If you have ever shaken a snow globe, you know the mesmerizing scene that unfolds. As you shake the globe, the snow swirls all around in a frenzy before gently settling on the ground, slightly disturbed. But you can only shake the snow globe so much until parts of it get destroyed.
You can apply the same line of thinking to ecosystems: some disturbances, say an earthquake or a storm, can be slight, allowing the ecosystem to recover and remain much the same, while others can destroy an ecosystem entirely.
So, without further ado, let's explore ecosystem disturbances!
- First, we will look at the definition and examples of ecosystem disturbances.
- Then, we will dive into the types of disturbances in an ecosystem and look at ecosystem disturbance biodiversity.
- After, we will learn about ecosystem disturbance succession.
- Finally, we will look at the relationships between organisms following an ecosystem disturbance.
Ecosystem Disturbance Definition
Before we get into the concept of ecosystem disturbance, let’s review what an ecosystem is.
An ecosystem is all the interactions between biotic (living organisms) and abiotic (non-living) factors in a particular area.
It can be easy to get habitat, ecosystem, and niche confused. You can think of it like this ecosystem < habitat < niche. An ecosystem is like a city and its people living together, habitat is a specific location within the city, and niche is the way a specific person lives within that specific location. Check out "Niche" to learn more!
With this in mind, we define ecosystem disturbances as follows:
Ecosystem disturbances are events, such as storms, floods, fires, or human activities, that disrupt the functioning of an ecosystem and change its population, resources, or physical environment.
An ecosystem usually exists in a state of balanced equilibrium that allows it to thrive. When a disturbance occurs, it knocks the ecosystem out of its state of equilibrium, and it becomes unbalanced in species diversity or composition!
Ecosystem disturbance example
If you look at your weather app or news station, I bet throughout the course of a year, you will bear witness to several disturbances. Almost all communities can be disturbed by a storm, even those found in the ocean because of the action of waves. There are also streams and ponds that are disturbed by seasonal flooding and drying.
You might also hear about twisters, hurricanes, earthquakes, and forest fires that leave devastation in their wake. These are all prime examples of ecosystem disturbances.
We are going to zoom in on one in particular to explore the good, the bad, and the ugly behind disturbances: forest fires.
Let's think together for a moment about forest fires and their impacts. The most obvious ones have burned trees and displaced animals (including humans!), but in order to fully flesh out what an ecosystem disturbance is, we also have to consider the benefits of a forest fire.
Yes, you heard correctly, benefits!
The truth is that many natural ecological disturbances, including forest fires, are healthy and even sometimes necessary for an ecosystem to continue thriving.
Some benefits of contained forest fire include:
Some chaparral and grassland biomes even need regular burning to maintain their structure and species composition!
Fig. 1: A controlled burn to maintain grassland in Patuxent Research Refuge.
A real-life example of these benefits can be seen in the redwood forests that cover California. For a long time, the redwood ecosystem has been shaped by fires. Indigenous people have been practicing cultural burns as a form of traditional fire management, passed from generation to generation. These fires improve soil quality and spur the growth of some plant species.
However, European colonizers suppressed these practices. The lack of cultural burns allowed organic matter to build up, increasing the potential for severe fires which can permanently disrupt diversity and threaten the lives of humans.
Types of Disturbances in an Ecosystem
There are two broad types of disturbances in an ecosystem: natural and man-made.
Natural disturbances include:
The other type of disturbance comes at the hands of humans, usually from the overuse or overexploitation of an ecosystem. In recent history, the strongest disturbances are human activities, whether in marine or terrestrial ecosystems. This can be seen in events such as:
Regardless of the type, all ecosystem disturbances do the same thing, though to varying levels of intensity and duration: they alter the ecosystem and disrupt its normal functioning.
Ecosystem Disturbance Biodiversity
In our example of the redwood forest, we briefly touched on how a disturbance can affect the ecosystem. But let’s look into how a disturbance can affect biodiversity.
The impact of a disturbance depends on its type, frequency, and severity:
- A high level of disturbance is typically caused by frequent and intense disturbance.
