Sunday, 22 August 2010

Earth’s biodiversity is driven by more than just Survival of the Fittest

Sahney, S., Benton, M.J. and Paul Ferry 2010. Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land. Biology Letters 6:544-547.
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Published in Biology Letters today by myself, Michael Benton and Paul Ferry at the University of Bristol. Also an article at the BBC.

New research suggests that biodiversity is closely tied to the niches animals occupy, and the rich biodiversity we see on Earth today has grown out of expansion, not competition. Darwin cited competition among animals, coined ‘survival of the fittest’, as a driver of evolution in his book, On the Origin of Species; since then competition has been considered key to having grown Earth’s biodiversity. But while competition has been observed on a small scale, (eg. between species), there is little evidence of competition guiding large-scale shifts in biodiversity, such as the dominance of mammals and birds over reptiles and amphibians in today’s world. Our new research supports the idea that animals diversified by expanding into empty ecological roles rather than by direct competition with each other.

When vertebrates moved onto land millions of years ago, they filled empty niches further away from the water, and then they continued to invade new habitats evolved by other organisms such as forests, canopies, and grasslands. These animals began to burrow, climb, fly and take advantage of new food sources.

Our research shows that tetrapods (amphibians, reptiles, mammals and birds) have explored only one third of habitable ecological space and that without human influence, biodiversity would continue to increase exponentially.

Examining the biodiversity of tetrapods, we realized that their taxonomic diversity (the number families) closely matched their ecological diversity (the number of niches they occupied) through their 400 million years of evolution, and that there appears to be little evidence for competition as the driving factor for their great diversity.

Diversity was driven by the dominant animals at the time, which expanded into empty niches. Competition did not play a big role in the overall pattern of evolution. For example, even though mammals lived beside dinosaurs for 60 million years, they were not able to out compete the dominant reptiles. But when the dinosaurs went extinct, mammals quickly filled the empty niches they left and today mammals dominate the land.

Growing and shrinking biodiversity is closely tied to the niches animals occupy, so habitat destruction is a key aspect of extinction. In Earth’s past there have been incentives for animals to move into new modes of life, where initially resources may seem unlimited, there are few competitors and possible refuge from danger.

However, if niches are destroyed more often than created because of man’s influence on the environment, animals won’t have the opportunity to adapt and biodiversity won’t continue to grow.


Anonymous said...

I would imagine that space, in the niche sense, is fractal. The more species in an ecosystem, the more niches there are to exploit, though perhaps smaller and more specialised. As A. de Morgan expressed it,
Great fleas have little fleas upon their backs to bite 'em,
And little fleas have lesser fleas, and so ad infinitum.
And the great fleas themselves, in turn,
Have greater fleas to go on;
While these again have greater still,
And greater still, and so on.
- Budget of Paradoxes, 1872

If this idea is correct, then it is not just space, but also complexity that allows for evolution.

As humans go about the Great Simplification, however, the space will get ever larger and the complexity ever smaller. We are therefore setting about a planetary experiment that will go a long way to decide, one way or the other, which hypothesis can be rejected. Once the experiment has run its course, there will (perhaps fortunately) be no humans around to assess the outcome.

Anonymous said...

Dear Sarda,
I find it difficult to reconcile several points of your article. It seems that your major point is that there is no need for competition to explain variation in biodiversity. However, you write that "For example, even though mammals lived beside dinosaurs for 60 million years, they were not able to out compete the dominant reptiles." What, if not "competitively superior", do you mean by "dominant"? In other words, what made dinosaurs dominant over mammals? More generally, what, if not competition for limited resource, is the driving force for migration?

To summarize, while I find it obvious that biodiversity is strongly correlated to ecological diversity, I do not see at all why this should challenge the view that competition is the driving force behind adaptation to new niches.

James George said...

Very interesting post, your study is fascinating, although I don't know of any evolutionary mechanism which could facilitate this kind of diversification. The study is perhaps more concerned with highlighting the importance of habitat then with re-writing evolutionary theory? But the implications to evolutionary theory seem unavoidable and, in the view of some, detrimental.

