“Disturbance” (or perturbation) sounds anything but positive.
After all, who’d want to live everyday with the looming threat of fire?
Or the threat of storm surges?
Of earthquakes and volcanic eruptions?
Of any phenomenon that shatters the quiet and quaintness of everyday life?
We humans like stability. Anything that gets in our way to achieving stability is considered a hindrance to development.
Yet, many ecological systems – ecosystems and landscapes – don’t just live with disturbance.
In fact, they embrace it!
Pine Trees Like to Burn
For example, upper montane pine forests rely on fire to complete their life cycle.
Barks resistant to heat, many pine tree species can survive fires, which easily occur in the typically dry environmental conditions high in the mountains.
Other plants on the forest floor?
They burn.
Turn into ash that fertilizes the soil.
Rich soil, no competition. The perfect environment for the pinecones to germinate!
Additionally, pinecones release the seeds within them only when activated by heat.
In other words, fire is a disturbance that facilitates the development of upper montane forests landscapes.
Even other types of landscapes benefit from the occasional blaze.
Sure, a significant chunk of a forest may burn down. But, through ecological succession, the forest landscape could recover – nay, even become better.
How?
Through fire, a forest otherwise dominated by a few species opens spaces for other species to grow.
This creates patches of habitat that differ from the dominant habitat of a landscape.
The result: a landscape with greater ecosystem diversity and species richness.
Other Types of Disturbance
Take note that fire isn’t the only agent of disturbance.
Earthquakes can topple centuries-old trees, which creates openings in the thick canopy that give a chance for other plants to thrive.
A hurricane can damage a patch of reef dominated by a single species of coral. That opens up space and resources through which other coral species can establish their populations.
A virus that wipes out the population of a generalist omnivorous species may allow the populations of species pickier with their food to boom.
Humans Also Like to Burn…a Little Too Much
Disturbance is good for “shaking up the status quo”.
Yet, too much disturbance makes ecosystems collapse, creating an undesirable landscape.
Think of human-caused disturbances to landscapes: deforestation, shifting cultivation (a.k.a. kaingin), urban sprawl.
Let’s not forget natural disturbances intensified by the climate emergency, which is caused by increased GHG emissions by human activity.
Naturally occurring perturbations are enough for most ecosystems to handle. Yet, add the pressures of anthropogenic disturbances, and suddenly nature has too much on her plate.
The Goldilocks Zone
Too much disturbance overwhelms the regenerative capacity of ecosystems, leading to ecosystem collapse and a less-than-ideal landscape.
Yet, with too little disturbance, an ecosystem becomes “stale”.
This is because with minimal disturbance, a few species will outcompete other species for space and resources.
The result: a monotonous landscape with little ecosystem and species diversity.
And the lack of biodiversity makes an ecosystem less resilient, or more susceptible to disturbances. A vicious self-reinforcing feedback loop!
Basically, nature needs perturbations, but not too much.
Kind of like Goldilocks.
Not too hot, not too cold. Just the right temperature to eat her porridge.
And not too hard, not too soft. Just enough fluff for a nice nap.
Likewise, not too many disturbances, but also not too little. Just enough for ecosystems and landscapes to develop.
This is called the intermediate disturbance hypothesis (abbreviated as IDH).
The IDH was first proposed by an ecologist, Joseph H. Connell.
In a 1978 paper, he explored the relationship of disturbance to species richness of rainforests and coral reefs.
Connell concluded in the paper that biodiversity is highest in ecosystems subjected to moderate disturbances.
Or, moderate intensity and moderate frequency.
This graph illustrates the IDH:
The X-axis from left to right indicates decreasing frequency and intensity of disturbance. The Y-axis from bottom to top represents increasing biodiversity in a site.
With too frequent and intense disturbances, only species that can quickly colonize a newly disturbed area can be found in the site.
Yet, with too infrequent and small disturbances, competitive exclusion will take place.
This means species that can best outcompete other species for resources will dominate in the site.
