Earth Almanac: July/August 2008

Mosquito Killers?

Most everywhere in the nation save northern states, stout, big-eyed, guppy-sized gambusia (a.k.a. “mosquito fish”) are hovering near the surfaces of ponds and dawdling streams that are often too brackish or polluted for other fish. Look for them as they snap up insect larvae or as females protect fry they deliver live from eggs that hatch inside their bodies and which were fertilized by spermatophores injected by males. Gambusia are prolific, delivering about 50 fry in a single brood after a gestation period of barely more than three weeks. With just one spermatophore a female can produce six broods, and females born early in the season can breed when they’re barely more than six weeks old. Both the eastern species (native to mid-Atlantic and southern states) and the western species (native to the central and lower Mississippi River system and Gulf Coast drainages) have been introduced around the world as alleged mosquito control. The diet of gambusia includes all manner of invertebrates, as well as the eggs and young of fish and amphibians. Calling them “mosquito fish” because they sometimes eat mosquito larvae makes as much sense as calling coyotes “frog dogs” because they sometimes eat frogs. Almost everywhere outside their natural range, gambusia have become worse pests than the mosquitoes they invariably fail to control. They facilitate algae blooms by denuding a water body of herbivorous zooplankton, and they limit or wipe out native species. In the Southwest, for example, they have depressed the threatened Chiricahua leopard frog, helped to extirpate many pupfish populations, and contributed to the elimination of the Gila topminnow from almost all of its range.

Jaws of Life

Save for dimpling mackerel and the tide trails of lobster-pot buoys, the summer sea is still. Suddenly a dorsal fin cleaves the surface, then another and another. Clearly they belong to sharks, huge sharks. You start noticing the diagnostic features of great whites—the tail lobes almost equal in length, the stabilizer keels on the caudal peduncle, the lighter-colored underside—and you can’t get the famous line from Jaws out of your head: “You’re gonna need a bigger boat!” The cavernous, gaping mouth heading for you is at once startling and reassuring. Not even great whites can open that wide. These are basking sharks, the sole member of the family Cetorhinidae and the planet’s second biggest fish after the whale shark. Basking sharks, which can weigh 8,000 pounds and approach 30 feet in length, patrol the world’s temperate oceans, lazily filtering plankton through their gill rakers. Look closer and you’ll see the physical characteristics of this species—tiny, vestigial teeth, long gill slits that almost encircle the head, and a snout reminiscent of an elephant seal. For centuries basking sharks were commercially plundered for their oil-rich livers, which account for a quarter of their body weight. Today exploitation continues, but for shark-fin soup. The giant shark is listed on the International Union for Conservation of Nature’s list of endangered species.

Respecting Elders

Along fertile river banks and forest edges in the eastern half of the nation, American elders are draped with clusters of shiny, blue-black berries. Humans find them bitter, but dozens of bird and mammal species feast on them, particularly after the first frost. The name elder derives from the Anglo-Saxon aeld, which means “fire” or “to kindle a fire”—this from the traditional use of the dried pith for tinder and the hollowed twigs to blow the embers aflame. Elders are fast-growing shrubs that can approach tree size and frequently form dense thickets, especially where seeds have been spread by bird droppings. When the berries are cooked, it destroys their bitter taste, rendering ingredients for delicious jams, pies, syrups, and wines. The strong-smelling leaves, crushed and rubbed on skin or placed under a hat, are said to make an effective insect repellent.