Abstract ]]>
The spatial heterogeneity hypothesis has been invoked to explain the increase in species diversity from the poles to the tropics: the tropics may be more diverse because they contain more habitats and microhabitats. In this paper, the spatial heterogeneity hypothesis prediction was tested by evaluating the variation in richness of two guilds of insect herbivores (gall-formers and free-feeders) associated with Baccharis ]]>
(Asteraceae) along a latitudinal variation in Brazil. The seventeen populations of B. dracunculifolia selected for insect herbivores sampling were within structurally similar habitats, along the N-S distributional limit of the host plant, near the Brazilian sea coast. Thirty shrubs were surveyed in each host plant population. A total of 8 201 galls and 864 free-feeding insect herbivores belonging to 28 families and 88 species were sampled. The majority of the insects found on B. dracunculifolia were ]]>
Baccharris genus radiated in Southern Brazil. Other diversity indices and evenness estimated for both gall-forming and free feeding insect herbivores, did not change with latitude, suggesting a general ]]>
B. dracunculifolia. Thus it is probable that, insect fauna sample in each site resulted of large scale events, as speciation, migration and coevolution, while at local level, the population of these insects is regulated by ecological forces which operate in the system. Rev. Biol. Trop. 59 (3): 1419-1432. Epub 2011 September 01.
Key words: community organization, latitudinal diversity gradient, spatial heterogeneity hypothesis, species diversity. Resumen La hipótesis ]]>
B. ]]>
(Asteracea) a lo largo de un gradiente latitudinal en Brasil. Las diecisiete poblaciones de B. dracunculifolia seleccionadas para el muestreo de insectos herbívoros estaban en hábitats con una estructura similar, a lo largo del límite Norte-Sur de distribución de la planta hospedera, cerca de la ]]>
B. dracunculifolia estaban restringidos a un sitio específico en lugar de tener una distribución geográfica similar a la de la planta hospedera. La riqueza de especies de insectos de alimentación libre no se vió afectada por el gradiente latitudinal, por lo que se corroboró la hipótesis de ]]>
Baccharis se extendieron hacia el sur de Brasil. Otros índices de diversidad y equidad estimados no variaron con la latitud para ninguno de los dos grupos de insectos herbívoros evaluados, lo que sugirie una estructura general para diferentes conjuntos de herbívoros asociados con la planta hospedera B. ]]>
. Por lo tanto, es probable que, la muestra de insectos en cada sitio sea resultado de eventos a gran escala, como la especiación, migración y coevolución; mientras que a nivel local la población de estos insectos está regulada por fuerzas ecológicas que operan dentro del sistema. Palabras clave: ]]>
Latitudinal variations in species diversity are among the most conspicuous and universal patterns in community ecology (Pianka 1966). According Willig
et al. (2003), just with notable exceptions, the pattern of higher diversity at low latitudes holds true regardless of the biota’s taxonomic affiliation (e.g., mammals, fishes, arthropods and plants), geographic context (e.g., all continents and oceans) or time domain (recent and 70 Mya). Explanations for higher (Buckley et al. 2010) or habitat heterogeneity (Whitehouse et al. 2009) to explain the increase of diversity towards the tropics. ]]>
In this paper the focus is on the spatial heterogeneity hypothesis, and the study objects are different insect herbivore guilds associated with a single host plant. On the local scale, the spatial heterogeneity hypothesis suggests that the tropics are more diverse because they contain more microhabitats (Pianka 1966). On a regional scale, this hypothesis has been proposed to explain the increase of species diversity from the poles to the tropics: the tropics are more diverse because they contain more ]]>
et al. 1998, Thomaz et al. 2008). On the other hand, as a corollary, the spatial heterogeneity hypothesis ]]>
The most species alive are tropical arthropods associated with plants (Price et al. 1995). Herbivorous arthropods maintain many important ecosystem processes and form sizeable parts of terrestrial food webs (Lewinsohn & Roslin 2008). ]]>
et. al. 1998, Novotny & Basset 2005). In fact, comparatively to free-feeding insects, galling insects generally are ]]>
et al. 1998, Golden & Crist 1999). Despite the high ]]>
et al. 1995, Ribeiro et al. 1998, Majer et al. 2001). Moreover, many insect taxa as aphids (Dixon et al. 1987), Ichneumonidae (Sime & Brower 1998) and ]]>
for speciation within a region (Buckley et al. 2010). While some studies include herbivores from several insect orders, studies of more than one guild on the same hosts ]]>
Baccharis dracunculifolia.
