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Their work supports earlier findings, based on DNA markers and crossing experiments which suggested that F. xylarioides is a species complex containing distinct, host-specific populations.

And one of the most abundant milkweed-visiting aphids, the nonnative oleander aphid, is host-specific, meaning it doesn’t eat other plants.

“Parasites, particularly the host-specific species, are perhaps the most imperiled group of organisms on Earth,” he says.

All are host-specific fleas that parasitize one species each.

If a theoretical possum species were living in four populations—only two of which carried host-specific fleas—and the two flea-carrying populations disappeared, the possum species as a whole could survive.