Ecosystem Disturbances, Keystone Species, and Human Impact Study Guide
Salmon and the Forest Ecosystem Dynamics
The salmon-forest ecosystem is an interdependent system where carnivores feed on salmon and subsequently carry their nutrients into the forest.
In this balanced state, nutrients from the salmon are absorbed by the trees.
Matter and low-entropy energy flow consistently along these established biological paths.
The partial food web for this ecosystem includes:
Salmon
Bears
Berries and plants
Eagles
Trout
Decomposers
Nutrients
The Critical Role of Decomposers and Indirect Effects
If a single part of the salmon-forest cycle is disturbed, such as the removal of decomposers, the entire system faces decline.
Impacts of removing decomposers:
Trees and plants would not be provided with the essential nutrients they need to survive.
Without nutrient availability, trees would fail to grow.
The lack of tree growth results in a loss of shade and the disappearance of small ponds (which are typically maintained because tree roots prevent erosion).
These small ponds and shaded areas are critical habitats for baby trout.
Without trout, top predators like bears and eagles would lose a primary food source (salmon and trout).
This demonstrates how a disturbance to one part of an ecosystem affects the entire food web.
Ecosystem Disturbances and Human Impact
Ecosystem disturbances can be likened to a line of dominoes: once one is pushed over or disturbed, every subsequent part of the system is affected.
Numerous disturbances are the direct result of human activities, including:
Hunting
Deforestation (the cutting down of trees)
Unsustainable fishing practices
Pollution
Case Study: The American Alligator
The American Alligator was historically viewed as an annoying pest because they appeared in residential swimming pools, on golf courses, and consumed game fish that fishermen preferred to catch.
Consequently, the alligator was hunted without limit until it became an endangered species.
The disappearance of the alligator led to an unlikely and unintended result: the game fish population plummeted.
The Biological Chain Reaction:
The alligator's primary food source is a large fish called a Gar.
The Gar preys on smaller game fish.
When alligators were removed, the Gar population increased substantially due to a lack of predation.
The increased Gar population then consumed the game fish at a much higher rate.
Legal Action: In 1967, the alligator was placed on the endangered species list and protected from hunting to restore the balance of the ecosystem.
Case Study: The Sea Otter and Kelp Forest
Sea otters inhabit kelp forest ecosystems, where they use the kelp for hiding and protection.
Giant kelp characteristics:
It is a large seaweed capable of growing at a rate of () in a single year.
Kelp serves as a vital producer in the ecosystem.
It provides protection from predators for various species, including sea otters and hatching fish.
Historical Decline:
During the 18th century, sea otters off the coast of California were hunted extensively for their fur.
Fishermen also killed them, believing the otters were competition for fish.
The sea otter population was hunted nearly to extinction.
Keystone Species
A keystone species is defined as a species that is depended upon by all other species within an ecosystem.
When a keystone species is removed by a disturbance, the community changes drastically.
The Sea Otter as a Keystone Species:
Sea otters prey on sea urchins.
Without sea otters, the sea urchin population grows uncontrollably.
Sea urchins consume kelp.
An explosion in sea urchin numbers leads to the disappearance of kelp beds.
Without kelp beds, fish have no safe place to spawn, leading to the disappearance of fish populations.
Other Examples of Keystone Species:
Jaguar: Inhabits rainforests of Central and South America. It maintains balance by eating different types of prey.
Parrotfish: The only species on the Great Barrier Reef that cleans the coral; without this cleaning, the coral cannot remain healthy.
Atlantic Top Predators: Sharks and tuna are keystone species in the Atlantic Ocean.
Food Web Dynamics: Tuna Removal in the Atlantic
Based on a simplified food web of the Atlantic Ocean, the removal of tuna causes the following community shifts:
Mackerel populations increase (loss of predator).
Small fish populations decrease (increased predation by more mackerel).
Shrimplike creature populations increase (fewer small fish to eat them).
One-celled life (including photosynthetic plankton) decreases (increased predation by shrimplike creatures).
Shark populations decrease (loss of tuna as a food source).
Environmental Disturbance: Acid Rain
Acid rain is a consequence of air pollution that increases the acidity of the soil.
Hypothetical Scenario: If acid rain kills the salamanders and frogs in an ecosystem, the food web is affected as follows:
Snakes: Population decreases due to the loss of salamanders as a food source.
Dragonflies: Population increases because there are fewer salamanders and frogs to prey on them.
Phytoplankton: Population decreases because the increased dragonfly population consumes more of them.
Trout: Population decreases because eagles, having fewer salamanders to eat, will increase their consumption of trout.
The Salmon Fishery and Commercial Methods
Approximately of U.S. commercial salmon fishing takes place off the coast of Alaska during the summer as salmon return to spawn.
Common Fishing Methods:
Gillnet: A curtain of netting hung in the water. The mesh size allows a fish's head to pass through but not its body. Gills become stuck when the fish attempts to back out.
Purse Seine: A net used to encircle a school of fish. The bottom is "pursed" shut so fish cannot escape, and the entire net is hauled in.
The Problem of Bycatch
Bycatch refers to any organisms caught in a net that were not the intended target species.
Examples of bycatch include non-target fish, sea turtles, and large marine mammals such as dolphins and whales.
Consequences of Bycatch on the Community:
Animals are often dead or dying even if they are thrown back into the water.
Juvenile fish are caught, preventing them from reaching adulthood and reproducing.
Endangered species are frequently caught.
Keystone species may be inadvertently removed from the ecosystem.
Bycatch Reduction and Mitigation
Scientific and engineering solutions to reduce bycatch include:
Acoustic deterrents: Devices attached to nets that emit sounds to warn dolphins and whales, who have sensitive hearing.
Mesh size optimization: Making mesh small enough that marine mammals cannot get stuck.
Weak links: Inserting sections into gillnets that allow large animals to break the net and escape.
Educational efforts for fishermen:
Notification of locations where bycatch is difficult to minimize.
Education on the safe return of bycatch to the ocean.
Resilience and Ecosystem Recovery
Ecosystems are more likely to recover if a disturbance is temporary (e.g., occasional line fishing) rather than persistent (e.g., daily net fishing or the introduction of an invasive species).
High Biodiversity: Ecosystems with higher biodiversity are more resilient and more likely to recover because they have multiple food chains to support top predators if one food source fails.
Healthy ecosystems can absorb small or infrequent disturbances but will collapse if changes are too drastic.
Human Population Growth and Carrying Capacity
The human population has been growing exponentially for hundreds of years.
The Earth is considered a closed system regarding matter.
Carrying Capacity: The maximum population the Earth's limited resources can support.
Estimates for Earth's carrying capacity vary widely between experts, ranging from to people.
Limiting factors include fresh water, food availability, and disease.
Human Adaptability: Humanity has historically exceeded projected carrying capacities by modifying the environment and discovering new resources.
Musical Chairs Metaphor: Population growth is like a game of musical chairs. While there may be room at the "party," the number of "chairs" (available resources) shrinks as consumption increases, even as more people continue to arrive.