Introduction

Much of the pasture land area near Palo Verde has been converted into rice fields. Rice must be cultivated in human constructed and managed wetlands. Rice fields increase the amount of wetlands in an area. Wetlands are a significant habitat in Palo Verde, Guanacaste, Costa Rica attracting 40 different species of waterfowl throughout the year.

The quality of water in a wetland is critical to the existence of a diverse community of macro-invertebrates needed to support the numerous waterfowl. We wondered how the water quality in a rice field would compare to the water quality of a natural wetland. Would there be enough biodiversity in the macro-invertebrates in a rice field to support 40 different species of waterfowl?

We selected and examined samples from two areas. We contacted the owner of a rice field, Francisco Boniche. We took soil, water and vegetation samples from his field. This rice field was to be harvested within the next two weeks. We also sampled the natural wetland, Quebrada Mula in Palo Verde.

Macro-invertebrates are an essential part of the food web in a wetland area. Many waterfowl eat them directly. Macro-invertebrates also are useful as reliable indicators, by species and frequency, of overall water quality. We predicted that the Quebrada Mula natural freshwater wetland would have a larger variety of macro-invertebrates than the rice field. We also predicted that water quality measured for turbidity, temperature, and pH would be different at both sites. We thought that turbidity and pH would be higher in the wetland than in the rice field. We also thought that temperature would be higher in the rice field

Methods and Materials

We selected three sites at each area to collect data. Data collection took place on 31 July at the rice field between noon and 1:00 p.m. The data collection at Quebrada Mula took place on 1 August between 8:00 a.m. and 9:00 a.m. We attempted to collect data from areas with water depth between 15 – 20 cm. At each of these sites, we measured the water depth, water temperature, and pH, then collected water samples at the surface and at depth to determine turbidity. We did not to disturb the substrate when taking the deeper sample. We then measured a 1-square meter plot and collected all vegetation samples within the plot by cutting the plants off from the bottom or pulling these up by the roots. We folded the vegetation in Ziplock bags and sealed the bags to prevent loss of samples. In the center of each square meter plot, we collected a benthic sediments sample with a tapered, 18 cm diameter PVC pipe. The pipe was pushed into the sediments no more than 10 cm, and the sample was then emptied into a Ziplock bag.

Upon returning to the lab, we rinsed each vegetation sample with water. The water was collected in a basin and saved for further examination. Each piece of vegetation was closely examined and stalks were cut open in order to determine fauna content inside. The rinse water was examined thoroughly for any macro-invertebrates, as were the soil samples. Macro-invertebrates were preserved in ethanol and counted , and grouped according to class. The results were then entered into a spreadsheet.

Results

Our results are presented in detail in Tables 1 and 2. The pH of both the rice field and Quebrada Mula was in the range of 5.5 to 6.0. Temperature ranged from the high-20’s (oC) to low-30’s (oC) in the rice field to mid-20’s (oC) in Quebrada Mula. The water from the rice field had less sediment than Quebrada Mula where 1 – 1.5 mm sediment was found in all three locations at depth.

The data indicated that similar classes of macro-invertebrates occur at both the rice field and Quebrada Mula; however, the total number of individuals differed sharply. A total of 27 individual macro-invertebrates were found in rice field samples. Insect larvae similar to cut-worms were found located at the bottom of many of the rice stalks. These larvae were the most abundant type of organism found in samples from the rice field. None of these larvae were found at Quebrada Mula. More nematodes were found in rice field samples than those from Quebrada Mula. Spiders were found at both sites in similar abundance. Three small fish were found in the rice field, but none were seen at the other site. One insect, the Solubea pecila beetle, was the only insect found in the rice field. This organism is a major pest for rice farmers in the area.

At Quebrada Mula, we collected a total of 183 individuals. Small crustaceans which rapidly swam in the water were the most abundant individual group with 115 individuals. Thirty-four mollusks were collected at Quebrada Mula, whereas only 4 live mollusks were found in the rice field along with 5 dead individuals. There were also 11, 3 cm-long black leeches found at Quebrada Mula. The 2 similar organisms found in the rice field were beige in color and only 0.5 cm in length. Seven larvae were found at Quebrada Mula, along with 12 clumps of gellatinous material found in the same samples as the mollusks. The insect population at Quebrada Mula was much greater than the rice field. Twelve insects such as ants and beetles were found in our samples.

