1 Niche Partitioning between Eisenia foetida Lumbricidae and native earthworms in Monteverde, Costa Rica Christine Askham Department of Biology, Gustavus Adolphus College ABSTRACT With an increased susceptibility to invasive species due to deforestati on, exotic species in tropical rainforests are quickly gaining potential to alter these already disappearing ecosystems. This study examined whether the exotic earthworm Eisenia foetida Lumbricidae has the potential to become an invasive species. Niche preference for level of organic soil content, level of soil texture, and humidity level was tested between native and exotic species in Monteverde, Puntarenas, Costa Rica. There was a significant difference for organic soil content level between native e arthworms and E. foetida . This significant difference was present both when the exotics and natives were tested alone and in each other s presence. Native earthworms avoided high organic content while the exotic earthworms preferred it. No significant d ifference between preferences of native and exotic worms was found for varying levels of soil texture. Both native and exotic earthworms were most abundant in the low soil texture level when tested alone. Native worms resided an average distance farther from the water source than exotic worms . RESUMEN Con una creciente susceptibilidad a especies invasoras debido a la deforestaciÃ³n las especies exÃ³ticas estÃ¡n ganando rÃ¡pidamente potencial en los bosques tropical para alterar estos ecosistemas. Este estu dio examino si la lombriz exÃ³tica Eisenia foetida Lumbricidae tiene el potencial de convertirse en una especie invasora. Preferencia de nicho, con respecto a contenido orgÃ¡nico del suelo, textura del suelo, y humedad se comparÃ³ entre una especie exÃ³tica y una nativa en Monteverde, Puntarenas. Las lombrices nativas evitan contenidos orgÃ¡nicos altos, mientras que las exÃ³ticas lo prefieren. No hay diferencia por selecciÃ³n de textura del suelo, ambas prefieren el mÃ¡s bajo. Las lombrices nativas se movieron di stancias mÃ¡s largas de la fuente de agua que las exÃ³ticas. INTRODUCTION With 350 million hectares of tropical rainforests already deforested Lamb et al. 2005 and the worldÂ€s supply of rainforests disappearing at an alarming rate, conserving the remainin g forest is essential. Deforestation leads to an increased amount of edge effects and fragmentation, which make a forest vulnerable to exotic species Hendrix and Bohlen 2002. Hendrix and Bohlen 2002 found that exotic earthworms successfully invaded h abitats that were already occupied by native earthworms after a disturbance in soil or vegetation. With rainforests less resistant to change due to deforestation, exotic non native species will be more likely to become invasive. Eisenia foetida also known as the California Red Worm, California Red Wriggler, or Red Worm is an exotic earthworm native to Africa now commonly used in Costa Rica for vermicomposting worms decomposing organic material. E. foetida is
2 found throughout the Monteverde region i n home composts, gardens, and has been found in the forest Lapachin 1996. Many local farmers have acquired E. foetida on their land unknowingly because the worm eggs are in the compost the farmers use as fertilizer. The concern with this exotic speci es is whether or not the presence of E. foetida influences the functions of native earthworms in forest habitats. Because these exotic earthworms have been observed outside their original habitat compost they have the potential to become invasive. Earthw orms are critical components to soil systems because they influence the resources available to plants via nutrient cycling Lavelle 1987. A change in the earthworm composition could result in a change in nutrients available to plants, and, consequently, alter the flora and fauna of the forest. For instance, vermicomposted soil yielded an increased growth, sprouting, and survival rates of Mulberry plants compared to plants in untouched soil Sharma 1999. Gaechter 2004 found that native earthworm specie s richness increased with increased E. foetida abundance, but she also admitted this could be due to external factors. If E. foetida does increase species richness, niche partitioning must allow multiple species to coexist using different resources. Nati ve and exotic species can coexist by partitioning available resources in the soil Hendrix and Bohlen 2002. Coexistence may occur because the native worms and E. foetida use different resources Lapachin 1996. Certain earthworm species are known to have different soil type preferences