The line transect method for estimating densities of large mammals in a tropical deciduous forest: An evaluation of models and field experiments

We have evaluated techniques of estimating animal density through direct counts using line transects during 1988–92 in the tropical deciduous forests of Mudumalui Sanctuary in southern India for four species of large herbivorous mammals, namely, chital (Axis axis). sambar (Cervus unicolor). Asian elephant (Elephas maximus) and gaur (Bos gaurus) Density estimates derived from the Fourier Series and the Half-Normal models consistently had the lowest coefficient of variation. These two models also generated similar mean density estimates. For the Fourier Series estimator, appropriate cut-off widths for analyzing line transect data for the four species are suggested. Grouping data into various distance classes did not produce any appreciable differences in estimates of mean density or their variances, although model fit is generally better when data arc placed in fewer groups. The sampling effort needed to achieve a desired precision (coefficient of variation) in the density estimate is derived. A sampling effort of 800 km of transects returned a 10% coefficient of variation on estimate for ehital; for the other species a higher effort was needed to achieve this level of precision. There was no statistically significant relationship between detectability of a group and the size of the group for any species. Density estimates along roads were generally significantly different from those in the interior of the forest, indicating that road-side counts many not be appropriate for most species.     

Influence of habitat,litter type,and soil invertebrates on leaf-litter decomposition in a fragmented Amazonian landscape

Amazonian forest fragments and second-growth forests often differ substantially from undisturbed forests in their microclimate, plant-species composition, and soil fauna. To determine if these changes could affect litter decomposition, we quantified the mass loss of two contrasting leaf-litter mixtures, in the presence or absence of soil macroinvertebrates, and in three forest habitats. Leaf-litter decomposition rates in second-growth forests (>10 years old) and in fragment edges (<100 m from the edge) did not differ from that in the forest interior (>250 m from the edges of primary forests). In all three habitats, experimental exclusion of soil invertebrates resulted in slower decomposition rates. Faunal-exclosure effects were stronger for litter of the primary forest, composed mostly of leaves of old-growth trees, than for litter of second-growth forests, which was dominated by leaves of successional species. The latter had a significantly lower initial concentration of N, higher C:N and lignin:N ratios, and decomposed at a slower rate than did litter from forest interiors. Our results indicate that land-cover changes in Amazonia affect decomposition mainly through changes in plant species composition, which in turn affect litter quality. Similar effects may occur on fragment edges, particularly on very disturbed edges, where successional trees become dominant. The drier microclimatic conditions in fragment edges and second-growth forests (>10 years old) did not appear to inhibit decomposition. Finally, although soil invertebrates play a key role in leaf-litter decomposition, we found no evidence that differences in the abundance, species richness, or species composition of invertebrates between disturbed and undisturbed forests significantly altered decomposition rates.     

Historical diversification of a terra-firme forest bird superspecies: a phylogeographic perspective on the role of different hypotheses of Amazonian diversification

Among those few hypotheses of Amazonian diversification amenable to falsification by phylogenetic and population genetics methods, three can be singled out because of their general application to vertebrates: the riverine barrier, the refuge, and the Miocene marine incursion hypotheses. I used phylogenetic and population genetics methods to reconstruct the diversification history of the upland (terra-firme) forest superspecies Xiphorhynchus spixii/elegans (Aves: Dendrocolaptidae) in Amazonia, and to evaluate predictions of the riverine barrier, refuge, and Miocene marine incursion hypotheses. Phylogeographic and population genetics analyses of the X. spixiilelegans superspecies indicated that the main prediction of the riverine barrier hypothesis (that sister lineages occur across major rivers) hold only for populations separated by "clear-water" rivers located on the Brazilian shield, in central and eastern Amazonia; in contrast, "white-water" rivers located in western Amazonia did not represent areas of primary divergence for populations of this superspecies. The main prediction derived from the refuge hypothesis (that populations of the X. spixii/elegans superspecies would show signs of past population bottlenecks and recent demographic expansions) was supported only for populations found in western Amazonia, where paleoecological data have failed to support past rainforest fragmentation and expansion of open vegetation types; conversely, populations from the eastern and central parts of Amazonia, where paleoecological data are consistent with an historical interplay between rainforest and open vegetation types, did not show population genetics attributes expected under the refuge hypothesis. Phylogeographic and population genetics data were consistent with the prediction made by the Miocene marine incursion hypothesis that populations of the X. spixii/elegans superspecies found on the Brazilian shield were older than populations from other parts of Amazonia. In contrast, the phylogeny obtained for lineages of this superspecies falsified the predicted monophyly of Brazilian shield populations, as postulated by the Miocene marine incursion hypothesis. In general, important predictions of both riverine barrier and Miocene marine incursion hypotheses were supported, indicating that they are not mutually exclusive; in fact, the data presented herein suggest that an interaction among geology, sea level changes, and hydrography created opportunities for cladogenesis in the X. spixii/elegans superspecies at different temporal and geographical scales.     

