Hutchinson 1959 + Schneider et al. 2016
Blog author: Yuguo Yang
Citations
Hutchinson GE. 1959. Homage to Santa Rosalia or Why Are There So Many Kinds of Animals? The American Naturalist93: 145–159.
Schneider FD, Brose U, Rall BC, Guill C. 2016. Animal diversity and ecosystem functioning in dynamic food webs. Nature Communications7: 12718.
Author background
G. E. Hutchinson was a famous British ecologist and a professor in zoology at Yale University. He is highly reputed for his contribution on niche theory and species coexistence studies.
F.D. Schneider is an ecologist and environmental scientist at ISOE Institute for Social-ecological Research in Germany. He works on biodiversity valuation and ecological transformation. His recent studies focus on ecosystem degradation and biodiversity loss.
Hutchinson 1959
The goal of this paper was to figure out what drives the high richness of species in the world and what limits the species diversity. He derived his idea from Voterra-Gause principle that sympatric species evolve to occupy different niche (defined by food) due to natural selection, isolation, and invasion. He argued that the food relationship was not enough to explain the high diversity of species.
The theory of food chain was proposed by Elton (1927), in which species at the higher levels of food chain were larger and rarer than species at the lower levels. Hutchinson listed a couple of factors that would limit the length of food chains. First, it was assumed that no more than 20% of energy could pass through one link of the food chain. In this case, the food chain theory cannot support the high species diversity because of the length of food chains were strictly limited by energy. Also, the energy flow could not be higher because the increase of predators’ efficiency would cause the decrease of the prey population and ultimately extermination of the prey. Second, animals would change size during their life history, the term that he called “metaphoetesis”. In this case, animals may change their position in the food chain. He further argued that the diversity of plants (basal food source in the food chain) was not sufficient to explain the animal diversity because, in his point of view, the “kinds of food” provided by plants were limited.
Hutchinson started illustrating his ideas by introducing McArthur’s point of community evolution: more efficient species take over the less efficient one, but more stable community outlast the less stable one. In Hutchinson’s opinion, the reason for high species diversity was that diversified communities were stable and persisted longer than less-diversified communities. Then he also talked about the limitations of diversity: length of growing season, total biomass in the system, growth form of plants, environment rigor & stability, refugee availability in the harsh environment, etc.
3 main conclusions were drawn from his paper: 1. the reason for diversity is that a diversified community is stable. 2. Evolution of communities increase stability (a more complex system derives from the less complex one). 3. It is easier to have a high diversity for small than large organisms.
Schneider et al. 2016
Schneider’s paper focused on the consequences of animal species lost on ecosystem function. The authors tried to answer this question by studying what would happen after increased animal diversity. There are 2 theories of processes that counteract with each other: 1. Increased animal diversity will increase the complementarity of herbivores (herbivores become more exploitive to the resources), which has a top-down control of plant communities. 2. High animal diversity will increase feeding rates within the consumer guild (within animals), and thus release plants from grazing pressure. The goal of this paper was to investigate which of the processes is stronger after increasing animal diversity.
Methods
The authors developed a dynamic food-web model using body mass as the only differentiating parameter for species feeding traits and physiology. i.e. it assumed that animals with the same body mass were identical. Also, the model assumed that most animals were omnivores (50% strict herbivory were tested as a comparison). The authors investigated 6 major variables in response to increasing animal diversity: total animal biomass, total intraguild predation, total animal metabolism, total plant biomass, total feeding on plants, and total plant metabolism. They also simulated the response of average body mass of plants and animals.
Results
3 main results were reported as the response to increasing animal diversity: 1. total plant biomass stayed the same. 2. animal metabolism increased, possibly due to increased respiration and intra-guild predation, and average animal body mass increased. 3. plant metabolism decreased but average body mass increased, indicating that more small plants were consumed than large plants.
Discussion
The results of this study indicated that diverse animal species would gain more total biomass and become more exploitative on plants. But at the same time, high intra-guild predation existed. The diversified animal species would not reduce plant biomass (top-down control did not increase), assuming most of the animals are omnivores (when applying 50% of the animals to strict herbivores, total plant biomass decreased in response of increased animal diversity). However, the decreased plant metabolism and the increased average body mass indicated that plant communities shifted in favor of large plants.
Comments
Both papers studied the animal diversity and food webs. Hutchinson’s paper is more descriptive and qualitative about the ecological theories which helps me understand the background of these processes. However, I find it a little disappointed about his explanation of what drives diversity. To me, “stability” is the result of diversified communities rather than the cause. I would expect an explanation that focuses more on the process itself.
I chose Schneider’s paper as the companion paper because it covers a lot of similar concepts and the authors tested the balance of 2 processes using simulated data. The results were clear and impressive, although I expected more explanation on some of the details, such as why the diversified animal communities tend to consume more small plants than large plants. My another concern is the assumption that most animals are omnivores, which is not plausible, but I do not know a better way to modify this part of the model.
I found the Schneider paper to be a good modern modeling take on the more descriptive concepts presented by Hutchinson in his paper. It would be interesting to see an empirical test of the results presented in this theoretical paper. I don't believe that the paper is lacking from not having empirical tests, but it would be of interest for future studies of ecological communities.
ReplyDeleteSimilar to Yuguo's comment at the end of this blog post, I did find it strange to see the assumption of omnivory in Schneider, as there are many examples of obligate herbivore and obligate carnivore animal species in nature and I would imagine that these more specialized diets would have different effects on a community compared to the effect of a broad generalist omnivore. I would have liked a bit more of an explanation for the reasoning behind this choice and/or a discussion of the potential effects this assumption may have and how it may potentially skew the results.
