) each night. [...] the toad invasion rate has increased from seven miles per year in the 1950s to a whopping 30 miles per year today, report Shine and his colleagues in the Feb. 16 edition of Nature.