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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
Niche Partitioning between Eisenia foetida (Lumbricidae) and native earthworms in Monteverde, Costa Rica Christine Askham Department of Biology, Gu stavus Adolphus College ABSTRACT With an increased susceptibility to invasive species due to deforestation, exotic species in tropical rainforests are quickly gaining potential to alter th ese already disappearing ecosystems. This study examined whether th e exotic earthworm Eisenia foetida ( Lumbricidae) has the pot ential to become an invasive species. Niche preference for level of organic soil content, level of soil te xture, and humidity level was tested between native and exotic species in Monteverde, Puntarenas, Costa Rica. There was a significant difference for organic soil co ntent level between native earthworms and E. foetida This significant difference was present both when the exotic s and natives were tested alone and in each others presence. Native earthworms avoided high organic co ntent 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 fart her from the water source than exotic worms. RESUMEN Con una creciente susceptibilidad a especies invasoras de bido a la deforestacin las especies exticas estn ganando rpidamente potencial en los bosques tropical para alterar estos ecosistemas. Este estudio examino si la lombriz extica Eisenia foetida ( Lumbricidae) tiene el potencial de convertirse en una especie invasora. Preferencia de nicho, con respecto a contenido orgnico del suelo, textura del suelo, y humedad se comparo entre una especie extica y una nativa en Monteverde, Puntarenas. Las lombrices nativas evitan contenidos orgnicos altos, mientras que las exticas lo prefieren. No hay diferencia por seleccin de textura del suelo, ambas prefieren el ms bajo. Las lombrices nativas se movieron distancias ms largas de la fuente de agua que las exticas. INTRODUCTION With 350 million hectares of tropical rainfore sts already deforested (Lamb et al. 2005) and the worlds supply of rainforests disappe aring at an alarming rate, conserving the remaining forest is essential. Deforestation leads to an increased amount of edge effects and fragmentation, which make a forest vulnera ble to exotic species (Hendrix and Bohlen 2002). Hendrix and Bohlen (2002) found that exotic earthworms successfully invaded habitats that were al ready occupied by native earthworm s after a disturbance in soil or vegetation. With rainforests less resistant to change due to deforestation, exotic (nonnative) 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 earth worm native to Africa now commonly used in Costa Rica for vermicomposting (worms decomposing organic material). E. foetida is 1
found throughout the Monteverde region in 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 specie s 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. Earthworms 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, sp routing, and survival rates of Mulberry plants compared to plants in untouched soil (Sharma 1999). Gaechter (2004) found that native earthw orm species 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. Native 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 pr esence 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 investigate if E. foetida and native earthworms demonstrate niche partitioning by preferring di fferent levels of organic soil content, humidity level, and soil texture. If E. foetida does not negatively affect native earthworms, both worms types should demonstr ate different preferen ces within organic soil content, soil texture, and humidity leve l because the exotic and native worms occupy different niches. Therefore, either preference for soil type, water content, or organic matter content will differ between the natives a nd 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 Megs Stables; the property of Meg LaVal. The study was performed under a tin roof w ithout 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 e xperiment in a covered wooden box. Native earthworms were dug up from the forest edge of the near the Estacin Biolgica de Monteverde. Native worms were enclosed in an aquarium covered with a wooden plank filled with the soil they were dug up in. Na tive earthworms displaying the morphological 2
traits of Eisenia foetida were not used in the experiment Only worms longer than three cm were used in the experiments. Four dist inct native morpospecies were identified; a species with a white stripe below the head, a grey species, a species that twitched and wiggled when touched, and a small white specie s. The small white species died in the aquarium and were not used in the experime nts. 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 found on the forest edge in Monteverde. Low consisted of soil found on a sm all leafless but grassy area next to the forest. The three soil levels were then la yered in eighteen differe nt 250 mL containers with a 6 cm by 6 cm base. There were six tota l combinations 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 worms were placed on the top layer of the six native and exotic trials. Thr ee natives and three exotics we re placed on the top layer of the last six aquaria. I re turned after 24 hours separate d, counted, and identified the worms in each level. I repeated this procedure one time. 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 from San Luis coffee-co-op), medium (leaf littered soil on forest edge), low (soil from grassy area near forest edge). Humidity Level I tested for a possible humidity preference using a humidity gradient created on a flat tray, 40 cm in length, with paper towels that wicked water from a source at one end (a plastic trough with water). Using 45 centimet ers of double-layered brown paper towel I marked eight sections five centimeters apart st arting with 0 at the source and ending 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 stick. After five minutes the section in which each worm was located was recorded. I repeated this procedure eight times for both native and exo tic 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). 3
Soil Texture The experiment for soil texture preference us ed the same methodology as the experiment for organic soil content except three soil textur e levels varied instead of organic content levels. Low soil texture consisted of all cl ay obtained at an ex cavation site about 100 meters up the trail at Megs 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 each 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 th e roadside with sieves. Analysis A contingency table analysis was used to determine whether exotic and native species (alone or together) randomly choose soil textur e and organic soil content. A t-test was performed between native and exotic pref erences for distance from water source. RESULTS Organic Soil Content The native and exotic worms tested separately showed a significant difference in their preference for organic soil content level (X2 = 22.9, df = 2, P<.0.001; Figure 1a). Exotic worms were most abundant in high organic content (55%) while native worms a voided the high organic content (9.3%). 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%nativeexoticA 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%native exotic high mediu Earthworm t y Earthworm t y m lowB Figure 1: Organic soil preference for native a nd exotic earthworms a) alone (N = 43,N = 60 respectively) and b) in the presence of the other (N = 51, N = 53 resp ectively). Percentage native and exotic (E. foetida) earthworms found in each layer of organic soil; high (coffee c medium (soil from leaf-littered forest), low (soi l from grassy forest edge). Both w earthworms were together and separate was ther e a significant difference between org matter preference (Chi-squa red test, df = 2, P of total ompost), hen the anic soil <0.001). 4
