Are the Smallest Organisms the Most Diverse? (Kenneth P. Dial and John M. Marzluff)
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| Kenneth P. Dial University of Montana 1988-2008 UM Flight Lab Founder and Director - 105 Publications - 4,021 Citations - His research revolves around the evolution of animal design, locomotor development, & life history and biology. - No longer taking graduate students he still conducts research in his lab. |
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| John M. Marzluff Professor of Wildlife Science College of the Environment University of Washington - 184 Publications - 10, 391 Citations - Avian Conservation Lab - He studies the effects of forest fragmentation on the nest predators of the endangered Marbled Murrelet, and a variety of songbirds in Washington and birds overall response to urbanization. |
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| Alison G. Boyer - Commentary Department of Ecology and Evolutionary Biology Yale University - currently an Assistant Professor at the University of Tennessee |
Prevailing Theory and General Belief
The smallest organisms are usually the most speciose within a taxonomic category.
- Defended by May, R.M. in his paper "The search for patterns in the balance of nature: advances and retreats" in 1986. But according to the data May presented, the distributions show that the smallest body-size category is never the most abundant.
Definitions
Subunit - taxonomic category (i.e. "species") counted within a unit (i.e. "genus")
Assemblage - an entire collection of units and their subunits
Methods- Collated data on 46 assemblages, including mammals, birds, fish, reptiles, amphibians, plants, insects and marine invertebrates.
- The relative body size of the taxonomic unit showing the greatest diversity within an assemblage: the # of units with a smaller average body size divided by the total # of units then multiplied by 100. This produces a scale from 0-100%.
- Assumptions of the calculations: the range of body sizes measured within an assemblage represents the true range.
- According to the data presented in Figure 1, there is no monotomic relationship between body size and taxonomic diversity.
- Dial and Marzluff show that the most diverse unit was larger than 38% of the unit's in it's assemblage.
- (50%) of the assemblages were dominated by a unit 16-40% larger than the smallest unit in the assemblage.
- The authors go on to point out that sampling area, assemblage size, and taxonomic affinity had no affect on the relationship with dominant size.
Discussion
- The results were consistent with May's original research, but not with his conclusion.
- It is not believed that body sizes and taxonomic diversity are randomly related within an assemblage. 50% of the most diverse groups fell within a narrow size range (relative size of 16-40%) which was twice the number expected by the authors within this body size range due to chance alone.
- Dial and Marzluff present the question: Given the presented figures, why are not all assemblages dominated by small-to-medium-sized taxa?
- The fossil record and recent extinctions suggests that a larger body size can be a liability during unstable environmental conditions but can become an asset in a more stable environment.
- Figure 2 summarizes the affect of environmental fluctuation on small and large taxa.
Discussion Questions
1. Do you think 46 assemblages was enough to get a clear picture of diversity?
2. Were there species that you think should have been included?
3. Why might a variable environment effect the extinction rates of larger taxa vs the extinction of smaller taxa in a stable environment?
Comments
2. I think that plants only being represented by pine trees is a gross underrepresentation. There are many more plant assemblages that have very different trends and patterns in plants. I doubt it would have changed the overall result, but it was more reasonable to not include the pines and just talk about animals than include the pines and attempt to say this is true across all macro life.
3. Large taxa benefit from stability across the landscape because they need to have large amounts of stable resources to survive over hundreds of generations. Smaller taxa are more adaptable, but are generally outcompeted by larger taxa when conditions are to large taxa’s benefit.
3) Yep, resources. This is something I've been thinking about with my dissertation, with size selection in bird communities and who survived the Pleistocene megafaunal extinctions. Medium-sized raptors and scavengers survived, while the largest species went extinct.
One thing I liked about this paper, and generally about the papers that we're reading now, is that they build directly on the foundation of earlier papers that we read. It makes sense that in groups with larger body size generally, the most frequent body size will be larger, and in smaller groups the dominant size will be smaller (going back to what is the ideal body size). For example, in Artiodactyls, larger genera dominate since these herbivores need to achieve a certain size to get enough nutrients.
2. One word: MICROBES!! Estimates of microbial diversity are much higher than that of plants and animals. Unfortunately, with microbes there's still a lot of problems with undescribed species, unresolved taxonomies, and how to define "body size". I am not sure that it's yet possible to do a similar study with microbes but I think it's an important group of organisms to consider for this topic.
3. I agree with Kevin and Oona but I am wondering if that pattern holds up once microbes are included....
2. I'm sure our herp specialists will have something to say about the turtles-only reptiles category, and, as Kevin pointed out, pine trees may not be the best representatives for Plantae. Mammals seem pretty well-represented to me - some of the largest terrestrial and marine groups are there, as well as Rodentia and Insectivora on the small end.
3. I agree with Kevin and Oona. Animals with a larger body size have a higher metabolic rate, as well as longer generation times, which may reduce their ability to adapt to a variable landscape compared to small taxa. Also, in a stable environment, large taxa may outcompete small due to the relatively large energy requirements of small taxa.
2. I was happy to see Cypriniformes included, as they are an extremely diverse order of fishes. However, I would have liked to see that trend continue in the selection. For example, it would have been nice to see how this is expressed in Perciformes.
3. Increased variability in habitat and resources disproportionally impacts larger taxa, as Kevin stated, so it’s not surprising that they are affected by a lack of stability in their environment. Smaller taxa are better equipped to adapt to a variety of habitats and resources, but they are at a disadvantage when conditions are stable. During unstable conditions, the dispersal abilities and life history traits (i.e. lifespan) of smaller taxa are apparently advantageous for survival.
3: Larger taxa depend on stability and thus are disproportionately affected by a variable environment. This makes sense because larger taxa usually take longer to mature and produce less offspring (k-selected) so they will take longer to recover from changes to their environment. On the other hand, smaller taxa don't take as long to mature and produce more offspring (r-selected) so they can recover more quickly from variations in their environment.