In the 1960s, a guy named Bob Paine picked a small stretch of rocky beach in Washington state and evicted its sea stars, crowbarring them off the rocks and throwing them back into the ocean. Within a year, the beach’s demographics had changed dramatically: barnacles and then mussels replaced algae and limpets. Species richness, or the number of different species present, hadn’t gone down by the number of sea stars–it had halved.
Paine realized the reason behind the shift was that without the sea stars around to eat barnacles and mussels, their populations skyrocketed, and their demand for algae and limpets increased, causing those populations to crash. He called the three-step chain a “trophic cascade”; the sea stars at the top that shaped the chain he called an “apex predator.”
Since then, scientists have recognized plenty of other examples of trophic cascades. My favorite (for completely non-science-related reasons) is topped by sea otters–lose them, and sea urchins spiral out of control, decimating kelp forests. Wolves, cougars, and sharks are other popularly cited apex predators.
Trophic cascades share a general, top-down pyramidal structure: typically, the bottom level is a plant, the middle level is an herbivore, and the top level is a carnivore. In between are two inverse relationships, so when the carnivore population decreases, the herbivore population increases, which makes the plant population decrease.
Right about now, you may be thinking: “This sounds an awful lot like an elementary school class on food chains, just with longer words.” Touché. BUT, some points to consider:
- Food chains feel static. Even if you level up to food webs (much more realistic), you’re still looking at a snapshot. Trophic cascades factor in the dynamism that makes real-world science so cool.
- Trophic cascades and their implications are still being integrated into policy.Take wolves, for example: due to lose Endangered Species Act protection throughout the US, even though their recovery has been shaky–and what about species they affect, like elk and aspen?
- Or consider that the East coast is swamped with deer, whose populations soared after the disappearance of local apex predators. Both deer overpopulation and population management strategies are expensive.
- Just a few apex predators in an area can pack a lot of punch. Which is good, since when you trace it down through trophic levels to solar energy, each individual requires quite a lot to survive (Take your carnivore and think about all the herbivores it eats. Now multiply that by all the plants each of those herbivores eats, then recall that plants build themselves from sunlight.). But that also means that if we can identify where trophic cascades have been thrown off, we could theoretically rebalance them without needing to build up large or dense populations of apex predators.
- It turns out predators don’t necessarily even need to eat anyone. For example, just having wolves around can keep elk from having such a one-track mind about grazing. This heightened alert is referred to as “the ecology of fear.”
- When counting carbon is always encouraged, plants are a huge piece of the puzzle, since they pull CO2 out of the atmosphere and turn it into themselves. Where herbivore populations are high because of missing predators, how much healthier could forests be with reduced grazing–and how much carbon would that mean?
- Usually, the conversation is about disappearing predators. The other side of the coin is the ultimate predators: humans. Are trophic cascades topped by humans unique in any way? How does high human population density affect them? And how could off-kilter relationships be stabilized?