Paper 40 - K. Sullivan

Paper 40

by Kaitlyn Sullivan

The latitudinal gradient in geographical range - how so many species coexist in the tropics. American Naturalist 133:240-256.

Blog by Kaitlyn Sullivan

Paper Author: George C. Stevens

(Commentary by David J. Currie)

David J. Currie

·       BSc McGill University

·       PhD McGill University

·       Full Professor for the Biology Department at the University of Ottawa

Author Biography/Research Interests

“The goals of Dr. Currie’s research program are: 1) to identify broad-scale patterns in the distribution, abundance and diversity of life; 2) to determine which environmental variables exert the strongest control on those patterns, and 3) to determine how human activities influence them. Recent research focuses on influences of climate, habitat conversion and pesticide use on these properties of natural systems.”

David Currie. (n.d.). Retrieved February 07, 2017, from https://science.uottawa.ca/biology/people/currie-david

George C. Stevens

·       BSc University of Iowa

·       PhD University of Pennsylvania

·       Has worked within the scientific community for over 30 years

Author Biography/ Research Interests

“George has worked his entire adult life to foster collaboration that results in new knowledge and creates new funding for efforts that yield broad societal benefit. First as a scientist (Ecology, Entomology, Botany, and Biogeography funded through government grants and foundation awards), later as a fundraiser for science-related organizations (museums, research institutions, and universities), and later still as a leader of broad-based philanthropic organizations that depend on collaboration among donors to create change (community foundations and their on-line analogues).

He remains engaged as a scientist through his service on the Board of the International Biogeography Society, his Associate Editorship at Diversity and Distributions (a journal dedicated to academic research in conservation biogeography), and through the fieldwork associated with a 40-year longitudinal study of tropical trees.”

Stevens, G. C. (n.d.). George Stevens President/CEO The GoodCoin Foundation (Linkedin Account). Retrieved February 07, 2017, from https://www.linkedin.com/in/georgestevens

Paper- Summary/Main Points:

The purpose of the is paper is to introduce “Eduardo Rapoport’s observation that species’ latitudinal ranges tend to be broader at high latitudes than in the tropics”.  In other words, species latitudinal ranges become smaller at lower latitudes, but become larger at higher latitudes.  Stevens coined this correlation “Roapoprt’s Rule”.   Furthermore, Stevens suggested that the reason Roapoprt’s Rule is observed is because the average climate change in the tropics is much smaller than average climate changes at higher latitudes, is the reason at all.  He went on to propose that “Roapoprt’s Rule is yet another way of looking at the latitudinal gradient in species richness”.

Hypothesis:

Stevens hypothesized “that a largen number of “accidentals” (i.e., species that are poorly suited for the habitat) occur in tropical assemblages.  The constant input of the accidentals artificially inflates species numbers and inhabits competitive exclusion”. 

However, Stevens goals for this paper did not lie in predictive models or testing a hypothesis.  Rather his interest focused on interpreting correlations between:

·       Diversity

·       Range Size

·       Latitude

Methods

Using data originally presented by Eduardo Rapoport, Stevens attempts to justify his claims that “the ranges of organisms decline with declining latitude”.  According to Stevens, an underlying reason for this is may be fluctuations in climate, stating that “climatic variables show a rather simple relationship with latitude’.  I feel Stevens best summed this up when he wrote that “the tolerance of any individual organism must span the range of conditions to which it is exposed throughout its life. The large latitudinal extent of high-latitude organisms is a simple consequence of the selective advantage to those individuals with wide climatic tolerances, tolerances that are needed for the successful exploitation of any given high-latitude location. For tropical organisms, individuals that have wide climatic tolerances derive no great advantage”.

 Results/Conclusion

Stevens concluded his paper by stating that, “The latitudinal gradient in species richness is paralleled by a latitudinal gradient in geographical-range size called Rapoport's rule.  It is suggested that the greater annual range of climatic conditions to which individuals in high-latitude environments are exposed relative to what low-latitude organisms face has favored the evolution of broad climatic tolerances in high-latitude species. This broad tolerance of individuals from high latitudes has led to wider latitudinal extent in the geographical range of high-latitude species than of lower-latitude species”. 

Questions

Has anyone attempted to prove or disprove the hypothesis Stevens presented at the end of his article?

Has Rapoport’s rule been tested with more recent data sets?