A Continuation of --The Case of the Arboricidal Megaherbivores

Our story thus far:

The scientific investigators, Skylark Holmes,(great great grand niece of the famous detective) and her friend Dr. Janet Watson proposed a theory to account for the extinctions at the end of the Pleistocene in "The Pleistocene Holocene Transition - The Case of the Arboricidal Megaherbivores" since then Holmes been busy creating a simulation of their theory. We find them in their study.

"Well, Watson, the model is completed and it's time for it to be presented

"Great! I hope you're going to tell me all about it. But, Holmes, I thought you never did presentation of your work, Holmes"

"True enough, I'm going to get our scribe to do the presenting."

"Ah ha! That's why you never get the credit.

True enough, but I get to play the games I want and don't have to worry about all that talk. Anyway, here's the abstract. What do you think?

"Er.. umm. What does this mean?" she reads, "All current models assume animal populations decrease monotonically to extinction."

"It means that all of the current models make the assumption that the path of extinction looks like this (graph 1), when all we know is this (graph 2)" she sketches two graphs.

Watson looks and then says, "Ah, yes, I remember, we delt with that issue before. It was the word that confused me."

"Monotonic simply means that it goes in one direction without wobbling up and down. Scientists make the assumption because we only see the beginning and the end of the extinction event and we have to extrapolate the rest." answers Holmes.

"Well, I know that we looked at both climate change factors and overkill in our earlier investigation, (The Pleistocene Holocene Transition - The Case of the Arboricidal Megaherbivores) "Yes, and do you remember we did a computer simulation based on Whittington and Dyke (1989) we when we were working on the early stages of the model?"

"Yes, we focused on their notion of carrying capacity as an absloute instead of realizing that food is part of what constitutes carrying capacity"

"Yes they defined carrying capacity as a function of space. Carrying capacity is actually ah, er.. kind of "shorthand" for all the things organisms need for their survival. For H. sapiens food is an important part of carrying capacity. When we ran the model so that the carrying capacity was affected by the loss of food herbivores avoided extinction."

"Yes, that's where we were when last we talked."

"Right. When I went back to my computer I realized that trying to adapt the Whittington and Dyke (1989) model was too unwieldly and that it was important to rethink I thought about what a minimal ecosystem might be like. This brings us to the next slide Re-thinking Carrying Capacity Arguments "OK, Holmes. So what was the result of your re-thinking?"

"The next slide. Disturbances in the Predator / Prey / Vegetation Relationship

"OK, so now what happens when you introduce H. sapiens, Holmes?"

"Yes. The overkill situation. So, Watson, what do you think? "

"Er.. well.. I um.. think that with two players hunting instead of only one, the herbivore population goes down followed by the predator population. Yes, I think so."

"And the vegetation, Watson? Don't forget in the stable ecosystem all three were affected by the disturbance to the one."

"Well, I er.. I don't know, Holmes.

"Well, then, let's look at the next slide. Introduction of H. sapiens

"Oh, my, Holmes! It's totally counter intuitive. The Herbivores don't suffer much, the predators go down, and the vegetation goes up. I don't know what to think."

"Yes, it is but it is what happens in the current day ecosystem as well. Remember our example of how rinderpest reduced the number of lions and the number of lion prides in Africa and the lions turned to hunting man?"

"Yes, I do, now. Is that the same dynamic?"

"Yes, rinderpest was a new predator on wildebeast and on water buffalo just an H. sapiens was a new predator atthe end of the Pleistocene. The same dynamic occurs - the herds of herbivores are reduced slightly but the population of predators increases and the vegetation increases. We saw that in the African example too."

"Where, Holmes? I don't remember that."

"Remember all the papers on how a full complement of herbivores produces grazing lawns and the papers on the partitioning of the grazing environment?"

"Yes, but..."

"Well, think about it. Those papers observed that with the eradication of rinderpest grazing lawns were established where the herbivores neatly partitioned the vegetative environment into niches."


"What was there before the establishment of grazing lawns?"

"Well, umm.. a more weedy and shrubby and perhaps woody environment?"

"Yes, exactly. And here's the next slideWhat Kind of a World Would We Have Now?

"OK, but where do we go from here?"

"Well we've seen that with overkill we would expect a world very unlike the world we have. And we've hinted something about perdators."

"That they might become man-eaters?"

"Yes, and again it happened in Africa with predators who know how dangerous it is to hunt H. sapiens. With naive predators that might be even more likely. So I'm suggesting that H. sapiens started to reduce predator populations. And that brings us to the next slide Second Order Predation"

"Well? What do you say now, Watson?"

"Well, Holmes, there's two things I notice."


"First that the curves are different, dramatically different."