- Elizabeth
Hutchinson comments in his section about niche requirements that, in order for one of a pair of species to be larger than the other, it needs to breed before the smaller species (using water bugs in Corixidae as his evidence of this). It made me wonder, how often is the need for temporal separation in breeding (presumably, to allow the larger animal adequate resources to develop) the case for pairs of related animals that co-occur today? Is this a relatively common trend in other taxa, or was Hutchinson over-extrapolating based on the example he had observed?
ReplyDeleteSchneider et al. made a similarly broad comment in the introduction to their manuscript, regarding the idea that increasing the number of obligate herbivore species should consequently lead to a higher absolute herbivory rate. This is challenging to directly and precisely quantify in natural ecosystems, but again made me wonder – is this always the case? Could there not be a reduction in the population levels of the already present obligate herbivore species to compensate for the presence of a new herbivore, eventually leading to the same overall rate of herbivory (and an approximately equal number of herbivores) once equilibrium between species is reestablished? I also was curious to learn more about how they chose to include 30 plant species in the majority of their models. While this might be an adequate representation of plant diversity in some cooler temperature communities, this is a fairly low-diversity community (especially if all vegetation layers are considered) to extrapolate upon given the inclusion of anywhere from 10-100 animals!
Schneider et al.'s paper is a timely and interesting companion to Hutchinson's paper. I was especially intrigued by their finding that more diverse communities have larger organisms. However, I did find the assumed links between body size and trophic level to be questionable. They say that large animals occupy the the top of their modelled food web; they are carnivores. But although many large animals, like big cats, are indeed carnivores, I would argue that most large animals are herbivores, like elephants and other grazers.
ReplyDeleteI enjoyed Hutchinson's paper. Although the writing style is different, as we have talked about before, in this particular paper I didn't find the style to detract from the paper. It was told more as a story, and it wasn't too difficult to follow the various tangents.
In general, both papers made for an interesting read. The Hutchinson paper asked an interesting question and followed it by describing important ecological ideas (e.g food webs rather than food chains; the effect of invasive species on established niches). There were some parts that made me question Hutchinson (i.e. the terrestrial realm is more diverse than the marine realm), but I think this is just a product of his time (i.e. understudied systems). Most of the arguments he made seemed reasonable. However, it was clear that discussions about arctic diversity clearly stumped Hutchinson. What I did appreciate was his mention of time in this discussion. Though I am not 100% convinced on his argument (that we don't know much about the arctic because it is too young to have reached maximum diversity), he did seem to indicate that communities can rise over time. The idea of "maximum diversity" is a strange one to me, though. There seems to be this notion of filling niches to their capacity throughout the paper (if there is such thing as a "maximum" diversity, then there must be a limit). I am not sure how one knows when a community has reached its maximum diversity so I was unsure what he meant.
ReplyDeleteThe Schneider et al. paper was a good follow up to the Hutchinson paper. I look forward to discussing this paper in class, because I am a bit confused about the conclusions. It seems to me that it is implied animals are omnivores in this model and diverse systems tend to have increasingly large animals (especially predators). First of all, what does it mean to be a large animal. In a rain forest, a large animal may not be as large as one on the Savannah. Secondly, aren't there classically some large herbivores? I think Stella mentioned this above. All in all, I would very much like to talk about this in class today for my own understanding.
I've read the Hutchinson paper several times in the past, and loved it each time. It's a really beautiful illustration of the thought process that a lot of ecologists seem to experience, wherein examining the natural world inspires you to ask questions that become science. I was also immediately fascinated by Hutchinson's use of food web theory to demonstrate underlying constraints on diversity. I think it's about as close to a law as we can come in Ecology, and I've always appreciated it. I do, however, agree with Yuguo that Hutchinson likely had the direction of causation between diversity and stability backwards. The Schneider paper is also fascinating because it speaks to an often-overlooked dynamic in food web studies. The movement of biomass around food webs is often non-linear. Examining the dynamics in a biomass context reveals feedbacks and complexity that is not apparent otherwise. I do agree with others, however, that additional realism in the trophic dynamics would have been interesting.
ReplyDeleteThe Hutchinson paper introduced the concept of the Eltonian predator chain that assumes the predator at each level in the food chain is larger and rarer. Based on the energy dynamics at higher trophic levels, the concept of the rarer predators makes sense, but I struggled a bit more with the idea that predators had to be larger than the prey. Examples such as wolves hunting bison seem to violate this Eltonian idea.
ReplyDeleteThe Hutchinson paper concluded with the statement about hoping for a “limited reversal” where “man becomes aware of the value of diversity no less in an economic than in an esthetic and scientific sense.” At the time of this paper’s publication, a lot of the environmental protection legislation had not yet been published. If Hutchinson were alive today, do you think that he would be satisfied with the existing legislation and public attitudes regarding protecting biodiversity?
I was surprised by the results in the Schneider paper, especially those presented in Figure 6. My background in plant ecology is extremely limited, and I had a hard time understanding how the plant biomass was approximately the same in the communities that had very different amounts of animal biomass. The authors offered the explanation that it was because of a favoring of larger plants and a “slowing down of biomass turnover.” Would plants in an actual community adapt to the increased animal biomass in this way?
I really enjoyed the Hutchinson paper for its cohesive readability and logical progression. I appreciated that he was able to discuss multiple complex facets of niche theory and food webs while avoiding dense discussion and big leaps in logic. I think it is interesting to think about the historical progression of community theory in literature we have read up until now in the course. We have graduated from associations and predator-prey systems to food webs! For Schneider et al. (2016) I thought the contradicting result of diversity and animal size with Hutchinson's paper was interesting. I wonder if this result stemmed from the oversimplification of the model, where all animals were omnivores rather than including some strictly herbivorous animals and a top predator or two.
ReplyDelete