Blog Author: Elizabeth Chambers
Principles of Ecology
Hairston et al (1960) and Shochat et al (2010)
Citations:
Hairston, N.G, et al. 1960. “Community Structure, Population Control, and Competition.” The American Naturalist. Vol. 94 No. 879: 421-425.
Shochat, E. et al. 2010. “Invasion, Competition, and Biodiversity Loss in Urban Ecosystems.” BioScience. Vol. 60 No. 3: 199-208
Author Background:
Nelson G. Hairston was an American ecologist who worked at the University of Michigan and later at University of North Carolina. He was interested in trophic interactions in ecological communities and was a major proponent of the concept of trophic cascades. Additionally, he served as a World Health Organization advisor for the United Nations, where his research focused on disease dynamics and the factors that contribute to the spread of human disease.
Eyal Shochat is an ecologist currently working at the Ben Gurion University of the Negev in Israel, and is also connected to Arizona State University as an independent researcher. His research largely focuses on how wildlife populations change in response to urbanization and other anthropogenic global change in their habitats, with a focus on bird populations in particular.
Hairston et al:
The authors’ goal was to study what drives populations to be limited in their size. The authors note that previous studies had only focused on single-species populations and argue that it is important to study the community as a whole and to use a multi-species approach in analysis, taking into account the influences from different trophic levels on the system. The paper approaches the trophic levels by dividing organisms broadly into three groups: plants, herbivores, and predators. In modern ecological terms, these may also be described as primary producers, primary consumers, and secondary consumers, respectively.
They observe that (almost) all energy fixed by photosynthesis conducted by the primary producers flows through the entirety of the biosphere and, as such, the totality of all organisms in a community will be limited by the amount of energy that is produced and which flows through the system. For the primary producers, they identify light and water as the limiting factors to their growth.
They argue that herbivores are generally not directly limited by the amount of food available to them, citing the vast amount of green plants and the rarity of these plants becoming depleted indicating that herbivores typically do not over-graze. Rather, herbivores are limited by predation and not resources. They use examples of insect and rodent outbreaks following predator removal as evidence of predation acting as a limiting factor upon the primary consumers. On the other hand, predators and parasites are food-limited, with their population sizes being limited by the number of available prey/host species in their community. Finally, decomposers are food-limited as well due to the finite amount of organic debris in a community. Their overall conclusion drawn from this discussion is that all terrestrial organisms “are resource-limited in the classical density-dependent fashion.”
They go on to describe the impact of these resource limitations as a cause of competition between species. Many different species may inhabit the same trophic level and will have overlap in their niche space. This niche overlap, combined with limited resources, will results in interspecific competition and attempting to out-compete other organisms which will drive natural selection and evolutionary change as species must adapt in order to survive in the competitive environment.
Shochat et al:
This paper focused on urban environments and the impact that urbanization has at the community level, rather than simply focusing on the effect of habitat loss on the species level. Compared to wild areas, urban areas tend to have a higher population density of total animals, but lower biodiversity. The authors wanted to determine what effect interspecific competition for resources had on biodiversity in urbanized environments and how this may explain the loss in diversity in cities. They hypothesize that urban environments have conditions which favor a small number of synanthropic or invasive species which can strongly out-compete other native, non-synanthropic species. The population size of these successful urban species will increase—leading to the higher total community population—while the species that cannot compete in an urban environment will see population crashes—leading to the lower overall species evenness.
Methods: The authors used two Long Term Ecological Research sites, Phoenix, AZ and Baltimore, MD for sources of population abundance data. Areas within the cities and suburbs were classified as urban, and undeveloped lands outside of the cities were classified as the wild habitats. The Arizona portion of the study focused on two different taxa, spiders and birds, while the Maryland portion just focused on birds. They analyzed species distribution rank of these two areas in order to determine the evenness and overall diversity of the communities. Additionally, they begin by tying in a brief review of other ecological papers and theories which back up their hypotheses in this article.