Anonymous said...

FWIW Herbert Spencer is usually attributed with the coinage of "Survival of the Fittest" after reading the first edition of Origin of the Species. Darwin liked the phrase and included it in later editions.

Travis said...

Well, PZ Myers has just linked to you (which is how I came here) so hopefully you will gain a few new readers out of that horde now that you are blogging again. I am happy to be here and am certainly going to be coming back.

Your paper is certainly getting a lot of press coverage. Nothing I have done has ever generated this much buzz. Exciting!

Jerry Coyne said...

Why did the reporters all dwell on the fact that your results showed that "Darwin was wrong"? You guys didn't sell your paper that way, did you? Otherwise I fail to understand how all the reporters hit on the same hook.

caynazzo said...

Maybe this is the wrong place to ask, but is there any evidence, fossil or otherwise, that the cataclysmic event that brought about the extinction of the dinosaurs also impacted extant mammalian taxonomy?

Anonymous said...

Hhmm, I would have considered "radiating into empty niches" a form of competition. After all, if there's a free source of calories and some individuals are in a better position to capitalize on it (or have lower mutational hurdles to overcome in capitalizing on it), then we have organisms selected based on their ability to capitalize on a (particular) new source of calories.

In other words, I simply don't see how this study in any way decentralizes or mediates or lessens the influence of selection on evolution, unless it does so simply by playing silly semantic games. We can leave that to Jerry Fodor, don't you think?

Dan L.

Anonymous said...

The phrase "survival of the fittest" was coined by Herbert Spencer, not Charles Darwin.

Mike Haubrich said...

Well, this is much more helpful to the lay reader than what has appeared in the popular press, and I for one appreciate that you have clarified your team's results.


Suvrat Kher said...

I am not arguing against your analysis but my understanding is that for Darwin the major component of competition really meant reproductive competition between individuals within a species.

I think some of the media reports (including the BBC link you gave) are conflating Darwin's idea of competition and your analysis which focuses on competition between groups or species.

Anonymous said...

The "competition" element in the particular conventional (and, unfortunately, historically most popular) interpretation of Darwin's treatise has always struck me as an over-interpretation, to say the least.

Competition with WHAT? As if some presumed 'opponent' was involved. Worse, as if individual creatures somehow understand not only who their opponent antagonists, foes and enemies are, but actively address the "competition" with some strategy for overcoming them.

Organisms who are purported to 'covet' their lives and genetic progeny (not to mention their potential lebensraum - "living room" aka 'environmental niches' - a convenient misidentification with the notion of territoriality) are immediately subsumed into an ideological stance that tells us more about how humans steeped in their cultural upbringings are so capable of jumping to unfounded conclusions.

Alas, in most human cultures, nature or the 'environment' has itself acquired the role of 'opponent': a thing which organisms seek to 'fight against' or which higher beings such as we ever and again pretend to 'conquer'.

The 'moral' of this horror state of affairs is: WE DO NOT GET IT. (By "WE" I mean the vast and astonishingly undereducated majority, of which a major proportion is so immediately persuaded by irrational promises from the religious contingent which provides 'validation' through the empty constriction of popularity: if one encounters lots of people who think a particular thing, why, it MUST be true...etc)

On your final bold-faced statement in particular: Indeed, and it is quite as simple as that.

Nothing that a bold planet-wide reduction in the grotesque overpopulation couldn’t conceivably address, I’m sure. Of course, any such movement would inevitably trigger a precipitous increase in the whining of those who imagine that our excessive population is an indicator of how ‘successful’ we are as a species.

astrokid.nj said...

Our research shows that tetrapods (amphibians, reptiles, mammals and birds) have explored only one third of habitable ecological space

Can you give examples of what remains habitable but unexplored? Definetely not the deserts, nor the oceans I would assume..

marcas said...

What a pity your conclusions got lost in the "darwin was wrong" hype surrounding your work.

mikemagee said...

