This is a pleasant little tome by the master of the niche, G.E. Hutchinson. Having recently been named the president of Yale’s department of Zoology, his address at the annual meeting of The American Society of Naturalists begins with a recalling of a trip to Sicily, and observing two species of beetle in a small pond below the sanctuary of Santa Rosalita, whom Hutchinson proposes could be considered “the patroness of evolutionary studies” for the duration of the tale. The beetles are of the family Corixidae, C. punctata (the larger) and C. affinis (the smaller), and Hutchinson takes note that all of the observed C. punctata are female, and hence must be at the end of their breeding season, while there is an equal mix of sexes of the smaller beetle, which must be just beginning theirs. A series of questions – why are the breeding periods offset, why are there two beetles and not 200, finally leads us to the point of the story, an exploration on why there are so many kinds of animals. Rather than attempt to explain the existence of the magnitude of animals on the planet mathematically, Hutchinson decides to focus on some of the factors which control the number of kinds of animals.
Food Chains
Noting Elton’s work on predator chains and using as example what he terms the “Eltonian” food chain, in which each predator is successively bigger than its prey, Hutchinson theorizes on the number of links possible in a food chain. Generously assuming that 20% of the energy of one organism might be passed on to the next link in the chain, and that predators should reasonably be about twice the mass of their prey, Hutchinson proposes that 5 links is the maximum allowable by the Eltonian predator chain.
Natural Selection
Natural selection, Hutchinson states, is the abbreviator of food chains, noting that any increase of efficiency of a predator at the nth link in the chain may well cause the extinction of the (n-1)th link; and that this extinction would thereby force the nth link predator to adapt to eating the (n-2)th link or itself go extinct. He notes that it would be unlikely for a new terminal predator to form an (n+1)th link.
Effect of Size
Hutchinson briefly states his view on sympatric niche development in the beginning of his address, and from that understanding he expresses the limitation on the number of links in a food chain that might be filled by organisms that change drastically in size over the course of their lives. Should an organism normally occupying the nth link in the chain at adulthood inhabit the niche of a smaller animal in its youth, the number of different kinds of animals in that chain would be severely reduced due to competition.
Effects of terrestrial plants
Here Hutchinson simply points out the incredible variety of terrestrial plants, and the subsequently incredible variation of the insects that feed thereupon. However, while this increases the overall variety of organisms, this does not much increase the number of links in the Eltonian chain.
Interrelations of Food Chains
Here we step away from the direct contemplation of food chains and into food webs. Since realistically, any predator at the nth level would have more than one prey item at the (n-1)th level, they will not eat themselves into extinction, nor fully exterminate a single prey, as once the first prey item became scarce, it would be more feasible to hunt the other. As food webs do not represent a 1:1 interaction between predator and prey, there leaves room to examine how new organisms might join the web, and its effect upon the web. Hutchinson sites MacArthur (1955), which states that the stability of a community is directly related to the number of links in the food web, that efficient organisms will displace inefficient ones, and that stable communities will outlast the unstable. He further sites that there are three ways to add an organism to a community; by displacing an existing organism, by filling an empty niche, or it may partition a pre-existing niche. The first might increase the stability of the community if it itself is a more stable organism. The second and third could provide new links in the chain, thereby increasing stability. Siting Elton again, Hutchinson adds that the most stable communities are the oldest; those which have had ample time to replace inefficient organisms and add new links, and that as time goes on, it would be progressively more difficult to add new organisms because of this stability. This, he notes, explains a bit of the overall question of diversity- organisms are diverse because assemblages of diverse organisms increase the links in food webs and make communities more stable.
Limitation of Diversity
As we now have some answer for why there are so many kinds of animals, Hutchinson goes on to ask why there are not more kinds of animals. Using arctic species as example, Hutchinson notes that overall biomass is likely a limiting factor on diversity; if the basal members of the web can only support half of the predators that a more productive area might, there will obviously be fewer links in the web. Age of the community might also play a role, as communities become more stable over time, Hutchinson suggests the arctic communities may have simply not had enough time to evolve the diversity seen elsewhere. He also notes that competition for space might limit diversity in an area, as is reasonably the case for voles in the British Isles.
