The noun species, referring especially to a group of organisms sharing common characteristics, can be either singular (e.g., that species is purple) or plural (e.g., these species are yellow). This is the convention in scientific writing, and it is usually followed elsewhere.
The word does share a Latin origin with the singular noun specie, but species and specie have diverged in meaning over the centuries and are now unrelated in all their main uses. Specie now refers primarily to coin money, and it appears in the phrase in specie, which means in coin, in kind, or (in law) as specified.
Of course, some people do use specie as a singular backformation from species—for example, homo sapiens is a unique specie—but this sort of use generally doesn’t make it into edited writing, and readers conversant in science might see it as simply wrong.
Because the royal antelope is so small, the bottle’s nipple had to be specially designed for this species. [Globe and Mail (link now dead)]
In areas where only one species is found, the breeding seasons are broader. [Life, David Sadava, H. Craig Heller, David M. Hillis]
Their dwindling numbers warrant protection as an endangered species, federal officials say. [New York Times]
There are three species of hares and rabbits residing in Minnesota, and chances are at least one species lives near you. [Minneapolis Star Tribune]
Therefore, high insecticide use partially explains why resistance has evolved relatively often with these species. [Insect Resistance Management]
The temporary exhibit opens April 15 and runs to May 28, 2012 and showcases the 10 species of translucent sea jellies … [St. Louis Post-Dispatch]