What did bison once eat in November in Kansas?

Most people think the greatest mystery with bison is "How many bison were there before Europeans arrived"?

That number to me seems trivial. If it was 5 million or 20 million is not unimportant, but it's a quantitative question, not a qualitative question.

Today I was out at Konza today. It's November. A warm day, but almost everything is brown. A few rosette forbs were green in the uplands. Some scattered grasses and sedges in other places. Almost nothing good for bison to eat.

Except by the roadsides and some of the firebreaks. Those areas are kept mowed throughout the year and tend to be dominated by the annual grass Bromus arvensis--japanese brome.

The thing about japanese brome is that it greens up early and stays green late. It's an order of magnitude more nutritious than almost anything else out there right now.


The other cool-season grasses just aren't that similar to japanese brome. They are mostly bunch grasses and aren't green or at least vigorous throughout the Kansas winter.

As you can see, the bison really work hard to get it and it must provide an important part of their current diet. As you can hear, they really rip at the turf. The grass there might have been 15 mm tall.

The only thing is that japanese brome didn't use to be in Kansas. When it arrived is uncertain but probably not until the early 1900's.

One thing I've wondered is that if japanese brome wasn't introduced, what would the bison be eating? What would be different about our bison if the grass wasn't there?

Those are just two of the the qualitative questions I think are pretty interesting about bison these days.

Grazers in a warmer world

If the world gets warmer, what happens to grazers?

Not an easy question. There are many grassland climate change experiments, but these are of limited utility here. Grazed and ungrazed grasslands are starkly different such that the consequences of warming for ungrazed grasslands are unlikely to apply to grazed grasslands.

If experiments don't help, then we need to look at how grazers respond to short-term variability in climate and compare that with geographic patterns that might represent long-term patterns.

When I've looked at how bison respond to inter-annual variation in climate, hot years don't affect them.

Yet, when we look across temperature gradients, hot places have small bison. Across 22 herds and a quarter million weights of bison, it's clear that herds in hotter places have smaller animals. Sometimes up to 500 lbs lighter.

Why the difference between short- and long-term patterns?

This is where experiments come in handy. When exposed to elevated temperatures short-term, grasslands begin to lose nitrogen. Over the long-term, these losses accumulate which drives down the quality of grass for grazers to eat.

One hot year, no problem. Many hot years and grazers don't grow as big.

The differences among bison are dramatic.

Back of the envelope calculation shows that just a 1°C increase in temperature across the US could cost the cattle industry $1 billion. Considering projections are for multiple degree C increases, those costs would accumulate.

[Regarding details, I'm about to submit this paper. We'll see how it's met.]

Genetic future of bison

I've spent the last few days in Tulsa at a conference sponsored by the American Bison Society, which is part of the Wildlife Conservation Society. The conference was attended by a mix of scientists, government officials, ranchers, and tribal members. The conference centered around three panels, two of which focused on the genetics of bison. The third panel, which I helped put together, was on the ecology of the bison. We were largely intermission for genetic questions.

A quick bit of history. Bison once ranged in the millions in North America, but at the end of the 1800's had been reduced to about a thousand animals. Some of the remaining bison had been bred with cattle in an attempt to improve the performance of cattle and cattle DNA became part of collective genome of bison. It is not evenly distributed among modern bison--some lineages have more than others, some appear to have none. The presence of cattle DNA in bison has largely consumed discussion on bison for the past decade. How much is there? How is it distributed? Does it have functional significance? Can we get rid of it and repopulate our herds with "pure" bison?

The discussion on the topics were long, nuanced, and technological at times--lots of next generation sequencing and single nucleotide polymorphisms being discussed. ABS will draft official statements, but for me, I think the meeting will be known as a watershed in tolerance and understanding for the modern American Bison. In short, most bison in public herds have some cattle DNA. Quantitatively, less than 1% of the nuclear DNA might be from cattle, but it's there. It possibly could be culled out of the herd, but the bison DNA that we would lose would far outweigh the potential benefit of removing traces of cattle ancestry. Bison also aren't unique. Many of our remaining wild species have DNA from other "species" in them. Wolves, bactrian camels, Przewalski's horses all bear the genetic imprint of domesticated relatives. 