Niche Requirements
Here we examine the result of formerly allopatric species of similar size and niche becoming sympatric. Hutchinson provides a relatively famous metric for niche partitioning based on size and features of the “trophic apparatus”, now referred to as “Hutchinson’s ratio”. It simply states that for organisms to exist in the same niche space, they must be separated in size by at least 1:1.3. Remembering his beetles, Hutchinson explains that in their case the size difference is not enough, and C. punctata and C. affinismust therefore separate themselves by breeding season.
Mosiac Nature of Environment
Hutchinson takes a moment here to explore the nature of sympatry in various terrestrial fauna. I think it might be worthwhile to discuss why the interactions seen in various classes are such.
Hutchinson’s closing remarks are largely a re-hash of the points stated above, but in his last paragraph he raises an interesting question- if his assumptions on diversity and community structure are correct, it would mean that great diversity is more obtainable by small species than large; which would in turn mean that the evolutionary effects on each type are different.
Food Chains
Noting Elton’s work on predator chains and using as example what he terms the “Eltonian” food chain, in which each predator is successively bigger than its prey, Hutchinson theorizes on the number of links possible in a food chain. Generously assuming that 20% of the energy of one organism might be passed on to the next link in the chain, and that predators should reasonably be about twice the mass of their prey, Hutchinson proposes that 5 links is the maximum allowable by the Eltonian predator chain.
Natural Selection
Natural selection, Hutchinson states, is the abbreviator of food chains, noting that any increase of efficiency of a predator at the nth link in the chain may well cause the extinction of the (n-1)th link; and that this extinction would thereby force the nth link predator to adapt to eating the (n-2)th link or itself go extinct. He notes that it would be unlikely for a new terminal predator to form an (n+1)th link.
Effect of Size
Hutchinson briefly states his view on sympatric niche development in the beginning of his address, and from that understanding he expresses the limitation on the number of links in a food chain that might be filled by organisms that change drastically in size over the course of their lives. Should an organism normally occupying the nth link in the chain at adulthood inhabit the niche of a smaller animal in its youth, the number of different kinds of animals in that chain would be severely reduced due to competition.
Effects of terrestrial plants
Here Hutchinson simply points out the incredible variety of terrestrial plants, and the subsequently incredible variation of the insects that feed thereupon. However, while this increases the overall variety of organisms, this does not much increase the number of links in the Eltonian chain.
Interrelations of Food Chains
Here we step away from the direct contemplation of food chains and into food webs. Since realistically, any predator at the nth level would have more than one prey item at the (n-1)th level, they will not eat themselves into extinction, nor fully exterminate a single prey, as once the first prey item became scarce, it would be more feasible to hunt the other. As food webs do not represent a 1:1 interaction between predator and prey, there leaves room to examine how new organisms might join the web, and its effect upon the web. Hutchinson sites MacArthur (1955), which states that the stability of a community is directly related to the number of links in the food web, that efficient organisms will displace inefficient ones, and that stable communities will outlast the unstable. He further sites that there are three ways to add an organism to a community; by displacing an existing organism, by filling an empty niche, or it may partition a pre-existing niche. The first might increase the stability of the community if it itself is a more stable organism. The second and third could provide new links in the chain, thereby increasing stability. Siting Elton again, Hutchinson adds that the most stable communities are the oldest; those which have had ample time to replace inefficient organisms and add new links, and that as time goes on, it would be progressively more difficult to add new organisms because of this stability. This, he notes, explains a bit of the overall question of diversity- organisms are diverse because assemblages of diverse organisms increase the links in food webs and make communities more stable.
Limitation of Diversity
As we now have some answer for why there are so many kinds of animals, Hutchinson goes on to ask why there are not more kinds of animals. Using arctic species as example, Hutchinson notes that overall biomass is likely a limiting factor on diversity; if the basal members of the web can only support half of the predators that a more productive area might, there will obviously be fewer links in the web. Age of the community might also play a role, as communities become more stable over time, Hutchinson suggests the arctic communities may have simply not had enough time to evolve the diversity seen elsewhere. He also notes that competition for space might limit diversity in an area, as is reasonably the case for voles in the British Isles.
