Brown J.H. and P. H. Nicoletto. 1991. Scaling of species composition -body masses of North American land mammals. American Naturalist 138-1478-1512.
(We have already introduced Dr. Brown's biography)
Background
Biological diversity compositions are varied in different regions. Biotic compositions in different scales caused and affected by different biological processes such as colonization, extinction, and speciation. As we zoomed in, more micro-ecological processes affect in small patches of homogeneous habitats and species interaction and abiotic environmental effects influence species coexistence, and microevolutionary processes such as natural selection and genetic drift in a population level. Both macro and microscopic ecological and evolutionary processes composite the continental and regional community. This paper analyses the North American terrestrial mammal composition using body size and spatial scale.
The main question: what is the macro compositional pattern of the North American terrestrial mammal based on the body mass across different spatial scale?
Methods: Three different spatial scales of terrestrial North American mammals were used including the entire NA continent with Mexico, 21 biomes (Dasman, 1975) and 24 small patches of homogeneous habitats. The North American and Mexican species list was obtained from Hall 1981 and Ramirez-Pulido et al. 1968. Species list of local habitats were collected from the variety of sources including some unpublished work. Bats were not included. Single body mass was assigned for each species (obtained from field guides (Burt and Grossenheider 1976, Whitaker 1980). Kolmogorov-Smirnov Dn statistic was used.
Results/discussions: Body mass frequency distribution of the entire continent was highly modal and skewed right (465 species median size was 45g). Comparing to the larger distribution, local communities have similar sizes because of competitive exclusion, differential extinction of species of large size with small geographic range, and specialization of energy use and dietary constraints. 24 small patches have a uniform body size. 19-37 species ranged from 100g to 2,500 g. 21 biomes were intermediate which indicated that 20 to 250 g body sized species would not coexist.
Discussion question, why is this pattern useful and how can we use this to deal with current species extinction in North America?
Looking at body size distributions within certain habitats or latitudes may provide insight to better our conservation efforts. Addressing the importance of body size variation may prove beneficial in choosing species to focus conservation effort. Perhaps certain body size distributions are important in different trophic levels to create a stable ecosystem.
ReplyDeleteRandom Question.... But why are bats, pinnipeds, cetaceans, and sea otters excluded from this data? Actually why are bats excluded from a lot of data, not just this article but a few others we have read in the past.
ReplyDeleteI think that is because bats and aquatic animals have such a different ecology (aquatic and volant) that for some anlysis they are difficult to compare with other groups
DeleteAfter reading this paper, I can see why it is such a noteworthy paper in macroecology. Especially after discussion on Tuesday, I appreciated the authors's thoughts on reasons for the mid-body size gap visible in mammals.
ReplyDeleteTo try and answer Angel's question, bats are excluded from a lot of studies that look at body size, distribution, range size, etc. because of correlations between body size, diet, locomotion, range distribution. Bats are generally small bodied but have a wide range of diets, use flight as locomotion, and their physiology of torpor have the potential to produce wacky results from analyses. Marine mammals are excluded because they have very different environmental pressures acting on them and have adapted size and diet strategies that are tailored to those very different environments. Kate talked about how marine life selects for bigger body size due to baby's tolerance for the conditions or remaining safe. Gravity and the associated physiological problems of getting really large are not as strong in a water environment, which allows for the giant mammals like the blue whale. Adding in those mammals to analyses also presents a high likelihood of skewed results.
Are results as accepted by peers if some of the data is pulled from unpublished work? I'm also curious about the simulations and how reliable. I found the implications pretty interesting, especially the introduction of alien species to recover diversity.
ReplyDeleteI found the last comment about using alien species to restore lost diversity as a dangerous statement. I can see the author's point, but there have been many cases in which aliens species have had so many unpredictable impacts that I think if someone not familiar with invasion ecology reads it, and think it is a very simple solution, things might go dangerously wrong and you might have the opposite result of what you want: a decrease in local diversity instead of mantaining it. The idea seems interesting, but has to be taken with caution.
ReplyDeleteI liked how this kind of brought us back to the seven forms of rarity and why we don't see big animals with tiny ranges. I thought it was interesting they hypothesized climate change as a cause for mass mammal extinction- especially the large mammals with small ranges. I would have liked to know what species they thought this may have affected. I also liked that they recognized a potential human impact on future range studies, but am not entirely sure why they thought their data was untouched by humans.
ReplyDeleteBringing alien species into a habitat to restore diversity may not be a great option, like we discussed in class with the cane toad example. If not much research is done, there is no guarantee that the alien species would adapt at all to a new environment. Most alien species do not become invasive, but they can mate with species already there and create offspring with lower fitness.
ReplyDeleteI liked the paper and as Willow mentions, it also remind me of the rarity paper. I think it would be very interesting to see a similar analysis for other regions, specially in the tropics as differences in the scaling of body mass frequency distribution might influence diversity gradients.
ReplyDeleteI found the result that local body size distributions are flat (about equal numbers of all size classes) to be highly intuitive. Any given habitat can support a certain set of ecological niches. Then, as one moves between habitats, the overall size distribution remains the same, but the individual species may change. I would expect this to hold true regardless of continent.
ReplyDeleteI agree with several others that the authors' plan of introducing alien species is a bad one. Time and time again, humans have purposefully (or accidentally) introduced nonnative species, leading to catastrophic changes to local ecosystems. I wonder if the authors are also supporters of rewilding North America...