Words are so useful because they point to Truth. But if they don’t point to Truth, they point away from Truth. As such I think it is time that we ecologists have a sit down with the word “disturbance” and ask, ‘what have you done for us lately?’ While we’re at it, we can have the same little talk with his cousin, “succession.”
But before we do that, I need to sit down and have another little intervention, this time with myself, and this time concerning trailer maintenance. This means that it is time once again for the Almanac section where I discuss what happened on the ranch this week. We got a bunch of snow in the mountains and they were forecasting a bunch of rain. I thought this would mean that we were going to have a big, over-the-banks-flood, but so far not as much rain as they thought. Abel is still running around and hunting stuff. I don’t know where he learned this. I don’t think he did.
Meanwhile have been busy driving back and forth to Roseburg. My beautiful fiberglass posts arrived for the fence project this spring. They wanted 600 dollars to bring them from Roseburg. I said that I would happily drive to Roseburg for 600 bucks. I got there, loaded the six thousand pounds of posts into the trailer, put the truck in drive to drive off, and the trailer brakes locked up. Eight o’clock at night, raining, bunch of snow coming, I need to get back over the mountains. For one horrible moment I contemplated driving back with no trailer brakes and no trailer lights, cause the only way to move the trailer was to unplug it from the truck. Fortuneately I didn’t do that but dropped it off at the shop and went home. But that meant I had to go back yesterday, an hour and a half, and spend all afternoon waiting for it. Stupid problem, obviously, pin fell out of the saftey chain. Can’t believe I didn’t think of that. Ok, got that off my chest.
After that little interlude into the real world, we are back in the world of ideas, safe where no one can fly off an icy mountain road with a truck and trailer. So, I am both a practical naturalist, who makes a living (almost) off of the ecosystem and a theoretical ecologist that wants to understand larger ecological processes. The more I do each of those things, the less I understand these two terms (‘disturbance’ and ‘succession’). The abuse of these term gets really bad. Case in point, there are real live researchers that have been issued degrees from accreditted universitites that have written the following nonsense.
“Biodiversity is maximized at intermediate levels of production and intermediate levels of disturbance.”
Than that gets published in real live journals. Oh man, that is right up there with fingernails on a chalkboard as far as I am concerned. Productivity, we can measure that. But having an ‘intermediate level of disturbance’ suggests that disturbance is being measured. Is it being measured? How can it be measured if it has not been defined?
Disturbance has been a human centered idea. Fire and flooding always disturbed us, so we called them ‘disturbance.’ Made sense. But then people started looking and measuring. They figured out that, lo and behold, some organisms actually like that stuff. But the term was never refined and it didn’t go away. Words become dangerous when they become wallflowers.
That being said, I think there could be a way to repurpose the terms ‘disturbance’ and ‘succession’ and turn them from trash into treasure. But they would be harder to use, since doing things right is always harder than doing things wrong. The two important and measurable proxies that we can lean on in this process are biodiversity and productivity.
In the absence of important and measurable proxies, people just throw the word around however it suits their preconceptions. For example, in ecological literature, you often hear grazing described as a classic disturbance. Well “grazing” is hardly one thing. What is the severity of the grazing? The timing? The recovery? Might these different factors radically change our pronouncements about grazing? Yes, they do in fact.
For example, in California, with Grounded Land and Livestock and Sonoma Mountain Institute, our data indicates that our grazing management increases plant biodiversity by three or four X in four to eight years, when compared to the previous grazing managers on those properties. But our data also indicates that our grazing management increases plant biodiversity by three or four X in about the same number of years, when the property was ungrazed before we came along. From the perspective of biodiversity, the other guy’s grazing has more in common with exclusion than with our grazing. Are both grazing regimes ‘disturbance?’
In the Serengeti, in order to exclude grazing you have to take a very active and technology centered approach. When you do, diversity and productivity fall off a cliff. So, from the California Coast to the Serengeti, wouldn’t it make more sense to call exclusion the ‘disturbance?’ I would have to say so. This might be a surprising outcome for some people but one of the hallmarks of measurable truth is that it is often surprising. That is why we measure. Well, that’s why other people measure. I can’t seem to measure something to save my life. I prefer to discuss what other people have measured. If we don't know what we are talking about, we can't measure, we can't be surprised, we can't learn.
Lets look at flooding to flesh out what I mean. Grazing and flooding are a way of life on our ranch, so I have access to first hand information and I do a lot of thinking on the subject, both out of interest and in the interest of survival. Probably 80% of our pasture sits under water for more than a few weeks a year in the winter. This level of flooding is poisonous to most plant species. With six inches of elevation change we go from having 20 or thirty species of plants in a ten meter plot to having six or eight. The ones that do grow do pretty well, but compared to what? Compared to if it never flooded? If it never flooded what would that mean for ground water? We would be in a whole different universe so it’s hard to say. Compared to if it only flooded for two weeks a year? That would probably be the highest biodiversity and production, but thats threading the needle pretty close. Then, even if the plants don’t like the flooding, what about the aquatic animals? Even if we lose a bunch of plants, does the benefit to aquatic biodiversity cancel that out? Accounting for their preferences is much harder, so we tend not to do it.
Then to throw in one more kicker, if we have a flood for a week or two in the summer, than production and biodiversity go bananas. This is what we do when we we ‘irrigate’ (though I prefer the term ‘prescribed inundation’). This brings up the uncomfortable question, what if production and biodiversity are higher when humans do something, than when they do nothing, whether by irrigating, building, a dam, or completely altering the global climate? Is that a ‘disturbance? I have to say, ‘no,’ that is not a disturbance. I don’t want to spend a whole week writing about this, but I will just say that, I doubt that the highest levels of human impact will ever create both high biodiversity and high production. But that is just my hypothesis. We have to be open to the idea that I am wrong on that one, or else what is the point of managing?
So that is what disturbance is not. What is disturbance? I would say that a ‘disturbance is an event that reduces both the biodiversity and the net primary productivity of a defined area, as measured from one year to the next.’ We can say that ‘succession is proceeding’ when year on year biodiversity and Net Primary Productivity are increasing. In this way you can never again refer to general processes as ‘disturbance’ unless you can prove that things like fire and flooding always reduce biodiversity and production everywhere, from one year to the next. You have to be site specific, you need to measure, and you need to measure enough things to insure that you are catching a good subset of the winners and the losers. That won’t be easy. Perfect. Worst case scenario, no one uses those terms ever again.
I have to admit that this definition of disturbance and succession is very savanna focused. But I think that is a good counter balance. Until now this sort of research has focused almost exclusively on forests. I use Net Primary Productivity instead of Total Biomass because Total Biomass grossly underrepresents production in herbaceous ecosystems. My proxy choice would have an interesting effect. If there was a forest that has sat ‘undisturbed’ for a hundred years and is now losing both biomass and species as a result, than that forest is ‘disturbed.’
This is where an obscure rant has reworld implications. On average, our National Forests are losing biomass. I suspect they are also losing biodiversity on average. This is not because of clear cutting. But because of ‘no management.’ I think this is a problem, so I will label those forests ‘disturbed.’ This is a value judgement but it reflects my values. If your management creates an increase in biodiversity and productivity than you have a good outcome. Again, words matter.
This brings us back to the hated term, the one I need to hold my breathe and grit my teeth in order to utter. “Biodiversity is maximized at intermediate levels of production and intermediate levels of disturbance.” I would argue that this is a classic case of confusing cause and effect. In the modern world, intermediate levels of production and intermediate levels of ‘disturbance’ are where you find savannas. It is savannas that are biodiverse.
