Heraldo L. Vasconcelos

Matrix habitat and species richness in tropical forest remnants

The abilities of species to use the matrix of modified habitats surrounding forest fragments may aAect their vulnerability in fragmented landscapes. We used long-term (up to 19-year) studies of four animal groups in central Amazonia to test whether species’ abundances in the matrix were correlated with their relative extinction proneness in forest fragments. The four groups, birds, frogs, small mammals, and ants, had varying overall responses to fragmentation: species richness of small mammals and frogs increased after fragment isolation, whereas that of birds and ants decreased. For all four groups, a high proportion of nominally primary-forest species were detected in matrix habitats, with 8‐25% of species in each group found exclusively in the matrix. The three vertebrate groups (birds, small mammals, frogs) exhibited positive and significant correlations between matrix abundance and vulnerability to fragmentation, suggesting that species that avoid the matrix tend to decline or disappear in fragments, while those that tolerate or exploit the matrix often remain stable or increase. These results highlight the importance of the matrix in the dynamics and composition of vertebrate communities in tropical forest remnants, and have important implications for the management of fragmented landscapes. # 1999 Elsevier Science Ltd. All rights reserved.

Conservation: Rainforest fragmentation kills big trees

In tropical forests, large canopy and emergent trees are crucial sources of fruits, flowers and shelter for animal populations. They are also reproductively dominant and strongly influence forest structure, composition, gap dynamics, hydrology and carbon storage. Here we show that forest fragmentation in central Amazonia is having a disproportionately severe effect on large trees, the loss of which will have major impacts on the rainforest ecosystem.

Rainforest fragmentation kills big trees.

In tropical forests, large canopy and emergent trees are crucial sources of fruits, flowers and shelter for animal populations. They are also reproductively dominant and strongly influence forest structure, composition, gap dynamics, hydrology and carbon storage. Here we show that forest fragmentation in central Amazonia is having a disproportionately severe effect on large trees, the loss of which will have major impacts on the rainforest ecosystem.

Global decomposition experiment shows soil animal impacts on decomposition are climate-dependent

Climate and litter quality are primary drivers of terrestrial decomposition and, based on evidence from multisite experiments at regional and global scales, are universally factored into global decomposition models. In contrast, soil animals are considered key regulators of decomposition at local scales but their role at larger scales is unresolved. Soil animals are consequently excluded from global models of organic mineralization processes. Incomplete assessment of the roles of soil animals stems from the difficulties of manipulating invertebrate animals experimentally across large geographic gradients. This is compounded by deficient or inconsistent taxonomy. We report a global decomposition experiment to assess the importance of soil animals in C mineralization, in which a common grass litter substrate was exposed to natural decomposition in either control or reduced animal treatments across 30 sites distributed from 43°S to 68°N on six continents. Animals in the mesofaunal size range were recovered from the litter by Tullgren extraction and identified to common specifications, mostly at the ordinal level. The design of the trials enabled faunal contribution to be evaluated against abiotic parameters between sites. Soil animals increase decomposition rates in temperate and wet tropical climates, but have neutral effects where temperature or moisture constrain biological activity. Our findings highlight that faunal influences on decomposition are dependent on prevailing climatic conditions. We conclude that (1) inclusion of soil animals will improve the predictive capabilities of region- or biome-scale decomposition models, (2) soil animal influences on decomposition are important at the regional scale when attempting to predict global change scenarios, and (3) the statistical relationship between decomposition rates and climate, at the global scale, is robust against changes in soil faunal abundance and diversity.

Forest fragmentation in central Amazonia and its effects on litter-dwelling ants

We assessed responses of ants nesting in twigs in the litter layer to habitat changes associated with forest fragmentation in central Amazonia. Ants were collected along transects located at nine distances (5, 20, 40, 60, 100, 200, 300, 400, 500 m) from the edges of two isolated 100-ha fragments and two continuous-forest sites. In total, 2880 m2 of litter were examined for the presence of ant colonies. We detected a significant decrease in litter depth with increasing distance to forest edge, and an increase and then decrease in the average diameter-at-breast-height (DBH) of large trees (DBH⩾10 cm), and in tree basal area. Distance to forest edge significantly affected ant species composition and this effect was partly attributable to variation in litter depth. With the exception of one fragment, no significant changes in ant nest densities or species richness were found with increasing distance to forest edge. However, species richness of ants was greater in continuous forest than in both fragments. Furthermore, most species (65.8%) had greater nest densities in continuous forest. These results suggest that edge and isolation effects both play a role in structuring litter-dwelling ant communities in Amazonian forest remnants.

Effects of forest disturbance on the structure of ground-foraging ant communities in central Amazonia

This study evaluates biotic responses, using ants as bio-indicators, to relatively recent anthropogenic disturbances to mature forest in central Amazonia. The structure of the ground-foraging ant community was compared in four habitats that represented a gradient of disturbance associated with differences in land use. Ants were collected in undisturbed, mature forest, in an abandoned pasture, in a young regrowth forest (situated in a former pasture area), and in an old regrowth forest (established where mature forest was just cleared and abandoned). More ant species were found in mature and old regrowth forest than in the abandoned pasture. By contrast, ant abundance tended to decrease with forest maturity. Both pasture and young regrowth forest exhibited a distinct ant species composition compared to mature forest, whereas species composition in the old regrowth forest showed greater similarity to that of mature forest. In spite of differences in fallow time between former pasture areas and non-pasture areas, there is evidence that different land-management practices do result in different rates of recovery of the ant forest fauna after land abandonment. In any case, recuperation of the ground-foraging ant fauna appears to be faster than regeneration of the woody-plant community. In this sense, regrowth forests may be valuable for the conservation of ground-foraging ants and perhaps for other components of mature-forest leaf-litter fauna within the context of a fragmented landscape.

