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 micro-habitats. 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 dracunculifolia (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 restricted to a specific site rather than having a geographic distribution mirroring that of the host plant. Species richness of free-feeding insects was not affected by latitudinal variation corroborating the spatial heterogeneity hypothesis. Species richness of gall-forming insects was positively correlated with latitude, probably because galling insect associated with 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 structure for different assemblages of herbivores associated with the host plant 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.
Highlights
Latitudinal variations in species diversity are among the most conspicuous and universal patterns in community ecology (Pianka 1966)
The data were collected within homogeneous habitat and the arthropod sampled was associated with the host plant Baccharis dracunculifolia
General patterns: A total of 8 201 galls and 864 free-feeding insect herbivores were collected from B. dracunculifolia at the seventeen sample points
Summary
Latitudinal variations in species diversity are among the most conspicuous and universal patterns in community ecology (Pianka 1966). Many insect taxa as aphids (Dixon et al 1987), Ichneumonidae (Sime & Brower 1998) and gall-forming insects (Fernandes & Price 1988) do not fit the general pattern of increasing species richness with decreasing latitude Evolutionary explanations to these latitudinal diversity anomalies have focused on either variation in rates of diversification or the amount of time available for speciation within a region (Buckley et al 2010). In this study the prediction of spatial heterogeneity hypothesis was tested by evaluating the variation in diversity of two guilds of insect herbivores (galling and free-feeding) along the distributional limits of the host plant, near the Brazilian sea coast. The data were collected within homogeneous habitat and the arthropod sampled was associated with the host plant Baccharis dracunculifolia
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