We'll see what the official statements say, but American bison will always be a symbol of America's past. Yet, our bison are also a modern symbol--a bit mixed up, bearing the traces of past pain and hope and ambition, but one that probably should not be atomized any more. In some small way, the conference reflected a modern and sophisticated sense of tolerance.

So, when people visit bison in our parks and preserves, they are likely to be seeing a little bit of Hereford. But, it's a small price to pay if it reminds us to learn more about the animal's history. Much of the bison community seems willing to accept the mark of history and focus anew on continuing to restore  them.

Grassland Climate Change 3.0

Critical climate periods for ANPP, flowering of three grasses, weight gain of calves, yearlings, and adults, as well as calving rates the following year for Konza. Gray bars indicate a negative effect of precipitation on the process, black positive.

If you look at the development of climate change research in grasslands, there have been two main stages. Climate Change 1.0 was trying to understand the importance of changes in growing season precipitation on ecosystem dynamics. Wet years are compared to dry years. Experiments that test climate change in 1.0 modify total precipitation.

We're still largely using Climate Change 1.0. Climate Change 2.0 examines effective precipitation during the growing season. Effective precipitation calculations largely take into account event size and distribution. Light rain events might lower effective precipitation as they are intercepted by canopies. Heavy rain events might lower effective due to greater flow through or runoff. Too light or too heavy and plants might not ever get a chance to use all the rain, hence lower effective precipitation. Some early-adopters are investigating Climate Change 2.0, but it's not mainstream yet. Certainly the projections and climate change models are not built to forecast in a manner that promotes 2.0.

One of my goals has been to push Climate Change 3.0. With 3.0, it's not just how much rain falls during the growing season, nor how much effective rain falls during the growing season. but when the rain falls. If you look at the critical climate periods for aboveground net primary productivity (ANPP), they largely show that 1.0 works--the more precipitation in the growing season, the more ANPP. For flowering of the major grasses, it's largely 1.0. Growing season precipitation largely determines flowering, with some differences among the species in their sensitivity to rainfall.

For Konza bison, there is just no relationship between growing season precipitation and weight gain for any sex or age class. But factor in the timing of precipitation, and you can explain up to 80% of the variation among years in weight gain. Why? It's because mid-season precipitation suppresses weight gain, while late-season precipitation promotes it. The climate-nutrition-performance cascade hits bison hard. Most likely, the same thing applies to cattle, although it hasn't been shown.

Climate Change 3.0 is nothing new conceptually. But in practice, 3.0 is. Training our models to predict when precipitation falls can be more important than how much falls for humid grasslands. Training ecologists to start to examine this will be probably be harder.

Bison growth curves

Weight of female (lower) and male (upper) bison at Konza Prairie and Ordway with age.

The performance of bison—how much weight they gain, how many calves are produced—is the ultimate expression of the functioning of North American grasslands. If we can compare the performance of bison in different grasslands, we have a window in the functioning of the grassland. Interannual patterns of weight gain show responses to climate variation. Average weights of animals give general indices to the provision of the quantity and quality of grass produced. Yet, no one has ever compared the performance of bison across grasslands in North America. 

We're getting pretty close to doing that. For any one site, we can fit a growth curve to the weights of animals as they age. There are a number of growth curves that are used for these purposes, but a good one is a generalized Michaelis-Menten equation:

where W0 is the birth weight, Wf is the asymptotic weight, K is the age at which animals are half their asymptotic weight, c is a constant describing the shape of the curve, and t is time in years.

If you fit the weights of bison with age with this equation, for each herd you can extract essentially how heavy cows and bulls get, as well as a rate of maturity...or half-maturity as K would represent.

Right now, we have data on weight gain for about six bison herds. There are about 10 herds in the US that have weight data from roundups. 

So far, we see a few basic things about bison. On average, males level off at about 75% greater weights than females (855 vs. 484 kg). It also takes them about 1.5 y longer to reach half their maximal weight. 

We also see that some bison herds are heavier than others. For example, mature bison in Ordway Prairie in South Dakota are 50-100 kg heavier than mature bison from Konza. That's a lot of bison. Is it a fluke? Unlikely. Over 90% of Ordway adult cows produce calves. At Konza, it's only about 60%. 

There must be a big difference in the grass between Konza and Ordway.  Because their bison growth curves are quite different.

