Buzz Holling and the origin of ecological resilience

The full moon is bright and beautiful right now.  Yesterday as we sat in our meeting on the fifth floor at Ole Miss, the sun broke through the clouds.  The meeting was disrupted.  Everyone was so glad to see the sun back after several days in hiding.  We Southerners have a love/hate affair with the sun.  Especially after a months long drought.

One in the meeting said he even felt guilty for feeling glad that the sun had come out.  No chance of that for us. We can’t let guilt or anything else slow us down.  So we pulled him back into the discussion of vulnerability and resilience.  We had a lively and engaging meeting.  Got me wired for sure.  I’m so sorry if your meetings aren’t like that.

Somehow we even got around to discussing the scientific paper which first established the concept of ecological resilience.  Buzz Holling discovered the concept while working for the Canadian Department of Forestry.  He was born to Canadian parents in the United States, but his parents took him to Northern Ontario to grow up.  He grew to love nature there.  After going to school in British Columbia, he headed back to Northern Ontario to study problems in managing forests and predator prey relationships.

predator prey relaionshipsHolling was an early convert to nonlinear dynamics, chaos theory and complex adaptive systems.   He was led to this epiphany during his early research on relationships between predator and prey.

Predator prey interactions lead to population cycles, with the predator population cycle temporally tracking the prey population cycle. The explanation of this phenomenon is straightforward: as prey populations increase, the increased availability of resources allows a rise in predator populations a little later in time. But the increase of predators leads to an increase of prey consumption and, consequently, a decrease in prey populations. Then, the lack of prey resources leads to a decline of predator populations. As predator populations decline, prey populations increase initiating the cycle once again.

The predator prey model was first diagrammed by Volterra in 1927. There are two species, a predator species with a population, N2, which only feeds on a single prey species with population, N1. The model incorporates demographic chaotic behavior which, nevertheless, does not stamp out the basic cyclic pattern.

Volterra’s model mathematically predicts these cycles. It exemplifies the former explanatory ideal of ecology: a quantitative model which not only accurately predicts ecosystem behavior but does so through observable interactions of species.

Some would call this traditional view of predator  and prey as a balance of nature. The predator balances the prey and the prey balances the predator and all are fluctuating around an equilibrium which is never reached.

Holling was among the first to look at predator prey data and realize that the “balance of nature” concept was inhibiting understanding of ecological systems.  Ample empirical data now suggests that the balance of nature assumption is almost never correct: natural ecosystems are nearly always far from equilibrium.  

Many folks outside ecology still believe in a balance of nature and climax communities.  It’s easy to understand why.  We like stability.  We don’t like disruption and chaos.  We want any change to be incremental.  Little changes we can handle.

Holling pulled this all together when he was 43 years old in a paper about stability and resilience.  Both stability and resilience are required in natural systems, but they are far from the same thing.  Resilience involves innovation and adaptation of a system.

A stable system resists change in order to maintain the status quo.  A resilient system may dissolve into components when faced with a major challenge or disruption, but it can rebuild itself and always rebuilds itself in a way that is more adapted than before.  Like a forest after a forest fire or Japan and Germany after being destroyed in World War II.

Holling made the bold assertion (at least bold in ecology in those days) that the world is not deterministic.  He arrived at that conclusion after his research group had studied the spruce budworm for 28 years.  The spruce budworm devastates forests in Canada.  It absolutely destroys the beautiful balsam fir.  There have been six outbreaks since the early 1700s.  Between these outbreaks the spruce budworm is an exceedingly rare species.

When outbreaks occur, there is major destruction of balsam fir in all the mature forests, leaving only the less susceptible spruce, the nonsusceptible white birch and a dense regeneration of both fir and spruce.  More immature stands suffer less damage and more fir survive.  Between outbreaks, the young balsam grows together with spruce and birch to form dense stands in which the spruce and birch suffer from crowding.  Eventually a stand of mature and overmature trees develops with fir as a predominant feature.

This mature forest, plus a sequence of unusually dry years, are the triggers for a spruce budworm outbreak.  Between outbreaks, the fir dominates the spruce and birch, but during an outbreak, the spruce and birch rise to dominance as the fir is decimated.  The budworm maintains the spruce and birch in the system.  But for the budworm, the fir would overpower everything else.

You could view the budworm as a predator and the fir as prey.  But it’s not so simple.  The budworm outbreak only occurs after the rare event of several years of drought. The long delay between outbreaks enables the fir to grow back to provide the fodder for the budworm conflagration.

All these species are most clearly and succinctly explained as complex adaptive systems (CAS).  CAS compete and cooperate, wax and wane.  If one develops an innovation, a gene, which enables it to attack more fiercely, then the other must respond and adapt if it is to survive.

This is the foundation of ecological resilience.  You should really read the paper.  Everyone I give it to is really fascinated by it.  You will be too.

=========

Holling, C. S., 1973. “Resilience and stability of ecological systems”. in: Annual Review of Ecology and Systematics. Vol 4 :1-23.  Available on-line at: http://www.zoology.ubc.ca/bdg/pdfs_bdg/2013/Holling%201973.pdf

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