Perhaps I could take that one.
The constitutional authority for SARA was very heavily discussed when SARA first passed through Parliament. People as eminent as La Forest, the former Supreme Court justice, provided opinions.
The criminal law power is one of the heads under the Constitution that is considered to provide the federal government with jurisdiction over species at risk across the country.The “national concern” branch, under POGG, is another.
I should mention that I'm not a constitutional expert, but I would refer you to the debates from the last time, where this was thoroughly debated.