A Savanna Life in a Technological World

Tall Grass Biodiversity Depression Part IV

Tall Grass Biodiversity Depression Part IV

Ideas are powerful.

Wow, good blog post, Nate. Other people might have thought of that before. But most of the time when we think of the power of ideas, we think about the power of good ideas. As humans, I think it is much more important for us to think about the power of bad ideas, or ideas that aren’t quite right. Better to have no ideas than ideas that aren’t quite right. Those ideas lead to all sorts of bad relationships, lost wealth, and conflicts of all kinds. But I don’t deal with relationships and wealth. Obviously. I deal with biodiversity.

I usually just want to deal with ideas about biodiversity but before I do that I guess I will talk about real life for a minute. Sigh. I just sold this season’s first group of cattle from Oregon and I am shipping them tomorrow. I was worried about running out of grass when I was fighting with Oregon Fish and Wildlife over water. As it works out we are going to have plenty of grass; oh well, better to have too much than not enough. Although there are limits to that, as I will discuss once we get back to ideas. Since we are selling cattle, I sorted out the group to ship in the tiny corrals we inherited from the previous owner. Triangle sort pens; bad idea, don’t do that. Abel and I were fixing up the old corrals yesterday so they can limp along a little more. He used his obsession with trailers to pull the backhoe with the pickup truck. See Facebook for pictures. Hanna just talks about how pregnant she feels, though she looks as beautiful as ever.

There. With that out of the way, I can go back to talking about things that won’t step on your toes or kick you in the groin if you mess up. The cattle I mean, not that Hanna would do that. Though if I keep talking about how pregnant she is she might be tempted. Back to ideas, quick.

Most people have ideas about biodiversity that revolve around rainforests. Therefore when eco-people go ranching or ranching people go eco, they often try to make their pastures look like rainforests. Which is to say they create pastures that have ‘high standing crop levels.’ But succession (an idea I will deal with in a future post) happens very differently in savanna ecosystems than in rainforests. In rainforests, you can see the last hundred years of production, in the form of a whole bunch of carbon (vegetation) just sitting there. In rainforests, there is a reasonably good correlation between standing crop and production, and a reasonably good correlation between production and biodiversity. Not so in savannas. In fact, the correlation is often inverse.

The difference, as usual, comes down to mega-fauna. In savannas, a big percentage of the production gets diverted away from standing crop and towards putting back-fat on buffalo, and filling the guts of giant ground sloths. Herbivores push the carbon in savanna ecosystems into soil organic matter and below ground plant material. Over hundreds of millions of years, savanna ecosystems have developed a powerful mechanism to ensure that standing crop never got very high. This is a mechanism pulled by herbivores and pushed by competing for woody vegetation.

Over evolutionary timescales, ‘standing crop’ was just too valuable of a resource for herbivores to let it sit around. There were too many hungry mammoths, wildebeest, and kangaroos roaming around for standing crop to build up unused. Unlike in rainforests, productive savannas don’t necessarily wear their production in extra standing crop. Productive savannas just attract more grazing animals that eat that material, leaving standing crop relatively unchanged.

On the other hand, you had herbaceous plants that knew that if tall vegetation hung around for too long, pretty soon things were going to turn into rainforests, deep dark places with no room for herbaceous plants. As a result, herbaceous plants have been working hard over the last hundred million years to make themselves more palatable to herbivores so that the herbivores come and get rid of all that pesky rainforest. In the process, keeping the world safe for herbaceous plants. Want to know more? Don’t worry, I go on and on and on about it in my book, the galley proof of which is available here.

Last week I gave you my bio-physical heuristic to explain the ‘humpback curve,’ the idea that savanna ecosystems have the highest biodiversity when standing crop is medium to low. I said this was because shorter grasslands can have more plant stems, which means more individuals, which means more species. That sounds good, but I will be the first to admit it has problems. An acre of eight-foot-high canarygrass around here probably has tens of thousands of stems. Many more than any rainforest, probably more than any dry grassland. Yet all those stems will be from one species, canarygrass. This is an element of the ‘Pardox of the Plankton’ that I have not heard anyone address. It seems to me that very productive grasslands, even short ones, are less biodiverse than grasslands with medium to low production. In other words, tall grass inevitably gives you low biodiversity, but short grass doesn’t necessarily give you high biodiversity.

I can’t give a good causal mechanism for this phenomenon or an idea that fully explains this at this point. The best I can do is go macro. Eight foot high grass just wasn’t that common over the last hundred million years. Because herbivores kept the vegetation shorter, there was just not a lot of eight-foot-high grass habitat and so not a lot of selective pressure for plants to evolve ways to insert themselves into that sort of environment, as plants evolved to insert themselves into rainforests with a high standing crop.

Trying to make our pastures look like rainforests is a bad idea. It ignores the evolutionary history of savanna ecosystems. The irony is, as I mentioned last week, managed grasslands can be just as diverse as rainforests, and more diverse than plant communities that ecologists generally think of as diverse, such as California chaparral or South African fynbos. The amazing thing is that these hyper-diverse grasslands are intensively managed by humans, and probably have been for 10,000 years.

There is a few ways that we could take this information and make bad ideas out of it. On the one hand, we could say that we don’t need to worry about biodiversity. Sometimes humans ‘create’ high biodiverse ecosystems, so let’s just go back to watching cooking shows and not think about biodiversity. Not quite right. On the other hand, we could say that we better quickly protect these biodiverse grasslands from humans who are bound to poison them eventually. Also not quite right.

In place of those not quite right ideas, I would suggest that grasslands are most diverse when they mimic the evolutionary history of the last hundred million years on this planet. I would suggest this means that grasses should be (in general and most of the time) short. But not too short. I would also suggest that over virtually the entire planet, the only way that this will happen will be through careful management.

Unfortunately, the modern world is now full of things like this, things that used to take care of themselves, but now must be carefully shepherded. Again, I go on and on about that in the book. I don’t know if I am thrilled about that idea, but it’s the best I can do.


Next week in the final post in this series (a post I am calling “Tall Grass Biodiversity Depression Forever”) I will discuss some practical management tools for keeping grass short-ish when grass growth can change so quickly and stocking rate changes so slow. In addition I will discuss the vegetation monitoring data collected by Sonoma Mountain Institute and Grounded Land and Livestock that support these ideas. 


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