This paper concerns itself with population control. The authors begin at the bottom, so to speak, looking at the accumulation of fossil fuels vs the rate of energy fixation by photosynthesis. They conclude that the rate of photosynthesis far exceeds the build-up of fossil fuels and that the vast majority of energy fixed by autotrophs must therefore pass through the biosphere. Next we look at the decomposers, which the authors conclude, must be food limited, as by definition they exist to degrade organic debris. Even if some decomposers are instead limited by predation, population density etc, the remaining decomposers must consume whatever is left by those non-food-limited decomposers, and as such, as a group, the decomposers are food-limited. Furthermore, any population that is not food-limited must be limited by the limitations of the level below them.
Looking next to the primary producers. Since large herbivores are not decimating populations of plants, nor are plants controlled by natural catastrophe, they must be limited by abiotic factors in the environment. Light would be the most obvious, but in arid environments, the authors note, water would likely be a limiting factor. In connection, herbivore populations clearly cannot be food limited, as they will continue to grow out of control when protected by man, decimating the plant population of the area. Furthermore, weather does not seem to be a limiting factor, unless one supposes that populations of herbivores are not capable of removing themselves from an area that has been badly effected by weather. These assumptions leave the control of herbivore populations to predation, up to and including parasitism.
Interestingly, as predators would likely be immune to the limitations by weather and other abiotic factors that herbivores and producers are immune to, we must again assume that predators are food-limited as a group. The authors note that some predators are arguably territory-limited (although that in itself is limited by food availability).
The authors thereby reach the conclusion that terrestrial communities are resource-limited. We see the reasoning behind this by examining plant/herbivore interactions. Plants, being in competition for space, must exist in such an area where they are not regularly depleted by herbivores. Herbivores in turn, must be limited by predation in order to not deplete their food source. Interspecific competition is also a consideration, as any link in the web occupied by multiple organisms reduces the effects of predation on either.
Looking next to the primary producers. Since large herbivores are not decimating populations of plants, nor are plants controlled by natural catastrophe, they must be limited by abiotic factors in the environment. Light would be the most obvious, but in arid environments, the authors note, water would likely be a limiting factor. In connection, herbivore populations clearly cannot be food limited, as they will continue to grow out of control when protected by man, decimating the plant population of the area. Furthermore, weather does not seem to be a limiting factor, unless one supposes that populations of herbivores are not capable of removing themselves from an area that has been badly effected by weather. These assumptions leave the control of herbivore populations to predation, up to and including parasitism.
Interestingly, as predators would likely be immune to the limitations by weather and other abiotic factors that herbivores and producers are immune to, we must again assume that predators are food-limited as a group. The authors note that some predators are arguably territory-limited (although that in itself is limited by food availability).
The authors thereby reach the conclusion that terrestrial communities are resource-limited. We see the reasoning behind this by examining plant/herbivore interactions. Plants, being in competition for space, must exist in such an area where they are not regularly depleted by herbivores. Herbivores in turn, must be limited by predation in order to not deplete their food source. Interspecific competition is also a consideration, as any link in the web occupied by multiple organisms reduces the effects of predation on either.
I'm impressed by the universality of the HSS 1960 paper. By viewing energy as the ultimate resource in the tradition started by Lindeman, they are able to come up with the sweeping statement that "populations of producers, carnivores, and decomposers are limited by their respective resources in the classical density-dependent fashion." Still, this conclusion seems to fit well with Hutchinson's emphasis on spacial limitations. I appreciate how Hairston, Smith, and Slobodkin treat predators and parasites as part of the same group.
ReplyDeleteI agree with Julie that the conclusion reached by the authors seems to fit well with Hutchinson's emphasis on spatial limitations, and also can be applied to his questions of how/why there are so many species. I like the simplicity of the conclusion that authors Hairston, Smith, and Slobodkin make regarding interspecific competition. They say that this must exist between the three main trophic levels that they define (producers, carnivores, and decomposers) in order to control all of these populations. I also enjoyed reading their statements about herbivores and how they are seldom food-limited (instead are predator-limited), and so are likely to compete for common resources. This makes me think of the amount of large herbivores present in the African savannah, and provides some explanation for how they can coexist.