Materials and methods Study system: The Asteraceae comprises approximately 1 110 genera and 25 000 species. The genus Baccharis belongs to the subtribe Baccharidinae, which is restricted to the American Continent (Barroso 1976). The Baccharidinae probably ]]>
Baccharis is the largest genus of Baccharidinae (approximately 400 species) and most species are found in the South and Southeast regions of Brazil, suggesting that this region represent the center of the genus origin (Jarvis et al. 1991). B. dracunculifolia (alecrim vassourinha) is a widespread, perennial and woody dioecious ]]>
et al. 2003, Fagundes et al. 2005). It grows in open and disturbed habitats, especially along highways and in abandoned pastures. Because it is an evergreen plant, its leaves and branches are important food resources for a variety of insect herbivores, especially coleopterans, heteropterans, hemipterans and orthopterans (Fagundes et al. 2005). ]]>
B. dracunculifolia also supports the largest known fauna of galling insects (17 species) in the Neotropics (Fernandes et al. 1996). The degree of habitat disturbance did not influence the richness of galls associated with the host plant B. dracunculifolia (Julião et al. 2005). ]]>
Study areas: The B. dracunculifolia shrubs selected for insect samples belonged to seventeen populations located along the Brazilian sea coast, from the Southern to the Northern distributional limit of the host plant inside Brazil. All plant populations occurred in disturbed environments along highways, within 500m of the seashore and ]]>
Table 1, Fig. 1). The surrounding vegetation ranged from 0.3 to 2.0m height and was comprised by grasses and other invasive species belonging principally to the Asteraceae, Convolvulaceae, Malvaceae, Melastomataceae and Solanaceae botanical families. At each host plant population, thirty B. dracunculifolia shrubs were selected to take herbivores samples. The shrubs selected for this were young, non-flowering or fruiting plants with 1.5-2.0m high and stem diameters <15cm. ]]>
Herbivore samples: Herbivore insects associated with B. dracunculifolia were censused by direct sampling on the host plant. Thus, each shrub was inspected during ten minutes and all free-feeding herbivore insects and galls observed were handling collected. The herbivores were sampled during the summer season (October of 2001 through January of 2002) in order to minimize ]]>
B. dracunculifolia. Therefore, potentially transient insect herbivores could have been included in data collection and analysis. ]]>
The effects of latitudinal variation on the two guilds of insect herbivores were tested using simple linear regressions. For each guild, linear regression analyses were performed using latitudinal variation as the independent variable and the Hill’s number of diversity (N0, N1 and N2) or evenness as dependent variable. The Hill’s number of diversity N0, N1 and N2 represent, respectively, the ]]>
a species assemblage. Thus, the unity of Hill’s numbers is species, making easy data interpretation and comparisons with other results. The number of abundant species (N1) represent the exponential function of Shannon’s index (N1=eH’), while the number of very abundant species (N2) is the inverse of Simpson’s index (N2=1/S). In addition, evenness was determined by the ratio of ]]>
Results
General patterns: A total of 8 201 galls and 864 free-feeding insect herbivores were collected from B. dracunculifolia at the seventeen sample points. The fauna of herbivorous insects associated with B. dracunculifolia was composed by 88 species from 28 families. The insect families with more species were Chrysomelidae (12 ]]>
unidentified species of Scolytidae deserve further studies due to their high abundance and frequency in the samples. This species was commonly observed feeding on meristems and new leaves of host plant (Appendix). ]]>
The guild of free-feeding insects represented 90.99% of total insect herbivore species associated with B. dracunculifolia, while gall-forming contributed with 9.01% of the sampled species. On the other hand, gallforming insects represented the most abundant feeding guild (90.48%), whereas free-feeding insects represented just ]]>
Appendix). Latitudinal gradient ]]>
The number of gall-inducing insect species associated with B. dracunculifolia decreased significantly towards the North, low latitude, limit of the host plant distribution (F=31.563, p=0.007, r2=67.78). However, the number of abundant species (F=1.469, p=0.245, r2=9.49), very abundant species (F=2.289, p=0.152, r2=14.05) and evenness (F=0.234, p=0.636, r2=1.642) did not vary ]]>
Fig. 2). When free-feeding insect herbivores were analyzed, none of the measures of species diversity (specie richness: F=1.988, p=0.179, r2=11.70; number of abundant species: F=0.372, p=0.551, r2=2.42; number of very abundant ]]>
2=0.34 and evenness: F=0.234, p=0.636, r2=1.64) were significantly related to variations in latitude (Fig. 3). Discussion ]]>
The number of gall-inducing insect species associated with Baccharis dracunculifolia decreased towards the equator. The observed result did not fit the more common pattern of increasing species diversity as latitude decreases (Willig et al. 2003, Lewinsohn & Roslin 2008). Factors such as geographic distribution, center of origin, taxonomic isolation and local diversity of the host plants have been used to justify the absence of correlation between ]]>
