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Diferencias en la composicin de la comunidad del escarabajo de corteza (Coleoptera: Curculionidae) dentro de los peciolos Cecropia obtusifolia (Cecropiaceae) en dos hbitats en Monteverde, Costa Rica
Differences in bark beetle (Coleoptera: Curculionidae) community composition within Cecropia obtusifolia (Cecropiaceae) petioles in two habitats in Monteverde, Costa Rica
Cecropia obtusifolia trees shed leaves daily throughout the year, regularly dropping large, woody leaf petioles. These petioles serve as the perfect habitat for Scolytine bark beetles, which burrow in, lay their eggs, and feed on the moist, fibrous pith of the petiole (Wood 1983). Cecropia spp. are known to thrive in fragmented, edge habitats altered by human land use transformation (Bello et al 1996) but success rates of associated fauna, including Azteca spp. ants, scale insects and Scolytine beetles, are less understood. This study examined the bark beetle community composition, morphospecies richness, and abundance inside C. obtusifolia petioles. It also investigated the rate of petiole colonization by Scolytine beetles, comparing two habitats: a C. obtusifolia population occurring in a forest tree fall light gap, and a population occurring along a neighborhood road. Both populations were located in Monteverde, Costa Rica. Fallen, dead petioles were collected and dissected from each habitat type, and all fauna found inside the woody petioles were removed, identified, and recorded. They were identified as either a Scolytine larva, one of five observed Scolytinae morphospecies, or a non-Scolytine organism. Differences between observed beetle communities in the two studied habitats were then compared. This revealed equal species richness and equal rates of petiole colonization for beetle communities at each site. The human disturbed habitat had greater Scolytinae abundance, while the forest light gap habitat had greater diversity due to higher evenness of morphospecies abundances. This is explained by the particularly high abundance of one morphospecies in disturbed habitat beetle communities. Overall, there were few evident differences between Scolytinae communities in the two examined habitats, indicating that observed Scolytinae populations are adaptable to human-caused habitat alterations and can disperse to fragmented C. obtusifolia trees. Their maintained survival and reproduction rates indicate that C. obtusifolia associated Scolytinae beetles are poor indicators for biodiversity loss and impacts on species relationships that result from continued global deforestation and habitat degradation.
Este estudio examina la composicin en la comunidad de los escarabajos de corteza en cuanto a nmero de morfoespecies y abundancia de individuos dentro de los pecolos de la especie C. obtusifolia. Tambin se investiga el rango de colonizacin de los peciolos colonizado por los escarabajos de la subfamilia Scolytine comparando dos hbitats entre los individuos encontrados en poblaciones en los claros del bosque y las poblaciones encontradas en las orillas de los caminos. Ambas poblaciones fueron encontradas en Monteverde, Costa Rica.
Text in English.
Escarabajos de corteza--Comportamiento
Tropical Ecology 2007
Ecologa Tropical 2007
t Monteverde Institute : Tropical Ecology
1Differences in bark beetle (Coleoptera: Curculionidae) community composition within Cecropia obtusifolia (Cecropiaceae) petioles in two habitats in Monteverde, Costa Rica Amy Strauss Departments of Biology and Environmental Studies, Whitman College ABSTRACT Cecropia obtusifolia trees shed leaves daily throug hout the year, regularly dropping large, woody leaf petioles. These petioles serve as the perfect habitat for Scolytine bark beetles, which burrow in, lay their eggs, and feed on the moist, fibrous pith of the petiole (Wood 1983). Cecropia spp. are known to thrive in fragmented, edge habitats altered by human land use transformation (Bello et al. 1996) but success rates of as sociated fauna, including Azteca spp. ants, scale insects and Scolytine beetles, are less understood. This study examined the bark beetle community composition, morphospecies richness, and abundance inside C. obtusifolia petioles. It also investigated the rate of petiole colonization by Scolytine beetles, comparing two habitats: a C. obtusifolia population occurring in a forest tree fall light gap, and a population occurring along a neighborhood road. Both populations were located in Monteverde, Costa Rica. Fallen, dead petioles were collected and dissected from each habitat type, and all fauna found inside the woody petioles were removed, identified, and recorded. They were identified as either a Scolytine larva, one of five observed Scolytinae morphospecies, or a non-Scolytine organism. Differences between observed beetle communities in the two studied habitats were then compared. This revealed equal species richness and equal rates of petiole colonization for beetle communities at each site. The human disturbed