Results: In the wildland of Arizona’s Sonoran desert, species evenness of spiders and birds was greater than in urban Phoenix, while the city had a greater total population density for both taxa. The study of Baltimore’s birds had very striking results following the same pattern, as just over half the total urban bird population was comprised of 3 invasives. Statistical testing showed that all of these results were significant. Optimal foraging studies previously done by Shochat show that common urban bird species are more efficient at obtaining food compared to desert species. This increased foraging efficiency allows the common urban species to out compete other birds and to dominate the city environments.
Discussion: The authors stress the importance of considering species evenness and the community profile of environments rather than looking at the number of species present in an environment. Resources and predation can limit the populations of synanthropic species in the wild which maintains the evenness of the community. In urban environments, these pressures are lessened for the synanthropic species, which allows them to thrive and dominate their communities and competitively exclude others, leading to a loss of evenness.
My Thoughts:
Both papers discussed the biodiversity of ecological systems, limiting factors for populations, and the effects of competition. Hairston et al really stuck out to me as product of its time – many of the general ideas are used in ecology today, but it was presented in a fairly simple manner and broadly lumped organisms into three big trophic levels, without going into detail on the effect of dynamics such as predators which are themselves prey to another consumer. This is a stark contrast to the more complex food webs that are discussed in ecology today. Shochat et al takes a heavy focus on the Anthropocene and how humans are changing communities in vast ways, in contrast to the more “hands-off” observational way that Hairston et al describe ecological communities.
I enjoyed reading Hairston et al. because of how they argued for their ideas using a combination of inductive and deductive reasoning. However, this approach made me somewhat confused about the basis of some of their claims, e.g., predators are resource limited, since as far as I understand there is no logical reason that predators are necessarily limited by resources, rather, it may be that the systems that people tend to observe seem to have predators which are limited by there resources. I think Elizabeth raised an interesting point about considering communities as food webs rather than food chains as Hairston et al. did, but I'm unsure how that might change their conclusions. Is lumping different species into four trophic levels an appropriate level of abstraction?
ReplyDeleteI liked how mechanistic Shochat et al.’s approach was in their consideration of bottom-up and top-down regulation. I think it’s a good demonstration of how general some ecological concepts are, and that these concepts can be applied to understanding novel environments.
- David
Both papers made for a good read. The Shochat et al. paper especially made for an interesting look into conservation of species diversity in urban (i.e. human dominated) environments. At first, I was a little curious when Shochat et al. initially stated that urban areas are often characterized by higher animal densities and diversity than wildlands (even if by chance). I would never associate an urbanized area with either of these characteristics (other than humans and their pets in terms of density). As I read on, it began to make a bit more sense to me why people would think that way. Urban areas provide some animals with a lot of resources (e.g squirrels running around a college campus always seem to be well feed and not afraid of students for obvious reasons). I think the paper was interesting because it attempted to address steps that could be taken to preserve diversity in urban areas (e.g. the bird feeders with special squirrel deterrents on them). I am just slightly unconvinced things such as this could make a dramatic impact when it comes to increasing diversity, especially given the authors' description as urban areas expanding out from a central area. I would have liked to see more of a comment about the true toll of urbanization on areas and how it needs to slow down, but this is a personal bias. My one question:
ReplyDeleteWhat if birds in urban areas are better at foraging because there are more available resources compared to the resources available in the desert? Overall, I enjoyed reading the paper. Good pick.
The Hairston et al. paper was interesting as well but mostly because it seemed to divert from previous papers we have read. Instead of focusing on how full a niche could get, it focused on what limits population size and introduced the idea of interspecific competition. It was a short paper, but a good one.
As others have said, while reading the Hairston paper it occurred to me that grouping all organisms into a few broad categories (producer, herbivore, etc.) might be too much of a simplification. However, I think that when this paper was written these ideas were so new that it was necessary to simplify species interactions in this way. Now, we know more about variation between species of the same trophic level and we know that species don't necessarily belong to a single, discrete trophic level. This insights, plus the increased modelling power that has come with advances in computer technology, allow us to make more nuanced hypotheses than those presented by Hairston et al.
ReplyDeleteThe Shochat paper was an interesting read. I wonder how differences in the ultimate source of energy influence difference between natural and urban areas. In natural areas, basically all energy ultimately comes from the sun and is made available to the ecosystem via plants and other producers. Although much energy in urban systems also comes from the sun, it is much more dependent on humans and not necessarily directly from plants. A good example of this is food waste that birds might eat, such as french fries or sunflower seeds. Although these energy sources originally came from plants, they were not produced in that area. They were brought there by humans.
