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1 Effects of detritus abundance and diversity on the composition of detritivore communities within tank bromeliads Callie Brown Department of Environmental Studies, University of Richmond ABSTRACT Detritivores are important in the cycling of nutrients w ithin bromeliads as they help release nutrients from detritus, making them available to the plant. This study set out to explore how the amount and diversity of detritus influenced the detritivore community structure within bromeliads. Results indicated that a greater amount of detritus supported a greater number of detritivore individuals and by some indices, a richer detritivore community. However, detritus diversity appeared to have no significant effect on the richness or abundance of detritivores. This could indicate that detritus is in fact a homogeneous resource and resource partitioning is not occurring among detritivores. It is also possible that detritivores are partitioning detritus according to factors other than the detritus source material which was the factor considered in this study. RESUMEN Los detritvoros son importantes en el reciclaje de nutrientes dentro de bromelias ya que contribuyen con la liberacin de nutrientes en los detritos, proporcionndoselos a la planta. Este estudio determin cmo la cantidad y diversidad de detritos influye la estructura de la comunidad de detritvoros dentro de las bromelias. Los resultados indicaron que una cantidad ms grande de detritos sostuvo un nmero ms grande de detritvoros y, de acuerdo con ciertos ndices, una comunidad ms rica. La diversidad del detrito no pareci tener un efecto significativo ni en la riqueza ni en la abundancia de detritvoros. Esto podra indicar que los detritos son un recurso homogneo de recurso y que la divisin de recursos no ocurre entre detritvoros. Es tambin posible que los detritvoros dividan los recursos de acuerdo con factores diferentes al material de la fuente de detritos,que fue el factor considerado en este estudio. INTRODUCTION Detritivores, or o rganisms that feed on dead organic matter, play an essential role in the cycling of nutrients through an ecosystem. Dead organic matter, termed detritus, contains a great deal of organic compounds that are inaccessible to the surrounding environment until broken down into inorganic elements. Detritivores work in conjunction with decomposers to return the nutrients stored in detritus back into the ecosystem. When detritivores digest organic material they shred it into smaller particles, which increases the surface area exposed to attack by decomposers. Decomposers, such as bacteria and fungi, release enzymes that break down organic molecules and return them to the environment as inorganic elements. In terrestrial systems, this process regenerates nutrient reservoirs in soils and enables future plant production (Ricklefs 1990).
2 Nutrients derived from detritus are particularly important for the survival of bromeliads. Bromeliads are an epiphytic family of plants that can be distinguished by their rosette arrangement of leaves that creates a tank in the center of the plant. The vast majority of bromeliads establish themselves on other trees, out of contact with the soil, and therefore must cope with limited access to nutrients as well as water. Bromeliads are able to survive in these conditions by impounding water and detritus in their tanks (Benzing and Renfrow 1974). Rainfall and fallen leaves, moss, bark, flowers, twigs, and l nutrients and water to the plant. Modified hairs (known as trichomes; Utley and Burt Utley 1983) on the surface of the leaves as well as on the base of the tank allow the bromeliad to absorb the water and nutrients, thus acting as roots. Due to the fac t that detritus is the principal source of nutrients for bromeliads (Bromeliaceae), detritivores can be expected to be all the more essential in the cycling of nutrients within bromeliads. The detritus that accumulates in bromeliad tanks supports a variet y of detritivores who feed on the organic matter. In doing so, the detritivores speed along the break down of the detritus and the release of its nutrients, which are taken up by the bromeliad (Benzing and Renfrow 1974). A total of 53 families of insects are recorded to have been found in plant held waters such as bromeliad tanks (Kitching 2001). While this number includes insects other than detritivores, some detritivores common to bromeliads include arthropods such as Diptera, Coleoptera, crustaceans a nd some members of the phylum Annelida (Kitching 2001). Since detritivores are important to the functioning of bromeliads, it is important to understand the factors that determine the detritivore community structure within tions of community structure point to the quality and quantity of resource base as strong determinants of community structure. This approach stresses that the resource base must be sufficient enough to sustain organisms and that it ultimately affects the presence and absence of organisms in the community (Kitching 2001). A study by Yanoviak (2000) investigated bottom up control in water filled tree holes by looking at the effect that leaf litter addition and removal had on macroorganism communities