and according to Lavelle 1993 type of soil system can be indicated by earthworm community structure. Different preferences by different species can be a result of genetics obligate niche partitioning or an adaptation to the presence of the other species facultative niche partitioning. Facultative niche partitioning was observed when the earthworm Enchytraeus albidus impaired the growth of E. foetida at high temperatures Haukka 1987. The purpose of this study is to i nvestigate if E. foetida and native earthworms demonstrate niche partitioning by preferring different levels of organic soil content, humidity level, and soil texture. If E. foetida does not negatively affect native earthworms, both worms types should dem onstrate different preferences within organic soil content, soil texture, and humidity level because the exotic and native worms occupy different niches. Therefore, either preference for soil type, water content, or organic matter content will differ betwe en the natives and exotics when tested alone, or when the two types of worms are put in the same habitat. METHODS This study was conducted between October 19 and November 16, 2007 in Monteverde, Puntarenas, Costa Rica at Meg s Stables; the property of Me g LaVal. The study was performed under a tin roof without walls with access to E. foetida and forest soil. One kilogram of E. foetida was obtained from the San Luis coffee co op and enclosed throughout the duration of the experiment in a covered wooden box. Native earthworms were dug up from the forest edge of the near the EstaciÃ³n BiolÃ³gica de Monteverde. Native worms were enclosed in an aquarium covered with a wooden plank filled with the soil they were dug up in. Native earthworms displaying the m orphological
3 traits of Eisenia foetida were not used in the experiment. Only worms longer than three cm were used in the experiments. Four distinct native morphospecies were identified; a species with a white stripe below the head, a grey species, a specie s that twitched and wiggled when touched, and a small white species. The small white species died in the aquarium and were not used in the experiments. Native species were randomly taken out of the aquarium for experimental use disregarding morphospecies. Organic Soil Content To test worm preference for organic soil content three different levels of organic soil matter were used; high, medium, and low. High consisted of compost from the San Luis coffee co op. Medium consisted of leaf littered soil foun d on the forest edge in Monteverde. Low consisted of soil found on a small leafless but grassy area next to the forest. The three soil levels were then layered in eighteen different 250 mL containers with a 6 cm by 6 cm base. There were six total combin ations of soil layer order Table 1 to control for earthworm preference for a certain location within the container. Three sets of these six combinations were used; one set for native worms only, exotic worms only, and native and exotic combined. Five wo rms were placed on the top layer of the six native and exotic trials. Three natives and three exotics were placed on the top layer of the last six aquaria. I returned after 24 hours separated, counted, and identified the worms in each level. I repeated this procedure one time. Humidity Level I tested for a possible humidity preference using a humidity gradient created on a flat tr ay, 40 cm in length, with paper towels that wicked water from a source at one end a plastic trough with water. Using 45 centimeters of double layered brown paper towel I marked eight sections five centimeters apart starting with 0 at the source and endi ng at 40 cm. Once the water line reached the 30 cm mark I would remove the paper towel from the water source to slow the leaching. Then, one worm was set on each of the eight segments. If a worm tried to leave the paper towel I would nudge it with a sti ck. After five minutes the section in which each worm was located was recorded. I repeated this procedure eight times for both native and exotic worms using a total of 55 native and 62 exotic worms. To determine level of humidity on each section I used a moisture meter with a scale of 0 driest to 4 wettest. Layer 1 2 3 4 5 6 Top High High Med Med High High Middle Med Low Low High Low Med Bottom Low Med High Low Med Low Table 1: Order of organic soil content layering. Each number 1 6 is a different container with varying levels of organic soil content; high compost fr om San Luis coffee co op, medium leaf littered soil on forest edge, low soil from grassy area near forest edge.
