Saturday, February 9, 2013

Bison and drought

Crude Protein concentrations of bison at Konza over 4 years. X-axis is day of year.

Drought in grasslands is typically visualized in the media with cracked-soil watering holes or dust clouds. Severe droughts might show emaciated cattle or even carcasses of the stricken.

Drought is more nuanced than that, though.

Few droughts generate no rain. Hence, there is typically some grass growth. Unless managers remove animals from the landscape quick enough, drought generates “overgrazing”.

The consequences of there being too little grass are elementary. Animals starve. Too little drinkable water and animals dehydrate.

But what are the consequences of a drought when there is sufficient quantity of grass and still sufficient water? Is it even a drought?

At Konza, 2012 was under drought. Yet, because the density of grazers (here, bison) is relatively low, there was sufficient quantity of grass for them. So did they even notice there was a drought?

For four years, Gene Towne and I (mostly Gene) have been collected fecal material from Konza bison. The bison herd of ~300 are allowed to graze wherever they want over approximately 1000 ha. For the first three years, the seasonal cycle of dietary quality was pretty consistent. When temperatures warmed up in the spring, dietary quality peaked and then declined to maintenance levels by late July. Quality was relatively low until the next spring.

But, 2012 was different.

Rainfall through late June was fairly sufficient, but drought hit soon after. Over a 2-month period, just 50 mm of rain (2 inches) fell. It was dry and it was hot, with temperatures routinely hitting 37°C (100°F).

And then, on August 24th, 100 mm of rain fell (about 4 inches).

Visually, some of the grasses started growing again. It looked like a second spring. Bouteloua seemed to respond the most, covering the uplands in a thin coat of green. 

And the bison? 

The quality of their diet declined through the growing season like almost every other year. But, then in early August, it started to get better.

By the end of September protein concentrations were almost twice as high as normal. For them, it was June all over again.

We had never seen patterns like 2012 before and we still have a bit more to learn to flesh out the story. 

We still have to analyze the fecals for dietary composition, so we don't know what plant species the bison were concentrating on. 

I also haven't asked Gene for all the weight data yet, but the weights of bison calves were actually heavier than average last year.

I guess if a grassland isn't too crowded, droughts might not be so bad for grazers.


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