habitat had greater Scolytinae abundance, while the forest light gap habitat had greater diversity due to higher evenness of morphospecies abundances. This is explained by the particularly high abundance of one morphospecies in disturbed habitat beetle communities. Overall, there were few evident differen ces between Scolytinae communities in the two examined habitats, indicating that observed Scolytinae populations are adaptable to human-caused habitat alterations and can disperse to fragmented C. obtusifolia trees. Their maintained survival and reproduction rates indicate that C. obtusifolia associated Scolytine beetles are p oor indicators for biodiversity loss and impacts on species relationships that result from continued global deforestation and habitat degradation. RESUMEN Los rboles de la especie Cecropia obtusifolia diariamente a travs del ao, dejan caer largos y leosos pecolos de las hojas. Estos pecolos sirven como perfecto hbitat para los escarabajos de la subfamilia Scolytine que habitan en la corteza de los rboles. Estos escarabajos hacen cuevas y pones sus huevos en estas, adems se alimentan de la madera del pecolo (Wood 1983). Cecropia spp. se encuentra en habitats alterados por los humanos como fragmentos o bordes de bosque (Bello et al. 1996), pero los grados de sucesin asociados a la fauna que viven dentro de esta especie como los presentados por las hormigas Azteca spp. y los escarabajos de la subfamilia Scolytinae, son menos conocidos. Este estudio examina la composicin en la comunidad de escarabajos de la corteza en cuanto a numero de morfoespecies y abundancia de individuos dentro de los pecolos de la especie C. obtusifolia Tambin se investiga el rango de colonizacin de los escarabajos de la subfamilia Sco litynae comparando entre individuos encontrados en poblaciones en los cl aros de bosque y a la orilla de los caminos. Ambas poblaciones estuvieron localizadas en Monteverde, Costa Rica. Se recolectaron y se disectaron pecolos cados en el suelo en cada tipo de hbitat y luego se identifico la fauna encontrada adentro de la corteza de lo s pecolos. La fauna encontrada fue identificada a nivel de morfoespecies los escarabajos adu ltos, larvas de estos escarabajos y los individuos no pertenecientes a esta subfamilia. Se encontraron diferenc ias entre las dos comunidades estudiadas. Estas revelan
2similar riqueza de especies y rango de colonizacin entre comunidades. En el hbitat alterado se encontr mayor abundancia de individuos pero el sitio de los claros de bosque tiene una diversidad mas alta, debido a la equidad encontrada entre en la abundancia de las morfoespecies Esto se puede exp licar a la alta abundancia de una morfoespecie en el hbitat alterado. Sobretodo, se encontraron pocas diferencias entre las dos comunidades, indicando que las poblaciones de scolitidos son adaptables a las alteraciones en el hbitat causadas por los humanos y se pueden dispersar a poblaciones fragmentadas de rboles de C. obtusifolia La supervivencia y el grado de reproduccin de estos escarabaj os de la subfamilia Scolytinae indican que estos son indicadores pobres de la perdida de biodiversidad y del impacto de la deforestacin y la degradacin del hbitat en la relaciones entre las especies. INTRODUCTION Tropical forest biomes have the highest biodiversity, most complex habitat specialization and highest rates of endemism across the globe. They harbor nearly 50% of described species, while covering less than seven percent of the earths surface (Plotkin et al. 2000). These ecologically important and rich landscapes have been and are continuing to be transformed into a deforested mosaic of urbanization, cattle pastures, agricultu ral plots, monoculture plantations, logged forests and secondary growth with only scattered fragment s of primary growth remaining (Nepstad et al. 1996). Rich tropical communities, due to their hi gh rates of specialization and endemism, are particularly vulnerable to the im pacts of fragmentation and habita t destruction. These impacts are often amplified for organisms reliant on close inter-species relations hips (Sankaran 2001). Cecropia trees are common pioneers of secondary growth forests and benefit from natural or human caused canopy disturbances. As a result, their populations increase along with deforestation rates (B ello et al. 1996). Cecropia interact closely with fauna such as Azteca ants and scale insects, as well as Scol ytine bark beetle s that use the large, woody petioles from the tree as their obligatory reproductive site (Wood 1983). Cecropia shed dead leaves year-round, and therefore are good hosts, despite low nutri ent content and patchy community distribution (Wilkinson 2002). These beetles are entirely dependent upon Cecropia while Cecropia neither benefit nor are compromised by the commen salist relationship (A. Masters, personal communication). Adult male Scolytine beetles burrow through the cortex of th e petiole (typically three to five days after the leaf has fallen) and leave a pheromone trail for a female to