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1 Bromeliad arthropod diversity in a regenerating pasture adjacent to a tropical montane forest, Costa Rica Ann Willow Jorgenson School of Oceanography, University of Washington ABSTRACT Tank bromeliads (Bromeliaceae, Vriesea sp.) harbor diverse faunal communities. A total of 426 arthropods assigned to 62 different morphospecies were collected and identified to taxonomic order. The composition of bromeliad arthropod fauna was investigated in an open pasture to see whether arthropod diversity or richness correlated with distance from the forest edge. Neither arthropod richness nor diversity was found to significantly correlate with distance from the forest edge. These results are discussed in relation to island biogeography theory and the ecological impli cations of arthropod diversity are considered RESUMEN Las bromelias (Bromeliaceae, Vriesea sp.) dan refugio a comunidades diversas de artrpodos. Un total de 426 artrpodos asignados a 62 diferentes morfoespecies fueron colectadas e indentificadas a or den taxonmico. La composicin de artrpodos en bromelias fue investigada en un potrero abierto para ver si la diversidad de artrpodos o riqueza se correlaciona con la distancia del borde del bosque. Ni la riqueza ni diversidad estuvieron significativam ente correlacionadas con la distancia del borde del bosque. Se discuten las observaciones de la diversidad en relacin con la teora de biogeografa de islas y se consideran las impicaciones ecolgicas de la diversidad de artrpodos. INTRODUCTION Epiphy tes in tropical cloud forests provide microhabitats that harbor diverse arthropod communities (Yanoviak et al 2004). Tank bromeliads (Bromeliaceae) are common epiphytes in many tropical forests. Research on the associated fauna found in tank bromeliads s uggests that the bromeliad interior may approximate a miniature ecosystem (Janzen 1983). The faunal relationship may be mutuality beneficial. Several studies strongly indicate that faunal associates provide nutrients that are readily available for uptake by the bromeliad host (Janzen 1983). As predators and prey, insects and other arthropods are of prime importance in moving energy through food chains and food webs (Price, 1997). Larger organisms, such as birds also benefit and even partially rely on the food resources bromeliads provide. In a study conducted in the Talamanca mountain range, Costa Rica, the foraging ecology of epiphyte searching insectivorous birds was quantified (Sillett 1994). The most substrate specific forager was Pseudocolaptes law rencii : 74% of foraging attempts were on bromeliads (Sillett 1994). Insects inhabiting bromeliads may also be integral to pollen dispersal and gene flow for many plant species near the bromeliad host (Howell et al, 1998). Understanding the dynamics of b romeliad faunal diversity is essential to qualifying human impacts on tropical ecosystems. Bromeliads have often been hypothesized to exhibit characteristics of island ecosystem dynamics (Phaeton1992,
2 Shaw 1993). A structurally and functionally different habitat surrounds the pasture trees and the bromeliads, creating an island like environment. Since bromeliads naturally occur in the forest, it has been predicted that forest fauna are the major source of colonizers (Phaeton1992). According to the theor y of island biogeography, an increase in distance from the forest edge, and thus the pool of potential colonizers, would result in a decrease in immigration and bromeliad faunal richness. This research analyzes bromeliad arthropod diversity in pasture tre es to understand the impacts of isolation created by the conversion of forest to pasture. MATERIALS AND METHODS The study site was located in an abandoned pasture adjacent to the Estacin Biolgica, Monteverde, Costa Rica (10 18 N, 84 48 W), betwe en 23 July and 3 August 2004. The site is located on the Pacific slope at 1530 m, about 300 m below the Continental Divide. The pasture was surrounded on all sides by montane wet forest that extended at least 10 m from the pasture edge. The interior of the pasture exhibited an uneven distribution of small to medium trees, small shrubs, and grasses. Twenty two tank bromeliads (Bromeliaceae, Vriesea sp.) were selected from tree trunks and logs at varying distances from the forest edge. Bromeliad size a nd height above the ground were kept relatively constant. Distance from the nearest forest edge, type of substrate, and the presence of nearby bromeliads were documented. The height of the bromeliads was measured from the base of the rosette directly to the ground. Once detached from the substrate bromeliads were immediately contained in a plastic bucket with a sealing lid and transported back to the