Thursday, 18 June 2009

Climate Change and Rangifer

Here's a little something I wrote while sipping tea in my brother's home, obviously on vacation (that's why I had so many work related emails open! Because I was on vacation!). As you possibly remember, I have confidence in Global Warming as a actual phenomenon as it's born out by a variety of data sets. I can't comment on the mechanism, except to say I don't second guess nuclear physicists, so why should I treat climatologists any different.

That said, I was somewhat... appalled by the treatment of a finding I've known about for a while:
The scientists were shocked to discover that 34 of the herds were declining, while no data existed for 16 more. Only eight herds were increasing in number. Many herds had been declining for a decade or more.

"We were surprised at the ubiquity of the decline," says Vors.
Surprised? I'm not sure they should have been. The data's been around since 2004ish (more on that in a moment).

"When we delved into the status of European reindeer herds, we were surprised that so many were declining. We expected them to be in better shape than North America herds because reindeer, namely the semi-domestic herds, are closely managed by humans."

[...]

"If global climate change and industrial development continue at the current pace, caribou and reindeer populations will continue to decline in abundance," says Vors.
Oh dear. I suppose I should start with the initial statement. Reindeer herding on a whole is on the decline due to human factors. Namely, fewer people are doing it. In North America, little market exists for reindeer meat, and historical factors have sabotaged the development of herding as an industry. In Eurasia, the industry is on the wane for a variety of reasons.

As for the out-of-nowhere statement about Climate Change, I present this graph from Kofinas and Russel:
You can probably see the issue right from the get go.
I'll refrain from begrudging the point. After all, I haven't seen the paper yet (I am writing this while on vacation!), but it would take strong evidence to convince me that the competing hypotheses are incorrect.

2 comments:

CabinDweller said...

Hrm. As a person who is not a biologist, a couple of ignorant questions:

1) What is prop. of maximum herd size? If I don't know what the axis label means, how am I to evaluate the chart?

2. And what is maximum herd size anyway? The management goals? The maximum recorded herd size?

3. Herd estimates are often done (always done?) by photo census. Weather conditions and other factors affect the ability to get a good count. How accurate are the counts?

4. Herd numbers fluctuate and different things might cause declines. One herd might be affected by an extreme icing event and lose a large percentage of its population in one winter. Other declines have been linked to disease. Others might have major predator pressure, while others do not. How can you lump them all together and come up with something to say about herds continent wide?

4. Why does the chart only go to 2003? They've been counting AK herds more frequently than that.

TwoYaks said...

Cabin Dweller:
Good questions! I'll address them in the order you presented them.

First, Prop. of maximum herd size is the proportion of the maximal size observed for a given herd. If in 1999, they observed the herd to be 500, and in 2001 they observed it to be 250, the value for 1999 would be 1, and 2001 would be .5 (corresponding to 100% of the maximal and 50% of the maximal correspondingly).

If there is a weakness here, this would be it. These numbers are frequently artificial, and are prone to errors such as accidentally counting overlapping herds, or so forth. Further, a single high reading - a high outlier - seriously effects all subsequent measurements. I've never been fond of theses maximal readings, especially when invoked for management, but haven't put the effort into finding a better measure myself. It's put-up-or-shut-up, and I haven't put-up.

Second, I believe I'd answered. It's the maximal observed. The assumption is that this level is the maximal the land can support - very likely a bad assumption!

Third, photocensuses tend to be very accurate, however, they have one superb advantage, in that there is a way in which you can express the uncertainty in measurements. There is a way in which you can quantify your sightability factor, and use that to correct your results. This gives a confidence interval, in which 95% of the time your measurements will lay. As an example, if they say it's 100±10, 95% of the time the value will lay between 90 and 110. Each count is of varying accuracy, and any good manager will express his uncertainty.

One of the big problems in some methods (especially pellet counts) is that their uncertainty is very great. Often times, the confidence interval may exceed the actual measurement - I once heard the estimate of Sitka Black-tail Deer on P.O.W island expressed as "100,000 deer ± 1 million deer," though mostly jokingly.

Next, you appear to have hit the heart of the matter! We call those sort of random events "stochastic events," and we assume that they're distributed randomly among herds, and distributed randomly within herds. Lumping herds without mutual environmental factors should produce a flat trend line across time, as the fluctuation of one would not impart any `signal` on other herds.

When we find that this isn't true (And there are tests to find out whether we have a significant result, or if the linkage would be expected by chance), and that they do seem to have some relation between them, we can say we've "falsified" the independence hypothesis. Now, we haven't proven what has caused it, but we have shown that there's some common predictive element between herds.

This final question is perhaps the easiest. This report that I've taken the figure from was compiled in 2004 (if I recall correctly) and the new data simply failed to make it in before it was published. I would like to talk to Gary Kofinas and get an updated version of the graph some time soon, but that's low on my list of priorities. That's why I was saying that I'm shocked people seem to think this is a new result. I've known there's some relation in herd size for some time now, anyhow.