“Chad Gadya”: an ecologist’s favorite seder song
Passover is soon upon us, and many Jews will celebrate by singing rounds of their favorite classic Passover songs. “Chad Gadya” is a song that ensnares the singer in an increasingly complex chain of causality. Starting with one’s father who buys a goat that gets eaten by a cat that is bitten by a dog that is beaten by a stick, etc., etc. But “Chad Gadya” is not simply an incredibly fun shanty, it is also a great lesson in food chain ecology.
A food chain describes the complex interconnections that occur in nature from different organisms that eat each other. At the start are plants that absorb energy from the sun. The plants are then eaten by herbivores, that are then eaten by predators, that are then eaten by higher-level predators and so on. Each step up the food chain is called a trophic level, from the Greek word trophikos, meaning food.
One phenomenon documented by ecologists is called a trophic cascade. When you remove a key species from the food chain, say from the top, the effects will cascade down the food chain, affecting every organism along the way. One classic example involves the sea otter, a charismatic marine mammal that lived off the coasts of North America as far south as California. Unfortunately, the sea otter was once prized for its lovely fur and, by the early 20th century, they had become extinct south of Alaska.
Sea otters happen to eat a lot of sea urchins. Sea urchins eat a lot of kelp. When the sea otters disappeared, the effect cascaded down the food chain. Sea urchin populations skyrocketed, and began feasting on the kelp. Entire forests of kelp began to disappear, only to be replaced by barren underwater fields full of spiky urchins. This spelled trouble for the many fish and other animals that depended on the kelp forests for shelter and food. Bald eagles, for instance, would normally eat fish hiding in the kelp forests. Without those fish, the eagles had to search elsewhere for food.
Fortunately, sea otter hunting was banned, and populations from Alaska were shipped in. Those new immigrant sea otters have begun to repopulate our coasts, restoring the kelp forests and returning balance to the ecosystem.
Understanding how ecosystems work can be very helpful. For instance, in Israel, on Kibbutz Sde Eliyahu, they grow many useful crops, such as alfalfa and oats. But voles (a type of rodent) can be a huge pest problem. Thousands of burrow openings can accumulate per hectare, causing severe damage to their crops. Poisons are expensive, can be dangerous for consumption and for other animals, and are often ineffective anyway because more voles can just immigrate from nearby fields. Barn owls eat voles, but they only nest in pre-existing cavities, so farmers began putting up nesting boxes to encourage the owls. The strategy worked, the owls keep the voles under control, and they end up being more cost effective than poisons. Manipulating ecological systems for pest management in agriculture – known as biocontrol – is an increasingly common strategy used around the world to improve yields.
So, when sitting down for your Passover seder, eating your favorite foods, reclining and singing your beloved songs, take a moment to reflect on the complex chains of events that brought that food to your table.
Ben Leyland is an ecologist at Simon Fraser University, a musician and an Israeli-Canadian resident of Vancouver.