The survey method and first results

The first few months of 2012 were spent investigating the forest, learning to identify the Riodinids and studying their habits. Then a survey method had to be chosen. It had to be not overly complicated to do, e.i. no excessive collecting or measurements, but still produce informative and reliable results. The most adequate appeared to be a Pollard transect count (Pollard, 1977) modified by Caldas and Robbins (2003). The protocol is walking a fixed transect through the study area for a predetermined time and recording the species and numbers observed, the modification being that you can net individuals for identification purposes. This step was important for the difficult to identify Riodinids like the brown and white Nymphidium-Synargis-Juditha group  and the red and orange Mesene. A 150 m existing path through the woods was chosen for the transect. This path traveled the length of the stream and crossed over to the opposite side, continuing for half the length of the site. The remaining 1/4 of the area was too difficult to survey, being swampy in part and with a very closed vegetation over the rest.

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Practice surveys were done during the last two weeks of April, 2012. After several trials I found that the time to walk the transect was 40 minutes. Everything looked in order, so I started the survey on May 1, 2012.  Two separate walks were made each sample day. The first between 9am and 11am and the second between 11:30am and 1:30pm.  Except during the dry season the second daily survey almost always added a few species not seen during the first run, but care was taken not to duplicate counts. The counts of the most common species seen in the two daily surveys were averaged or the highest count used. Due to the small size of the butterflies and the habits of many, the effective observation radius was about 2 meters. After each daily survey the data was entered on an Excel spreadsheet with the species names in columns and the days in rows. Each entry was either 0 for not observed or the number of individuals seen. Every month a new sheet was started.

Fig.1  Below is a summary of the number of species seen each day in the first year. The sampling rate was 26.6 days/month; 319 days were sampled. An index of  the number of species observed/sample days was calculated for each month to compare trends between the months.excel_days_2012

 

Fig.2     All the raw data of the numbers of species observed per day.yr_1_Page_2

Fig. 3    Species/month. The Y axis is the monthly mean of species observed.yr_1_Page_3

Summary

For the whole 12 month period 78 species were observed with 19 (24%) as singletons. Combining singletons and doubletons, 32% of the species were rare. Running the data in EstimateS (Colwell, 2013) to estimate species richness, a Chao1 mean of  108.08 (CI 87.53-172.95) was calculated. From the Sobs and Chao 1 we can estimate that only 72% of the species in the local assemblage were observed, 30 species went undetected. Considering the intensive sampling effort, 30 undetected species appears high but then again only 1 hour and 20 minutes were spent sampling on any given day, just 10% of the daylight hours and the subjects we are dealing with are very good at concealing themselves.

Species richness increased from May, the early part of the rainy season, until it peaked in the middle of September and then declined and stabilized during Oct. and Nov. A further drop in the daily number of species occurred in December which coincides with the beginning of the dry season. The lowest levels of richness were seen in the dry season (Jan.-Feb) and this low level continued through the next months as the rainy season began. Only a few species reproduce and maintain populations in the forest throughout the year, immigration, emigration and extinction drive the fluctuations in species richness.

 

References

Caldas, Astrid, and Robert K Robbins. 2003. “Modified Pollard Transects for Assessing Tropical Butterfly Abundance and Diversity.” Biological Conservation 110: 211–219.

Colwell, R. K. 2013. EstimateS: Statistical Estimation of Species Richness and Shared Species from Samples , Version 9.1, Persistent URL <purl.oclc.org/estimates>.

Pollard, E. 1977. “A Method for Assessing Changes in the Abundance of Butterflies.” Biological Conservation 12 (2): 115–134.

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