The Greatest Debate in Ecology
What SLOSS Tells Us About How to Build Nature Preserves
The field of ecology was utterly shattered in 1967, and it rose from the ashes of its former self a renewed and revolutionized science.
Part of what ensued from 1967 was a debate that has been going on for decades, and it’s about conservation. Simply put, is a single large preserve area better than several small ones?
As we tear down forests for farmland, we’re faced with the necessity of creating natural conservation areas. The purpose of these areas is to maintain some of the rich biodiversity nature had before we interfered.
What happened in 1967 was the publication of a book called The Theory of Island Biogeography by Robert MacArthur and E. O. Wilson. The book brought focus to conservation biology and led to mathematical predictions about biodiversity and area.
Most importantly, however, it offered invaluable evidence favoring the species-area relationship, which was an increasingly popular concept at the time.
Simply put, larger areas harbor more biodiversity. That’s an established fact and there is no debate about that.
The SLOSS (Single Large or Several Small) debate is a clash between different viewpoints, but neither of them ignores the species-area relationship.
Single Large
The reason to favor a single large conservation area over several small ones is simple — it will have more resources and house an array of different species.
Island biogeography fits into this because it’s essentially what we’re doing — we’re creating artificial islands and calling them natural preserves. Bigger islands have more biodiversity, and so do bigger preserves. It’s the species-area relationship in practice.
Let’s illustrate this with Microsoft Paint, shall we?
Imagine the blue triangle is a single wolf population that goes around the conservation area looking for rabbits to prey upon. They must go around the area to get all the resources they need because a smaller zone would not harbor enough rabbits to sustain them.
Now imagine the area is broken up into four smaller zones, none of which connect with the others:
Now the wolves can no longer move between areas, and so there aren’t enough rabbits in their zone to maintain the wolf population. They would hunt the rabbits to near-extinction and go extinct themselves. From there, the rabbits are few and their population is at a high risk of going extinct — they’re a small epidemic away from complete annihilation.
But what about the rabbits in the other three squares?
Imagine, if you will, that the rabbit population is represented by grey circles. In the terrible illustration above, the white areas around them represent the vegetation they have eaten.
If you imagine the wolves going around the zone counterclockwise, you can see that they leave behind smaller rabbit populations and control any impending population explosions through predation.
If that area is divided, the rabbits will see themselves suddenly free from predation and their population will suddenly rise uncontrolled. The rabbits will eat all the vegetation available to them, and their huge populations will collapse and bringing the local flora down with them.
This is a very simple example to illustrate a very complex problem. It’s not just a matter of helping the wolves stay alive, the rabbits and vegetation are also threatened by slicing up the preserve.
And there is also the edge effect. A single large preserve has less “edge” than any number of smaller preserves equating to the same area.
Imagine the brown area as the edge. Any edge like this gives rise to a transition zone that effectively separates the two landscapes.
These transition zones will generally be in the area assigned to the preserve since any outside area will have already been developed into farmland.
If the transition zone separates farmland from forest, it will be an area with more biodiversity than the farmland but less biodiversity than the forest.
Several Small
Proponents of this side of the debate have a very simple contradiction to their opponents — you cannot assume that several small areas will host the same species as a single large one.
The example of the rabbits and wolves isn’t always the case because people don’t usually chop up a big natural area into four separate small ones all next to each other.
Several small areas can, under certain conditions, harbor more species than large ones.
You can’t boil the natural world down to neat abstract theories and species-area curves. As Barbara Zimmerman argued when studying frog ecology, area isn’t a big factor when dealing with amphibians. If you want to preserve amphibians, a smaller area with more streams and ponds will do better than a large area with a scarcity of them.
Simply, if multiple small preserves have different species, then they have different ecosystems and may be perfectly capable of sustaining themselves.
And then there is the benefit of redundancy. If something devastates one of several small preserves, it’s not as big of a problem as it would be had it been one large continuous preserve:
So what’s the verdict?
The debate is nowhere near as fervent as it used to be. The general consensus is that it depends on the situation — sometimes single large preserves are preferred, and sometimes several small lines will do just fine.
Sometimes we do slice big natural areas into smaller ones right next to each other — building a road through a forest would cause exactly this scenario.
That doesn’t necessarily spell ecological doom. There are clever solutions that can be applied to situations like this.
Building a road through a forest can cause fragmentation, but that could be avoided if, for example, a wildlife bridge was installed.
There are actual bridges like that in places like Canada and The Netherlands, I highly recommend you check these images that I am afraid of showing here thanks to copyright law.
What the SLOSS debate ultimately led to was the realization that there is no best model that applies to every circumstance. Sometimes a single large preserve is better, sometimes several small ones are better.
Through a combination of single large and several small preserves, we can achieve a very reasonable level of conservation. It’s important to remember that both sides of the debate have the same interest, they want to preserve as much biodiversity as possible.