Mutualism between Maieta guianensis Aubl., a myrmecophytic melastome, and one of its ant inhabitants: ant protection against insect herbivores

SummaryThe hypothesis that ants (Pheidole minutula) associated with the myrmecophytic melastome Maieta guianensis defend their host-plant against herbivores was investigated in a site near Manaus, Amazonas, Brazil. M. guianensis is a small shrub that produces leaf pouches as ant domatia. Plants whose ants were experimentally removed suffered a significant increase in leaf damage compared with control plants (ants maintained). Ants patrol the young and mature leaves of Maieta with the same intensity, presumably since leaves of both ages are equally susceptible to herbivore attack. The elimination of the associated ant colony, and consequent increase in herbivory, resulted in reduced plant fitness. Fruit production was 45 times greater in plants with ants than in plants without ants 1 year after ant removal.

Do herbivores exert top-down effects in Neotropical savannas? Estimates of biomass consumption by leaf-cutter ants

Abstract Question: Plant communities in Paleotropical savannas are regulated by a combination of bottom-up and top-down effects. However, the paucity of ungulates and other large herbivores in Neotropical savannas has led to speculation that these communities are primarily structured by physical factors such as fire, precipitation and soil chemistry. We addressed the following question: How much plant biomass is consumed by leaf-cutter ants in Neotropical savannas, and is it comparable to the amount of biomass consumed by herbivores in Paleotropical savanna sites? Location: Our study was conducted at the Estação Ecológica do Panga, located 30 km south of Uberlândia, Minas Gerais, Brazil. All field work was conducted in the vegetation type known as cerrado sensu stricto. Methods and Results: Using direct measurements of herbivory, coupled with estimates of plant productivity and ant colony density, we found that leaf-cutter ants (Atta spp.) consume 13–17% of the foliar biomass produced annually by woody plants in a Neotropical savanna (Brazilian cerrado). Although comparisons with other savanna systems are complicated by methodological differences among studies, the proportion of biomass consumed by Atta species is about 25% of that consumed by the entire ungulate community in some African savannas and greater than or comparable to the total herbivory observed in some terrestrial ecosystems. Conclusions: We hypothesize that this intense biomass consumption by Atta will have important ecological consequences for the cerrado ecosystem, because leaf-cutter abundance increases in fragmented or degraded habitats. These effects are likely to be exacerbated as anthropogenic pressure in this biodiversity hotspot increases.

Cheating the cheater: domatia loss minimizes the effects of ant castration in an Amazonian ant-plant

Abstract. We studied the relationship between Hirtella myrmecophila (Chrysobalanaceae), a common but little-studied Amazonian ant-plant that produces leaf-pouches as domatia, and its obligate ant partner, Allomerus octoarticulatus. Field observations revealed that H. myrmecophila drops domatia from older leaves, a characteristic that is unique among myrmecophytes. The physiological mechanism for abortion of domatia is currently unknown, but this characteristic allows for the existence, within the same plant, of branches with and without ants. Older branches generally bear only old leaves with no domatia and therefore have no ants, whereas younger branches have leaves of various ages. Ants forage mainly on new leaves, and experimental removal of ants showed that A. octoarticulatus is crucial for defense of these leaves against insect herbivores. However, A. octoarticulatus also acts as a castration parasite, severing the plants inflorescences. Mature flowers and fruits were only found on older branches with no ants, and flower production was 8 times greater on plants whose ants were experimentally removed than on control plants. Given the reproductive costs inflicted by its mutualistic partner, we suggest that abortion of domatia is a strategy developed by H. myrmecophila to minimize the effects of cheating by A. octoarticulatus. These results support the view that evolutionary conflicts of interest between mutualistic species often impose selection for cheating on the partner, as well as for mechanisms to retaliate or to prevent super-exploitation. Opposing selection pressures, operating independently on the two partners, probably help to maintain the evolutionary stability of this mutualistic relationship.

Dynamics of the leaf-litter arthropod fauna following fire in a neotropical woodland savanna.

Fire is an important agent of disturbance in tropical savannas, but relatively few studies have analyzed how soil-and-litter dwelling arthropods respond to fire disturbance despite the critical role these organisms play in nutrient cycling and other biogeochemical processes. Following the incursion of a fire into a woodland savanna ecological reserve in Central Brazil, we monitored the dynamics of litter-arthropod populations for nearly two years in one burned and one unburned area of the reserve. We also performed a reciprocal transplant experiment to determine the effects of fire and litter type on the dynamics of litter colonization by arthropods. Overall arthropod abundance, the abundance of individual taxa, the richness of taxonomic groups, and the species richness of individual taxa (Formiciade) were lower in the burned site. However, both the ordinal-level composition of the litter arthropod fauna and the species-level composition of the litter ant fauna were not dramatically different in the burned and unburned sites. There is evidence that seasonality of rainfall interacts with fire, as differences in arthropod abundance and diversity were more pronounced in the dry than in the wet season. For many taxa the differences in abundance between burned and unburned sites were maintained even when controlling for litter availability and quality. In contrast, differences in abundance for Collembola, Formicidae, and Thysanoptera were only detected in the unmanipulated samples, which had a lower amount of litter in the burned than in the unburned site throughout most of our study period. Together these results suggest that arthropod density declines in fire-disturbed areas as a result of direct mortality, diminished resources (i.e., reduced litter cover) and less favorable microclimate (i.e., increased litter desiccation due to reduction in tree cover). Although these effects were transitory, there is evidence that the increasingly prevalent fire return interval of only 1–2 years may jeopardize the long-term conservation of litter arthropod communities.