follow. Copulation takes place inside the burrowed tunnel, and the female subsequently carves a pocket in the petiole pith for egg deposition. Eggs hatch three to five days later, and pupation occurs within the petiole. Adult beetles then bore out through the cortex to repro duce in a new host petiole. The duration of this cycle is generally around 25 days (Wood 1983). Multiple Scolytinae species have evolved the same Cecropiadependent reproduction patterns and can share petiolesup to four adult pairs can inhabit the same petiole. Di fferent beetle species within one petiole often exhibit niche partitioning to reduce resour ce competition (Wilkinson 2002). Inbreeding is common due to the closed petiole ec osystem, in which the majority of the Scolytinae life cycle is spent, because siblings frequently mate before exiting their birth petiol e (Beaver et al. 2001). In this study, I assess differences in co mmunity composition and colonization rates of various Scolytinae morphos pecies in petioles of C. obtusifolia in areas of high human disturbance versus natural primary forest light gaps. My objective was to determine how these bark beetles respond to habitat fragmentation. Given that Cecropia shed leaves year round, beetle popul ations are not accustomed to dispersing far, and upon leaving the birth petiole, young adult beet les often choose petioles in
3 very close proximity to use for mating and colo nization (Jordal and Kirkendall 1998). For this reason, despite host success, I expect that rate s of colonization of Scolytinae beetles will be lower in highly disturbed human habitats due to their infrequent need for far dispersal and the resulting inability of the beetles to trav el successfully between highly fragmented C. obtusifolia communities. Following the same logic, I anticipate lower species richness of beetles in petioles from human-altered habitats, but higher abunda nce of each present species due to increased availability of petiole material for oviposition and less resource competition between species. MATERIALS AND METHODS Study Sites Two sites in Monteverde, Costa Rica were used for this study, each with a population of at least four mature C. obtusifolia trees that shed leaves daily. One site was located in the forest of the Bosque Eterno de los Nios, at the Bajo del Tigr e entrance in a natural tree fall light gap. The other site was located along the roads in the Bajo del Tigre neighborhood, a once-forested habitat that was altered by humans. The two sites were within 50 meters in elevation of each other (1340 m 1390 m) to control for potential variation in beetle colonization and composition caused by changes in elevation. In each study site, four suitable trees in clos e proximity to each other were selected and recorded. From each tree, ten petioles of simila r age, moisture content, color, length, and diameter were gathered from the ground or from vegetation up to three meters off the ground. Samples were immediately placed individually into larg e Ziploc plastic bags, which were sealed and labeled; 80 petioles were coll ected in total. According to W ood (1983), leaves that fell three to 15 days prior to collection are ideal for anal yzing Scolytinae communiti es because the petioles during this time are in adequate condition to supp ort these beetle populations. Ideal petioles are moist and fresh, moderately stiff to the touch, an d medium brown in color (Jordal and Kirkendall 1998)these criteria were used in selecting petioles of approximately the same age. Length from the petiole base to the base of the leaf blade was measured using a tape measure and was between 25 cm and 50 cm for each examined petiole. The diameter of each, measured with a caliper at the midpoint between petiole base and base of leaf blade, was between 0.6 cm and 1.2 cm for each petiole. These criteria control for potential variance in community dynamics within each petiole caused by varied age and size of leaf. Within ten days of collection, each petiole was removed from its sealed bag and dissected using tweezers and scissors, by cutting down the cen ter of the petiole alo ng its length. Tweezers were used to peel back strips of bark along the petiole cortex, a nd the petiole was additionally cut into 0.5 cm long discs horizontally down the entire length of the petiole. A ll fauna present in the petiole or in the Ziploc bag were removed usi ng tweezers and placed into labeled vials filled with 70% alcohol solution to maintain the insects until identification. Petioles that were not colonized by any Scolytinae were recorded as S = 0. For each petiole, all fauna were removed one by one and identified to morphospecies of Scolytinae using a dissecting sc ope. Morphospecies was disti nguished based on color, size, pubescence and elytra characteristics. Scolytine larvae present in the petioles were recorded, however morphospecies identities of observed larvae were not determined. Any non-Scolytinae insects found in the petioles, such as an ts or spiders, were noted as well.
