Ecosystem Regulation and Keystone Species

Overview

This lecture examines how apex predators regulate ecosystems through top-down control, introduces the concept of keystone species, and explores trophic cascades with detailed examples from marine environments. It highlights the shift in ecological thinking from bottom-up to top-down regulation and demonstrates the profound effects predators have on community structure, biodiversity, and ecosystem stability.

Food Chains and Ecosystem Regulation

Traditionally, ecologists believed that populations were regulated from the bottom up: the number of producers (plants) determined the number of herbivores, which in turn limited the number of predators.
In a University of Michigan class, Professor Fred Smith challenged this view by asking why herbivores do not consume all available plant life, even though they have the capacity to do so.
Smith, along with Nelson Hairston and Lawrence Slobodkin, proposed the "Green World Hypothesis": predators keep herbivore populations in check, preventing them from destroying plant communities and maintaining ecosystem balance.
This idea was radical at the time, as it suggested that predators play a crucial role in regulating ecosystems, not just the availability of food at the base of the food chain.
The Green World Hypothesis shifted ecological focus from solely bottom-up control to recognizing the importance of top-down regulation by predators, fundamentally changing how scientists understood ecosystem dynamics.

Robert Paine's Starfish Experiment

Inspired by the Green World Hypothesis, Robert Paine set out to test the role of predators in regulating ecosystems.
On the Pacific coast, Paine identified the starfish Pisaster ochraceus as the top predator in the intertidal food web, preying mainly on mussels and other species.
Paine conducted a landmark experiment by removing starfish from a rocky outcrop and monitoring the changes over time.
The removal of starfish led to a rapid decline in species diversity: the number of species dropped from 15 to 8 within a year, and eventually to 7, resulting in a near-monoculture of mussels that crowded out other species.
Paine introduced the term "keystone species" to describe species that have a disproportionately large impact on their ecosystem relative to their abundance. Removing a keystone species like the starfish caused dramatic changes in community structure, while removing most other species had little effect.
His work demonstrated that some species are critical for maintaining the diversity and stability of ecosystems, and that the loss of such species can lead to ecosystem collapse.

Trophic Cascades and Further Experiments

Paine extended his research to tide pools, where he observed that sea urchins, when abundant, overgrazed on kelp and prevented its growth, leading to a loss of kelp forests.
By manually removing sea urchins from some pools and leaving others untouched, Paine found that kelp quickly began to flourish in the absence of urchins, showing that unchecked herbivore populations could control the entire ecosystem.
This experiment revealed that not only predators, but also certain herbivores, can have strong effects on community composition when not regulated by predators.
Paine’s findings highlighted the interconnectedness of species and the cascading effects that changes at one trophic level can have on others, a phenomenon known as a trophic cascade.
These experiments provided clear evidence that the removal or addition of a single species can trigger a chain reaction throughout the ecosystem, affecting many other species and altering the structure of the community.

James Estes and the Role of Sea Otters

James Estes, influenced by Paine’s work, compared islands in Alaska with and without sea otters to study their ecological impact.
On islands with sea otters, urchin populations were low and kelp forests thrived; on islands without otters, urchins were abundant and kelp was scarce or absent.
Estes’ research showed that the presence or absence of sea otters created a "trophic cascade," where the effects of a predator (otter) extended indirectly down the food chain, influencing both urchin and kelp populations.
The history of sea otters in the North Pacific was shaped by the fur trade, which nearly drove them to extinction. Their recovery was uneven, creating natural experiments across different islands.
Estes’ work provided clear evidence that sea otters are another example of a keystone species, regulating the structure and health of coastal marine communities. The removal of otters led to widespread changes throughout the ecosystem, affecting not only kelp and urchins but also fish, birds, and invertebrates that depend on kelp forests.
This research demonstrated the far-reaching and indirect effects that predators can have on entire ecosystems, reinforcing the importance of top-down control.

Killer Whales and Four-Level Trophic Cascades

In the 1990s, Estes observed a sudden decline in sea otter populations and investigated possible causes, ruling out starvation and disease.
Evidence pointed to killer whales (orcas) as the new predator of otters. Historically, killer whales fed on large whales, but after commercial whaling drastically reduced whale populations, orcas shifted to preying on sea otters.
This change added a fourth trophic level to the system: killer whales (apex predator) → sea otters → sea urchins → kelp.
The removal of otters by killer whales led to an increase in urchins and a decline in kelp forests, illustrating how human actions (such as whaling) can have far-reaching, indirect effects on distant ecosystems.
Estes and his team confirmed this by comparing otter populations in areas accessible and inaccessible to orcas, finding that otter populations only declined where orcas could reach them.
This example powerfully demonstrated the concept of trophic cascades and the interconnectedness of ecological systems, showing that changes at the top of the food chain can ripple down and transform entire ecosystems.

Broader Impacts and Conservation Insights

The concepts of keystone species and trophic cascades have been observed in many other ecosystems, including those involving wolves, sharks, and lions.
The removal of apex predators often leads to dramatic changes in the abundance and diversity of other species, sometimes destabilizing entire ecosystems and reducing biodiversity.
These insights have fundamentally changed ecological theory, shifting the focus from bottom-up to top-down regulation and informing modern conservation strategies.
Recognizing the importance of apex predators has provided ecologists and conservationists with new tools for managing and restoring ecosystems, emphasizing the need to protect keystone species to maintain ecosystem health.
Ignoring the role of top-down effects and apex predators can lead to unintended consequences and even ecosystem collapse, highlighting the importance of understanding and preserving the complex interactions within natural communities.

Key Terms & Definitions

Producer: An organism, such as a plant, that creates energy-rich compounds from sunlight.
Herbivore: An animal that feeds on plants.
Predator: An animal that hunts and eats other animals.
Green World Hypothesis: The idea that predators keep herbivore populations in check, preventing the overconsumption of plant life.
Keystone Species: A species with a disproportionately large effect on its ecosystem relative to its abundance.
Trophic Cascade: A series of indirect effects across multiple levels of a food chain, triggered by changes in the population of one species.
Top-down control: The regulation of ecosystem structure and function by predators at the top of the food chain.
Apex Predator: The top predator in a food web, with no natural predators of its own.

Action Items / Next Steps

Review additional examples of keystone species and trophic cascades in various ecosystems to deepen understanding of these concepts.
Prepare for discussion on the ecological and conservation impacts of removing apex predators from ecosystems, considering both direct and indirect effects.
Consider how human activities, such as hunting, fishing, and habitat alteration, can indirectly affect distant ecosystems through changes in predator populations and food web dynamics.
Explore current conservation strategies that incorporate the concepts of top-down control and keystone species, and evaluate their effectiveness in maintaining ecosystem health and biodiversity.
Reflect on the importance of preserving apex predators and keystone species to prevent ecosystem collapse and ensure the long-term stability of natural communities.