More ways to be different

Diversity is important in more senses than just having many species living together, says S.Ananthanaryanan.

There are many senses of diversity. There can be diversity in the molecules of a gas may move, in the profiles of customers of a product or service or in the age of individuals in a population. A paper by an international group of a scientists reports the finding that a healthy mix of age groups in fish populations is useful for the stability of numbers.

Molecules of a gas

The accepted model of a gas is of a huge number of microscopic, but identical particles in incessant motion, ricocheting both off the sides of their container as well as off each other. The force they exert on the container is the pressure of the gas and by bouncing off each other, they maintain a distribution of different numbers of particles at different speeds.

The interesting thing is that for the particles all to be moving in some fixed band of speeds is not naturally possible, and if imposed, the imposed uniformity soon breaks down. The natural distribution is of a large number of particles in some level of speed and progressively smaller numbers at speed greater or less than the mean speed. If fast particles are removed, for example by escape into outer space at high altitudes, the particles that remain readjust their speeds to again have a number, though a lesser number, at the higher speeds.

The reason for the bulk of the particles to choose the middle speed band is that for a given total energy of the gas, there are overwhelmingly more ways where most particles to occupy the middle band, with lesser numbers in the outer bands, than where many particles are very fast or very slow. One could say that the natural state is the state that has the greatest diversity of forms.

The marketplace

There is a French saying that a mouse with only one hole is soon trapped. (souris qui n’a qu’un trou est bientôt prise). A business with a specialised customer base is similarly  in risk of getting wiped out. A business should ideally have different categories of customers. If one category changes its preference, the loss could be replaced by customers of other categories, so that the customer mix may settle down to a new distribution in the new conditions.

On the other hand, if the market were of a few categories only, it would take time to replace a lost group of customers and the business may not have the stamina to hold out. Or, at least, it would do badly for a long time before it recovers. This is an instance of stability that comes from diversity in the customer population. It is similar in the case of those successful business houses that have a diverse product mix – so that obsolescence of one of them has little impact and the business house is able to develop new lines or to revamp the outdated one.

Fish populations

Fish populations left undisturbed settle down to a distribution of older fish, adult fish and young fish. Unchecked build-up of older fish would discourage survival of young fish, but this, in time, would ensure less older fish, and hence more young fish. More young fish would result in more losses to predators and increase in the proportion of experienced ones and slow recovery of a stable mix.

Decline in the fisheries, world-wide has made it important to understand what is happening to these populations. The work of the George Sugihara, at San Diego and colleagues, reported this week in Nature, suggests that the challenge is not just to rebuild the biomass, but also to restore the age distribution.

The collapse of the California sardine industry in the late1940s had been blamed by some on commercial fishing and by others on reducing sea surface temperature or on changing wind patterns. In an attempt to disentangle man-made and environmental causes, the Cooperative of fisheries in California has collected data of both exploited and unexploited fish assemblages. The differentiated information, data was collected not from the figures of landed fish but of levels of fish eggs and larvae.

Ichthyoplankton

These are fish eggs and larvae found in the upper 200 metres of the sea. The eggs stay passive while larvae develop swimming ability when a little older. Ichthyoplankton are an important component in the water column and the larvae feed on smaller larvae and are fed upon by larger animals. But the abundance of eggs and larvae is a good indicator of the spawning population present in the area at the time. Sampling the egg and larva populations is an easier and faster way of assessing adult population levels.

The San Diego group analysed data covering 50 years and has come to important conclusions. The first is that while commercial fishing does have an affect on fish stocks, the variability of the stock level does not fully correlate with the intensity of fishing. The other effect of fishing, which eliminates larger fish, is to increase the number of younger fish.

This reduction can affect survival in two ways. One is that younger fish, being less experienced and hardy, fare badly under environmental attack. The other is that demographic measures, like intrinsic growth rate, are affected by a population of younger fish. These 2 effects can be separated using statistical methods.

[the writer can be contacted at simplescience@gmail.com]