Disturbance of Ecosystems and Keystone Species

Disturbance of Ecosystems

  • A disturbance is defined as an event that alters the biodiversity of a community by either removing organisms or changing the availability of resources.

  • Disturbances can be either positive or negative in their effects.

  • Examples of Disturbances:

    • Fire

    • Human Activity

    • Flood

    • Drought

Ecosystem Resiliency

  • Resilient Ecosystems: Some ecosystems demonstrate high resiliency, meaning they can recover from disturbances relatively quickly.

    • Example: Forest fires are often considered healthy disturbances, leading to secondary succession that can benefit the ecosystem.

  • Less Resilient Ecosystems: Other ecosystems take a long time to recover, or may never be fully restored, after damage.

    • Damage to habitats can lead to a reduction in diversity, as many species cannot survive degradation.

    • Example: Water pollution can cause widespread death of aquatic life, significantly reducing diversity.

Disturbing an Ecosystem: Salmon-Forest Food Web Example

  • This example illustrates a partial food web in a salmon-forest ecosystem, which is typically in balance.

    • Carnivores (like bears, eagles, trout) feed on salmon, transferring nutrients to the forest.

    • These nutrients are absorbed by trees, which provide shade for ponds where baby trout can grow.

    • Matter and low-entropy energy flow along these established paths.

  • Impact of Removing Decomposers:

    • If decomposers are removed, trees and plants would not receive necessary nutrients.

    • Lack of nutrients would hinder tree growth, reducing shade for ponds and increasing erosion, which negatively impacts baby trout habitats.

    • Consequently, bears, trout, and eagles would have less salmon to eat, leading to a decline across the entire food web.

Disturbances and Ecosystems: The Domino Effect

  • A disturbance in one part of an ecosystem has ripple effects throughout the entire system, similar to a line of falling dominoes.

  • Many disturbances are caused by human activities, including:

    • Hunting

    • Deforestation (cutting down trees)

    • Unsustainable fishing practices

    • Pollution

The American Alligator: A Keystone Species Example

  • Historically, the American alligator was viewed as a nuisance, often hunted without limit, becoming an endangered species.

  • Unintended Consequence of Alligator Disappearance:

    • The game fish population (fish caught for sport) unexpectedly plummeted.

    • The alligator's primary prey was a large fish called a Gar, which preys on game fish.

    • With alligators removed, the Gar population exploded, leading to a significant increase in predation on game fish, causing their decline.

  • The alligator was protected under the endangered species list in 19671967.

The Sea Otter: Another Keystone Species Example

  • Sea otters inhabit and hide within kelp forest ecosystems.

    • Giant kelp, which can grow 9090 m (300300 ft) in one year, are crucial producers and provide protection for many species, including sea otters and hatching fish.

  • Historical Hunting and Its Effects:

    • In the 18th18^{th} century off California, sea otters were hunted extensively for their fur and by fishermen who perceived them as competition.

    • This hunting pushed sea otters close to extinction.

  • Ecosystem Changes Due to Otter Loss:

    • Fishermen observed profound changes in the kelp forest ecosystem following the loss of sea otters.

Keystone Species Defined

  • A keystone species is a species that other species in an ecosystem depend upon heavily.

  • The removal of a keystone species by a disturbance causes drastic changes to the entire community.

  • The Sea Otter as a Keystone Species: In just a few years after the decline of otters, fish populations vanished.

    • Without sea otter predation, the sea urchin population rapidly increased.

    • Sea urchins consume kelp, so their surge led to the disappearance of kelp beds.

    • The loss of kelp, which provides safe spawning grounds, caused fish populations to decline.

  • Other Examples of Keystone Species:

    • Jaguar: In Central and South American rainforests, jaguars prey on approximately 8787 different species, maintaining ecosystem balance.

    • Parrotfish: On the Great Barrier Reef, parrotfish are the only species that clean coral, a vital process for coral health.

    • Top Predators (Atlantic Ocean): Sharks and tuna are keystone species.

      • If tuna are removed, mackerel populations increase, small fish decrease, shrimplike creatures increase, one-celled life decreases, and shark populations also decline.

The Salmon Fishery

  • Approximately 9595\% of commercial salmon fishing in the U.S. occurs off the coast of Alaska during the summer spawning season.

  • Common Commercial Fishing Methods:

    • Gillnets: A