The Elusive Climax

Image Credit: CC BY-SA 3.0 US, Image Cropped

Somewhere in my education, I distinctly remember a video that explained ecosystem succession moving towards a climax condition. The film depicted the gradual filling of a lake and subsequent encroachment of saplings as the system aged towards its inevitable end as a hardwood forest in the eastern United States. I remember thinking even then, “but where do lakes come from?” I couldn’t work out how there could be a mosaic of habitats if there was a steady progression towards a single endpoint.

The theory of an ecological climax, proposed by Frederick E. Clements in 1916, was a central theme in my university textbook and ecology education in the 1990s. The end of the successional road, the ‘climax community’ was described as self-perpetuating, with inputs balanced by exports.

After college, I stuck my ecology textbook (and Clements’ climax concept) in a box in my parent’s basement and didn’t think about it until years later after I had become a firefighter and been trained in fire ecology. I was preparing to talk with high school students about forestry in the American Southwest and I wanted to address the natural role of fire. To do that, I needed to conceptually reconcile my climax education with my fire experience. I dug the climax concept out of storage, dove into the contemporary literature, and learned that climax theory had been mothballed, elbowed out by disturbance ecology.

‘Disturbance’ is defined as a discrete event (like a fire or a storm) that causes mortality and disrupts ecological community structure and/or function. Disturbance has also been defined as any process that removes living biomass from a community. I prefer some hybrid of these definitions. 

Ecosystems perpetually dance between biomass accrual and removal processes. In arid ecosystems, culling by fire, insects, drought, and disease (or some combination thereof) is what keeps ecosystems in balance with their limited water supply. In tropical forests, physical (e.g. cyclones) and biological disturbance mechanisms create openings that allow new access to limited light. Disturbance prevents any one species from dominating an ecosystem, maintaining diversity. 

Clements’ climax is mostly mythical because ecosystems are defined by dynamic oscillations between troughs and peaks rather than a linear progression towards a set endpoint. Ecologists refer to the boundaries of these peaks and troughs as an ecosystem’s ‘historic range of variability’.  These parameters of the past outline the ‘natural disturbance regime’ in terms of how big and how often disturbances hit. Since the disturbance regime is characterised by the magnitude and frequency of disturbance events it can be plotted as a wave. For rivers, the magnitude of the disturbance is the flood level, while the fire regime is characterised by fire severity. In both cases, frequency is described by the interval between disturbances.  

Beavers are natural disturbance agents in North American riparian ecosystems. (Image Credit: Krista Bonfantine, CC BY-SA 2.0)

Like most ecological processes, disturbance occurs at multiple scales of space and time. The daily activities of bark beetles in conifer forests do not become categorised as a ‘disturbance event’ until the bark beetles change the demographics and ecological function of the forest. The population surges that get dubbed ‘outbreaks’ may be well within the historic range of variability or they may be a symptom of a sick, out-of-whack ecosystem. It’s not always easy to tell the difference.

The patterns of disturbance that may seem random and chaotic are (or were) predictable in their rhythms and, as such, serve as behavioural triggers. It is well established that increasing streamflow triggers migration and spawning in freshwater fish, but I was astonished to learn that plants can ‘smell smoke’. Plants in fire-prone ecosystems use smoke as a cue to set and release seeds because favourable growing conditions may be produced by fire and, in some places, monsoon rains can be expected on the heels of fire season. 

The climax concept has generally been replaced by an equilibrium paradigm that describes ecosystems as existing in a steady state, maintained by the natural disturbance regime. This conceptual foundation, however, is built on a pile of shifting sand. In the Anthropocene, humans are disturbing ecosystems in new ways and pushing ecosystems to places where they have never been before. The climatic conditions that have oscillated so predictably through millennia have changed in the blink of an evolutionary eye.

Megafires are the most dramatic examples of mutating disturbance regimes. Five years ago, models were predicting an increase in fire frequency and fire intensity in the western United States and Australia. In the U.S., the combination of hot and dry conditions that used to occur once every 75 years have occurred three to six times over that period. The increased likelihood of dry, superheated conditions was expected to produce fast and furious fires in Australia. Oh, how right they were. During 2020, more than 4 million acres burned just in California and in Australia, a similar area was torched by intense, severe fire. With an increased likelihood of favourable fire conditions, the grim records from the past year likely won’t stand for long.

Severe fire in a U.S. woodland. (Image Credit: Krista Bonfantine, CC BY-SA 2.0)

Stephen Pyne has dubbed our current epoch the Pyrocene because our burning of lithic fuels has so fundamentally shifted the global system that fire is in the driver’s seat in ways it never has been before. As fire becomes bigger and badder in places where it has always been and moves in to shape places where it has never been, disequilibrium rules. Climax may be difficult to achieve but even equilibrium is elusive.

Krista Bonfantine is a PhD student at Deakin University studying the effectiveness of environmental flows using DNA metabarcoding and citizen science. Her background includes water and fire management and her passion is connecting science and society for a better, wetter world. Learn more on her website or follow her on Twitter.

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