Visualizing AmphibiaWeb Data with Continuous Cartogramsby Michelle S. Koo, Vance T. Vredenberg, Joyce Gross, Carol L. Spencer, Tate Tunstall, David B. Wake
Visualization is a powerful tool that conservation biologists must harness to not only better understand the patterns of decline themselves but to effectively convey the information. Applying AmphibiaWeb data and those of collaborators to density-equalizing cartograms sheds new light on global amphibian diversity and decline. These new perspectives will hopefully present the data in more intuitive and compelling fashion for scientists and the environmentally-aware public alike.
Global Amphibian Diversity
A cartogram of the world's amphibian diversity (above) is a marked departure from the conventional map of the world. The countries of the northern latitudes disappear while countries of Central and South America reveal the concentration of amphibian species diversity. In particular, Brazil (833 species), Colombia (747), and Ecuador (484) lead the world in total number of amphibian species. China (358), Mexico (359), and the United States (295) are almost equal in size; similarly, Madagascar (244) and Australia (227) are equals in the eyes of the cartogram. India (296) and Sri Lanka (109) are prominent in their region. Indonesia (315), Papua New Guinea (308), Malaysia (217), and the Philippines' (104) large number of amphibian species lends weight to Southeast Asia, not a region easily emphasized at the global scale.
Using Global Amphibian Assessment (GAA) Status Data
The cartogram above displays data from the Global Amphibian Assessment, a partner of AmphibiaWeb, which tracks the IUCN Red List of Threatened Species for the status of amphibians. Eight categories are assessed: Extinct, Extinct in Wild, Critically Endangered, Endangered, Vulnerable, Near Threatened, Least Concern, and Data Deficient (see WWW.GLOBALAMPHIBIANS.ORG details; for criteria, see WWW.REDLIST.ORG). Threatened species are considered to be Critically Endangered, Endangered and Vulnerable.
Applying percentage of threatened species per country to a cartogram emphasizes Central America, the Caribbean, and northern South America where threatened species make up to 93% of native amphibian species as in Haiti. Adjacent Dominican Republic has over 86% of its amphibian species threatened. El Salvador, Costa Rica, Colombia, Honduras, Mexico, and Guatemala have 28% to 55% threatened, respectively. Countries of the Mediterranean are notable for their prominence in percent of threatened species; Turkey and Greece lead the region (35% to 23%, respectively).
The cartogram draws attention to another area of concern: South East Asia, where the Philippines have 49% of its amphibians listed as Threatened or Endangered, and Sri Lanka has almost 70%. Throughout the region, there are elevated levels, from Malaysia (23%) to India (28%). Places such as Burma (Myanmar) and Thailand have low values possibly due to lack of data.
The cartograms above illustrate the differences in the distribution patterns of the three orders of amphibians. Anurans (frogs and toads), the most affected of amphibians, are also the most numerous, and species are concentrated in the New World tropics. In sharp contrast, the Caudata (salamanders and newts) are concentrated in North America. The legless order of Gymnophiona (caecilians) are less well known and distributed mainly in the tropics of South America and India.
New Species Discovery
The amphibian taxonomy / decline paradox: It might seem counterintuitive that with increasing documentation of amphibian extinctions and population collapse, the number of new species being discovered continues to rise. Since 1985 when Frost's Amphibians of the World was published, well over 2,000 new species have been discovered, which represents a 50% increase in total recognized species. The cartogram above illustrates where the new species have been discovered in the most recent years. The top four countries for new species discovery are Brazil (68 new species), Papua New Guinea (46), Sri Lanka (45), and Peru (41).
What is a Cartogram?
Here, a cartographic technique is used to visualize the most up-to-date data compiled on amphibian species, their geographic distribution, and endangered status using continuous or density-equalizing cartograms. We use the diffusion-based method of Gastner and Newman (2004) in which area is purposely distorted in proportion to an attribute while maintaining recognizable topology. Areas of high value diffuse away from areas of low value until the density is equalized uniformly thus distorting national boundaries while maintaining zero flow between boundaries. This allows us to recognize countries even though their shape is distorted.
To implement the cartograms, ArcView 3.3 and ArcInfo (ESRI) were used as well as a windows program by Jesse Adams and tested against a Java applet by Frank Hardisty.
Why Use Cartograms?
Cartograms present geographically relevant, numerical data in an efficient diagram. Recognizable political typology and proportional distortions are balanced to achieve an effective cartogram. Usually applied to social and economic data, we present cartograms of biodiversity data to highlight the state of amphibian diversity, decline, and discovery specifically for use in conservation planning and assessment. While amphibians do not recognize political boundaries, conservation planners and biologists necessarily do.
Cartograms such as these reflect political realities. Diagramming biodiversity data
by country demonstrates as much about the state of knowledge as the effort being
conducted in biodiversity work. For example, countries that are expected to have a
larger representation of species diversity, such as Peru and Bolivia in South America
and countries of the African Congo basin, reflect a deficiency of data rather than
low biodiversity. The New Species cartogram reflects where in the world field and
taxonomic work is being done. Cartograms are a novel way for conservation biologists
to gauge the effectiveness and balance of field work, data on amphibian biodiversity,
and conservation policy on a global scale.
Gastner, M.T. and M.E.J. Newman 2004. Diffusion-based method for producing
density-equalizing maps. Proceedings of the National Academy of Sciences of
the USA. 101 (20): 7499 - 7504.
Updated January 7, 2010, by Kellie Whittaker