laboratory where the diameter of the rosette was measured and bromeliads were carefully dissected in the bucket. All arthropods were removed by hand, tweezers, or an aspirator and placed in plastic Ziploc Baggies. Morphospecies were identified and their abundances recorded for each bromeliad. A Shannon Weiner index was used to calculate diversity for each bromeliad. A simple regression was used to test for a significant correlation between diversity and two separate variables: distance from the forest edge and diameter of the bromeliad rosette. Diversity and distance were compared using Kruskal Wallis. A Jaccard similarity index was used to determine species overlap between distance intervals (0 10 m, 10 20 m, 20 30 m). RESULTS Bromeliad height ranged from 0.85 3.75 m above the ground. The diameter of bromeliad rosettes ranged from 0.35 0.53 m. A to tal of 426 arthropods assigned to 62 different morphospecies were collected. Most morphospecies were identified as members of 14 orders, although 5 morphospecies could not be identified (Table 1). The most species rich order was Coleoptera, which contai ned 19 morphospecies (Table 1). The second richest order was Araneae, which contained 11 different morphospecies (Table 1). Diversity index (H') ranged from 0.494 to 1.60 (Figure1). A simple regression showed no significant correlation between distance f rom the forest edge and diversity (R 2 = 0.119, P = 0.1160) (Figure1). Species richness (# of morphospecies bromeliad 1 ) ranged from 4 to 15 (Table 1). No significant correlation was observed between bromeliad size
3 and diversity (R 2 = 0.017, P = 0.0790). N o significant difference in species richness or diversity between three distance intervals (0 10 m, 10 20 m, 20 30m) was found. There was a low number of species overlap between distance intervals, with the greatest overlap occurring between 0 10 m and 20 30 m (Jaccard:0.36). Species overlap was very low between 0 10 m and 10 20 m, and between 10 20 m and 20 30 (Jaccard = 0.28: 0.27, respectively) DISCUSSION A correlation was not observed between distance from the forest edge and bromeliad arthropod rich ness or diversity. According to the theory of island biogeography, a negative correlation would be expected between distance from the forest edge and bromeliad faunal richness. This correlation was not observed in previous studies on bromeliad arthropods c onducted in the same area (Phaeton 1992, Shaw 1993). There are several potential explanations for the lack of correlation between diversity and distance from the forest edge. The spatial scale may have not been great enough to observe an existing correlat ion. Most of the observed species had wings and may have been capable of easily traveling the furthest distance sampled, 30.45 m. Bromeliad environments were not identical: the trees were not evenly spaced, the bromeliads experienced various levels of su nlight availability, surrounding epiphytic flora varied in composition and abundance, and wind conditions may have varied significantly. Although it appears there is little species overlap between distance intervals measured from the forest edge, this may not be significant. There was a very high number of species distributed over a relatively low number of bromeliads with many species occurring only once. It is possible that randomness rather than specialization influenced the apparently low overlap. Al though an attempt was made to hold the size of the bromeliad constant, diameter of the rosette may not have been an accurate measurement of the available habitat space. It was observed that bromeliads with similar rosette diameters had varying numbers of a ctual leaves and consequently different amounts of available habitat space. Past studies have found a clear correlation between habitat space and diversity (Shaw 1993). Although there was no correlation between rosette diameter and arthropod diversity, th e variation in number of leaves could have had an effect on diversity. Another potential source of error in the methodology of the experiment that may have affected the results, was that although samples were contained as quickly as possible, due to diffi culty of detachment, prolonged physical disturbance may have allowed some arthropods to escape capture. The number of morphospecies found was interesting. A recent study examined arthropod diversity in epiphytic mats, primarily bryophytes and mosses (Ya noviak et al, 2004). Of the 1943 arthropods found, only 42 different morphospecies were assigned (Yanoviak et al 2004). Comparatively, bromeliads exhibited a much greater species richness, containing 62 morphospecies out of only 426 arthropods. Bromelia ds provide a habitat for a diverse assemblage of arthropods. It is likely the ecological niche they provide plays an important role in maintaining biodiversity in pastures that contain trees. Due to rapid habitat destruction and fragmentation concern rega rding the preservation of insect species has developed rapidly (Price 1997). The arthropods inhabiting bromeliads are a key factor in moving energy through the food
