The thistledown effect: Planktonic dispersal

One of the reasons why it is so difficult to manage marine life properly is the way it reproduces and disperses. Most of our experience is on land, and nearly all life on land has reproduction within the same population. With land plants and animals, the parents and the offspring occur closely together, and the young are born into the same population as their parents. Dispersion also occurs, but this spreading out is either slow or happens mainly to adults.

None of this is generally true in the sea. Most marine species have very large numbers of very small eggs and these are dispersed by drifting away from the parents in the currents. Most marine animals also have drifting (planktonic) larval stages as well. So, when settlement and/or metamorphosis occurs the young are a long way from their parents.

These are general rules ( and there are exceptions). Everyone is familiar with thistles and thistledown drifting in the wind. This is one of the examples of plant seed dispersal on land. The reproduction and dispersal of thistles can be used as a model of what happens generally in the sea. Thistles have many small seeds with a fine hairy down which are dispersed by the wind. Even if we control thistles rigorously over most of the country, the dispersal of windborne seeds from scattered patches of thriving thistles will keep producing new thistles everywhere. The thistledown effect is similar to what happens with the planktonic dispersal of marine species.

This is both bad news and good news for marine reserves The first bit of bad news is that no single marine reserve can be self-sufficient, unless it is gigantic. In any reserve of practical size some of its species will be totally dependent on recruitment of juveniles from outside. For many other species in the reserve much of the recruitment will be by larvae, eggs or spores which drift in from somewhere else. So if marine reserves are to be sustainable there has to be a network of them.

The second piece of bad news is that we cannot calculate the result of this planktonic dispersal in precise terms. Or, more accurately, even if we could learn how to calculate it, the result would be different for each species (different lengths of time in the plankton), it would be different in each year (current speeds and directions vary a lot), and it would be very different for each arrangement of reserves (varying with their spacing, size and precise position). In short we cannot calculate in precise terms where we should have the marine reserves.

The first piece of good news is that we don't need precision calculations. We know the principles and the trend of their effects. A farmer does not need to calculate which way or how far thistledown will travel, to know what to do. If he can't or won't do it the Noxious Weeds Board will. Thistles are pests, so we have to reverse the conclusions for marine reserves. It is clear that the precise position of marine reserves is not the crucial question. What we need is a network of maximally reproducing areas scattered around the country thickly enough to have a mass effect on recruitment of juveniles everywhere. Maximal reproduction from an area is generated by full protection i.e. marine reserves. The process can be started anywhere, and local and secondary principles can be used for precise positioning of the first reserves in each region.

The second bit of good news is that we already know how to handle situations like this. We do it all the time to create networks of hospitals, schools, fire brigades, and other systems which the community has decided are important. Precise locations have to be decided, but they are not the point that governs action.. Maximal delivery of the required effect is what we arrange. If we believe the effect is important we already know how to arrange it in political and social terms.

Bill Ballantine