open intermidate distrubance hypothesis

The Intermediate Disturbance Hypothesis

ECOBEAKER #2 ALTERNATE ACTIVITY

What is an ecological disturbance?

• A sudden, temporary event that causes a pronounced change in an ecosystem

• Examples of natural disturbances include heavy storms, flooding, forest fires, hurricanes, and earthquakes

• The effects of disturbances on ecosystems depends on their:
• Intensity – how much of an effect each individual event has on the ecosystem • Frequency – how often the disturbance events occur

What is “ecological succession”?

• Process by which the community of species living in an ecosystem changes over time

• The time for succession to occur varies by habitat as well as by the starting conditions

• Can take anywhere from several years to several decades

• Large-scale succession (e.g. after mass extinction events) can take several millennia

Succession – key terms

• Pioneer species – The subset of species that begin the process of succession; generally hardy species that can survive in barren environments

• Climax community – the composition of species that would eventually populate an environment during the last successional stage
• Primary succession – Succession starting on newly-formed habitat
• Secondary succession – Succession starting on previously occupied habitat that had been recently cleared of organisms

Question Set A

1. Give an example of anthropogenic (human-caused) disturbance (1).
2. The process by which ecosystems repopulate after a disturbance would best be described as (primary/secondary) succession (1).

Intermediate Disturbance Hypothesis

• A hypothesis stating that the highest levels of biodiversity for an ecosystem will be achieved when there is an intermediate level of disturbance occurring

Question Set B

1. Why would high levels of disturbance result in lower biodiversity? (2)
2. Why would low levels of disturbance result in lower biodiversity? (2)
3. Which succession term best describes the community composition that would be seen in conditions resembling the left side of the IDH graph? (1)

Working with Real Data

• The following dataset is from an ongoing research project on San Clemente Island (SCI) which is characterized as a California grassland.
• In 2012, there was a lightning ignited wildfire on SCI which spread across the grassland. However, the fire was stopped by a large road with no vegetation that slowed the spread.
• This created a natural experiment: burned and unburned areas in the same ecosystem located very close to each other.
• From 2013-2017, the United States Geological Survey (USGS) and the San Diego State Soil Ecology Restoration Group (SERG) monitored the changes in the plant community in the burned and unburned (control) plots.

Working with Real Data

• We have provided a subset of this data to look at how the intermediate disturbance hypothesis works in this ecosystem.

• We have provided plant density data (number of stems) for four common plant species.

• Sipa pulchra (STIPUL) is a perennial native grass and many of the USGS and SERG researchers were hoping the wildfire would increase the population.

• Bromus hordeaceus (BROHOR) is an annual, nonnative, invasive grass and researchers are attempting to control its spread on SCI and across Southern California.

• Lepidium nitidum (LEPNIT) is an annual flowering plant native to the west coast of North America.

• Erodium botrys (EROBOT) is an annual, nonnative, invasive flowering plant common in much of North America. It thrives in disturbed landscapes.

Fire in 2012

Fire in 2012

Fire in 2012

Fire in 2012

Question Set C:

1. Compare/contrast the population densities for burned and unburned plots for each species (1)
2. Hypothesize why some species perform better after a disturbance. (2)
3. In addition to fire, what are other factors that may be contributing to the population fluxes? (2)

Fire in 2012

Question Set D:

1. Describe the pattern in the Shannon Diversity (H index) over time. How does this pattern differ between disturbed and undisturbed plots? (2)
2. Describe how to calculate the Shannon Diversity Index. Why is this an important index to measure? (2)
3. Hypothesize what would happen to the diversity index if there was a fire every year. (1)

Fire in 2012

Annual Fires

Question Set E:

1. Hypothesize what would happen to the Shannon Diversity Index if a fire occurred every five years? Every ten years? Do you think there is an optimal period between fires in this ecosystem to maintain biodiversity? (2)
2. Why is maintaining biodiversity important? (2)
3. The U.S. Forest Service has been actively suppressing forest fires since the early 1900s. How might these efforts have changed the biodiversity of forests in the U.S.? (2)
4. Within the framework of the intermediate disturbance hypothesis, how might climate change affect biodiversity? (2)