A Savanna Life in a Technological World

Tall Grass Biodiversity Depression Part I

Tall Grass Biodiversity Depression Part I

I need to practice what I preach.



This week I am going to take a deeper dive into some of the themes discussed last week, “taller vegetation” and the biodiversity depression that goes along with it. I got some pushback on Facebook around this issue and it will take a series of blog posts to address those questions and comments. But I am sticking to my guns, in fact, I am raising the ante. I am going to say that if you are a grazing manager and you are reading this blog post (therefore, you are a nature geek) the most important thing that you could do to improve biodiversity on the land under your management is to maintain shorter vegetation. All this controversy is making me feel tingly all over. 

Last time I mentioned that we were clipping pastures to try to manage vegetation composition. I stated that we were trying to mimic the missing effects of very large herbivores (ie elephants). I stated that “very tall” vegetation, over all of the landscape, all the time, has a negative effect on biodiversity. I also stated that when vegetation gets to this point managers have very few ecological tools, as most of the animals that eat tall vegetation disappeared from the landscape a long time ago. So when we find ourselves in this situation, sometimes the only recourse is a technological tool. So we mow. Which brings us to the first thread in the Facebook discussion. It seems that some people would disagree with these statements.

Part of our disagreement comes from the fact that all these terms are relative, I am talking about “tall grass” and “short grass” instead of being more specific. Most everywhere that we (Grounded, SMI, MBG) manage ranches, the neighbours would laugh if they knew I was advocating for keeping pastures ‘short.’ I think everywhere our grass is taller than any of our neighbours. That being said, I am not sure it always should be.

Right now we in Oregon are about to start grazing a bunch of grass (~20% of the ranch) where one species is largely in bloom, and another has ripe seed, though we are mowing the ripe seed before the cattle get to it. I refer you to the picture. And I am very glad to have all that more mature grass as growth slows down and I want to lengthen rotations. But I definitely don’t want the whole ranch to look like that, for animal welfare, production, and biodiversity reasons.

So to generalize about all grasslands, while still being a little more precise, I might say that:

Rule #1

Our grass should not be as tall as it would be if we had no grazing animals at all, (at least not over all of the ranch, all of the time) if we want to increase levels of biodiversity and ecological production.

But I think there is more than just a miscommunication at play here. I think that even people deep in the ‘eco-grazing’ camp hold an assumption that grazing animals are not ‘natural’ and therefore the less noticeable their impacts, the better. But if grazing animals are natural (and I would say that they most certainly are) than if your grazing is unnoticeable, than your grazing is unnatural.

I want to point out that these statements are not that controversial in the ecological research community. The fact that they are controversial with some managers points to a problematic disconnect. Ecological researchers could learn a lot from ecological managers and ecological managers could learn a lot from ecological researchers. Neither party should make the mistake of assuming that because the one statement is true, the other statement is not.

Next week I will discuss my impression of the state of the art in this field of research. Until then I want to address another thread in this discussion and I will give my citations there.

Aaron commented on Facebook that it might be better to talk about “deep root grazing” instead of “tall grass grazing.” This is a reference to widely held assumption on the part of us managers that there is an exact 1:1 relationship to grass height and root growth, taller grass equals deeper roots. I highly suspect that this is at least partially true, but the exact relationship between above ground growth and below ground growth in grass is unclear to me (especially once the grass gets beyond phase 2.5). Root growth is a harder proxy for us to measure, but if we look at very short pastures, they definitely dry up faster than taller pastures. However, does that relationship continue all the way through to mature seed? I don’t think so. I think a grass plant that is firmly inside phase II stays green longer than one that has gone over to phase III. If we go with the heuristic of ‘as above so below, only more so,’ than should we also assume that grass roots also become more lignified, harder and less responsive, as the phenology matures? Would that explain that phenomenon? I don’t know. It would be an interesting question for researchers, but for us managers, we can’t wait for a conclusive consensus and I don’t think we need to. Here’s why.

I would suggest that we not pay much attention to ‘bio-physical’ rules of thumb when it comes to grazing management, rules of thumb like taller vegetation=deeper roots=better management. There are going to be all sorts of pitfalls involved with that sort of thinking. These are ecological systems. Our management philosophies need to be guided by ecological principles, which means they need to be rooted (ha) in evolutionary biology. If you are a mechanical engineer you need to tie your management to Newtonian physics. If you are a chemical engineer you need to tie your management to the periodic table. If you are an ecological engineer you need to tie your management to the last 30 or 300 million years of evolution on this planet.

Ariel made a related comment about perhaps not all components of biodiversity react the same, that while plant diversity might increase with shorter vegetation, maybe animal diversity is higher in taller vegetation. While different species of animals (and plants) definitely prefer very tall vegetation, some animals also prefer absolutely bare ground. Most are somewhere in between. While grassland nesting birds need taller vegetation for nesting cover, they need much more foraging area then they need nesting area. Many birds disappear from very tall grassland with lots of thatch. They have a hard time getting around to catch insects and they have a hard time getting at soil invertebrates. I would predict that insects (components of biodiversity themselves and drivers of avian diversity) prefer forage in phase II just like the livestock. For instance, for the week and a half this spring in California when the grass around our house was in Phase II, our chickens hardly touched the grain we fed them. In contrast, when the grass got tall and full of seeds, our chicken condition took a big slide, even with us feeding them all they could eat.

If I had to guess, I would suspect that animal biodiversity would perfectly mirror plant biodiversity, also exhibiting a ‘humpback curve’ (which I will discuss next week). The reason I predict this comes back to a theory. Plant diversity and animal diversity are not arbitrary. They are the way they are for a reason. That reason is as follows; most species are going to be adapted to the average condition over evolutionary history, call it 30 million years. Some species prefer conditions on the margins. Their ecological niche involves them capitalizing on conditions that are less common, but still occurred. However, most species are going to prefer the average. As such I would say that the vegetation heights preferred by plant species are going to equal the vegetation heights preferred by animal species. What’s more, the vegetation height that maximizes plant/animal biodiversity is going to be the vegetation height that was most common over the evolutionary history of mega-fauna maintained ecosystems (30-300 million years).

All makes for very gripping reading, I know. But there are important takeaways for all you nature geeks out there. Namely, that it is just as easy (maybe easier) to destroy biodiversity with vegetation that is too tall as with vegetation that is too short. And I think that we come to that tension point sooner than most nature geeks assume. I am ready to have things thrown at me.

Next week I will elaborate on my take on the state of the art in ecological research around biodiversity in herbaceous (grassland) ecosystems. By doing that I will try to paint a picture of what an evolutionary basis for grassland management might look like and the implications of that management for biodiversity more generally.


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