But, with disturbances in the “Goldilocks Zone” – just the right frequency and intensity – a site is dynamic enough to keep competitive species from dominating yet is relaxed enough to avoid a total wipeout of species.
Several studies have presented evidence in favor of IDH.
However, some ecologists like Jeremy W. Fox are skeptical of the concept and have written papers refuting it (See: Fox, 2013).
Of course, ecologists supporting the IDH wrote counter-critiques, such as this 2014 paper by Michael A. Hutson (See: Hutson, 2014).
Point is, the IDH is not irrefutable, much like many scientific theories.
However, such theories tend to be the best explanation we have for various phenomena, given the information we have for now.
Application in Landscape Architecture
The landscape architects’ work is a part of nature, not apart from it.
Landscape design is subject to the dynamism of nature. Thus, we can observe IDH manifested in landscape architecture.
Intended Disturbance
Landscape maintenance is an agent of disturbance.
Think about it: what we consider as “weeds” are merely opportunistic plants trying to colonize an open area.
Left alone, a landscape design undergoes ecological succession. Eventually, it becomes a wild, biodiverse forest.
This clashes with the intended state of manicured landscapes.
Consider a clubhouse outdoor amenity area: the need to look fancy and prestigious pressures landscape architects and homeowners associations to keep it that way.
Caretakers mow the lawn, weed out unwanted plants, apply insecticides, and chlorinate the swimming pool.
All disturbances that disrupt colonization by weeds, booming of insect pest populations, and algal blooms in pool water.
A few species do survive, namely those intended by the owner or the landscape architect.
In other words, the design intent promotes the leftmost part of Connell’s IDH graph: high frequency and high intensity disturbances.
Minimizing Disturbance
The previous example shows how disturbance is used to prevent unwanted species colonization.
But, many landscape maintenance issues can be analyzed and resolved from the lens of the IDH concept.
For example, trampling.
Turfgrass cannot develop when subjected to frequent foot traffic.
What’s the design intervention? “Keep Off the Grass” signages?
No, but install hedges.
Buffer planting that guides pedestrian traffic along an intended path.
This minimizes disturbance by trampling and allows the turfgrass to grow and establish itself. (Of course, weeds can enter the picture now that there’s less disturbance.)
To add, the IDH concept can be applied in establishing ecological systems.
As windbreakers, trees buffer against strong winds. Minimized disturbance, hence more varieties of plants and wildlife can thrive within the site.
The maintenance regime for pollinator gardens is in stark contrast to conventional manicured gardens.
Using pesticides? Inadvisable; it’ll kill both pests and pollinators.
Intensive mowing and pruning? Careful, or the pollinators may become collateral damage.
Lastly, a concept in ecotourism is to limit the number of visitors in an ecologically delicate site.
Maybe through higher ticket prices. Maybe through a queuing system supported by landscape design.
But why bother? With less human activity comes less disturbance, and that aids in conserving the ecotourism site, which benefits everyone in the long-term.
Conclusion
The intermediate disturbance hypothesis or IDH suggests there is a “Goldilocks zone” for disturbance.
Too much disturbance, and most species in an ecosystem or landscape are wiped out. Too little disturbance, and a few competitive species will dominate the site.
With just enough disturbance, an ecosystem or landscape is dynamic enough to encourage new species to establish themselves, but stable enough so that it doesn’t collapse.
Besides those mentioned, can you think of other applications of the IDH? Let me know in the comments down below.
Thank you for your time, fabled readers!
Literature Cited:
- Connell, J. (1978). Diversity in tropical rainforests and coral reefs. Science, 199(4335), 1302-1310.
- Fox, J.W. (2013). The intermediate disturbance hypothesis should be abandoned. Trends in Ecology & Evolution, 28(2), 86-92.
- Hutson, M.A. (2014). Disturbance, productivity, and species diversity: empiricism vs. logic in ecological theory. Ecology, 95(9), 2382-2396.
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