Mechanisms: Population Dynamics and Interspecific Interactions (Week 10)
Chapter 8
Brown, J.H. 1995. University Chicago Press, Chicago.Brown begins the chapter with discussing some general population-level mechanisms. In order for us to understand the mechanisms that drives species-level patterns it is key to understand population interactions. Where we find species and how abundant they are depends upon the environment, the interactions with one another, and the interactions with other organisms in their habitat.
Brown then goes into a couple studies done on North American bird species. The first study was done on North American breeding land bird populations using BBS data. What Brown and colleagues found was that some species increased in certain strata while they decreased in other strata and vice versa. These findings he attributed to Gleasonian Individualism because species in the same genus or subfamily differ enough in niche requirements that their response is completely individualistic to spatial and temporal variation in the environment. The second study he discusses the effects of climate change and food supply on wintering bird populations using CBC data. What they found was that nest predation/brood parasitism on the breeding grounds, deforestation on the wintering grounds of species that winter in the tropics, severe weather on the wintering grounds of species that winter in temperate NA, and other unmeasurable factors, all effect the decade-long trends seen in these populations of songbirds.
The population dynamics among mammals is very similar to those of birds. Brown gives us the Dobzhansky-MacArthur hypothesis which is the tendency, as species diversity increases, for biotic interactions to become increasingly important in determining the abundance, diversity, and ultimately the diversity of species. This hypothesis offers us an explanation that entails all the other patterns seen in population dynamics.
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The Evolution of Ecological Specialization in Southern African Ungulates: Competition or Physical Environmental Turnover?
Codron et. al.
OIKOS vol. 117 no. 3 p. 344-353 March 2008
The authors of this study asked what environmental an organismal changes were taking place in the mid-late Pleistocene grassland ecosystems that allowed for the southern African grassland biome present today. They thought it could have been due to environmental conditions or by competition between other ungulates. They chose three study times in the mid-late Pleistocene: the Cornelian, Florisian, Holocene/Modern. The sites they obtained fossil records from were the Cornelia-Uitzoek, Florisbad, and the Kareepan respectively. From the fossils and current species tooth enamel was obtained to run C13/C12 and O18/O16 radioisotope dating. This was done to determine whether they were grazing on C3, C4, or a mix of the two, and to tell whether they were competing for resources that were abundant. What they found was that the climate at the time these species were thriving was the main driver in determining their range. However, competition, facilitation, and predation all play roles in the organisms range and existence. The climate drove many grasslands to become primarily C4 and in turn this caused many ungulates to switch from browsing on C3 or a mix, to primarily grazing on C4. However, some species were incapable of adapting to C4 which lead to their extinction or were able at the time to exploit the available C3 plants that were left. The primary concern that comes from this study is that the continuous increase in CO2 will possibly drive many species in African grasslands extinct due to competition or due to the inability to adapt to a C4 diet only.
From reading the chapter, I wonder at what measure of diversity does abiotic factors matter more than biotic factors?
ReplyDeleteI found it incredibly surprising that the red-eyed vireo responded similarly across different regions. I would have expected every population to behave differently.
Another study that would be cool, although I don't know how you'd set up the experiment, would be to look at how plastic species' interactions are/have been through time. It would also be interesting to test if past extinctions (not the mass extinctions) are more correlated with climate change or increased diversity.
Some context: Elisabeth Vrba, proponent of the habitat theory of macroevolution and the resource-use hypothesis, also studies African ungulates, so the findings of this study reflect strongly on her work.
ReplyDeleteUnfortunately, I have had a family emergency (it includes my mother and her inability to ride a motorcycle), so I don't think I'll make class today.
ReplyDeleteI found the article quite interesting, and the chapter doubly so. I wonder how the modern classification (or rather, not so modern now with the advent of molecular phylogenetics) is effected by the differences seen in populations such as those discussed in the chapter. Would these animals, that differ in between the strata, be classified as different sub-species? Or simply as different populations. I've always been confused about this.
The idea of parasitism was also interesting to me. Do the abiotic factors that effect a host also effect the parasite, or are the parasites independant of these changes as long as the host is available?
I'll try to make it back to UNM as soon as possible, but if I don't make it, I'll see you all tomorrow.