that our results are related to the evolutionary history of the host plant and the highly specialized feeding habit of gall-inducing insects. The genus Baccharis is more species-rich in the Southern region of Brazil, that probably corresponds to the genus origin center (Jarvis et al. 1991). Moreover, ]]>
Baccharis genus radiated into the Southern region of Brazil, justifying the decrease of galling insect richness towards the North, limit of host plant observed in this study. ]]>
Species’ ranges may be constrained by both climatic tolerances and barriers to dispersal (e.g. mountains and rivers). In the former case, species can disperse to a new habitat but fail to become established, whereas in the latter case species might never have opportunity to reach the new habitat even though they have attributes needed to persist there (Buckley et al. 2010). In this study, physical barriers are uncommon among the sample sites, but the ]]>
over time (niche conservatism hypothesis) might drive species distributions in space (Buckley
et al. 2010, Kozak & Wiens 2010). However, adequate experimental design is need to test the effectiveness of niche conservatism to determine the regional distribution of galling species associated with B. dracunculifolia. The variation in latitude did not ]]>
associated with B. dracunculifolia. This result supports the spatial heterogeneity hypothesis prediction because species richness should not change within structurally similar habitats along a latitudinal gradient. However, Dawidowitz & Rosenzweigh (1998) present four examples (ants, grasshopper, scorpions and mammals) where diversity trends within single biome types change along the latitudinal variation. In contrast, no significant ]]>
Acacia trees in Eastern Australia. The findings suggest that different mechanisms operate for different taxa and that it is not wise to generalize the effects of latitude on ]]>
Despite of different patterns of species richness observed for gall-inducing and free feeding insects, neither the diversity indices represented by Hill’s number N1 and N2 or evenness of these two guilds were ]]>
B. dracunculifolia (Fagundes et al. 1996, Andrew & Hughes 2005). In fact, there are strong evidences that bottom-up (Faria & Fernandes 2001, Espírito-Santo & Fernandes 2002) and top-down (Fagundes et al. ]]>
B. dracunculifolia and regulate diversity at the local level. The majority of tropical insect herbivores are relatively rare (Price et al. 1995). Generally, just a ]]>
et al. 1995), Homoptera (Dansa & Rocha 1992) and galling insects (Fernandes et al. 1996), can be found predictably and at high densities on specific host plants in tropical regions. However, the composition of herbivorous families observed in this study corroborates the general insect composition patterns described in other tropical areas. Usually, Chrysomelidae and Curculionidae are folivores recurrently important components of ]]>
et al. 1997, Basset 2001, Neves et al. 2010), while Cecidomyiidae is dominant among the galling insects (Carneiro et al. 2009). The general pattern of low density and frequency, specially observed in free-feeding insect guild, ]]>
et al. 1984, Lawton 1991, Lewinsohn & Roslin 2008). The majority of insect herbivores, especially free-feeding insects found associated with B. dracunculifolia, showed low population density and frequency. Low density could be associated with generalist habits of herbivores permitting migration to a variety of host plants, while low species ]]>
found on B. dracunculifolia were restricted to a specific site rather than having a geographic distribution mirroring that of the host plant. The variation in ]]>
and time and across different environments (Novotny 2009). Thus, it is probable that local events, such as biological interactions, regulate the population dynamics of herbivores associated with B. dracunculifolia. In contrast, the insect ]]>
B. dracunculifolia. Thereby, the insect fauna sample from each site, resulted of large scale events, as speciation, migration and coevolution, while at local level, the population of these insects is regulated by ecological forces which operate in the system. Understanding the core of the relative role of ecological and evolutionary processes will be essential to predict general community ]]>
strategies for insect conservation (Perry et al. 1998, Lawes et al. 2000). Acknowledgments ]]>
We thank B.G. Madeira, P.W. Price and T.M. Lewinsohn for insightful comments on earlier drafts of the manuscript, and two other anonymous referees who made valuable contributions to this paper. Financial support by the Fapemig (the Minas Gerais Research Foundation) and CNPq (the National Research Council of Brazil). ]]>
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Correspondencia a: Marcílio Fagundes. Laboratório de Biologia da Conservação DBG/ CCBS, Universidade Estadual de Montes Claros, Montes Claros, MG, Brazil, 39401-089; marcilio.fagundes@gmail.com G. Wilson Fernandes. Ecologia Evolutiva & Biodiversidade/DBG/ICB/Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil; gwfernandes@gmail.com
Received 21-VII-2010. Corrected 07-I-2011. Accepted 08-II-2011. ]]>