4 Morphospecies richness (S) and abundance of individuals in each morphospecies (N) were recorded per petiole. Shannon-Weiners diversity index (H) and evenness (E) were calculated comparing the two habi tat types using the total beetle abundance and richness values from the examined 40 petioles from each site. A ttest was applied to the H value to compare the habitat diversity differences per site. Margalefs index (Smarg) was calculated to compare the difference in sample size per site. Sorensons quantitative index was applied at th e tree level to discern whether similarity values of the beetle composition at the two sepa rate habitats differed from similarity values within the same habitat. This was used to account for compositional variance in beetle communities between trees at each site, using th e mean calculated index value per comparison category (within forest habitat, within disturbed habitat and betw een the two distinct habitats). Colonization rates per site were calculated as percentage of petioles containing at least one Scolytine beetle, and co mpared between sites. RESULTS From both the human disturbed habitat site and th e forest habitat site, 38 of the 40 examined petioles contained Scolytinae individuals so that each site showed an equal colonization rate of 95%. A total of 1042 beetles were found and identifie d as either Scolytine larvae or one of five observed morphospecies of adult beetle. All five morphospecies were observed in petioles from each habitat (Table 1). A total of 237 observed non-Scolytinae fauna were found between both sites. TABLE 1. Abundance (N), Morphosp ecies Richness (S), Diversity (H) and Evenness (E) values of Scolytinae beetles found in to tal per habitat, excluding larvae. Forest Site Disturbed Site N 336 457 S 5 5 H 1.40 1.24 E 0.87 0.77 Patterns of Compared Community Composition between Habitat Types Abundance of total observed Scolytinae beetles was higher in the human-distu rbed habitat (Table 1). There was a difference in diversity of Scolyt ine beetles found between th e forest habitat (H = 1.40) and the human disturbed habitat (H = 1.24; Modified t-test, t = 4.43, P < 0.05). In the petioles collected from the forest hab itat site, 70 individuals of morphospecies one, 50 individuals of morphospecies two, five individuals of morphospecies three, 103 individuals of morphospecies four and 108 individuals of morphospecies five were found, along with 109 Scolytinae larvae and 158 non-Scoly tinae individuals. In the petio les collected from the human disturbed habitat site, 100 indi viduals of morphospeci es one, 33 individuals of morphospecies
5 two, two individuals of morphospecies three, 104 individuals of mor phospecies four and 218 individuals of morphospecies five were found, along with 139 Scolytine larvae and 79 nonScolytinae individuals. Distribution of the abundance of each morphospecies found per tree within each habitat is shown in Figure 1. Morphospecies one, two, three and four s howed no difference in occupancy between habitats (Figure 2a, 2b, 2c, 2d). Morphospecies fi ve showed a higher incidence of occupancy in C. obtusifolia petioles of the human-disturb ed habitat (P = 0.0058; Figur e 2e). Scolytine larvae abundance per petiole was not different between sites (Figure 3). Th e non-Scolytinae fauna species found were more abundant in forest habitat C. obtusifolia petioles (P = 0.0443; Figure 4). 0 50 100 150 200 250 1234ABCD Disturbed Trees Forest TreesAbundance Morphospecies 5 Morphospecies 4 Morphospecies 3 Morphospecies 2 Morphospecies 1 FIGURE 1. Scolytinae (Coleoptera) abundance of each of five morphospecies found per C. obtusifolia individual in both disturbe d and forest habitats. a) 0 0.5 1 1.5 2 2.5 3 Disturbed Forest Habitat TypeAbundanceb) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 Disturbed Forest Habitat TypeAbundance
6 c) 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 Disturbed Forest Habitat TypeAbundanced) 0 0.5 1 1.5 2 2.5 3 3.5 Disturbed Forest Habitat TypeAbundance e)* 0 1 2 3 4 5 6 7 Disturbed Forest Habitat TypeAbundance FIGURE 2. Average abundance of each mor phospecies (Coleoptera: Scolytinae) per C. obtusifolia petiole in human disturbed versus forest habitats. a) Morphospecies one (t = 0.990, df = 78, P = 0.325, Std Error = 0.758) b) Morphospecies two (t = -0.934, df = 78, P = 0.353, Std Error = 0.455) c) Morphospecies three (t = -0.924, df = 78, P = 0.358, Std Error = 0.081) d) Morphospecies four (t = 0.014, df = 78, P = 0.99, Std Error = 1.844) e) Morphospecies five (t = 2.838, df = 78, P = 0.0058) *Indicates p<0.05. 0 0.5 1 1.5 2 2.5 3 3.5 4 Disturbed Forest Habitat TypeAbundance FIGURE 3. Average abundance of Scol ytine (Coleoptera) larvae found per C. obtusifolia petiole in human disturbed versus forest habitats (t = 0.538, df = 78, P = 0.592, Std Error = 1.393)
