Showing posts with label ecology. Show all posts
Showing posts with label ecology. Show all posts

Friday, May 28, 2010

Preferential attachment in ecosystem mutalism



Tree, vine, and bromeliad

In a previous post I made an analogy to illustrate the evolutionary pattern of "attenuated parasitism" -- a model for how antagonistic agents can evolve cooperation. I created that analogy while sitting on the edge of the Puerto Rican rain forest thinking about how it is that vines and trees can co-exist -- after all, being a vine is such a good strategy that it seems like they should have killed all the trees (and themselves) by now! That same day I had another idea that I didn't get around to writing down: such cooperative mechanisms should, given time, lead the forest to become one giant interconnected web of mutualist interactions.

The argument is simple. If a vine and a tree form an alliance while the same tree specie and some other specie, say, a bromeliad also form an alliance then it is logical that the vine and the bromeliad will have an increased probability of forming an alliance of their own owing to the simple fact that they co-occur in the same location (around the tree) more frequently than other random specie pairings. A small interaction network like this would likely grow by accumulating more and more interactions by the same mechanism.

There is a mathematical theory of such network growth called "preferential attachment" (also known by about half a dozen other names). The study of such networks dates back to at least Yule, 1925 who shows that such processes build what came to be called "scale free networks". Such networks end up with many more hyper-connected nodes than one might intuit. For example, the tree in the previous discussion is likely to become one such hyper-connected node with many, many mutualistic interactions both literally and metaphorically hanging off of it.

It is easy to imagine how the growth of such cooperative interdependence would tend to drive an ecosystem towards a single giant interconnected web of specialists. It is conversely hard to imagine how an outside generalist specie could successfully invade such a well-connected ecosystem. Therefore one would expect to see more internal differentiation from species whose interconnections into the web are already established as opposed to generalist invasive species whose ancestors came from the outside.

I haven't studied the field evidence well enough to know if this idea is supported or not. Following are a few random Google hits on the subject that I've only glanced at long enough to think that there's plenty of room for theory development in this field!

Saturday, May 2, 2009

Vaccinate your child or gramps gets it in the stomach!

There seems to be a growing ignorance about vaccination. From my informal queries of friends and acquaintances who have chosen not to immunize their children or who do not get flu vaccines, I have found that few people understand that vaccination is part of a greater social compact, not merely a personal cost/benefit analysis. The effect is called Herd Immunity. When we vaccinate ourselves, and especially our children, we are adding to the communal common defenses. Obviously, everyone would like to have a defensive wall built to protect a community yet everyone would prefer to not contribute. But that's not the way a good society works; we share the costs of doing things that benefit the common good.

Immunization of children is particularly important for two reasons: 1) Children's immune systems respond to vaccination much more effectively than do others, especially the elderly who are the most likely to die of viral diseases such as influenza. 2) Children are responsible for much of the transport viruses throughout a community owing to their mobility and lack of hygiene.

For example, a controlled experiment conducted in 1968 by the University of Michigan demonstrated that large-scale vaccination of children conferred a 2/3 reduction in influenza illnesses to all age groups. For a nice article on the subject, here's a Slate article from 2008.

I propose an old-fashioned poster campaign to inform about the social benefits of vaccination. Here's a couple of prototype posters I photoshopped up this afternoon. (Apologies to Norman Rockwell!)






(Original photo Adam Quartarolo via WikiCommons)

Wednesday, April 29, 2009

Tree and Vine: An Allegory of Attenuated Parasitism.



The town of Forrest has been around for centuries. It’s the kind of place where sons inherit their father’s businesses and nobody can remember when things were too different. The town has always been so small that it supported only a single shopkeeper; the current proprietor of this humble store is a tall and stable fellow named Woody, the descendant of a long line of tall and stable men just like himself who have worked hard to build and maintain what’s always been a social focus of Forrest.

Small towns like Forrest might seem peaceful to visitors, but internally there are the inevitable gripes, grievances, and grudges. For example, a recent family feud over the inheritance of their grandfather’s property has split Woody from his cousin Trey. As a consequence, Trey has recently opened a competing store directly across the street from Woody thus ending Forrest’s long-established one-shop monopoly. This, as you’d suspect, has been terrible for Woody.