ReplyDeleteI like the emphasis this paper places on the importance of energy transfers and trophic level in determining population size. Resources are generally limiting in most systems and thus interspecific competition is a key driver of population size. I do think that intraspecific competition is important here as well in regulating population size in the context of resource limitation.
ReplyDeleteThis article speaks of the importance of energy transfers through trophic levels in a system with each population of consumers being limited by the resources/food available to it. This creates an interspecific competition for these available resources. The exception to this resource limitation is herbivores which are instead limited by predation and therefore do not exhibit resource competition. Since herbivores are limited by predation not resources vegetation is only limited by light exposure and water allowing vegetation to flourish, this is known as the green earth hypothesis. There are exceptions mentioned however, in cases where predators have been reduced as in Kiabab deer, where they are allowed to increase in numbers until they become resource limited creating what is known as trophic cascade. When a predator is removed from a system the lower trophic level is allowed to increase in size due to its lack of predation causing its resource to become depleted this in turn causes the next lower trophic level to increase with its resource then becoming depleted and so on.
ReplyDeleteIn my opinion, the one thing missing from the HSS 1960 paper is intraspecific competition as a limiting factor because competition for space is not exclusive to interspecific relationships. For instance, old stands of ponderosa pines will out compete most of the younger saplings for resources (light and water). On the other hand, a clear cut can change the stand's dynamics. If the second growth saplings come back in high numbers, then the competition is initially reduced. Each new sapling will grow until intraspecific competition limits resources, like water and light, and thereby limits growth. We can see this example in the Jemez Mountains, where 40-50 year old trees are vast in number and stunted in growth.
ReplyDeleteGood observation Sami. I also had trouble following the argument that consumers are generally predator-limited, rather than weather limited. In fact it would seem that weather, rather than predators or resources, could control populations at any trophic level in some cases. Extreme high or low temperatures can cause mortality that limits populations, and yet temperature is neither a resource nor a predator. Perhaps this is more an exception than a rule though.
ReplyDeleteI was also wondering what the reference to the Kaibab deer herd was.
Overall I really like this paper for the simplicity of it's logic and generality of it's scope.
I agree with Nelson et. al when stating that it is very hard to separate out trophic levels because many species occupy each level and there is a lot of overlap. He relates this to inter-specific competition and how this can be a "powerful selective force." My only question is: we tend to not really think of plants competing for resources with animals, but could we look at water and availability and question if plants compete with animals for water? Trees take up tremendous amounts of water, could trees be limiting population sizes of larger animals?
ReplyDeleteThis is an interesting consideration. We could probably come up with other examples of competition between distantly-related organisms for various resources.
DeleteBut animals produce metabolic water by eating the plants.
DeleteBut animals produce metabolic water by eating the plants.
DeleteCommunity Structure, population control and competition. The authors provided a summary of a wide range of common observations seeing in nature for understanding control of populations within communities . The framework used to define population control in natural communities are interactions among member of very tropic level (e.g, competition, predation). The author's shed lighted on how trophic level members are limited by their resources using the concepts of density dependent.
ReplyDeleteI felt like this paper was almost Fermat-like. It has a lot of really good examples and it makes sense but I assume there has been a large amount of work done to try and prove these concepts with actual evidence. I can see why it is considered a very important paper.
ReplyDeleteAfter reading the HSS 1960 paper again, I have a couple of questions: 1) does resource-limited always mean density-dependent? 2) What's so special about herbivores? Aren't most predators also limited by predation?
ReplyDeleteTo answer one of your questions- as the authors point out, organisms that are limited by predation (ie. herbivores) are not as influenced by interspecific competition. You can have multiple herbivores in the same area eating the same food without issue. However, carnivores are highly affected by interspecific competition, which may be why they tend to have functional territories, or divergent niche space. Since carnivores are subject to competition, we can reason that they are food-limited.
DeleteNo idea what's so special about herbivores. This would be fun to discuss.
To answer one of your questions- as the authors point out, organisms that are limited by predation (ie. herbivores) are not as influenced by interspecific competition. You can have multiple herbivores in the same area eating the same food without issue. However, carnivores are highly affected by interspecific competition, which may be why they tend to have functional territories, or divergent niche space. Since carnivores are subject to competition, we can reason that they are food-limited.
DeleteNo idea what's so special about herbivores. This would be fun to discuss.