withi n water filled tree holes on Barro Colorado Island, Panama. Yanoviak (2000) found that leaf litter addition increased macro organism richness and abundance in water filled tree holes, suggesting resource base as a strong community determinant. A related study conducted by Bernal (2002) found that there was a positive correlation between the turbidity of bromeliad tank water and the diversity of protists within the tank. This finding suggests that resource base is affecting protist community composition as turbidity may reflect nutrient quantities. If the diversity and abundance of micro invertebrate detritivores, such as protists, is affected by the nutrient base within bromeliads, it could be expected that macro invertebrate detritivore communities in br omeliads would be affected by the resource base as well. The aim of this study is to explore the effect that detritus richness and biomass have on the richness and abundance of macro invertebrate detritivore communities within bromeliad tanks. According to some, detritus is a heterogeneous resource that differs according to variations in source material, chemical composition and particle size (Moore et al. 2004). The heterogeneity of detritus should allow for competing detritivores to coexist by the par titioning of different types of detritus resources. If this is the case, a greater diversity of detritus should be able to support a greater diversity and number of
3 detritivores. Therefore, this study expects to find that not only a greater amount of detr itus but also a greater diversity of detritus within a bromeliad will support a higher abundance and diversity of detritivores. MATERIALS AND METHODS This study was conducted in the pasture behind the Estacin Biolgica Monteverde in Puntarenas Costa Ric a between April 13 th and May 9 th 2005. The site is surrounded by Lower Montane Wet Forest at an elevation of 1570 m. Twenty bromeliads with a center tank diameter between four and eight centimeters were sampled. For each sampled bromeliad, the tank di ameter was measured and recorded and all leaf litter as well as macro invertebrates were removed using tweezers and placed directly into plastic bags. The contents from the plastic bags were then transferred into burlese funnels for one to two days in ord er to dry out the leaf litter and siphon out the invertebrates. Once dried, the leaf litter was sorted into the following categories based on source material: leaves, twigs/bark, seeds, roots, moss, baby bromeliads, galls, flower parts, dead insect parts, and unidentifiable. The mass of each category was measured for each sampled bromeliad. The invertebrates collected from each bromeliad were placed in petrie dishes and were identified to order and morphospecies. The differentiation between detritivores and non detritivore specimens was also noted. A number of calculations were made for each bromeliad. Overall morphospecies richness (S), Margalef diversity (Smarg), and abundance of individuals were calculated for the detritivores as well as all inverte brates collected for each bromeliad. Total detritus biomass (N) and the number of represented detritus types (S) were calculated for to adjust for sample size. Las tly, for each bromeliad the proportion of the total detritus biomass that each detritus type represented was calculated and then used to calculate the Shannon series of regression s. RESULTS A total of 143 macro invertebrate individuals were collected from the 20 bromeliads. Of these 143 individuals, 111 were detritivores. A total of 65 morphospecies were represented. Insect orders represented in the collected macro invertebrate s included Diptera, Dictyoptera, Hemiptera, Coleoptera, Orthoptera, Lepidoptera, Psocoptera, and Dermaptera. Isopods, belonging to the order Crustacea were also common among collected macro invertebrates. In addition there were representatives from the p hylum Annelida and Platyhelminthes (Fig. 1 and Fig.2). Perhaps the most surprising organisms found included two salamanders, a frog, and the snake species Imantodes cenchoa. and detritivore richness (p = 0.59, R 2 = 0.016, n = 20), Smarg (p = 0.937, R 2 = 0.000, n = 20) or abundance of detritivores (p = 0.926, R 2 = 0.000, n = 20). To test whether bromeliad width has an effect on detritivore communites richness and diversity a regression test was run. The size of bromeliads (as measured by tank diameter) and the species richness (p = 0.156, R 2 = 0.109, n = 20), Smarg (p = 0.54, R 2 = 0.021, n = 20) and
