Wednesday, November 7, 2007

"Nets versus Nature": an interesting News and Views

David Conover writes an interesting News and Views piece in the current Nature about a paper published this week in PNAS (here). Here's what he has to say about it:

"People like to catch big fish, sometimes so much so that fish sizes overall become greatly diminished. According to one view, the continual removal of large fish from a population sets the stage for rapid, undesirable evolutionary changes, including slower growth, earlier adult maturation and permanently smaller size1, 2. This occurs because removing the largest fish directly opposes natural selection, which tends to favour large size.

What happens when these two forces simultaneously oppose one another? Can evolution respond quickly enough to track changes in fishing selection, or does natural selection counteract it? Writing in Proceedings of the National Academy of Sciences3, Eric Edeline and colleagues illustrate the outcome of this dynamic tug-of-war between the forces of natural selection and fishing selection."

He points out that the role of natural selection is often not considered in fishery's management because of the assumption that humans are just another predator in the system and we just need to regulate how much that one predator harvests. However, this paper shows that the size selective nature of human predation can have really different effects than other sources of mortality (including nonhuman predators), which tend to impact smaller, slower, weaker individuals. Thus, typical predators create selection for large size because not only is fitness generally higher at large mass but fewer things will eat you - whereas humans create pressure against it. The authors of the PNAS paper demonstrate these opposing forces empirically with a unique and high resolution data set on Pike in Lake Windermere, England. Humans did indeed selectively take large individuals whereas other sources of mortality weeded out the small. They also showed the predicted life history relationships between fishing intensity and growth rate, which are consistent with the models we discussed earlier in the semester.

This paper isn't written from the perspective of human ecology (although I'm sure lots of human ecologists are very interested in this) but it fits the aims of this class very well because it uses life history theory to demonstrate a relatively simple but previously somewhat overlooked feedback that underlies human predation and and a key ecological pattern. ...er something like that anyway...




5 comments:

Dan said...

Addressing Paul's argument in class, there seems to be an underlying theme in this and several other topics we've discussed, including sustainability, about the distinction between what is natural and what is artificial. the distinction between the two is difficult to establish with consistency. One side may argue that anything man-made by definition is artificial. While the other side may argue that man is part of nature and anything that he creates is thus part of nature and therefore natural. The later view would contend that natural processes dictates human's ability to capture the big ones and thus this is the best of all possible worlds. The former view would contend that humans are screwing the natural selective process of body size in fish, and thus this world is full of woe. But what is really the significance? Well are children won't be catching 20 in. trout like me (sucks to be them). Will they cultivate their garden? But what do I know, I'm no philosopher.

Anonymous said...

Good post Dan,
This is something I noticed as well in our recent readings. I interpret it as being a little bit of both natural and artificial changes that human society causes. Although I have no clear definition of either, I think what many of these papers are encouraging a new way of looking at sustainability via human behavior patterns. Your children may not be catching 20 inch trout, but they probably will be practicing behavior which gives them the optimum amount of resources and access to reproducing at that time. One thing I wonder is what activities are driving these varying optimality behaviors-humans or human interaction with the environment? What is an optimality behavior that is good for the individual but when practiced by the group it becomes detrimental to the environment, animals, or human society? Is it possible for humans to be conscious of "overall optimality behavior" that does not negatively affect the environment but also positively benefits quality of life for humans, so that we can truly have a more sustainable environment? I don't know if that is just repeating what you said or what, but I am also really interested in the topic and like your post :)
Myra

Human macroecology admin said...

Hey.
As you have probably noticed, I am usually pretty skeptical of claims that make stark contrasts between 'natural' and 'artificial' where anything not human is natural and anything that is is artificial. While humans are definitely fairly different a lot of the time, we need to be careful in how we view this business of what is 'natural.' In the context of this blog post - I think they are showing that the underlying life history tradeoff, that follows from our predictions, makes this a 'natural' process. Just because humans happen to take more prime age adults doesn't make them unnatural - they are changing the pressures from the typical background forces though. There may be a lot of other cases where this same argument would apply.
As for the trout around Silver City - if folks let 'em be a while, then we'll have more 20 inchers again. Does the fact that we cold decide not to harvest a resource for a while make us unnatural? I'd say no but that's a pretty unusual behavioral capacity just the same...
Oskar

Human macroecology admin said...

Myra raises some interesting questions: What is individually optimal behavior that harms the environment or human society generally? And can humans behave in a generally optimal manner (sustainable manner?), considering the human-env system? I think a key difficulty in achieving sustainability is the tendency for humans to optimize their individual behavior to capture as much energy as possible while giving up as little as possible. The overuse of "common pool resources," such as the air, water, soil, and public lands, is a prime example. Using these pools to deposit your waste benefits you directly, but you effectively "share" those costs with others, as with everyone else breathing the air around your car. So unless those resources are given monetary value, people have too little incentive to consistently behave in the public good. As a society, we have to decide what to optimize in order to have any shot at doing so.

Bill

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