Introduction
Welcome to AP Biology topic 8.6: biodiversity! In 8.6, we’ll review how biodiversity impacts an ecosystem’s resilience and the role keystone species serve in ecosystems.
Resilience to Change
Imagine a lively little town where many people live, and each individual has special skills that they use to help each other out and benefit their neighbors. There are doctors, construction workers, firefighters, and so on and so forth. Suddenly, a series of severe storms sweeps through the area, leaving many homes destroyed. The town springs to action, and everyone does what they do best to get the town back on its feet. The firefighters rescue trapped townspeople, the doctors nurse them back to health, and the construction workers rebuild what was blown down by the storms. This town is like a diverse ecosystem, with many component parts, and such ecosystems are resilient to changes in the environment.
Now imagine that our town is replaced with a few small families living in a handful of houses. If the storms were to roll through again, it would be very difficult for these people to rebuild. This is what happens when an ecosystem has low biodiversity. Biodiversity is a term that describes the variety of life in an ecosystem, and ecosystems with low biodiversity and fewer component parts are more susceptible to sudden change. This applies both to natural ecosystems, such as marshlands and tundras, and artificial ones, like urban areas or farmland.
The Role of Biodiversity
The diversity of an ecosystem directly influences its structure, organization, and function. High biodiversity leads to:
- Complex food webs, where multiple species interact in different ways (predator-prey relationships, decomposers, symbiotic relationships, etc.)
- Resource partitioning, where different species use resources in different ways, such as eating different parts of the same plant or living in different parts of a tree, allowing multiple species to coexist without directly competing for the same resources
- Redundancy, meaning that different species can perform similar roles in the ecosystem, so that if one species is affected by sudden change, others can take over its role and maintain the ecosystem's stability.
Keystone Species
Arches are one of the strongest shapes in architecture. Each stone in an arch is supported by its neighbors, making them very good at supporting loads. The most important stone in an arch is a keystone, the one at the top that connects the two halves of the arch.
Image Source: 2.3b Keystone Species | YouTube
The keystone (highlighted in red)locks the two halves together. It is the most important stone, and if it is removed, the arch will collapse.
This arch is like an ecosystem, with each stone representing a species and its niche. Species whose effect on the ecosystem is disproportionately large relative to their abundance are called keystone species. Just like the keystone in an arch, if the keystone species is removed, the ecosystem collapses.
That may sound a little confusing, so let's take a look at a real keystone species. You may have heard of the grey wolves of Yellowstone, but did you know that sea otters are also a keystone species? Indeed, sea otters prey on sea urchins, which graze on kelp. In the 18th and 19th centuries, many otter species were hunted for their fur, nearly driving them to extinction. Without the otters to keep them in check, sea urchin populations skyrocketed, devouring all the kelp. Kelp forests that once teemed with life and offered shelter and breeding grounds for many marine species turned into wastelands covered in sea urchins, both dead and alive.
Top Image Source: Australia's marine heatwaves provide a glimpse of the new ecological order | The Guardian
Bottom Image Source: As Oceans Warm, the World's Kelp Forests Begin to Disappear | Yale Environment 360
A kelp forest in Tasmania before and after sea urchins stripped the rocks clean.
Thanks to recent conservation efforts, however, sea otter populations have begun to rebound. With their reintroduction, the kelp forests have begun to regrow and return to their former glory, increasing the diversity of the ecosystem and becoming more resilient to sudden change.
Summary
- Ecosystems with fewer parts and little diversity are more vulnerable to sudden changes in the environment.
- Keystone species, producers, and essential abiotic and biotic factors contribute to maintaining the diversity of an ecosystem.
- An ecosystem's diversity can influence its organization.
- The effects of keystone species on the ecosystem are disproportionate relative to their abundance in the ecosystem, and when they are removed from the ecosystem, the ecosystem often collapses.