Tag: The Song of the Dodo

Immigration, Extinction, and Island Equilibrium

Equilibrium is an important concept that permeates many disciplines. In chemistry we think about the point where the rate of forward reaction is equal to the rate of backward reaction. In economics we think of the point where supply equals demand. In physics we can see how gravity is balanced by forward velocity to create things like planetary orbits.

No matter which discipline we are examining, the core idea remains the same: Equilibrium is a state where opposing forces are balanced.

In biology, equilibrium is so important that it can mean the difference between life or death; for a species, it can decide whether they will thrive or become extinct.

In The Song of the DodoDavid Quammen dives into how equilibrium affects a species’ ability to survive, and how it impacts our ability to save animals on the brink of extinction.

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Historically, the concept of island equilibrium was studied with a focus on the interplay between evolution (as the additive) and extinction (as the subtractive). It was believed that speciation, the process where one species becomes two or more species, caused any increase in the number of inhabitants on an island. In this view, the insularity of islands created a remoteness that could only be overcome by the long processes of evolution. 

However, Robert MacArthur and E.O. Wilson, the co-authors of the influential Theory of Island Biogeography, realized that habitats would show a tendency towards equilibrium much sooner than could be accounted for by speciation. They argued the ongoing processes that most influenced this balance were immigration and extinction.

The type of extinctions we’re referring to in this case are local extinctions, specific to the island in question. A species can go extinct on a particular island and yet be thriving elsewhere; it depends on local conditions.

As for immigration, it’s just what you’d expect: The movement of species from one place to another. Island immigration describes the many ingenious ways in which plants, animals, and insects travel to islands. For instance, not only will insects hitch rides on birds and debris (man made or natural, think garbage and sticks/uprooted seaweed), animals will do the same if the debris is massive enough.

Seeds, meanwhile, make the trip in the feces of birds, which helps to introduce new plant species to the island, while highly motivated swimmers (escapees of natural disasters/predators/famine) and hitchhikers on human ships (think rats) make it over in their own unusual ways.

We can plot this process of immigration and extinction graphically, in a way you’re probably familiar with. Quammen explains:

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The decrease in immigration rate and the increase in extinction rate are graphed not against elapsed time but against the number of species present on a given island. As an island fills up with species, immigration declines and extinction increases, until they offset each other at an equilibrium level. At that level, the rate of continuing immigration is just canceled by the rate of continuing extinction, and there is no net gain or loss of species. The phenomenon of offsetting increase and decrease – the change of identities on the roster of species – is known as turnover. One species of butterfly arrives, another species of butterfly dies out, and in the aftermath the island has the same number of butterfly species as before. Equilibrium with turnover.

So while the specific species inhabiting the island will change over time, the numbers will tend to roll towards a balanced point where the two curves intersect.

Of course, not all equilibrium graphs will look the like one above. Indeed, MacArthur and Wilson hoped this theory would be used not just to explain equilibriums, but to also help predict potential issues.

When either curve is especially steep – reflecting the fact that immigration decreases especially sharply or extinction increases especially sharply – their crossing point shifts leftward, toward zero. The shift means that, at equilibrium, in this particular set of circumstances, there will be relatively few resident species.

In other words, high extinction and low immigration yield an impoverished ecosystem. To you and me it’s a dot in Cartesian space, but to an island it represents destiny.

There are two key ideas that can help us understand the equilibrium point on a given island.

First, the concept of species-area relationship: We see a larger number of a given species on larger islands and a smaller number of a given species on smaller islands.

Second, the concept of species quantity on remote islands: Immigration is much more difficult the further away an island is from either a mainland or a cluster of other islands, meaning that fewer species will make it there.

In other words, size and remoteness are directly correlated to the fragility of any given species inhabiting an island.

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Equilibrium, immigration, evolution, extinction – these are all ideas that bleed into so many more areas than biogeography. What happens to groups when they are isolated? Jared Diamond had some interesting thoughts on that. What happens to products or businesses which don’t keep up with co-evolution? They go extinct due to the Red Queen Effect. What happens to our mind and body when we feel off balance? Our life is impoverished.

Reading a book like The Song of the Dodo helps us to better understand these key concepts which, in turn, helps us more fundamentally understand the world.

The Founder Principle: A Wonderful Idea from Biology

We’ve all been taught natural selection; the mechanism by which species evolve through differential reproductive success. Most of us are familiar with the idea that random mutations in DNA cause variances in offspring, some of which survive more frequently than others. However, this is only part of the story.

Sometimes other situations cause massive changes in species populations, and they’re often more nuanced and tough to spot.

One such concept comes from one of the most influential biologists in history, Ernst Mayr. He called it The Founder Principle, a mechanism by which new species are created by a splintered population; often with lower genetic diversity and an increased risk of extinction.

In the brilliant The Song of the Dodo: Island Biography in an Age of ExtinctionDavid Quammen gives us not only the stories of many brilliant biological naturalists including Mayr, but we also get a deep dive into the core concepts of evolution and extinction, including the founder principle.