Niche Requirements
Here we examine the result of formerly allopatric species of similar size and niche becoming sympatric. Hutchinson provides a relatively famous metric for niche partitioning based on size and features of the “trophic apparatus”, now referred to as “Hutchinson’s ratio”. It simply states that for organisms to exist in the same niche space, they must be separated in size by at least 1:1.3. Remembering his beetles, Hutchinson explains that in their case the size difference is not enough, and C. punctata and C. affinismust therefore separate themselves by breeding season.
Mosiac Nature of Environment
Hutchinson takes a moment here to explore the nature of sympatry in various terrestrial fauna. I think it might be worthwhile to discuss why the interactions seen in various classes are such.
Hutchinson’s closing remarks are largely a re-hash of the points stated above, but in his last paragraph he raises an interesting question- if his assumptions on diversity and community structure are correct, it would mean that great diversity is more obtainable by small species than large; which would in turn mean that the evolutionary effects on each type are different.
I really like how this paper almost reads as a literature review towards the question "Why are there so many kinds of animals?" It seemed pretty modern in that way. This idea of a multi-prong approach to answering the question also seemed more contemporary as Hutchinson really tries to answer the question from a number of different points. I did feel like he could have take a couple of steps further with some of the math stuff that he was trying to do. I almost felt like he could have taken his ideas almost to the point of saying that there are so many species because the Earth is the size that it is, but he didn't take it that far and probably couldn't take it quite that far. Still, it is interesting to think of the synthesis of all of these ideas (and here again, a paper that explains the title of the section) and think about how with our knowledge now, how we could delve even further into these ideas.
ReplyDeleteHutchinson 1959:
ReplyDeleteAs Kat has pointed out, Hutchinson concludes that more diversity leads to more stability. This does seem to follow, since he proceeds on the basis of Volterra-Gause's competitive exclusion principle, along with Lack's emphasis on density-dependent population levels which are the product of evolution. It's fascinating how Hutchinson is able to move from a simple description of two insect species in a pond to the general question of why there are so many species, and a consideration of the process of speciation. It is noteable that Hutchinson's current estimate of the total number of species in 1959 is 10^6, the same order of magnitude as the number of named species (2.3 million) in the open tree of life, a just-announced website: https://opentreeoflife.org. It is only one order of magnitude less than current estimates of the total number of species (8.7 million).
I enjoyed reading this paper, but do agree that he almost could have used a little more math to back up his statements (almost). It seems like this would be the perfect instance for a model that could offer some explantation of how the Earth can sustain so many species. That being said, I really enjoyed considering each different factor he went through, and how he somewhat systematically eliminated some of the factors he at first considered. He ultimately comes to the general conclusion that a "complex trophic organization of a community is more stable than a simple one", and this could be a key driver to the sheer mass of species on our planet. I did think it was interesting how he ended the paper. He mentioned that all of this can take place because of "cooperative behavior", which gives light to a philosophical driver of life in both the ecological community and within humans.
ReplyDeleteInteresting that Hutchinson didn’t pick St. Francis, but I suppose the Santa Rosalia story is what led to his question, it’s definitely a catchy hook to the synthesis. It is interesting to me that we know ~99.9% of all species that inhabited the planet at any given time are extinct and yet the world is still incredibly diverse regardless of what organism class you look at. I really like the last body paragraph dealing with the observance of more small organisms than large organisms this is a fascinating phenomena (though not surprising). I wonder if humans have or will change this rule?
ReplyDeletePaper 16: Hutchinson
ReplyDeleteI thought this paper was super interesting. I hadn’t up to this point looked at diversity from the perspective that Hutchinson offers in this paper – that more diversity leads to a more stable community and thus high diversity is something that would be favorable and be maintained under natural conditions. Could this work as a positive feedback loop where once a community reaches a stable point it can then continue to drive more diversity or is it only unidirectional where it’s not possible to have stability without first having diversity and then once stability is reached a maximum amount of diversity is also reached similar to a climax? I also thought it particularly interesting when he states that there is less need for species diversity if existing species have highly modifiable behavior. So basically a single species could fill multiple niches in the same community by modifying its behavior to exploit resources in different ways. Are there any species like this or have any communities been identified that may follow this idea?
Humans would be one example that comes to mind. How many species' niches have we come to occupy and competitively excluded? I also found this interesting, especially the potential relations between behavioral flexibility, brain-body allometrics.