Climate-nutrition-performance cascade

Critical climate periods for precipitation for ANPP, flowering of 3 grass species, bison weight gain, and the calving rate of adult females the following year.  

Some ranchers around here know that "a dry June is money in the bank". Supposedly when precipitation in June is low, cattle gain more weight. More cow means more money. I haven't heard it too much around here and I had never seen data to support it (until the work on bison weight gain at Konza), but it exemplifies the climate-nutrition-performance cascade and is a cautionary lesson in understanding climate change.

The performance of grazers--their weight gain and calving rate--is dependent on both the quantity and quality of the grass they eat. The interannual determinants of quantity seem fairly straightforward (for some sites). Quality is less so. Quality encompasses a lot of things, but primarily is protein. And protein is nitrogen.

The cascade links climate to performance through protein. The proximal and distal drivers of variation in protein are complicated enough that they are still being worked out. But as we work through this, it will be important to ask whether there protein at certain times is more important than others. And if so, maybe interannual variation in climate at certain times is more important than others. Rain in June might be more important than May, for example.

We've used the critical climate period approach to begin to tease some of this out at Konza. The figure above shows a broad CCP for ANPP--rain that falls in June or September still is important for determining growing season biomass. The three C4 grass species have different, but overlapping, CCP's. Each is about 80 d. In contrast, bison weight gain and calving rate seems to respond to variation in precipitation for just relatively short periods. One in late June, early July. The other mid- to late August. Calving rate depends on just mid- to late August precipitation.

Between climate and bison performance is protein. And the controls on protein we're still figuring out.

Until we do, we'll have a hard time understanding the dynamics of grazers, no less their fate in a world where climate has changed.

Natural history of bison dispersing seeds

Bison heads carry more than horns.

Bison do a lot in grasslands. They eat, poop, pee, rub, trample, and wallow, which fundamentally can restructure how a grassland functions. If you spend enough time watching bison, you'll see them eat some unusual things. For example, early in the season, I've seen a cow systematically nip off sumac buds. Not something we typically associate with bison, at least not overly curious bison. Another thing bison do is disperse seeds. And a close look at seeds makes us rethink a bit about what they eat.

Researchers at Oklahoma State recently published a paper where they analyzed the seeds attached to bison forehead fur in the fall and fecal material over the year. In all, they found the seeds of 76 species on the fur of bison. Turns out males and females had different seeds stuck to them, which related to where they spent time.

More interesting was what was found in the fecal material. There really is only one way for seeds to get into bison pies--they have to eat them. Half the seeds were grasses, which means half weren't. This is surprising because plains bison are thought to predominantly eat grass. Yet, in the spring there were seeds of Viola. In July, there was Solanum and Lepidium. In October, there was Lepidium.

Most of the generalizations from the grass dominance of diet comes from either microhistological studies (leftover plant parts) of bison fecal material or changes in species composition. Yet, microhistological studies might underestimate forbs if their cell walls are easily degradable. Changes in species compostion with grazing show increases in forbs, but cannot rule out which forbs they might eat.

Figuring out what they eat has never been easy. Here, some simple natural history might just reset one of the fundamental assumptions about bison.

Rosas, C. A., D. M. Engle, J. H. Shaw, and M. W. Palmer. 2008. Seed dispersal by Bison bison in a tallgrass prairie. Journal of Vegetation Science 19:769-778.

Bison and seasonal protein

Forage crude protein concentrations (%N * 6.25) for male and female bison over the season from Konza.

Bison are the largest native grazers in North America left. Their history is interesting, having almost gone extinct with the Pleistocene megafauna, and not having evolved into their modern form until about six to eight thousand years ago. Most of the attention on the evolution of the animals has been regarding changes in their morphology. Most of the attention to the modern animals has been their genetics and the introgression of cattle genes—finding “pure” bison. Most of the interest in their modern ecology has been on their role as a keystone in ecosystems.

Almost entirely missing from the study of modern bison has been their nutrition. There has been some work on diet—do they eat forbs or grasses; cool- or warm-season grasses. Yet, animals that ranged throughout North America and never had access to grasses like the progenitors of modern cattle would have found in northern Europe would likely face strong nutritional stress throughout much of the year. The adaptations of bison to low forage quality, no less the basic patterns of the availability of energy and protein to bison have gone all but unasked.

