Paper 31: Brown 1984
On the Relationship between Abundance and Distribution of Species
Blog Author: Lam Le
Blurb Author: Christy M. McCain
-Ph.D., University of Kansas, 2003
-Associate Professor & CU Museum Curator of Vertebrates
-Research interest:
“I am interested in the mechanisms producing and maintaining patterns of species distribution, abundance, and diversity. To address these processes, I consider three levels of ecological organization to be equally important: species-level autecology, population-level dynamics, and community-level processes and interactions. My research so far has highlighted small mammal range dynamics, abundance patterns across altitudinal ranges, and species richness patterns along latitudinal and elevational gradients.”
Paper Author: James H. Brown
-Distinguished Professor of Biology at the University of New Mexico (2001)
-Received a bachelors with honors at Cornell University in 1963 before obtaining his PhD at University of Michigan in 1967.
-Awarded the Robert H. MacArthur Award by the Ecological Society of America for his work towards metabolic theory of ecology (2005)
-Focus on 3 aspects of ecology: population and community ecology of rodents and harvester ants in Chihuahuan Desert, large scale questions relating to distribution of body size, and metabolic theory of ecology
Summary/Main Points:
Main Question: Is there a pattern of spatial variation in abundance within the area in which a species normally occurs?
-Background: Although abundance and distribution are known to be correlated, further research is needed to investigate the pattern over the range of spatial scales from local populations to entire geographic ranges of species.
-Assumptions:
1) Abundance and distribution of each species are limited by the combination of physical and biotic environmental variables
2) Environmental niche variables are spatially autocorrelated
3) Closely-related species differ in no more than few niche dimensions
Methods:
-Plot variation in population density over ecological gradients within local region (figure 1)
-Plot density as a function of distance along four transects through the widest part of the range in four major compass directions (figure 2)
-Look at r to determine correlation between abundance and distributions
Results:
-Distributions of rarer species exhibit more sampling error. As sample size increases, plots appear smoother. Distributions for common species resemble normal distribution, differ from random distributions.
-Abundance decreases toward northern or southern boundaries.
-Closely-related species that have the highest population densities have wider geographic ranges.
Species that are rare have restricted spatial distribution.
Conclusions:
-There is a gradual decline in abundance from the center to the boundaries of geographic ranges of species with exceptions include: abrupt changes in abundance and multimodal patterns of abundance created by patchiness
-There is a positive relationship between abundance and geographic ranges.
-Suggests normal probability density distribution as a useful model of spatial variation in abundance, with some exceptions.
Comments/Questions:
-I like how the author attempts to find a general pattern for abundance vs distribution and also acknowledges exceptions.
-This paper analyzes a lot of data already been gathered, with most of them concentrated on birds and plants. Why is it difficult to gather quantitative data on organisms other than these? This paper was written in 1983, so I’m hoping a lot more data has been obtained since then.
I think working with birds and plants is much easier than mammals/insects/amphibians/reptiles because plants can't run and hide and birds are easy to locate both by visual and oral tactic so they are good groups to begin with. Other groups of animals would require more people and more time to locate species to determine ranges and abundance. But because plants and birds are very different in their dispersal ability, they would be a great comparison to find patterns or differences in patterns based on dispersal ability and strategies. But I think these are important concepts to understand from a conservation standpoint.
ReplyDeleteI can see why this is a paper that has lead to so many more ideas and research over the years. Brown lays the foundational theoretical work, but encourages further modeling and application of his theories. I did find his writing style hard to read, as he uses a lot of compound sentences that follow his string of thought. Combined with a lot of citations and figure references, it was hard for me to follow the text.
ReplyDeleteI like how with a seemingly simple theory for explaining two general patters, Brown is able to find ways to relate fields such as biogeography, community and population ecology.
ReplyDeleteRegarding the first observation and its explanation, it is interesting to see how ecological niche models have provided an other way to explore these issues and test wether the position of a site in the niche space (not geographical) of a species defines its density. In fact there is a current debate about the generality of this pattern from an ecological niche model perspective (https://rethinkingecology.pensoft.net/article/24827/download/pdf/).
With respect to the explanation of the abundance-geographic range extension relation, its hard for me to understand why achieving higher densities under certain conditions necessarily implies positive fitness under a broader range of conditions.
I found it interesting how he supposes that if his ideas might have some impact, they would help to develop new models that would take into account multidimensional niche effects in the distribution of species. And basically, those are the Ecological Niche Models that have been developed in the last few years. I wonder if he would feel comfortable with the limitations of those models, also related to the fact that we still do not know the specific constraints that different environmental and biological factors might have in each taxa, including those analyzed by Niche modeling, so many assumptions have to be made in those cases.
ReplyDeleteI might be mistaken, but did you mention in a previous class that this idea was debunked or something? That species density is actually equally distributed in their region? Also, are there other studies that explore this theory using different species? I'm curious whether there would be any changes.
ReplyDeleteThe importance of this paper is that presenting the pioneering find of conceptual pattern "density is greater near the center of the range and declines, usually gradually, toward the boundaries". This general foundational informaiton helps many modern biological, ecological and biogeographical research by giving a hint to other unique patterns of population and species distribution density, and abundance. However, we have to remember that most of this pattern shows normal distributions that focus on the middle but most evolutionary interesting scenarios usually happen on the edge of the distribution. Therefore, we cannot ignore the boundaries. Especially, we are now living in a very interesting time period of earth's history that we have to expect the most unexpected.
ReplyDeleteWhile his arguments are logical and seem to be supported, it’s difficult to distinguish between the figure results and statistical artefact. They are all significant, but the best fit lines seem arbitrary and the r values seem to barely fit. I was working on my assignment 1 before reading these and had dismissed plots that had similar shapes to some of these. When is an r value too low or is there a rule of thumb for significant trends?
ReplyDeleteI didn't particularly enjoy this paper. One of the sentences I disagree with is: "As a consequence of their multiple niche requirements, almost all species have highly restricted distributions". North American birds are good examples of species that are not "highly restricted". For many species, distributions have an ebb and flow (they aren't rigidly defined and change slightly from year to year).
ReplyDeleteI thought the author explained what was supposed to be a simple theory but the way he wrote it... there was a lot of fluff or more so a lot of thing that made it harder to read. Also did we talk about the idea that Wide spread populations are large numbers of small groups in various locations and that they are suitable, basically being trapped into a certain area for survival means in earlier this semester?
ReplyDelete