4 abundance of detritivores (p = 0.091, R 2 = 0.150, n = 20) demonstrated no sig nificant correlation. When regressions were run on detritus richness and detritivore community composition, detritus richness showed a significant positive correlation with detritivore species richness (p = 0.021, R 2 = 0.262, n = 20) (Fig. 4a). However, the data did not show a significant relationship between detritus richness and Smarg of detritivores (p = 0.062, R 2 = 0.1807, n = 20) (Fig. 4b). The relationship between detritivore abundance and detritus richness was nearly significant (p = 0.054, R 2 = 0 .1906, n = 20) (Fig. 4c). Total detritus biomass was positively correlated with the species richness as measured by S (p = 0.001, R 2 = 0.4399, n = 20) (Fig. 3a). However when the data were adjusted for sample size by calculating Smarg, there was no longer a significant correlation between detritus biomass and detritivore richness (p = 0.098, R 2 = 0.1448, n = 20) (Fig. 3b). The regression ran on detritus biomass and detrivore abundance showed a significant positive correlation (p = 0.000, R 2 = 0.6107, n = 2 0) (Fig. 3c). DISCUSSION The results show that the abundance of detritus plays an important role in determining detritivore communities within tank bromeliads. The data support the original hypothesis that a greater amount of detritus will support a gr eater abundance of detritivores (Fig 3c). These results are congruent with findings from the Yanoviak 2000 study of the role that resource availability has on water study on the effects of nutrient a vailability and protist abundance. These three studies offer a strong indication that the resource base within bromeliad tanks largely determines the size of communities within bromeliads. Nonetheless, the protist study by Spaulding (2005; this volume) d id not confirm these findings but rather found that an increase in resources had no effect on protist diversity within bromeliads. Thus the relationship between resource base and detritivore community composition is neither definite nor consistent. In addi tion to affirming the hypothesis that detritus abundance supports detritivore abundance, the results of this study indicate that detritus abundance may also play a role in determining the richness of the detritivore communities within bromeliads. The data show that as the amount, or mass, of detritus within a bromeliad increased, the richness of the detritivore community increased when measured by S (Fig. 3a). Therefore, detritus abundance had a greater effect on the detritivore community composition than was initially expected. It is also interesting to note that the abundance and richness of the entire macroinvertebrate community increases with abundance and richness of detritivores (Fig. 3a and 3c). This indicates that detritivores are the basis for a more complex community and that diversity of detritivores begets diversity of higher trophic levels. Thus there is reason to believe that detritus abundance has effects on the larger community assemblage and structure within bromeliads. The results refut ed the second of the original hypotheses, that greater diversity of detritus would beget greater diversity of detritivores. Detritus diversity as measured by the Shannon Weiner index had no significant effect on the richness or abundance of detritivore co mmunities. There was a significant positive relationship between detritus richness (S) and detritivore species richness (S; Fig 4a) but this significance was lost
5 when the data was adjusted for sample size with the calculations of Smarg (Fig 4b). The regr essions also showed that detritus richness had no significant effect on detritivore abundance (Fig 4c). There are a couple possible explanations for these results. The results could support the view that detritus is not heterogeneous but is in fact a ho mogeneous resource no matter what its source material may be (Moore et al. 2004). It could be expected that if detritus were a homogeneous resource, no resource partitioning would occur among detritivores. Therefore, variations in source material would h ave no affect on the detritivore community composition, as was the case in this study. A different explanation for the lack of influence detritus diversity had on the detritivore community is that detritivores are partitioning detritus based on a factor o ther than source material. This study measured detritus diversity and richness in terms of source material, but it is possible that detritivores are partitioning detritus based upon its particle size, chemical composition or perhaps by its quality as meas ured by its stage of degradation. Therefore, in the future it would be beneficial to investigate how the quality of detritus affects detritvore communities within bromeliads. ACKNOWLEDGEMENTS Thank you to Karen Masters for her enormous help in developin g and carrying out this study. I would also like to thank Matt Gasner, Javier Mendez and Ollie Hyman for their assistance throughout the course of my study LITERATURE CITED Benzing, D.H. and A. Renfrow. 1974. The mineral nutrition of Bromeliaceae. B otanical Gazette. 135(4): 281 288. Bernal, S., 2002. The effects of volume and turbidity on protist communities in tank bromeliads. CIEE Tropical Ecology and Conservation. Fall: 49 59. Utley, J.H. and K. Burt Utley. 1983. Bromeliads In: Costa Rica Nat ural History D.H. Janzen, ed. The University of Chicago Press, Chicago, IL, pp.197 198. explanations for community structure. Annual Review of Entomology. 46: 729 760 Moore, J .C. et al. 2004. Detritus, trophic dynamics and biodiversity. Ecology Letters. 7(7): 58 Ricklefs, E.R., 1990. Ecology, W.H. Freeman and Company, New York. Spaulding, J., 2005. Protist community diversity in relation to resources in bromeliads. CIEE T ropical Ecology and Conservation. Fall. Yanoviak, S.P. 2001. Predation, resource availability, and community structure in Neotropical water filled tree holes. Oecologia. 126: 125 133.