Quammen begins by outlining the basic idea:

When a new population is founded in an isolated place, the founders usually constitute a numerically tiny group – a handful of lonely pioneers, or just a pair, or maybe no more than one pregnant female. Descending from such a small number of founders, the new population will carry only a minuscule and to some extent random sample of the gene pool of the base population. The sample will most likely be unrepresentative, encompassing less genetic diversity than the larger pool. This effect shows itself whenever a small sample is taken from a large aggregation of diversity; whether the aggregation consists of genes, colored gum balls, M&M’s, the cards of a deck, or any other collection of varied items, a small sample will usually contain less diversity than the whole.

Why does the founder principle happen? It’s basically applied probability. Perhaps an example will help illuminate the concept.

Think of yourself playing a game of poker (five card draw) with a friend. The deck of cards is separated into four suits: Diamonds, hearts, clubs and spades, each suit having 13 cards for a total of 52 cards.

Now look at your hand of five cards. Do you have one card from each suit? Maybe. Are all five cards from the same suit? Probably not, but it is possible. Will you get the ace of spades? Maybe, but not likely.

This is a good metaphor for how the founder principle works. The gene pool carried by a small group of founders is unlikely to be precisely representative of the gene pool of the larger group. In some rare cases it will be very unrepresentative, like you getting dealt a straight flush.

It starts to get interesting when this founder population starts to reproduce, and genetic drift causes the new population to diverge significantly from its ancestors. Quammen explains:

Already isolated geographically from its base population, the pioneer population now starts drifting away genetically. Over the course of generations, its gene pool becomes more and more different from the gene pool of the base population – different both as to the array of alleles (that is, the variant forms of a given gene) and as to the commonness of each allele.

The founder population, in some cases, will become so different that it can no longer mate with the original population. This new species may even be a competitor for resources if the two populations are ever reintroduced. (Say, if a land bridge is created between two islands, or humans bring two species back in contact.)

Going back to our card metaphor, let’s pretend that you and your friend are playing with four decks of cards — 208 total cards. Say we randomly pulled out forty cards from those decks. If there are absolutely no kings in the forty cards you are playing with, you will never be able to create a royal flush (ace+king+queen+jack+10 of the same suit). It doesn’t matter how the cards are dealt, you can never make a royal flush with no kings.

Thus it is with species: If a splintered-off population isn’t carrying a specific gene variant (allele), that variant can never be represented in the newly created population, no matter how prolific that gene may have been in the original population. It’s gone. And as the rarest variants disappear, the new population becomes increasingly unlike the old one, especially if the new population is small.

Some alleles are common within a population, some are rare. If the population is large, with thousands or millions of parents producing thousands or millions of offspring, the rare alleles as well as the common ones will usually be passed along. Chance operation at high numbers tends to produce stable results, and the proportions of rarity and commonness will hold steady. If the population is small, though, the rare alleles will most likely disappear […] As it loses its rare alleles by the wayside, a small pioneer population will become increasingly unlike the base population from which it derived.

Some of this genetic loss may be positive (a gene that causes a rare disease may be missing), some may be negative (a gene for a useful attribute may be missing) and some may be neutral.

The neutral ones are the most interesting: A neutral gene at one point in time may become a useful gene at another point. It’s like playing a round of poker where 8’s are suddenly declared “wild,” and that card suddenly becomes much more important than it was the hand before. The same goes for animal traits.

Take a mammal population living on an island, having lost all of its ability to swim. That won’t mean much if all is well and it is never required to swim. But the moment there is a natural disaster such as a fire, having the ability to swim the short distance to the mainland could be the difference between survival or extinction.

That’s why the founder principle is so dangerous: The loss of genetic diversity often means losing valuable survival traits. Quammen explains:

Genetic drift compounds the founder-effect problem, stripping a small population of the genetic variation that it needs to continue evolving. Without that variation, the population stiffens toward uniformity. It becomes less capable of adaptive response. There may be no manifest disadvantages in uniformity so long as environmental circumstances remain stable; but when circumstances are disrupted, the population won’t be capable of evolutionary adjustment. If the disruption is drastic, the population may go extinct.

This loss of adaptability is one of the two major issues caused by the founder principle, the second being inbreeding depression. A founder population may have no choice but to only breed within its population and a symptom of too much inbreeding is the manifestation of harmful genetic variants among inbred individuals. (One reason humans consider incest a dangerous activity.) This too increases the fragility of species and decreases their ability to evolve.

The founder principle is just one of many amazing ideas in The Song of the Dodo. In fact, we at Farnam Street feel the book is so important that it made our list of books we recommend to improve your general knowledge of the world and it was the first book we picked for our learning community reading group.

If you have already read this book and want more we suggest Quammen’s The Reluctant Mr. Darwin or his equally thought provoking Spillover: Animal Infections and the Next Human Pandemic. Another wonderful and readable book on species evolution is The Beak of the Finch, by Jonathan Weiner.