Holmes is obviously excited, sitting on the edge of her chair, "And...?"

"And.. that they don't end up in that different a place. There's a few less predators and a few more people and a little less food for herbivores but there's more herbivores. Holmes, there's more herbivores in your Second Order model than in the Overkill model!"

"Right, Watson! Fewer predators and more people -- in the Introduction of H. sapiens model, I don't like to call it Overkill when that is not what happens, predator populations are reduced through competition with H sapiens. We can call that one the competitive model if you like."

"Yes, I like that."

"OK, then, in the Competitive Model predators have a slight advantage because their recruitment rate is greater and because they are already established. But, in the Second Order Model, when H. sapiens begins to reduce predator populations H. sapiens becomes more dominant. Because the dominant predator in the Second Order Model has a slightly lower recruitment rate we end up with slightly more herbivores when the model stabilizes and, naturally, more herbivores means less food."

"Yes, I see but, Holmes you're supposed to be explaining extinctions"

"Ah yes, so I am. So, Watson, in that time when the model is oscillating - when Food for Herbivores drops and herbivores boom, what is happening there?"

"I don't know, Holmes."

"Well, herbivores are booming because predators are falling off and there aren't yet enough people to take their place."


"Food drops because the herbivores eat it all"

"OK, I see that."

"Now, in this time there would have to be intense competition between herbivores for the available food. Only the most efficient - those who could exploit the scarce resources best would be able to survive and reproduce."


"This leads to a selection regime where we would expect to find certain kinds of animals being favored over others - grazers over browsers, ruminants over monogastrics and small over large. Here, look at the next slide What Happens During the Cycles?"

"And that describes our world better than the Competition Model."

"Yes, I think so. I'd like to be in contact with a climate person who had read it."

"Oh, that's easy, Holmes. They should send e-mail to your scribe by clicking here: elin@quaternary.net

"So, Watson, you like it."

"Yes, it's fine, but why have you put in this last line in the abstract? The one that says, 'An interactive system dynamics computer simulation will be available for demonstration and testing'? I thought you had done the simulation and were happy with the result"

"Well, there's two answers to that.

"First, yes, I am happy with the work so far. But, it is a work in progress. All I have done to date is show that it is plausable on the most highest, most simplified general level. I still have work to do.

The second reason for having a model available is that I have found that people learn more by doing things themselves than they do by just being told."

Watson, who has been taking care of a young niece and nephew agrees feelingly, "Yes, that's true."

"So," Holmes continues, "If people can put in their own starting values they can see the impact of their intuitions or feelings about the model."

"Excuse me, Holmes, but I thought this was science. What do you mean when you say intuitions or feelings"

"Yes, Watson, it is science. But, in dealing with extinctions we have to start somewhere and we have to 'construct' what we think is appropriate. If you like it better you can call it formulating hypotheses and testing them within a constrained environment. The computer model I have constructed is like a simulation lab where people can test their hypotheses.

Do you remember the Whittington and Dyke (1989) paper? The one we talked about before when we were talking about carrying capacity"


"Do you remember how they determined what hunting rate to use -- how much each person would have killed per year?"

"They derived it, didn't they. They did simulations unitl they found a hunting rate that made the herbivores go extinct."

"Yes, exactly! It was 3.862 a.u. (animal unit) per person per year. It was in the chart we used too."

"Yes, Holmes. What are you driving at?"

"OK. Do you think that's reasonable?"

"Well... I.. er, uh. I suppose so. How am I supposed to know?"

"Ah ha! My point exactly. How are you supposed to know. You don't, and I don't, or I didn't have any context for thinking about whether or not that is reasonable. But, if you had put in the numbers yourself and seen the result you might have developed some notion of what is or is not reasonable. I did a small calculation to see what I thought of the number. Do you remember what an a.u. (animal unit) is?"

"Ah, yes, I do, Holmes."


"It is 1000 lbs."

"Right! So if we look at this in terms of something we all can understand, like Quarter Pounders from Macdonald's, it means that each man woman and child ate 42.3 quarter pounders a day. Even taking waste into consideration... say ... half ... or even two thirds each person ate 14.1 quarter pounders every day."

"Really, Holmes that's a bit much."

"Yes, but you see. If you don't interact with the values yourself it is much harder to know what is reasonable and what is not."

"So having a model to play with will help people think about what is reasonable?"


"But Holmes, how are you going to do that on the Web which is where we are now?"

"Well, first I'm going to put this part up and by the end of June 1998 I'll get an interactive version up."

"OK, Holmes. People could send e-mail to your scribe to be reminded by clicking here: elin@quaternary.net"

"Good idea, Watson. And now let's go out and celebrate!"