Native and exotic worms tested in each others presence also showed a significant difference between preferences of each worm type (X2 =37.8, df = 2, P<0.001; Fig.1b). Exotic worms were again most abundant in the high organic content level (73%) whi natives were the leas t abundant (13.7%). le Humidity Level There was a significant difference between pref erences of exotic and native worms (t-test = 2.80, df = 115, P = 0.006; Fig. 2). The mean distance 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 of 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 wa s observed within the parts of the paper towel in which 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 500510152025303540 Distance (cm) native exotic0 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 differen ce in distance preference was found between native and exotic earthworms (t-test, df = 115, P = 0.006). 5
Soil Texture There was no significant difference between pr eferences of exotic and native earthworms in either the alone or togeth er 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 frequenc y was not significantly different from that observed in other soil types. When the exotic and native worms were tested together, the medium texture level appeared to har bor the lowest proportion of worms for both types (Fig. 3), but this frequency was not significant. Twen ty-seven percent of the 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%). 0% 20% 40% 60% 80% 100%native exotic Earthworm tyA 0% 20% 40% 60% 80% 100%native exotic Earthworm ty high medium lowB DISCUSSION Figure 3: Percent of exotic ( 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 (cla y) after 24 hours. A) exotic (N = 59) and native (N = 57) worms were tested separatel y. No significant diffe rence was found between texture level preference between exotic a nd native worms (Chi-squared test, df = 2, P >0.05). b) exotic and native worms were tested in the presence of the other. No significant difference was found between preference type between native and exotic worm (Chisquared test, df = 2, P > 0.05). In order for the exotic earthworms to not ha ve a negative impact on native earthworms as Gaechter (2004) claims, the two types of earthwo rms cannot be found in the same niche. 6
Differences in preferences between native a nd exotic earthworms were evident between levels of organic soil matter, but not with in soil texture or humidity level. The exotic and native earthworms preferred different levels of organic soil content. Niche partitioning explains the e xotic earthworms pref erring the high 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 organic matter is present even compared to preferen ces of many native species. This difference in preferences between natives and exotics remained the same when the earthworms were tested in each others presence. Because the preference stayed the same, obligate niche partitioning is evident. This niche partitioning between organic so il 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 co mposts 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 vari ables 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 determine if E. foetida will remain in the compost or travel into the forest. No niche partitioning within soil textur e is evident between the exotic and native earthworms. Without niche partitioning, co mpetition is likely to occur between native and exotic worms within the same soil texture. Exotic worms compete for resources with native worms resulting in decreased native worm abundance. For example, Ha milton et al. (1988) found the presence of E. foetida decreased cast stability and weight gain in another earthworm species ( Lumbricus terrestris ). Competition can change soil profiles, nutrient and organic matter dynamics (Hendrix and Bohlen 2002) and ultimately affe ct entire ecosystems. Because habitat alteration leaves forests more vul nerable to invasive 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 worms pr efer 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 textures. 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 due to differences in activity levels. The exotic worms resided an average distance closer to the water source than the native worms. The e xotic worms appeared to be more active than the native worms. The native worms set on the dry parts of the paper towel 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. 7
According to Yach (1999) worms need to be wet to remain active. It could be possible 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 fort y four percent of the exotic worms found in the section cl osest 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 result of testing multiple native species at th e 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 concern. 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 fast er than the native worms. Competition between natives and exotics would preven t natives from inhabiting the forest. Although niche partitioning between native a nd exotic earthworms is evident in regards to organic soil content; many other fact ors 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 pa rtitioning is only found within one variable so far, E. foetida needs 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 th is project. Thank you Jordan Cohen for allowing me to steal this project idea from you. This project wouldnt have been possible without Meg LaVals help obtaining worms and permission to conduct my study on her property, thank you. Thank you Julia and Sadie for tolerating my alwa ys entertaining sleep talking. Also, I am forever grateful to Alan Masters, Pablo Allen, and Taegan Mc Mahon for doing all they coul d to keep me healthy in this minefield of tropical diseases. Gracias Jose Luis y Juan Jose y mi amigo especial para hablar conmigo cuando estaba trabajando con mis lombrices. Finalmente, quiero deci r gracias a mi familia; Vilma, Gerardo, y Lilli Jimenez por sus comida, casa, y chistes. Muy interestante. LITERATURE CITED Gaechter, L. 2004. Colonizing li mitation 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. M itchell. 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: Lumb ricidae) in relation to temperature, moisture and presence of Enchytraeus albidus (Henle) (Enchytraeidae). Biology and Fertility of Soils 3:99-102. 8
9Hendrix, 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. Re storation of Degraded 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. Earthworm 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: 74. Yach, R. 1999. Worming the Way to Finished Compost. BioCycle 40:34.