Discussion

We were surprised at the high water temperature in both areas and the lack of suspended sediments in the water. The similar pH (slightly acidic) at both sites was not what we expected. We initially thought the rice field would more acidic than the wetland because of the chemicals on the field, yet the difference in average pH was only 0.15. We expected to find more macro-invertebrates in the Quebrada Mula. Our data supported this hypothesis; however, biodiversity in an area is based on both the number of individuals and the number of species present. A diverse community should have relatively equal numbers of individuals for each species present. Keeping this in mind refer to Figure 1; it appears that the rice field is more diverse than the wetland because of the evenness of the individual numbers in the different groups. This interpretation is skewed because we were unable to key out individuals to species. Instead, we grouped many different species into the same category. If one compares the total numbers of individuals in the rice field to Quebrada Mula, the natural wetland is significantly higher in macro-invertebrates indicating that the natural wetland is able to support a higher number of organisms.

We also must consider that our collections took place at different times of day, during the rainy season, and near the time for harvesting of this particular field of rice. We suggest that comparative tests on a regular basis during the year would allow a more accurate study of the actual species diversity. The macro-invertebrate individuals need a more detailed taxonomic study, so that a Shannon-Weiner Index can be applied to the number of species and individuals. This would show a more realistic distribution of species and diversity.

The question remains whether rice fields are "bad" habitats compared to the natural wetlands. The rice fields are monoculture, chemicals are used to control pests and weeds, and high amounts of fossil fuels are used in the entire process from planting to harvesting to mulching. Our data shows that rice fields do support a range of organisms including many species of waterfowl which we observed while we collected our samples. We did not see such a wide diversity of waterfowl at Quebrada Mula. But, the essential question remains. What will be the long-lasting effects of the chemicals used in the rice fields for all of the represented species?

Anecdotal Information

During our project, we had the opportunity to work with a local rice grower who willingly let us work in his field. He rents the rice plot, and has been cultivating it for 3 years. A good crop can yield as much as 1.2 million colones (288 colones/dollar, July 1999). There is the potential for two crops per year, but this does not happen very often for assorted reasons. In order to compensate for the uncertainty of annual success, the farmer has a bank loan, insurance, etc. The crop that we examined was planted 23 April and will soon be ready for harvest. One of the problems that the grower has is pests, which are many. One of them is a small beetle, Solubea pecila, that sucks the leaves and stems. These insects can retard the rate of growth in the rice plant. Another pest is the rice rat, which does great damage to the crop. The farmer controls the rat population by keeping rice mounds clean by burning, as well as keeping a boa constrictor for biological control. A pesticide known as Karate is applied when the rice flowers. Other problems include broadleaf weeds, such as coyolillo, tamarindillo and joyunquilo, which were obviously abundant in the field. The fertilizer the farmer uses is 30-0-15 at the first stage of growth, then he applies 15-0-30 for the flowering. The fertilizer and pesticide is applied by airplane. Each flight costs the farmer 30,000 colones, and each crop needs 2 applications of these treatments from the time it is planted to harvesting. The used water in the plot goes into the ditches and canals, then into the Tempisque River. The farmer rents a machine to harvest the rice in the third month.. After the harvest, the plot is prepared for further cultivation by mulching the residues into the soils. There are many expenses in the cultivation of rice by small farmers.
 

Table 1
 

Sample	pH	Temp	Depth	Turb Top	Turb Deep
RP-1	5.5	30	17.5 cm	0	0
RP-2	5.5	31	16 cm	0	0
RP-3	6	29	21 cm	0	0
QM-1	6	27	18 cm	1 mm	1 mm
QM-2	5.5	27	28 cm	0	1.5 mm
QM-3	6	27	20 cm	0	1 mm

Table 2
 

Group	Rice Field	Quebrada Mula
Nematodes	4	1
Annelids	2	11
Mollusks	4	34
Spiders	2	3
Insects	1	12
Larvae	11	7
Crustaceans	3	115
Total Individuals	27	183

 

Figure 1

Literature Cited

"Research at Palo Verde: Past and Future", Schnell, Charles E., Associate Executive Director, OTS, 1994

Essential Question

The activity that we did at Palo Verde can be applied to any ecosystem: temperate deciduous forest, savanna, marine or desert. In our study, we restricted ourselves to a very minimal identification process; however identification could be taken to a much more detailed level. Our study was not detailed enough to perform a Shannon-Weiner Index because of our broad classification procedure, but again with a more detailed classification process this could be done in the classroom. Biodiversity can be studied at any grade level. Our approach would probably be better suited for the middle school classroom. In high school, more detailed classification and statistical analysis should be used.