Decadence is a concept in ecology that is very distinct in meaning, but sometimes similar in spirit, to the use of the word to describe societal excess and decline. The basic idea is that ecosystems require disturbance to maintain productivity and biodiversity. Fire, flood, and grazing are probably the most frequently invoked examples of disturbance. An ecosystem that is declining due to a lack of disturbance is decadent. At the level of individual plants in the ecosystem, a plant with a high proportion of dead to live biomass is decadent. There is no question that this conceptual framework is accurate and useful in many cases. Fire-dependent ecosystems, especially, have been very thoroughly documented. However, there is also no question that it is not universally applicable. A plant with a high proportion of dead to live biomass could be a mature tree in old-growth forest, since wood is mostly dead. While they should not be envisioned as completely steady-state, disturbance-free ecosystems, applying the concept of decadence uncritically would lead us to believe old-growth forests are fundamentally diseased. Mature plants are often a good thing, not an indicator that something is wrong. We should think critically about what we know about the role of disturbance in an ecosystem and make informed decisions about when and how we apply the concept of decadence. I think there are several factors that tend to bias us in favor of overestimating the importance and frequency of disturbance, and assigning inappropriate importance to decadence, in semiarid ecosystems.
Most grasslands are adapted to both fire and grazing. The tendency for most grasses to keep their buds (apical meristems) at or a little below ground level is a great adaptation for tolerating events, like fire and grazing, that remove large amounts of above-ground biomass. Many grasses also tend to accumulate energy reserves until that kind of disturbance event comes along. A plant that still has its buds intact and stored energy available can quickly convert stored energy into new growth to replace what was lost. In the absence of disturbance, accumulations of dead biomass on the ground make it more difficult for new growth to emerge, and the stored energy reserves remain unused and are eventually lost to decay. This pattern of grassland features is so common and well-known that it has become the established paradigm. In this paradigm, decadence is a very useful concept. A high proportion of dead to live biomass is strong evidence that something is going wrong. Semiarid grasslands, however, often deviate from this pattern. For instance, the dominant grasses in the Chihuahuan Desert are suffrutescent. The above-ground stems persist and branch for multiple years, with new growth coming from buds well above ground level. This makes them less tolerant of both fires and grazing, and means that accumulations of dead biomass have no little or no inhibitory effect. New growth does not need to push through a layer of dead plant matter, the grasses can simply continue growing above old biomass indefinitely. The presence of suffrutescent grasses in an ecosystem should cause us to immediately recognize that we are not operating under the standard grassland paradigm, and that decadence is probably not a useful concept. However, people are not good at dealing with exceptions. Once a pattern has been established in our minds, it becomes decoupled from the evidence on which it was based. Instead of recognizing changes in the underlying evidence as a cause to rethink the paradigm, we accept the paradigm as factual and see new evidence only by its light. We have been presented with very strong evidence, from the 1890s to present, that our grassland paradigm is fundamentally, and sometimes catastrophically, wrong in semiarid grasslands dominated by suffrutescent grasses. The paradigm has, nonetheless, been remarkably resilient. For instance, there is a series of papers from 2000 to 2010 on the role of prescribed fire in suffrutescent-dominated grasslands on the Jornada Experimental Range in which researchers have continued to advocate for an important ecological role of fire in the maintenance of these grasslands. They do so despite more than a century of evidence strongly rejecting the traditional grassland paradigm in these ecosystems, and a strong rejection of this ecological role for fire in their own data, which they present in these papers. When exceptions to the standard paradigm are so strongly resisted by professional scientists, it should be no surprise that this paradigm also continues to be accepted by most non-academics living and working in these ecosystems.