4 chain and may provide a substantial food source for larger organisms like birds (Price 199 7). Arthropods from the order Blattodea are scavengers that break down waste and aid in the cycling of nutrients (Evens 1984). Species of the order Coleoptera occupy many foraging niches as predators, scavengers, and herbivores (Borror and DeLong 1964). Winged arthropods such as those belonging to the order Coleoptera may aid in the pollination systems of local plants. Arthropods are an integral part of ecosystem dynamics. Felling of tropical forests certainly has dramatic an often devastating ecologica l effects, but the trees and bromeliads that are left will certainly provide many ecological benefits. ACKNOWLEDGEMENTS Thanks to the Estacin Biolgia for the opportunity to conduct research. Thanks to Maria Jost and Oliver Hyman for their expert bromeliad collecting assistance and technical support with statview and excel. Thanks to Carmen Rojas and Carlos Guendon for their help along the way. Thanks to Emily Matheson for listening to me complain. LITURATURE CITED Borror D. J. and D. M. De Long. 1964. An introduction to the study of insects. Holt, Reinhart and Winston. New York Evens, E. H. 1984. Insect Biology: A text book of entomology. Addison Wesley publishing company inc. Massachusetts Howell, D. V., J. T. Doyen and A. H. Purcell III. 1998. Introduction to insect biology and diversity. Oxford University Press, New York. Janzen D. H. 1983. Costa Rican natural history. The University of Chicago Press, Chicago. Phaeton, L. 1992. Macroinvertebrate bromeliad fauna in forest and pasture trees in a Montane cloud forest of Costa Rica. UCEAP Fall Price, P. W., 1997. Insect Ecology 3 rd Edition. John Whiley & Sons, Inc., New York. Shaw, T. 1993 Species diversity of arthropod fauna in tank bromeliads. Tropical biology and conservation. Su mmer Sillett, T. S. 1994. Foraging ecology of epiphyte searching insectivorous birds in Costa Rica. The Condor 96: 863 877 Yanoviak, S. P., Walker, H. and Nadkarni, N. M. 2004. Arthropod assemblages in vegetative vs. humic portions of epiphyte mats in a neotropical cloud forest. Pedobiologia 48: 51 58.
5 Table 1. Numbers of individuals and morphospecies categorized by taxonomic order. An egg sack contained 101 Araneae in a single bromeliad. Order #individuals #species Araneae 154 11 Blattodea 69 1 Chilopoda 6 4 Coleoptera 38 19 Dermaptera 23 3 Diplopoda 1 1 Diptera 2 1 Hemiptera 6 1 Hymenoptera 9 2 Isopoda 43 2 Lepidoptera 2 1 Orthoptera 25 8 Symphyla* 10 1 Thysanurans 3 1 Unidentified 35 5 Total 426 62 *Was identified to class
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Jorgenson, Ann, Willow
Diversidad de artrpodos de bromelias en un pastizal regenerativo junto a un bosque montano tropical, Costa Rica
Bromeliad arthropod diversity in a regenerating pasture adjacent to a tropical montane forest, Costa Rica
Tank bromeliads (Bromeliaceae, Vriesea sp.) harbor diverse faunal communities. A total of 426 arthropods assigned to 62 different morphospecies were collected and identified to taxonomic order. The composition of bromeliad arthropod fauna was investigated in an open pasture to see whether arthropod diversity or richness correlated with distance from the forest edge. Neither arthropod richness nor diversity was found
to significantly correlate with distance from the forest edge. These results are discussed in relation to island biogeography theory and the ecological implications of arthropod diversity are considered.
Las bromelias (Bromeliaceae, Vriesea sp.) dan refugio a comunidades diversas de artrpodos. Un total de 426 artrpodos asignados a 62 diferentes morfoespecies fueron colectados e identificados al orden taxonmico. La composicin de artrpodos en bromelias fue investigada en un potrero abierto para ver si la diversidad de artrpodos o riqueza se correlaciona con la distancia del borde del bosque. Ni la riqueza ni la diversidad estuvieron significativamente correlacionadas con la distancia del borde del bosque. Se discuten las observaciones de la diversidad en relacin con la teora de biogeografa de islas y se consideran las implicaciones ecolgicas de la diversidad de artrpodos.
Text in English.
Monteverde Biological Station (Costa Rica)
Diversidad de especies
Estacin Biolgica de Monteverde (Costa Rica)
Tropical Ecology Summer 2004
Ecologa Tropical Verano 2004
t Monteverde Institute : Tropical Ecology