Forrest is not entirely full of hard working capable souls – for example, consider Vinnie the thief. Vinnie, like Woody, is descended from an ancient line of Forrest inhabitants. Vinnie, like Woody, pursues the same occupation as his father and his father’s father. But Vinnie, unlike Woody, isn’t exactly a clone of his ancestors.

Vinnie’s father was a notorious scoundrel. An aggressive thief and burglar, he was nevertheless as dimwitted as he was ruthlessness. It doesn’t take a genius to know that if you continue to steal from the same store over and over that there might eventually be nothing left to steal. This concept seemed totally lost on Vinnie’s father and as a result he almost caused Woody’s father to close the only store in town.

But, as suggested, Vinnie was not cut out of the same aggressive yet witless stock as his father. Indeed, Vinnie is more bargainer than terrorist -- a theme established early in his life. When Vinnie’s father began to push him into the family business, his father told him: “Go into Woody’s store, show him who you are, break a few things then take what you want and stroll out like you own the place. That’s how it works for guys like us. That’s how it has always worked.”

Young Vinnie tried. He walked into Woody’s store and looked around. He picked up a few items that looked breakable and considered tossing them to the ground. But, soon he became aware of Woody’s suspicious gaze following him around and found himself placing the stock back on the shelf and adverting his eyes. Finally, Vinnie decided just to come clean.

“Do you know whose son I am?” Vinnie asked Woody naively.

“Of course.”

“Then how about you just give me a hundred bucks?”

Woody thought about this. A hundred dollars was actually quite a small price to pay compared to the usual cost in damage and theft. But, a hundred dollars for what exactly? A hundred dollars just to make some kid walk away? All things being equal, Woody would just assume he didn’t have Vinnie’s small-time extortions nor his father’s grand theft, but that really wasn’t one of the available options and therefore the proposed agreement would be the lesser of two evils.

“I’ll tell you what”, said Woody pulling out the cash, “I’ll give you one hundred dollars a week for doing absolutely nothing as long as you don’t make the same deal with my cousin Trey across the street. This will be your territory, but the store across the street stays your father’s territory. Deal?”

“Deal.” Vinnie said, shaking Woody’s hand three times.

And with that simple verbal contract, an arrangement was made. Each week Vinnie would come in, shake Woody’s hand three times, and earn a hundred dollars.

Over time, their relationship became, if not exactly friendly, at least routine. Little by little they forgot about the initial circumstances of the arrangement and found themselves acting like civil gentlemen considering the issues of the day.

One day, a small force of bandits from a nearby town attempted to invade, seeking to steal supplies and animals. Obviously, both Woody and Vinnie were desperate to repel this invasion and during the crisis all past discord was forgotten. Not surprisingly, between the two of them, Vinnie was the better fighter owing to the weapons and viciousness inherited from his violent family. That’s not to say that Woody didn’t engage the enemy, but violence is clearly Vinnie's comparative advantage.

A few months later, a fire broke out. As before, both Vinnie and Woody had a mutual interest in stopping this mortal threat. While Vinnie pitched in to fight the fire, this time it was Woody – with his access to buckets and hoses – who played the comparatively larger role in extinguishing this mutual threat.

And so it went. As the relationship normalized, they found that their common needs were greater than their distrust and consequently they found more and more ways that it was profitable to depend on each other’s specializations. Vinnie became not only the defender of the neighborhood but also the store’s out-of-town sales representative and Woody paid him a commission on his sales. Meanwhile, Woody’s freed resources meant that he was able to invest more in a nicer shop with more stock to the profitable benefit of both.

Generation after generation inherited the agreement and the benefit of specializing and working together turned out to be great. The paltry hundred dollars became not so much an extortion as just one part of a complex set of mutual exchanges of goods and services. In fact, Vinnie and Woody’s sons didn’t even know why they engaged in this weekly routine of thrice handshakes and an exchange of cash -- maybe it was some sort of ritual of friendship; maybe it had to do with some old debt now long since irrelevant; whatever, it seemed a quaint part of their past. To outsiders, it was hard to imagine the shop running without two employees, and most assumed that it had always been that way and always would.