"Competition did not play a big role in the overall pattern of evolution. For example, even though mammals lived beside dinosaurs for 60 million years, they were not able to out compete the dominant reptiles."

There seems to be a glaring contradiction between these two adjacent sentences.

Surely occupying empty niches is a strategy for escaping competition in saturated ones.

Dalton said...

You have to compete in order to obtain more space.

Anonymous said...

What really surprises me is how often people misunderstand evolution and in so doing claim that it is not correct. Evolution by natural selection (Darwin's complete thought) is about advantage, not competition. So if a trait offers an advantage, the species will survive better. Entering a new niche is about adaptation and the species that does it first has the advantage of being able to exploit something no other species has. Competition is the wrong way to approach evolution, it is about advantage, which does not necessarily imply competition.

Anonymous said...

I agree completely.
Read Stephen Gould.
Evolution is not solely 'up', but in 'all directions at once'.

Anonymous said...

Read Stephen Gould.
Evolution is not solely 'up', but 'in all directions at once'.

Anonymous said...


William Tooke said...

Please.....address Jerry Coyne's question.

Anonymous said...

I am so glad I stumbled across this. I have a few arguments against it and they may only be based in semantics. In general I find this, upon first inspection, a logical and accesible explanation the process of new species evolution. I was taught that evolutionarily speaking, species remained relatively stable for long periods of time with sudden drastic changes occurring to rapidly develop new species. However, no explanations gave me a sense that it was well-understood just how and why that occurs. This brings me a little closer to understanding. For that, I thank you. Also, as a teacher, I love it when our academics feel it necessary to make their findings accessible to the lay person - it is a great help to educators!

Sarda Sahney said...

Hello everybody,

Thank you for your comments, unfortunately I have had hundreds of spam emails, about 20 spam for every genuine comment, so I am slow in replying. I will do my best to moderate them and respond as soon as I can.

Thanks, Sarda

Richard D said...

Hi Sarda.

Just to say I enjoyed reading about your research, in spite of the fact that I only really got here because of all the sadly manufactured hype surrounding the 'Darwin was wrong' thing.

I really hope nothing I ever do ends up being so misrepresented!

From an ex-Bristol palaeo person I wish you all the best in your PhD and hope you are spared too much spam as a result of this.

Anonymous said...

I see nothing here that is really new or that hasn't been espoused by ecologists for the past 40 years. Ecologists often speak of the "ghost of competition past". Organisms that are most suited to their environment are best at AVOIDING competition. In fact, competitive displacement is a fundamental way that ecological niches are created and subsequently partitioned. Finding a correlation between biodiversity and the number of niches is also obvious and well known for the last 40 years or more since competition CAUSES organisms to shift in their niche space to avoid subsequent competition. Adaptive radiation could be considered nothing more than "fleeing from the constraints of competitive niche spaces. The rest is nothing but semantic arguments and marketing ploys for the paper. Of course there is also INTRAspecific competition which is ALWAYS present given that individuals of the same species, barring cases of trophic dimorphism (e.g. mosquitoes) and developmental dimorphism (species that undergo metamorphosis), always are under fairly intense competition because they always occupy the same niche space. If anything, I would say this supports the already well-established literature on the topic but shows that it works for deep time as well. Of course, I don't understand how you can calculate that most of the ecological spaces of tetrapods had been explored. The (probably incorrect) assumptions that are buried in that are too numerous to count. For example it assumes that tetrapods are only in competition with other organisms that are tetrapods that have fossilizable parts. To use a contemporary equivalent--over 1/3rd of the food we grow for human consumption is eaten by insects and nematodes and yet we rarely consider nematodes to be a competitor for food. Needless to say bacteria and fungus are also excellent competitors for what we eat.

Danny said...

astrokid.nj - I think the point is that tetrapods have only explored 1/3 of habitable ecological space. The other 2/3 is inhabited by non-tetrapoda.

Anonymous said...

Related article in this week's Science:

Alroy 2010: "The Shifting Balance of Diversity
Among Major Marine Animal Groups" p1191...