Is it useful to categorise disturbances as pulses or presses?

In the 1980’s Edward Bender coined the terms ‘pulse’ and ‘press’ to describe different types of disturbance to ecological communities. These terms were originally intended to be used to classify experimental perturbations. However, in the 30 years since they were first used this division between pulse and press has become the dominant way of thinking about perturbations in real-world ecosystems. Bender defined pulse disturbances as perturbations from which the community is subsequently allowed to respond. Now pulse disturbances are seen as any relatively short-term, easily delineated disturbance.  Press disturbances, on the other hand, were defined by Bender as those in which ‘densities of perturbed species are altered and maintained at predetermined levels by adding or removing individuals as needed.’ This has since been altered with press disturbances defined as those that continue at a similar intensity following their initial occurrence*. Common examples of pulse disturbances are fire, drought, or selective logging, while press disturbances may represent conversion to agricultural use. Importantly, deciding whether a disturbance is classified as a pulse or a press comes down to whether a researcher considers its duration to be short-term or not.

As a result, this definition  of what category a disturbance falls into is inherently subjective. Pulse disturbances are often events that happen in a single year, but is a drought that lasts for three years still a pulse? How about conversion to agriculture that is abandoned after a decade, is that still a press disturbance? What if it is abandoned after a year, is it a pulse disturbance now? All of this results from an imprecise definition, which can lead to serious misunderstandings in ecology. However, this is not an issue that can be solved a better definition. Rather, I think that dividing disturbances into categories is unhelpful and the categories pulse and press do not communicate all the details required to compare disturbances.

As I’ve written before, I’m not a big fan of categorising things in ecology when it can be avoided. My opposition comes from the fact that I think it removes a lot of the nuance regarding what is going on in ecological systems (Brian McGill has written a great post on this issue). Comparing the impact of pulse and press disturbances on biodiversity may be superficially interesting, but it’s much more interesting to look at how the duration of a disturbance influences biodiversity change and any subsequent recovery . Ideally, when investigating the influence of disturbances it would be useful to know how (i) long a disturbance lasted (or how long since it started in the case of ongoing disturbances) and (ii) when a disturbance has ceased, how much time has passed since the last disturbance. In a perfect world, we would also like to know the intensity of the disturbance, but we can usually only find this out by analysing what happens to an ecological community in response to the disturbance.

Another reason not to classify disturbances as pulse or press is that this doesn’t take into account any of the ecological differences in taxonomic groups you may be investigating. For example, if a drought occurs in a temperate forest in a single year it will be experienced as a temporary change for a tree which may survive for hundreds of years. However, this drought may represent the entire lifespan of an invertebrate species. So even though the disturbance length for the two species may be the same, the drought may account for <1% of the lifespan of the tree while accounting for ~100% of the invertebrate’s lifespan. Therefore, the same disturbance may have very different effects on different species groups. If the disturbance inhibits reproduction this would result in steeper declines for species with a naturally shorter lifespan**.

The same issues arise when explaining the effects of longer lasting change, such as agricultural conversion. All else being equal declines would be expected to be more pronounced in species with shorter life spans, and recoveries in their populations would likely be faster. This has already been recognised by work on extinction debt, which suggests that species with short generation times are less likely to show lags between disturbance and population decline than longer-lived species.

So, although dividing between pulse and press disturbances may be superficially appealing, reporting the duration of disturbances is likely to allow greater potential for generalisation. Combining this data with information on species traits such as mass/longevity will then help to answer the question of how the effect of disturbance duration differs amongst species and which species might be most vulnerable to anthropogenic change. This approach could also help to improve our understanding of d recovery following disturbances, a process which will likely become more widespread as humans continue to move from the countryside to cities.

* The term ramp disturbance was also introduced by Lake in 2000 to describe disturbances that increase in intensity over time, such as climate change, but I didn’t want to get into that here.

**Some of what I’ve written here is a result of some back-and-forth I had with Martin Jung and Dale Nimmo on Twitter, you can see a summary of that conversation here.

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