Learning target: I can work with a team to classify real interactions from evidence, write rules that predict what happens to one partner when the other partner's population changes, and apply those rules to an ecosystem none of us has studied.
Twelve evidence cards: what happened when one partner changed
1 Card 1 (Set A): Sea otters, urchins and kelp, California coastSea otters dive for purple sea urchins and eat them. The urchins graze giant kelp. Fur hunters killed nearly all of California's sea otters by the early 1900s. On reefs where the otters were gone, urchins multiplied and chewed the kelp down to bare rock. A small group of otters survived along the Big Sur coast; as it spread, urchins became scarce on those reefs and kelp forests grew back.
2 Card 2 (Set A): Wolves and moose, Isle RoyaleOn Isle Royale, an island in Lake Superior, wolves are the moose's only large predator, and moose are most of the wolves' food. Over sixty years of counts, wolf numbers often rose a few years after moose numbers rose and fell a few years after moose numbers fell, though disease and hard winters also moved both lines. When disease and inbreeding left only two wolves in the late 2010s, the moose population grew quickly.
3 Card 3 (Set A): Deer and young oaks, oak woodlandDeer browse the leaves and shoots of young oak trees, usually without killing them. Ecologists in many oak woodlands have fenced small plots so that deer cannot enter. After several years, fenced plots hold far more young oaks tall enough to escape browsing than unfenced plots nearby, where the young oaks stay short and are bitten back every year.
4 Card 4 (Set B): Yucca and yucca moth, southwestern desertsA female yucca moth gathers pollen from one yucca flower, flies to another, lays her eggs inside its seed chamber and presses pollen onto the flower so that seeds form. Her caterpillars eat some of the seeds and leave the rest. The moth caterpillars eat nothing but yucca seeds, and no other insect pollinates these yuccas. Yuccas planted in gardens on other continents flower but set no seed unless people pollinate them by hand.
5 Card 5 (Set B): Figs and fig wasps, warm regions worldwideEach kind of fig tree is pollinated by its own kind of tiny wasp. A female wasp crawls into a young fig, which is really a closed cluster of flowers, pollinates the flowers and lays eggs in some of them. Her young grow up inside, eating a few developing seeds, and the new females fly off carrying pollen. In the 1880s and 1890s, California farmers planted a Turkish fig whose young fruit kept dropping off; the crop succeeded only after its wasp was brought from the Mediterranean and settled in 1899.
6 Card 6 (Set B): California poppies and their many visitors, grasslandCalifornia poppies are visited by many different kinds of bees, beetles and flies. Each visitor gets pollen or a place to rest in the warm flower and carries some pollen to the next poppy. Ecologists find that a flower served by many kinds of visitors usually keeps setting seed when one kind of visitor becomes rare, because the others fill in. A flower that depends on a single kind of visitor is hit much harder.
7 Card 7 (Set C): Winter ticks and moose, New England forestsWinter ticks climb onto moose in autumn and drink their blood all winter; one moose can carry tens of thousands. In years when autumn is long and snow comes late, tick numbers boom, and many moose calves are too weakened by blood loss to survive until spring. In years with fewer ticks, more calves survive. Each winter tick spends its whole feeding life on a single large mammal, such as a moose, deer or elk, and cannot develop without one.
8 Card 8 (Set C): Gray whales and gray whale barnacles, Pacific OceanOne kind of barnacle settles only on the skin of gray whales and nowhere else. The adult barnacles filter tiny food from the water as the whale swims along its migration route. Scientists have not measured any harm or benefit to the whales from carrying them. Because the barnacle lives on no other surface, the number of places it can live depends on how many gray whales there are.
9 Card 9 (Set C): Cattle egrets and grazing animals, pasturesCattle egrets are small white herons that walk beside grazing cattle, horses and wild grazers, catching grasshoppers and other insects that the grazers' feet flush from the grass. Egrets feeding beside grazers catch more food with fewer steps than egrets feeding alone. The grazers show no measured gain or loss. Over the last 150 years, as cattle ranching spread, cattle egrets spread from Africa to the Americas, including California, and became common wherever there were pastures.
10 Card 10 (Set D): Barred owls and spotted owls, Pacific Northwest and northern California forestsBarred owls spread west from eastern North America during the 1900s and now share old forests with northern spotted owls. Both hunt flying squirrels, woodrats and other small mammals, and both need big trees with nest sites; the larger barred owls drive spotted owls out of their territories. In a large experiment, barred owls were removed from some study areas. Spotted owls there nearly stopped declining, while in comparison areas where barred owls stayed, spotted owls kept declining.
11 Card 11 (Set D): Starlings and bluebirds, parks and farmlandEuropean starlings, released in New York in 1890, spread across North America. They nest in tree holes, as bluebirds do, and there are only so many holes; starlings are larger and take over many of them, and both species spend time and energy fighting for holes. Volunteers have put up thousands of bluebird nest boxes with entrance holes too small for a starling. Along these 'bluebird trails', bluebird numbers rose.
12 Card 12 (Set D): Yellow starthistle and native plants, California grasslandYellow starthistle, a spiny weed from the Mediterranean, now covers millions of acres of California grassland. Its deep roots use up soil water in summer, leaving less for native grasses and wildflowers growing in the same soil. Where land managers have controlled starthistle, more water stays in the soil and native plants can increase. Where native perennial grasses are thick, starthistle seedlings have a harder time getting started.