4 Soil Texture The experiment for soil texture preference used the same methodology as the experiment for organic soil content except three soil texture levels varied instead of organic content levels. Low soil texture consisted of all clay obtained at an excavation site about 100 meters up the trail at Meg s Stable. Medium texture soil consisted of the clay soil mixed with 250 mL of granules measuring less than 2 mm in diameter mixed in for ea ch 1145 mL of clay soil . High soil texture was the same as medium treatment except the granules were between 2 mm and 6 mm in diameter. Correct sized granules were obtained by sifting dirt collected on the roadside with sieves. Analysis A contingency table analysis was used to determine whether exotic and native species alone or together randomly choose soil texture and organic soil content. A t test was performed between native and exotic preferences for distance from water source. RESULTS Organi c Soil Content The native and exotic worms tested separately showed a significant difference in their preference for organic soil content level X 2 = 22.9, df = 2, P<.0.001; Figure 1a. Exotic worms were most abundant in high organic content 55% wh ile native worms avoided 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% native exotic Earthworm type Percent found in each level A 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% native exotic Earthworm type Percent found in each level high medium low B Figure 1: Organic soil preference for native and exotic earthworms a alone N = 43,N = 60 respectively and b in the presence of the oth er N = 51, N = 53 respectively. Percentage of total native and exotic E. foetida earthworms found in each layer of organic soil; high coffee compost, medium soil from leaf littered forest, low soil from grassy forest edge. Both when the earthwo rms were together and separate was there a significant difference between organic soil matter preference Chi squared test, df = 2, P<0.001.
5 Figure 2: Percent of earthworms found at each distance interval along a water gradient from the water source at 0 cm. Native N = 55 and exotic E. foetida N = 62 were tested separately. Significant difference in distance preference was found between native and exotic earthworms t test, df = 115, P = 0.006. the high organic content 9.3%. Native and exotic worms tested in each other s presence also showed a significant difference between preferences of each worm type X 2 =37.8, df = 2, P<0.001; Fig.1b. Exotic worms were again most abundant in the high organic content level 73% while natives were the least abundant 13.7%. Humidity Level There was a significant difference between preferences of exotic and native worms t test = 2.80, df = 115, P = 0.006; Fig. 2. The mean di stance of the exotic species 12.0 cm was closer to the water source than native species 18.3 cm. The highest abundance in one segment of the exotic worms was 44% of total exotic worms found 2.5 centimeters from the source while the highest abundance o f native worms was 18% of total worms 17.5 centimeters from the source. Measuring with the moisture meter resulted in a distribution of moisture content ranging from 0.9 2.75. No moisture gradient was observed within the parts of the paper towel in whic h the water had leached. The only gradient was found between the source 2.5, middle of paper towel 2.0, and end 0.9. 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 Distance cm Percent of worms native exotic 0
6 Soil Texture There was no significant difference between preferences of exotic and native earthwo rms in either the alone or together trials Fig. 3. Although both the exotic and native worms, occurred in apparently higher frequencies at the low texture soil when alone 40.3 and 57.6% respectively, the frequency was not significantly different from t hat observed in other soil types. When the exotic and native worms were tested together, the medium texture level appeared to harbor the lowest proportion of worms for both types Fig. 3, but this frequency was not significant. Twenty seven percent of t he exotic worms and 26% of the native worms were found in the medium level. When the exotics were tested alone they were most abundant in the low level 40.3%, but when tested with native worms the most abundant level changed to high 46%. DISCUSSION In order for the exotic earthworms to not have a negative impact on native earthworms as Gaechter 2004 claims, the two types of earthworms cannot be found in the same niche. 0% 20% 40% 60% 80% 100% native exotic Earthworm type Percent found in each level A 0% 20% 40% 60% 80% 100% native exotic Earthworm type Percent in each level high medium low B Figure 3: Percent of exot ic Eisenia foetida earthworms compared to percent of native earthworms in each level of soil texture; high clay with granules less than 6 mm, medium clay with granules less than 2 mm, and low clay after 24 hours. A exotic N = 59 and native N = 57 worms were tested separately. No significant difference was found between texture level preference between exotic and native worms Chi squared test, df = 2, P >0.05. b exotic and native worms were tested in the presence of the other. No significan t difference was found between preference type between native and exotic worm Chi squared test, df = 2, P > 0.05.
