When is an island an island?

[james_davis_nicoll]

Useful factoid: If recall Wilson correctly, if you assume all other factors stay the same, the number of species a given region per unit area can support is proportional to the square root of the total area. If, for example, you have two islands, one 10 units in area and the other 1000 units, the second one will usually have about 10x as many species per unit area and presumably about 1000x as many species in total. 100 small islands might have as much area as one big one but they will only a tenth as many species.

Does anyone here know what conditions are necessary to disconnect adjacent regions? Obviously water will do it, esp deep water. Roads and wide clear cuts can, since they make migration more difficult.

Obviously, this rule of thumb implies that a few large preserves are probably a more effective tool for species preservation than a lot of little ones.

It also implies large cities should have more species/area in them than small ones. I wonder if that is the case or if other factors (like the relative youth of large cities as common habitats) dominate?

It also means that in the Reefs of Space setting, bigger rocks will tend to be more various places to live (in terms of life) than little rocks. OTOH, delicate lifeforms might find small islands nice places to live, thanks to the lack of various competators and predators.

Comments

[ellarien]

Somewhat relevant, maybe: the mountains around here in the Southwestern desert are called 'sky islands'. A few thousand feet in altitude makes a big difference; the environment on top is very different from that of the desert, resulting in isolated populations. (Googling 'sky island' brings up some helpful-looking links.

[redbird]

The city thing only works if the city is biologically isolated from its surroundings, and has no significant internal barriers. I doubt, for example, that New York City would fit any such rule--Manhattan is more cut off from the rest of the city than Brooklyn and Queens are from Nassau or the Bronx is from Yonkers and points north on the mainland.

[dd-b.livejournal.com]

Roads and clearcuts may not persist long enough to have a big effect.

In analyzing cities by this rule, it's interesting to look at the human population as well as the other species. There does seem to be a tendency for big cities to be a lot more diverse in human population.

[nancylebov.livejournal.com]

I'm not sure that the rule is going to apply as strongly in space as it does on earth--on earth, most living things are where they are because they were able to travel there on their own. People in cities are a special case, but I'm not sure how you'd define the different types--it's fair to say that you get more diversity in cities but how would you measure it?

In space, most living things get to whereever as the result of human action. This means that you could have some asteroids set up as nature preserves (and you probably should--now, how might they go entertainingly wrong?) and others of the same size with relatively simple ecologies, though no doubt not quite as simple as the powers that be would prefer.

It's not species per unit area

[(Anonymous)]

It's total number of species. IOW, the island of 100 square miles will have 10x as many species as the island of 1 square mile.

IMS they first noticed it when looking at bird and lizard species in the Caribbean, which has a nice mix of islands across four or five orders of magnitude. Subsequent testing has shown it to be empirically robust.

A few large preserves... Okay. James, this is known as the Single Large Or Several Small debate. SLOSS, for short. And it's had population and conservation biologists at each others' throats for about the last 15 years.

The book you really want to read here is _The Song of the Dodo_, by David Quammen. It's a high-end popular science book on island biogeography. Almost a decade old now (1997 IMS) but still well worth reading. It discusses MacArthur-Wilson and its consequences for long-term diversity and conservation at a pleasantly crunchy level of detail, and then gives cites that will show you where to find more, more, more.

Trust me, you're going to like this one.


Doug M.