Stevens, G.C. 1989. The latitudinal gradient in geographical range - how so many species coexist in the tropics. American Naturalist 133: 240-256.
Blog author: Maria Goller
Blurb author: David J. Currie
Professor, Department of Biology, University of Ottawa
B.S. and PhD from McGill University
Dr. Currie wants to understand global patterns of biodiversity and the environmental variables that influence these patterns. He is especially interested in how human influences (climate change, habitat destruction, and pesticides) affect communities.
Paper author: George C. Stevens
I couldn’t find any information on George C. Stevens other than that he was part of the Department of Biology at Gustavus Adolphus College in Minnesota.
Main Points:
1) Rapoport’s rule can be applied to a variety of organisms
2) Rapoport’s rule can explain the pattern of more species at lower latitudes
Background:
There are more species at lower latitudes
Rapoport described a pattern in 1975 that species have larger ranges at higher latitudes
Assumptions:
Length of a species’ range (north-south) rather than total area is a better indicator of the range of climates experienced
Main Question: this was less of a hypothesis-driven paper and more of a descriptive paper detailing an observed pattern.
Methods
Data:
Species richness and range data from a variety of papers
Methods:
Plotted occurrence/range data for different taxa across latitude
Looked for correlations
Results:
· Rapoport’s rule: the geographic range of organisms decreases with latitude
· Pattern holds true for North American trees, marine mollusks, freshwater and coastal fishes, reptiles and amphibians, and mammals.
· This pattern also can be applied to tropical organisms
· Exceptions to Rapoport’s rule reinforce the pattern:
o Migratory bird breeding ranges do not fit Rapoport’s rule, and should be expected to do so
o Other organisms with migrant behavior should also be expected NOT to follow the rule
· Argues against what others have called exceptions to the latitudinal gradient in species richness
· Organisms at higher latitudes must have greater tolerance to a wider range of climates (and are therefore lessrestricted to a particular area)
· In the tropics, organisms are more specialized for a particular microclimate (and therefore morerestricted in area)
· Because tropical species are less tolerant to climatic variation than species at higher latitudes, the landscape appears more heterogeneous in both microclimate and species
o Populations easily “spill over” into unsuitable habitat
o Do not compete directly with species endemic to these unsuitable habitats
o Therefore, these spill-over species add to richness without establishing themselves there
o (In brief, none of these habitats are closed systems, and foreign species enter and exit)
· Rapoport’s rule should not be linked to competition
Questions and Comments
I am intrigued by his idea that, in many areas, tropical species are only found in immigration-sustained populations rather than in self-sustained populations.
For this to be true, there would have to be regular, extremely frequent dispersal of accidentals.
How many individuals are in these immigration-sustained “populations”?
I would like to see empirical evidence to support this idea.
While I enjoy the “old school” style of papers that are largely thought experiments, they sometimes also frustrate me.
This was an important paper to trigger future studies to investigate this theory further. I think one factor that encourage larger ranges for higher latitudinal species is the ability to withstand changes over time. Natural selection favors the ability to withstand climatic changes and tolerance to extremes. Over time species in in temperate areas die out as habitats and climate changes while those with tolerance can expand their range with these extinctions or range reductions. So overtime these tolerant species develop large ranges. And this would be more likely to occur in higher latitudes because with more extreme seasonality, climatic tolerance would be more selected for than in lower latitudes.
ReplyDeleteI agree with Alex that a major point in this paper was linking range sizes and their pattern with latitudinal gradient to climatic factors that are seen with the gradient. I appreciated the authors's attempt to explain what part of these climatic gradients affect the gradients seen in both species richness and range size. I also like that the author called out the point that previous research collected data and tried to make their data fit to Rappaport's Rule, rather than the other way around. This is something that could have more discussion, as this is a very easy way to insert biases into research, by looking for data that you want to fit a certain pattern.
ReplyDeleteI appreciate how much easier this paper was to read than some of our previous pieces. Echoing Alex and Devra, I thought the evolutionary basis for the range differences was interesting, and makes a lot of sense. I noticed that some of the range slopes (Figs. 1 & 4) are greater than some that taper off (Figs. 3 & 5). Does this also have to do with climatic tolerance?
ReplyDeleteI agree with the previous comments about how organisms living in more variable climatic conditions should have more physiological tolerance than those living in less variable conditions. I wonder if it has been tested, as some tropical species living in high elevations may be able to face drastic climate changes too, so maybe an issue regarding isolation is to be considered too.
ReplyDeleteAlso, as Maria said, it also caught my attention this idea about how tropical populations might be sustained by the "recue effect". I also wonder if it has been tested.
I was also very intrigued by the idea that the mechanism linking smaller range sizes and higher species richness is the dispersal ans establishment of individuals from suitable patches into surrounding unsuitable habitat, resulting in an higher local diversity. Im wandering if there is any empirical data supporting this as an important mechanism for species richness in the tropics.
ReplyDeleteThe author didn´t mentioned it, but I think that another possible mechanism for an effect of range size on diversity, would be an increased beta diversity with decreasing geographic range extension.
This is another good paper that explains the latitudinal gradient in species richness with Rappoport's rule. The language is easier to follow and the points were systematically laid. Also, all the counter-arguments were explained and judged/compared with the phenomenon. While reading this paper, I have been comparing the species richness in Siberia vs Amazon. The importance/use of this paper is to understand the biogeographical patterns of species distribution and their conditions (tolerance/sensitivity to climate) to generate predictions for conservation management of each area under different possible climatic conditions. Great paper!
ReplyDeleteBecause of my academic interests, I of course advocate putting a little paleo in everything, so I liked that he alluded to evolutionary history as a driver. I thought it was neat he tried to synthesise all of the previous work under this “rule” and addressed where he fell into pitfalls.
ReplyDeleteIn the result section where you wrote "these spill-over species add to richness without establishing themselves there". I'm a little confused about what happen to these spill-over species when they get to unsuitable habitats. How do they survive without establishing themselves?
ReplyDeleteI enjoy how straight forward this paper is compared to a lot of the papers we have read in the past. I appreciate Alex for clarifying why certain latitudes affect climate tolerance. Are migratory birds the only exception to Rapoport’s rule?
ReplyDelete