Trophic cascades

I learned a new phrase recently, tropic cascade. I had learned about this concept before, but I love the mental image I get from tropic cascade. It describes the ripple effects of one change in a food web system. I imagine a small ripple of water gathering energy slowly to become a big wave and then finally an uncontrollable cascade of water changing everything in its path.

“Trophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed. For example, a top-down cascade will occur if predators are effective enough in predation to reduce the abundance, or alter the behavior of their prey, thereby releasing the next lower trophic level from predation.”

A backyard example of a trophic cascade is when local cats are kept indoors. Cats are the top predators in many backyard ecosystems killing song birds, frogs, moles, and anything else. With the cats removed, the birds flourish and are able to raise more babies. The birds are able to eat more insects keeping those populations in check. In turn, less insects means that the plants have less herbivores eating them and are able to grow stronger. The act of keeping cats indoors cascades down the trophic levels leading to a healthier garden.

These trophic cascades can happen in any direction in the food web. Poisoning moles in the lawn can cascade in the direction of the moles prey but also in the direction of the creatures that eat the moles. Moles are tunneling through the soil looking for grubs to eat. Uneaten grubs would be able to cause more damage to the plants roots they feed on. So, we need the moles to eat the grubs and keep their populations low. In the opposite direction of the cascade, moles are food for hawks, foxes, owls and other predators. The poison in the mole can move into these predators and kill them. If we have less hawks, them our backyard rabbit and squirrel populations may explode. The cascade flows in multiple directions that we may not be able to anticipate from the start.

Sometimes a backyard can start a tropic cascade that effects much bigger ecosystems. Over fertilizing our lawn can lead to fertilizer running off into our storm drains. From the storm drain those chemicals are washed into the local creek. Extra fertilizer in a creek causes algae blooms which drains the oxygen out of the water causing the fish to die. Fish eat mosquito larva that float on the water’s surface.

These unintended consequences may be difficult to see the cause and effect without looking closely at the entire system and following the tropic cascade from start to end. Ecosystems are very complicated and many times it takes several negative inputs to throw the system out of balance enough for us to notice. However, the more we study the interactions in our world and outside our front door, the better we can understand our role in protecting and supporting the plants and animals we care about.