7 Differences in preferences between native and exotic earthworms were evident between levels of organic soil matter, but not within soil texture or humidity level. The exotic and native earthworms preferred different levels of organic soil content. Niche partitioning explains the exotic earthworms preferring the hi gh organic soil content and the natives preferring the low and medium organic soil content. The fact that more than one native species was tested makes this exotic worm preference even more conclusive. The E. foetida organic soil preference for high organ ic matter is present even compared to preferences of many native species. This difference in preferences between natives and exotics remained the same when the earthworms were tested in each other s presence. Because the preference stayed the same, obli gate niche partitioning is evident. This niche partitioning between organic soil levels could be a significant finding because E. foetida are transported between countries for vermicomposting and arrive to their new destinations already in compost of high organic matter Yach 1999. Since the exotic earthworms prefer this high organic matter, they would be expected to remain in the compost rather than travel outside of composts and gardens and invade the forest. Although natives earthworms and E. foetida do portray niche partitioning within the controlled realm of organic soil content, in nature many more variables such as pH and temperature can drive an earthworm away from a certain soil type. All of these variables must be taken into account to determi ne if E. foetida will remain in the compost or travel into the forest. No niche partitioning within soil texture is evident between the exotic and native earthworms. Without niche partitioning, competition is likely to occur between native and exotic wor ms within the same soil texture. Exotic worms compete for resources with native worms resulting in decreased native worm abundance. For example, Hamilton et al. 1988 found the presence of E. foetida decreased cast stability and weight gain in anot her earthworm species Lumbricus terrestris . Competition can change soil profiles, nutrient and organic matter dynamics Hendrix and Bohlen 2002 and ultimately affect entire ecosystems. Because habitat alteration leaves forests more vulnerable to invas ive species, deforestation could lead to a change in forest dynamics if E. foetida do co exist within the same soil texture as native worms. Having no significant differences between soil texture preferences does not necessarily mean exotic and native wo rms prefer the same niche and compete against each other. It is possible the experiment does not provide enough variation between textures to be deemed different niches. Testing for soil texture preference should be repeated with natural varying soil tex tures. Despite lack of a moisture gradient on the paper towel, a difference between average distances from the water source still existed between exotic and native worms. This significant difference is not due to changes in humidity level but could be d ue to differences in activity levels. The exotic worms resided an average distance closer to the water source than the native worms. The exotic worms appeared to be more active than the native worms. The native worms set on the dry parts of the paper tow el tended to curl into a ball and not move at all. The more active exotics when set on dry paper would crawl around and may be more likely to find the water saturated portions.
8 According to Yach 1999 worms need to be wet to remain active. It could be p ossible the native worms need to be wetter in order to be active and, therefore, remained stationary. Even though the average exotic worm distance from the water source was closer, this difference could actually be the result of the native worms requiring more water in order to be active. The more even distribution of the native worms on the paper towel Fig. 2; could be explained by the native worms not moving from the spot they were set on. The activity of the exotic worms is evident by forty four perce nt of the exotic worms found in the section closest to the water source. If no worms moved from their original spot, only 12.5% would be expected in any given section. Also, the more even distribution of native worms in the humidity test could be the resu lt of testing multiple native species at the same time. Each of these native species could have different preferences or activity levels. If the exotic worms are more active than the natives, the threat of E. foetida to become an invasive species is of co ncern. If a recently degraded forest area comes into contact with native and exotic species, the higher amount of activity of the exotic worms would allow them to invade the forest faster than the native worms. Competition between natives and exotics wou ld prevent natives from inhabiting the forest. Although niche