7 0 1 2 3 4 5 DisturbedForest Habitat TypeAbundance FIGURE 4. Average abundance of non-Scolyt inae (Coleoptera) organisms found per C. obtusifolia petiole in human disturbed versus forest habitats. (t = -2.045, df = 78, P = 0.0443, Std Error = 1.668) Similarity between individual C. obtusifolia trees within each separate habitat showed no difference from the similarity between the two over all habitats (n = 8; Table 2). Standard error values reveal high variability of beetle com position between individual trees of both habitats (Table 2). TABLE 2. Sorensons quantitative index of similarity calculated between every Cecropia individual combination within bot h habitats separately and betw een habitats. Recorded indices reflect mean index value of each tree combina tion index calculation per comparison category. Comparison Localities Sorenson's Quantitative Index Standard Error Within Disturbed Habitat 0.518 0.100 Within Forest Habitat 0.463 0.0756 Between Habitats 0.511 0.0549 DISCUSSION The findings of this study show higher overall Scolytinae abundance in the habitat altered by humans than in the forest light gap, indicating the ability of bark beetles to successfully find host populations in patchy open areas, as well as in forest ha bitats. This is also implicated by the equal species richness between th e two observed habitats with the same five morphospecies present, as well as the equal colonization rates of 95% that are evident at both sites. These trends
8 oppose the previously hypothesized differences in both species richness and colonization ability between habitats, demonstrating that the impacts of habitat fragmentation do not significantly alter either species richness of the associated beetles or their ability to oc cupy petioles. One possible explanation for this adaptability relies on the fact that tropical bark beetle populations are known to thrive even with extrem ely low brood sizes and thus are resilient when their population densities are thre atened (Jordal and Kirkendall 1998) Therefore, even if only a few beetles of each species arrive at C. obtusifolia trees in areas affected by human land use, their populations can survive to r ecover to normal densities in th e new site. This is in part because mortality caused by predation and parasitism are low in the closed and protected petiole system (Jordal and Kirkendall 1998). Additionally according to a study performed by Beaver et al. (2001), inbreeding insect populations such as bark beetles have more successful distribution to islands than outbreeding populations, because they are less challenged by mate location and have quicker generational turnover. Thus they ha ve a higher rate of popu lation size increase due to more highly female-biased populations. Such tr ends on island systems can be applied to this study in that human disturbance causes Cecropia populations along roadsides to be isolated and therefore act as habitat islands. While colonization and richness between the tw o habitats did not differ and abundance of Scolytine beetles was greater in the disturbed habitat, data indicate a significant difference between the diversity of the two habitats, showing greater diversity in the forest habitat. Because species richness is identical between sites bu t diversity differs, the compositional difference between populations lies in the evenness of each morphospecies. These results agree with the study by Didham et al.(1998) that found no change in beetle community species richness as a result of tropical forest fragmentation, but did find differences in relative success rates of various beetle species showing differential adaptability between species. Nearly half of the observed Scolytine beetles in the human altered habitat were members of morphospecies five, while the forest habitat had a much more even distribut ion of morphospecies, with the most abundant morphospecies appearing less than one third of th e time. It could be that morphospecies five thrives particularly where the landscape has b een altered by humans. Ri bera et al. (2001) demonstrate that beetles small in size and with high dispersal ability are most likely to do well in highly degraded habitats. This concurs with my study in that morphospecies five was distinguished in part by its particularly small size. The higher overall beetle abundance in the disturbed habitat may be a result of the particul ar success of morphospecies five, because each of the other four morphospecies maintained relatively similar population sizes between sites. While abundance of Scolytinae was found to be greater in the human disturbed habitat, the