Research

Mapping Biodiversity in Central America

Results

Taxonomic Level	Costa Rica	Panama	NW Colombia	LCA
Order	6	6	6	6
Family	14	24	25	27
Genus	46	70	84	110
Species	89	129	202	322

The above map was prepared by drawing a square grid over the landscape and counting the number of species/grid cell. The highest species diversity by this analysis was observed the Choco and Tuira basins - a finding that has reported by World Wildlife Fund in their studies (Dinerstein et al, 1997). Maps of each individual species were produced (not shown).

Summaries of counts for each country in the region of study were produced and are shown above. Simultaneously, examination of genetic analysis seemed to indicate that the drainages of the LCA landscape could be coalesced into geographically contiguous regions based on similarity at a population level within species.

Both the Venn diagram and the table show a fall in diversity from south to north. The number of species shared between the various named regions also reflect a turnover in the species composition. This can be further seen by comparing the pie charts for each region showing the composition at the order level.

Roeboides is a primary freshwater fish found in many parts of lower Central America. The 'tree' diagram next to the picture of the fish represents the relationship of fish from the different regions within LCA (colored blocks). The tree is produced by measuring similarity distances between a stretch of mitochondrial DNA 842 bases in length representing the genes ATP synthase subunits 8 & 6. Note that groups of branches occur within a single biogeographic region and that distances between these groups are much larger in comparison.

The situation for Aequidens is similar. However since Aequidens is a secondary freshwater fish, the branches in the Pacific slope that face the Bay of Panama are shorter and not isolated from each other. This suggests that the ability of secondary fishes to cross short distances through salt water has allowed populations in these regions to share genetic information.

Both of these examples do however confirm our work with species counts and identification that there are identifiable ichthyological regions within lower Central America with their own distinct fauna and endemic species.

An interesting independent way we arrived at the same biogeographic region classification was by Detrended Corresponded Analysis. This is an application of multivariate analysis to a matrix of species by area based on the dataset. The results shown here indicate that 33% of the total variance could be explained by the first 3 axis, the most important two of which are shown below. The clustering of the individuals from the various regions indicate that these regions have greater species similarity within them and differences between them.

For further summary of the data, we decided to use these biogeographic regions instead of individual river drainages.

Species counts at various levels of taxonomy were obtained by running an ArcView script --- Species/Region Counter developed for the purpose. This script is available from the author by correspondence.

Endemic species were defined as species found in only biogeographic region within the LCA landscape. Caution was needed in interpreting these results as single misidentifications and erroneously located collection points could give spurious results. By an iterative process of revision and running the ArcView script, a list of endemics for each region were produced..

Area:species curves are an important way of noting how complete the sampling was and consequently, whether actual species counts could be higher than estimated. Preliminary species curves were generated using the ArcView script Species:Effort Curve. It is hoped to modify this script to include statistical testing and curve-fitting to randomized data. The curves show that while Panama and Costa Rica had been adequately sampled (flattening of the curves), NW Colombia did not yet plateau out with sampling. This indicates that the actual diversity measures for Colombia are under-estimated by the numbers in this study, due to inadequate sampling.

Results

Our goal of producing a Data Product: A GIS of neotropical freshwater fish of Lower Central America was obtained.

• The system is established at the Smithsonian Tropical Research Insitute and will be made accessible through the internt in the future.
• 6 Biogeographic regions were identified based on species composition and, we suggest, shared historical biogeography.
• Species counts for countries and biogoegraphic regions were calculated and species lists have been generated.
• A clear pattern of species composition and distribution was evident from the data:
• Species counts fall from East to West. This is particularly true for species known to have evolutionary origins in South America. This suggests that the primary factor explaining current distribution patterns is dispersal of species from parent drainages in South America.
• An opposite pattern emerges for the cichlids and poecoielids - having greater distributional ranges in Costa Rica and Panama. This is consistent with the view that these groups have radiated from basal lineages within Central America.
• Pacific and Atlantic drainages show significant differences in species composition. This shows the strong vicariant nature of the central cordillera - a prominent geographic barrier in lower Central America. It also shows the shorter, swifter nature of the Atlantic drainages that could explain in ecological terms the different composition.
• The fact that there are greater number of shared species where the cordillera is lower further strengthens this point of view.

Conclusions

1. Freshwater fish diversity in lower Central America is largely a function of dispersal from the south and from the north.
2. The bulk of the species have originated from the south. Primary fish versus northern secondary species).
3. Almost no southern species group has crossed over completely to North America.
4. Sufficient evolutionary time has elapsed to enable different biogeographic regions to produce their own endemic lineages. Our genetic data show that in several cases, the genetic distance between these populations suggest that they should be species in their own right.