partitioning between native and exotic earthworms is evident in regards to organic soil content; many other factors need to be evaluated before the impact of E. foetida on native earthworms can be determined. More research needs to be done on the other factors that drive a species to exist in a certain location such as pH, temperature, available nutrients, etc. Because niche partitioning is only found within one variable so far, E. foetida need s to be used with caution. Since E. foetida has been found in the Monteverde forest, the threat of invasion is still of concern. ACKNOWLEDGEMENTS I would like to thank Karen Masters for her guidance, advice, and support with this project. Thank you Jor dan Cohen for allowing me to steal this project idea from you. This project wouldn t have been possible without Meg LaVal s help obtaining worms and permission to conduct my study on her property, thank you. Thank you Julia and Sadie for tolerating my al ways entertaining sleep talking. Also, I am forever grateful to Alan Masters, Pablo Allen, and Taegan McMahon for doing all they could to keep me healthy in this minefield of tropical diseases. Gracias Jos Ã© Luis y Juan Jos Ã© y mi amigo especial para habla r conmigo cuando estaba trabajando con mis lombrices. Finalmente, quiero decir gracias a mi familia; Vilma, Gerardo, y Lilli JimÃ© nez por sus comida, casa, y chistes. Muy interestante. LITERATURE CITED Gaechter, L. 2004. Colonizing limitation of the composting earthworms, Eisenia foetida. CIEE Tropical Ecology and Conservation Spring 2004.161 168. Hamilton, W.E., D.L. Dindal, D.M. Parkinson, M.J. Mitchell. 1988. Interaction of earthworm species in sewage sludge amernded soil microcosms: Lumbricus terrestris and Eisenia fetida. The Journal of Applied Ecology 25:847 852. Haukka, J.K. 1987. Growth and survival of Eisenia fetida Sav. Oligochaeta: Lumbricidae in relation to temperature, moisture and presence of Enchytraeus albidus Henle Enchytra eidae. Biology and Fertility of Soils 3:99 102.
9 Hendrix, P.F., and P.J. Bohlen. 2002. Exotic Earthworm Invasions of North America: Ecological and Policy Implications. Bioscience 52:1 11. Lamb, D., P.D. Erskine, and J.A. Parrotta. 2005. Restoration of Deg raded Tropical Forest Landscapes. Science 310: 1628 1632. Lapachin, O. 1996. Contrasting vermicomposting rates between two worms: Eisenia foetida and a native worm. CIEE Tropical Ecology and Conservation; Summer 1996. 93 104. Lavelle, P. 1988. Earthw orm activities and the soil system. Biology and Fertility of Soils 6: 237 251. Neuhauser, E.F., R. Hartenstein, and D.L. Kaplan. 1980. Growth of the Earthworm Eisenia Foetida in Relation to Population Density and Food Rationing. Oikos 35: 93 98. Reinecke, AJ, and J.M. Venter. 1985. The influence of moisture on the growth and reproduction of the compost worm Eisenia fetida Oligochaeta. Rev. Ecol. Biol. Sol 22: 473 481. Sharma, S. 1999. Composting Silkworm Culture Waste. Compost Science and Utilization 7: 7 4. Yach, R. 1999. Worming the Way to Finished Compost. BioCycle 40:34.
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Lugar de particin entre Eisenia foetida (Lumbricidae) y las lombrices nativas en Monteverde, Costa Rica
Niche partitioning between Eisenia foetida (Lumbricidae) and native earthworms in Monteverde, Costa Rica
With an increased susceptibility to invasive species due to deforestation, exotic species in tropical rainforests are quickly gaining potential to alter these already disappearing ecosystems. This study examined whether the exotic earthworm Eisenia foetida (Lumbricidae) has the potential to become an invasive species. Niche preference for level of organic soil content, level of soil texture, and humidity level was tested between native and exotic species in Monteverde, Puntarenas, Costa Rica. There was a significant difference for organic soil content level between native earthworms and E. foetida. This significant difference was present both when the exotics and natives were tested alone and in each others presence. Native earthworms avoided high organic content while the exotic earthworms preferred it. No significant difference between preferences of native and exotic worms was found for varying levels of soil texture. Both native and exotic earthworms were most abundant in the low soil texture level when tested alone. Native worms resided an average distance farther from the water source than exotic worms.
Con un aumento de la susceptibilidad hacia las especies invasoras debido a la deforestacin, las especies exticas estn rpidamente ganando potencial en los bosques tropicales lluviosos para alterar estos ecosistemas que se estn desapareciendo. Este estudio examin si las lombrices exticas Eisenia foetida (Lumbricidae) tienen el potencial para convertirse en una especie invasora.
Text in English.
Tropical Ecology 2007
Native earth worms
Ecologa Tropical 2007
Lugar de particin
t Monteverde Institute : Tropical Ecology