abundance of non-Scolytine fauna was greater in the forest habi tat. This may be due to the fact that forest light gap habitats are in much cl oser proximity to continuous forest than are areas highly impacted by humans, such as the road side site examined in this study. The C. obtusifolia individuals in the disturbed habitat were isolate d, and surrounded by little non-Cecropia vegetation. In the natural fo rest light gaps, however, C. obtusifolia individuals were in very close proximity to high quantities of forest vegetation. Under the assum ption that non-Scolytinae fauna are not specialized to rely on Cecropia petioles, this trend can be explained by a decreased likelihood that non-Scolytine individu als could survive successfully in areas with very little plant diversity or abundance aside from C. obtusifolia Additionally, this pattern may suggest that the increased presence of non-Scolytine beetles inside C. obtusifolia petioles in the forest habitat explains the lower abundance of Scolytine beetles in this hab itat, possibly due to predation,
9 parasitism or competition for resources. Further research focused on interactions between Scolytinae and non-Scolytinae would add in sight to the patterns observed here. Community similarity at the tree level reve als values that do not vary substantially between within-site tests and betwee n-site tests. This indicates that there is nearly equivalent variability in beetle community composition from all C. obtusifolia individuals, despite which C. obtusifolia population the tree is from. This may s uggest that microhabi tat differences in C. obtusifolia trees impact beetle community compositi on as much as macrohabitat differences between the two sites. This overa ll similarity of beetle composition in petioles be tween trees and between habitats may be due to the fact that ab iotic conditions of each site are similar in how they differ from abiotic conditions in continuous forest. Light gaps and human-impacted edge environments have increased light, decreased hu midity and higher temperatures than continuous, closed canopy forest conditions (Williams-Linera 1990; Denslow et al. 1990). Scolytine beetles associated with C. obtusifolia which are light gap pioneers, may be particularly adapted to such abiotic conditions. The maintenance of beetle reproductive rates a nd population densities at both sites may be explained by the similarity in e nvironmental conditions of light gaps (natural canopy disturbance) and altered la nd-use habitats (disturbance caused by humans). In order to more thoroughly compare and contrast forest habitat C. obtusifolia associated beetle communities with human altered habitat communities, a greater sample size of trees per site would be useful. Increased sample size would be useful to discern whethe r between tree variance within one habitat is a common characteristic of these particular communities, or whether there are distinguishable habitat base d differences in beetle commun ity composition between sites. As deforestation, habitat de gradation and global climate ch ange continue to destroy primary forest habitats, it is essential to identif y and understand trends in biodiversity as they are impacted by these large-scale cha nges. While global biodiversity is declining rapidly in response to anthropogenic changes to the natural landscape (Sala et al. 2000), this study shows that Scolytinae species associated with C. obtusifolia can successfully adapt to such changes. For this reason, these beetles would serve as poor indicator species for th e impacts of deforestation on biodiversity and species interactions. ACKNOWLEDGEMENTS Id like to thank Tania Chavarria Pizarr o for advising me throughout the resear ch project process, Alan and Karen Masters for their help and support in project planning and the paper write-up process, Pablo Allen for originally introducing me to the Scolytinae-Cecropia relationship and for help in identification of beetle morphospecies differences, and Taegan McMahon for being accessible for s upport and guidance during this study. I would like to thank the Bosque Eterno de los Nios for allowing me access to the cloud fore st for collection of petioles. Thank you to the Araya Rodriguez family of Santa Elena for supporting me during the data collection process. Thank you to David Strauss for assisting me in th e field. Lastly, I would like to thank every member of the beautiful CIEE Fall 2007 programwere the bomb diggity. LITERATURE CITED Beaver, R.A., B. H. Jordal and L.R. Kirke ndall. 2001. Breaking taboos in the tropics: incest promotes colonization by wood-boring beet les. Global Ecology a nd Biogeography, 10(4): 345-357.
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