I think Hairston’s paper elaborated the limits of a community pretty well, although I had a hard time to understand what the authors mean “escape from control by the weather” at the very beginning. It is nice that among the classic papers we’ve gone through, this is the first paper that includes the decomposers into the big picture of a community dynamics. I agree with David that predators may not be limited by resources. I think predators are rather limited by the energy cost and gain during predation.
ReplyDeleteThe companion paper brings up a very interesting topic. I do feel a little concerned about comparing the species richness between desert and urban ecosystem. In my understanding, desert is not an ecosystem with high diversity but with some specialized species. It will make more sense to focus on some desert communities rather than the richness per se.
The Hairston work provides an overview of many of the ecological concepts taught in introductory level biology courses. The idea that herbivores are seldom food-limited seems to be supported by the complementary paper (Schneider 2016) from Monday’s class, which found through modeling that there was a consistent plant biomass despite increased animal biomass in the community. With our current understanding of ecosystems, some of the concepts about trophic levels don’t quite hold up because we know that species can span multiple trophic levels, but as an overview of general ecological principles, Hairston’s work still seems relevant and applicable.
ReplyDeleteThe study of urban birds in the complementary paper is interesting and seems to be supported in general observations of bird communities. The idea that the invasive species out-compete native species is clear especially in the cases of birds like European starlings and house sparrows. These two invasive species are so common that they are highlighted on the Audubon website as one of the 15 most common “backyard” species (https://www.audubon.org/news/get-know-these-15-common-birds). The invasive generalist species are an example of how the interspecific competition among members of the same trophic levels that Hairston discusses in his paper can play out in real-life scenarios.
I thought it was really neat for Hairston to give accumulation of fossil fuels vs. energy capture during photosynthesis as the starting point in illustrating energy flow and trophic effects on other organisms. I think that the “meteorological catastrophes” he refers to, that in 1960 may not have caused exceptional destruction to plant communities, are starting to have much more dramatic effects today that will influence entire food chains (e.g., the large-scale mortality events of a given tree species or genus that have occurred following intense multi-year drought). His statement that species occupying the same level within communities cannot avoid competition seems to contradict the idea of niche partitioning we have discussed in previous weeks – he briefly addresses that niche diversification can occur as a result of competition at the end, but I still think the over-simplification of his concept of trophic levels is perhaps one of the weaknesses of his argument.
ReplyDeleteThe Shochat et al. paper provided a very interpretable example of what I think we are seeing, not only in urban ecosystems, but in other ecological communities worldwide. Invasive species have numerous adaptations that make them able to outcompete/displace native species (earlier leaf-out in invasive plants, more generations per year in insects, lack of co-evolution with plant defenses allowing for more available food resources in herbivores). In the urban case, as Stella and Kat both mentioned, invasive species have an even greater advantage over native species that is facilitated by humans increasing food resources/foraging opportunities – perhaps native species might be less likely to take advantage of these.
I thought the Hairston paper gave a pretty fair overview of food webs, but made a couple of broad assumptions on the way that were not necessarily justified. For example, he talks about how all predator populations are always resource limited because territoriality is the only other limiting factor but it does not fit the story well enough. We know now that non-consumptive effects also reduce prey populations and that many carnivores are limited not by resources, but by factors like search and handling time (e.g., functional response). I also don't think I'm convinced that herbivores are rarely limited by food or competition. Lastly, I thought the "control by weather" concept that he brings up several times was too vague. Did he mean differences in climate at the end of the herbivores' range? Did he mean environmental stochasticity, like storms, causing high mortality?
ReplyDeleteShochat et al. was interesting and brought food webs into a very modern perspective with a common contemporary problem, which is that urban areas have reduced biodiversity and stamp out native species at high frequencies. They talked about food webs in the same framework at Hairston, so I agree that this was a good pick for a companion paper. I wish they had acknowledged other urban factors that can limit species abundance and biodiversity, such as habitat fragmentation, but that was probably a bit out of scope for a food web paper.