6 FIGURE 1. The distribution of the major taxa of all ma croinvertebrates collected. The total sample size of all macroinvertebrates was 143 individuals.
7 FIGURE 2. The distribution of major taxa of detritivores that were collected. Total sample size of detritivores was 111.
8 a b c FIGURE 3. The correlations between detritus biomass and the richness and abundance of detritivores within bromeliads. (a) There was a significant posit ive correlation between detritus biomass and detritivore richness as measured by S (p = 0.001, R 2 = 0.4399) (b) yet when measured by Smarg there was no significant correlation (p = 0.098, R 2 = 0.1448). (c) The regression shows a significant positive corre lation between detritus biomass and abundance of detritivores (p = 0.000, R 2 = 0.6107).
9 a b c FIGURE 4. The correlations between detritus richness and the richness and abundance of detritivores wi thin tank bromeliads. (a) There existed a significant positive correlation between detritus richness and detritivore richness when detriviore richness was measured by S (p = 0.021, R 2 = 0.262). (b) However when the data was corrected for sample size, detr itivore richness as measured by Smarg showed no significant correlation with detritus richness (p = 0.062, R 2 = 0.1807). (c) The relationship between detritus richness and detrivore abundance is nearly significant (p = 0.054, R 2 = 0.1906).
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Efectos de la abundancia de detritos y la diversidad en la composicin de las comunidades detritvoros dentro del tanque bromelias
Effects of detritus abundance and diversity on the composition of detritivore communities within tank bromeliads
Detritivores are important in the cycling of nutrients within bromeliads as they help release nutrients from detritus, making them available to the plant. This study set out to explore how the amount and diversity of
detritus influenced the detritivore community structure within bromeliads. Results indicated that a greater amount of detritus supported a greater number of detritivore individuals and by some indices, a richer
detritivore community. However, detritus diversity appeared to have no significant effect on the richness or abundance of detritivores. This could indicate that detritus is in fact a homogeneous resource and resource partitioning is not occurring among detritivores. It is also possible that detritivores are partitioning detritus according to factors other than the detritus source material, which was the factor considered in this study.
Los detritvoros son importantes en el reciclaje de nutrientes dentro de las bromelias ya que contribuyen con la liberacin de nutrientes en los detritos, proporcionndoselos a la planta. Este estudio determin cmo la cantidad y diversidad de detritos influye en la estructura de la comunidad de detritvoros dentro de las bromelias. Los resultados indicaron que una cantidad ms grande de detritos sostuvo un nmero ms grande de detritvoros y, de acuerdo con ciertos ndices, una comunidad ms rica. La diversidad del detrito no pareci tener un efecto significativo ni en la riqueza ni en la abundancia de detritvoros. Esto podra indicar que los detritos son un recurso homogneo de recursos y que la divisin de recursos no ocurre entre los detritvoros. Es tambin posible que los detritvoros dividan los recursos de acuerdo con los factores diferentes al material de la fuente de detritos, que fue el factor considerado en este estudio.
Text in English.
Monteverde Biological Station (Costa Rica)
Ciclo de nutrientes
Estacin Biolgica de Monteverde (Costa Rica)
Tropical Ecology Spring 2005
Ecologa Tropical Primavera 2005
t Monteverde Institute : Tropical Ecology