DeleteLike Martina and Ali, I found the idea of increased diversity leads to a more stable community, very thought provoking. I keep going back to the May paper, and he has the exact opposite idea. He states that increased diversity leads to a decrease in stability. In these examples I think it all comes down to how they defined stability, and what it means to have complex interactions between organisms. I really like Hutchinson's idea of looking at community interactions as links in a chain, its a nice visualization tool, made better in class when we thought of it as a web, where multiple links can attach.
ReplyDeleteYeah I think they say in the intro to the theoretical section that the diversity-stability relationship was debunked or undermined by May, and then kind of waved the idea of by saying these things are difficult to define. But I'd be curious to know more - Has this the diversity-stability relation really been debunked? Can't we just define these things better (i.e. mathematically - the lack of models that Noah and Ali mention)? How does this relate to all the modern research on biodiversity and ecosystem functioning? The diversity-stability relationship seems to be one of the more important reasons Hutchinson gives for why there are so many species, so is his explanation undermined?
DeleteHere's a review paper from 2000 that sheds light on the multiple definitions of stability used in ecology and how the ideas of Elton, MacArthur, Hutchinson and May can be resolved: http://www.nature.com/nature/journal/v405/n6783/full/405228a0.html
DeleteThanks Helen - that paper looks super interesting and helpful!
DeleteHutchinson’s “Homage to Santa Rosalia” is an interesting take on how to understand the magnitude of diversity on our planet. It is important to consider definition of species and the importance of morphology in phylogeny of Hutchinson's time. I wonder if Hutchinson’s point of view or reasoning would change, if at all, with modern genetic and phylogenetic understanding? Furthermore, this idea also goes hand in hand with “lumpers and splitters” in systematic, and perhaps the diversity of the planet is less straight forward than one might think. Rather, it is dependent on ideology and definition of terms.
ReplyDelete
ReplyDeleteHutchinson's is addressing a fundamental question in biology: Why there are such as an enormous number of animal species?. As it was expected I found 3034 citations for this paper in google scholar. I think any biologist trying to give an answer to this question will cite this paper in his research. I liked the way he introduced Santa Rosalia as the witness of changes in the diversity of animals found in Monte Pelegrino. In order to tackle this question he used the complexity of food webs in communities and interactions between different levels. I liked the way he used constraints in body size, predator -prey interactions, lost of energy within the food chain, abundance of terrestrial plants and abiotic factor to give us an insighful though about diversity of life forms
This was definitely a fun read. I was not surprised to see Hutchinson attribute stability to diversity, its only logical that more species, and more niches for that matter, beget more links and more connections between link and inherent stability. I liked his example of oscillations in boreal forests and tundra to demonstrate this idea. It would be interesting to look at biodiversity hotspots in a similar context to see if similar oscillations occur. Now that I think of it, biodiversity hotspots may be the answer to Martina’s question regarding positive feedback loops.
ReplyDeleteI thought this paper/talk reads like an extremely rich thought-piece. The reliance of his explanation for earth's diversity on an apparently questionable diversity/complexity-stability relationship was unfortunate, but I did find the idea of natural selection acting on communities, as well as on individuals and species, to be pretty neat. The strongest parts of this piece in my mind were Hutchinson's explanations for the limitations on diversity. He points out some very interesting relationships between food web complexity, productivity, body size, niche dynamics, and earth history. I kept trying to apply the concepts he was introducing to help understand the latitudinal diversity gradient, something that's long fascinated and baffled me, and for the first time I felt like it made some amount of sense. A few questions (H throws out a lot of interesting speculative hypotheses that I wonder what has come of):
ReplyDelete- What is the role of range size in determining diversity, and how does this relate to body size? H. alludes to this in several places, but doesn't discuss it much explicitly.
- What does H. mean when he mentions 'the continual occurrences of catastrophes...may keep the arctic terrestrial community on a state of perennial though stunted youth'?
- Has Hutchinson's ratio held up over time? Also, what is the mechanism by which having ~1.3x body size allows significant enough niche differentiation for coexistance?
- Has H.'s hypothesis that evolution acts differently on smaller organisms than larger organisms been tested and supported? How?
- Has the idea of 'autocatakinetic' diversification held up?