Decadence is very easy to assess quickly and subjectively. Fundamentally, you go outside during the growing season and look around you. How green does the vegetation look? Other ecosystem attributes for which decadence is a proxy, on the other hand, are quite difficult to measure. Measuring an ecosystem’s primary productivity requires that you clip and weigh new growth on the plants around you, dry them and weigh again to correct for variation in moisture content rather than productivity, correct for the time of year (how much of the year’s biomass has been produced at the time you measure), correct for consumption by herbivores, and correct for weather. Having collected this data, supervised others who are collecting it, and managed productivity data collected by various people over several decades, I know that not only is this data difficult and time-consuming to collect, without a constant focus on quality assurance and quality control it will not be collected with any usable accuracy and even with a strong focus on QA/QC the data will be less accurate than many of the other attributes of the ecosystem that we could measure. Biodiversity data is also very difficult to measure. Thinking simply of plants, accurate biodiversity data really requires the oversight and assistance of someone who has a decade or more of expertise with the taxonomy and identification of plants in that ecosystem. I say that as someone with that kind of expertise who is absolutely certain that he makes silly or careless errors in collecting this kind of data. I think I make those errors far less frequently than people without the same expertise, but I know I make them. I try to notice and correct myself when I do. In any case, if decadence is a good proxy for various ecosystem attributes we want to measure–and in some cases it is–it is a wonderful shortcut. When it is not a good proxy–and in some cases it is not–the sheer subjective obviousness of this variable will make it very tempting to use it anyways. In practice, that’s what most people do. You will hear, for instance, people say that a prescribed fire had worked really well. If you push a little to try to find out what evidence they’re looking at and how they’re evaluating it, rapid growth from grasses after the fire is almost always one of the indicators they’re using. Suppose before the fire you have a grassland with about 50/50 green new growth and brownish, older, mostly dead plant matter. After the fire, you have a carpet of fresh green. Subjectively, this is very appealing. You can’t help but notice a very dramatic change in decadence, and even without a grounding in traditional paradigms about grassland ecology most people are going to intuitively interpret this as a positive change. Does it actually correlate with positive changes in things like productivity, biodiversity, and so on? We can’t know that without actually doing the measurements. When faced with a subjectively obvious improvement, though, it is hard to convince people to put in the time and effort. So we tend to look at the easy variables and fill in the rest with assumption. It’s easy to look at decadence, and someone showed that it’s a good shortcut over there… close enough.
People have inherent aesthetic biases. Dead plant material on the ground in an ecosystem is called “litter”, and while ecologists all conceptually know that this use of the word is rather different than the “trash left by humans” usage, it’s probably not a coincidence that it’s the same word and our thinking is probably colored by the negative connotations. We think of cleanliness, orderliness, and uniformity as desirable attributes in our homes, cities, and gardens, and we don’t simply turn this bias off when we visit more natural environments. Decadent plants are untidy. In semiarid systems where both grasslands and shrublands are equally natural parts of the landscape, we simply like the looks of a grassland better. Further, some kinds of plant communities are simply easier for us to interact with than others. Shrubbier plant communities are often more difficult to walk through and see around in, especially if they have a lot of dead branches. In areas where spiny plants are common, shrubbier plant communities can be painful. Within a grassland, generally the shorter the grasses are and the less dead plant material there is, the easier it is to walk around in. In a forest, down limbs are a hindrance. Our aesthetic biases and physical means of interacting with the world push us towards a negative view of decadence.
Last but not least, one of our major uses of semiarid ecosystems is the grazing of livestock. Viewed through the lens of a rancher, dead plant biomass accumulating on the land is wasted feed for livestock. Fresh new growth after a fire is highly palatable. And people are very strongly biased towards interpreting their actions in a positive light. If you make your living by creating disturbance events that remove a lot of above-ground plant biomass, you will probably view that process as beneficial and adopt a worldview like the traditional grassland paradigm. When that paradigm happens to be correct, everything lines up nicely. In grazing-adapted ecosystems, the path to managing grazing in a way that is consistent with the long term health and productivity of the ecosystem is generally obvious and relatively easy to follow. People still make bad decisions, because people are people, but someone dedicated to making the right decisions can figure out how to do it and the system is relatively resilient to the occasional mistake. Believing that grazing is beneficial is not just the conceptual path of least resistance, it’s true across a substantial range of possible grazing management practices. People tend to take the conceptual path of least resistance regardless, however. When we find ourselves in a semiarid grassland dominated by suffrutescent grasses, we should be able to figure out that this is not a grazing-adapted ecosystem and change our concepts and behavior accordingly. If our livelihoods depend on grazing, that’s a strong force pushing us in the opposite direction.
The concept of decadence is regularly applied in semiarid ecosystems, but I think in most cases it is misapplied. Attempts to arrive at a better understanding face strong headwinds, and have not prevailed so far.