- A low level of disturbance is typically caused by a disturbance of either low frequency or low intensity.
According to the intermediate disturbance hypothesis: moderate levels of disturbance results in higher species diversity than either high or low levels of disturbance.
This is because a high-level disturbance can reduce diversity by creating environmental stresses that go beyond what many species can tolerate. It can also disturb the community so frequently that species that grow or colonize very slowly are eliminated. On the other hand, a low level of disturbance can lower species diversity by allowing dominant species to eliminate species that are less competitive.
So, how does an intermediate level of disturbance increase species diversity? It does so by opening up habitats that less competitive species can occupy!
In a study comparing the richness of invertebrates in streambeds exposed to flooding at different frequencies and intensities, New Zealand ecologists found that when floods were too frequent or too rare, invertebrate richness was low: when flooding was too frequent, some species found it difficult to become established in the streambed, and when flooding was too infrequent, species were often displaced by those that were more competitive.
As predicted by the hypothesis, richness was at its highest when there was an intermediate frequency or intensity of flooding.
Ecosystem Disturbance Succession
How exactly does disturbance lead to a change in species richness or diversity? After a severe disturbance strips away most, if not all, existing vegetation, the disturbed area undergoes ecological succession.
Ecological succession is when a disturbed area is colonized by different species, which are slowly replaced by other species, which are in turn replaced by yet another species.
It is during this period that changes in the composition and structure of communities are most noticeable.
When the process of ecological succession starts in a nearly lifeless environment where soil has not yet formed, it is referred to as primary succession.
Let’s take a look at an example of primary succession:
Say a new volcanic island was formed. This island is nearly lifeless: the only life forms present are prokaryotes and protists.
The first to colonize such areas are lichens and mosses.
Then, soil develops as rock weather and organic matter amass from the decomposed remnants of the first colonizers.
With the presence of soil, lichens and mosses are overgrown by grasses, shrubs, and trees. These could have come from seeds blown in from nearby areas or transported by animals.
Plants eventually dominate the area.
The above example can take hundreds to thousands of years.
But doesn’t ecological succession also happen in areas that have soil and life? When an existing community is cleared by a disturbance, but the soil is left intact, the area can still return to a state similar to how it was originally. This is called secondary succession.
Let’s look at an example of secondary succession:
A forested area is cleared for farming but is later abandoned.
The first plants to colonize are typically herbaceous plants that grow from seeds carried by the wind or by animals.
Over time, woody shrubs may replace most of the herbaceous plants.
Forest trees may eventually replace most of the woody shrubs.
The above example explains what happened following the 1988 forest fires in Yellowstone!
Ecosystem Disturbance Organism
Let's finish off by talking about the relationship between the early-arriving organisms and the organisms that arrive after an ecosystem disturbance. Their relationship can be described as one of the following:
The early-arriving species can help the later-arriving species by making the environmental conditions more favorable. For example, the former could increase soil fertility.
The early-arriving species can hinder the establishment of later species. That means later species are able to colonize the area in spite of–rather than because of–the early species.
The two groups can be completely independent of the other, with the early-arriving species neither helping nor hindering the other.
Ecosystem Disturbance - Key takeaways
- Ecosystem disturbances are events, such as storms, floods, fires, or human activities, that disrupt the functioning of an ecosystem and change its population, resources, or physical environment.
- There are two main types of ecosystem disturbances: natural and man-made.
- After a severe disturbance strips away most, if not all, existing vegetation, the disturbed area undergoes ecological succession.
- Ecological succession is when a disturbed area is colonized by different species, which are slowly replaced by other species, which are in turn replaced by yet another species.
References
- Jane B. Reece et al. Campbell Biology. Eleventh ed., Pearson Higher Education, 2016.
- U.S. National Park Service, "Fire & Redwoods—What Does the Future Hold for this Ancient Species?" July 17, 2022.
- Figure 1: Controlled Burn (https://flickr.com/photos/43322816@N08/6941238660) by U. S. Fish and Wildlife Service - Northeast Region (https://www.flickr.com/people/43322816@N08). Licensed by CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/deed.en).
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