Cost Rican Frogs

Tropic and microhabitat niche overlap in two sympatric Dendrobatids from La Selva, Costa Rica

by Rodrigo Cajade, Eduardo Federico Schaefer, Marta Ines Dure & Aurturo Ignacio Kehr
Cuaderna de herpetología published in 2010
http://www.scielo.org.ar/scielo.php?pid=S1852-57682010000200002&script=sci_arttext (web)
http://www.scielo.org.ar/pdf/cherp/v24n2/v24n2a02.pdf (pdf)

Dendrobates anuran (DA)

Oophaga pumilio (OP)

 

Introduction

Since I am moving to Costa Rica in August, I thought it might be interesting to learn about some of their amazing herptofauna! This article studied Dendrobates auratus (DA) and Oophaga pumilio (OP) in the La Selva Biological Station. These Dendrobatidae species that live in wet tropical and subtropical forests of the Americas of which Costa Rica is a perfect study location. DA and OP exist sympatrically, which means that they live in the same geographic region, and they both specifically like a leaf litter habitat. As with many areas of the rain forest, these species are being threatened by habitat loss. These gorgeous frogs have more in common than their beautiful, bright colors, but they are both diurnal and throughout many areas of Costa Rica new populations have been introduced for the sole purpose of tourism.


La Selva Biological Station in Costa Rica

Methods

Researchers in this study worked in a secondary forest at the biological station from March 2-17, 2008 and captured specimens by hand during the daylight hours. They would flush the frog’s stomach contents (preserved in 5% ethanol) for prey evaluation, and then they measured the frog’s body length and mouth width. Pictures of each specimen collected were taken (dorsally and ventrally) which allowed for individual specimen collection and as a way to prevent repeat collections of the same frog throughout the study. Where the frogs were found was a given a microhabitat designation with (1) wet ground covered with ferns, (2) leaf litter associated with the base of cacao plants and trees, (3) anthills, and (4) under fallen tree trunks. This allowed for a volume of the stomach prey items to be calculated, as well as an “importance factor” for each prey type to be calculated. Many other mathematical models and programs were used to quantify the data like Levin’s index, the EcoSim program, and Pearson’s correlation coefficients.

Results

In the established time frame for this study, researchers found 14 of each frog species. There were several incidences of specimen recapturing. The diets of the species were analyzed.

OP at 17 different prey types as shown by the stomach content analysis, and its favorites were formicids (ants), acarines (mites & ticks), and collembolans (springtails). The OP specimens had an average body length of 20.41±1.06mm and mouth width of 6.23±0.42mm. Mostly these frogs liked microhabitat (2) which is high in moisture, low in sun exposure, and has a wet ground surrounded with ferns.

DA showed 14 different prey types with the same favorites at OP. The DA specimens were larger than the OP with an average body length of 25.36±4.25mm and mouth width of 7.47±1.25mm. DA used all the microhabitats with 55% in (1), 12.5% in (2), 17.5% in (3), and 15% in (4). Even though DA was present in all of the microhabitats most were found in (1) which is an area with lower moisture, more direct sunlight, and with leaf litter around the cacao trees.

Even though these two frog species are found in the same area based on this research their niches do not really overlap. It ended up that both these species inhabit larger niche than that have been previously reported. These authors hypothesize that this is due the fact that DA and OP seek out the different prey than was mentioned in previous studies. Their niches, though, are still limited by their prey choices (considered to have low diversity index), but supports that both DA and OP are active foragers until they find a colony where they may sit-and-wait forage to take full advantage of the opportunity of available prey. Throughout this study researchers noticed that the same frogs would return to certain insect colonies to forage.

Many Dendrobatidae specialize their eating to ants and mites, which are associated with the alkaloids (why they are poisonous) in their dermal skin glands to ward off predators. This study’s data supports diet as the environmental source of alkaloids for both DA and OP. With the “importance factor” that was used in this study the calculation found that formicids were the most important for DA, but OP liked acarines the most. These shows have these frog species specialize on certain prey.

Conclusion

Overall, this study demonstrates that DA and OP do have overlapping niches, but they use their microhabitats differently so they do not compete. However, this lack of competition could be due to the large amount of resources in the area, under the niche complexity hypothesis. This article provides valuable information about these frog’s habitats that are continually being threated. There is much more that can be learned about Dedrobatidae frogs in Costa Rica. Personally, I cannot wait to see these species in real life and take some pictures of my own.