At Konza, Gene Towne has been collecting fecal material throughout 2009. Every two weeks, he has collected fresh pies from both males and females. Then we send the samples off to Texas A&M’s GANLab to see what the crude protein (nitrogen) and digestible organic matter (energy) was of the grass that they were eating.

If you look at the patterns from 2009, a few fascinating patterns stand out. First, the minimum protein requirements for mass gain for cattle are about 6% crude protein. Bison at Konza have about 100 day window to gain mass during the growing season. After that, there is little protein available beyond what is required for maintenance.

Second, the differences between males and females has never been observed before. Males tend to form “bachelor” herds and do their own thing until the rut—roughly August. After that, they often go off on their own again. The CP patterns show that the males are not selecting as high a quality forage early in the season, but the peak is broader. During the rut, quality is about the same as females. Afterwards, the males are selecting lower quality forage than the females. Why? Why wouldn’t the males feed in the same places on the higher forage quality. A mystery right now.

Lastly, by mid-October, CP had dropped to roughly 4.5%. Not much good green out there for anyone. Gene’s found that the bison lose about 10% of their weight during the winter, which can be up to 200 pounds for the large males. We’re beginning to see why.

Hopefully, data like this will continue to be taken at Konza for a couple of years. It’ll be fascinating to see the differences between wet and dry years on forage quality. With any luck, we can start similar measurements at a number of other TNC sites with bison to being broader comparisons.

Nutrient limitation, climate, and bison

Relationships between mid- (a, c) and late-summer (b, d) precipitation and bison weights for each year for Konza Prairie (a, b) and Tallgrass Prairie Preserve (c, d). Calf weights and yearling weight gain (YWG) were adjusted for differences in sex ratios to represent the average weight of an average male and average female bison. Midsummer weights were standardized for variation in late-summer precipitation and vice versa.

In RSWP, I write a lot about nutrient limitation of plants. One thing that comes out strongly as we look at how ecosystems function is that nutrient limitation in plants can induce nutrient limitation in herbivores. Especially for nitrogen which animals generally cannot access through other means except by eating plants.

We just had a paper published that illustrates a few important points regarding nutrient limitation in grazers and the distal controls on grazer performance. In the paper, we examined how interannual variation in the timing and magnitude of precipitation affected the weight gain of free-roaming bison. Every year, the bison herds are rounded up, and each individual weighed. Bison weights of calves and animals in their second growing season (yearlings) were analyzed for 14 years for Konza Prairie, Kansas, and 12 years for Tallgrass Prairie Preserve, Oklahoma. The sites are both native grasslands on the drier edge of what is considered humid grasslands. The records of weight gain for wild herbivores are among the longest known—only Marco Festa-Bianchet’s excellent work on mountain goats is comparable.

As we looked at how much weight the animals gained each year, we found was that more rain late in the summer increased the weight gain of the bison. Not much of a surprise there. More rain in August, which is often dry, means more green grass later for the animals, which would allow them to grow more. What was unexpected (to some) was that having more precipitation early in the middle of the growing season (late June, early July) caused animals to gain less weight. Why? Here, it’s not the quantity of food, but its quality. What we found was that having high precipitation in the middle of the growing season also increased flowering (see previous post on flowering). More flowering means more low-quality stems, which lowers the average protein concentrations of the grass, and lowers weight gain. This idea wasn’t entirely unknown—some ranchers say a dry June is money in the bank—but it was the first time quantified scientifically.

Now, it also might be the quality of the leaves that is low in high mid-summer precipitation years (we don’t have data on that), but the data illustrate a few key points. First, in grasslands, quality is as important to consider as quantity when considering grazer performance. As I describe in RSWP, there are glaring examples of this being glossed over (as well as great examples of its consideration). Second, as we think about how grasslands are structured and how they might change, the timing of precipitation can be as important as the amount. Increase or decrease the amount of precipitation by half and bison gain the same weight. Shift it from August to early July and weight plummets.

What we learn from simple observations continues to amaze me. By no means have we plumbed the depths of understanding the complexity of grasslands. Complements to both Konza’s Gene Towne and The Nature Conservancy’s Bob Hamilton for doing such a great job for more than a decade.

Craine JM, Joern A, Towne EG, Hamilton RG. 2009. Consequences of climate variability for the performance of bison in tallgrass prairie. Global Change Biology 15: 772-779.