Effects of flooding duration on species richness, floristic composition and forest structure in river margin habitat in Amazonian blackwater floodplain forests: implications for future design of protected areas

Rivers in central Amazonia experience annual water-level fluctuations of up to 14m, flooding vast areas of adjacent forest for periods ranging from a few to 270 days per year. At different sites, variation in the duration and type of flooding results in a mosaic of habitats that includes lakes, grasslands, forests, and streams. To study the effects of flood duration on plant species richness and floristic composition, two river margin sites were surveyed on the rivers Jaú and Tarumã-Mirim. Both areas are seasonally flooded by blackwaters, and plots were made at different topographic levels (lower, middle and upper slopes). All woody plants with DBH>5cm were inventoried in five 10 × 40m plots in each of the three topographic levels, which varied in length of flood duration and mean water level. Plant species richness did not vary significantly between topographic levels, but species composition varied substantially. At both study sites, the species composition exhibited distinctive distribution patterns with respect to the three topographic levels and river site. Differences in the distribution of dominant species in both sites probably relate to the ability of species to withstand seasonal flooding, although other edaphic factors associated with the topographic levels may also be important, especially for less-dominant, locally rare, and habitat generalist species. Species composition overlap among topographic levels at the two sites was highly variable ranging from 15% to 43%. Knowledge about the complex pattern of species composition and distributions between and among topographic levels and river sites is important for the preservation of the diverse flora of the blackwater forests and for the creation of future conservation management plans and design of protected areas in this ecosystem that will maintain the biodiversity.     

The enigma of antipredator behavior in lemurs: Evidence of a large extinct eagle on Madagascar

Numerous zoologists who study diurnal lemurs on Madagascar have noted that they react strongly to the presence of birds of prey. For two of the most intensively studied lemurs, Propithecus verreauxiand Lemur catta,there are few documented cases of raptor predation. Thus, the maintenance of this stereotypic response is enigmatic. Bird bones recovered from cave surface deposits in southwestern Madagascar include the remains of an eagle (Aquila),a genus that has disappeared from Madagascar and that would have been capable of hunting animals the size of adult P. verreauxi and L. catta.The stereotypic response of these two lemurs toward raptors may have been retained from the period when this extinct eagle inhabited the island and is reinforced by rare acts of predation by extant birds of prey.     

Regeneration of<Emphasis Type="Italic"> Rhizophora mangle</Emphasis> in a Caribbean mangrove forest: interacting effects of canopy disturbance and a stem-boring beetle

Current theory predicts that in low-density, seed-limited plant populations, seed predation will be more important than competition in determining the number of individuals that reach maturity. However, when plant density is high, competition for microsites suitable for establishment and growth is expected to have a relatively greater effect. This dichotomous perspective does not account for situations in which the risk of seed predation differs inside versus outside recruitment microsites. We report the results of a field experiment and sampling studies that demonstrate such an interaction between microsite quality and the risk of propagule predation in mangrove forests on the Caribbean coast of Panama, where it appears to play a key role in shaping the demography and dynamics of the mangrove, Rhizophora mangle. Rhizophora's water-borne propagules establish wherever they strand, but long-term sampling revealed that only those that do so in or near lightning-created canopy gaps survive and grow to maturity. These microsites afford better growth conditions than the surrounding understory and, as importantly, provide a refuge from predation by the scolytid beetle, Coccotrypes rhizophorae. This refuge effect was confirmed with a field experiment in which Rhizophora seedlings were planted at different positions relative to gap edges, from 5 m inside to 20 m outside the gap. Mortality due to beetle attack increased linearly from an average of 10% inside a gap to 72% at 20 m into the forest. The interaction between canopy disturbance and propagule predation may be having a large impact on the composition of our study forests. Being shade-tolerant, Rhizophora seedlings that escape or survive beetle attack can persist in the understory for years. However, the high rate of beetle-induced mortality effectively eliminates the contribution of advance regeneration by Rhizophora saplings to gap succession. This may explain why the shade-intolerant mangrove, Laguncularia racemosa, is able to co-dominate the canopy in low intertidal forests at our study sites.     

Performance and fate of tree seedlings on and around nests of the leaf‐cutting ant Atta cephalotes: Ecological filters in a fragmented forest

Habitat fragmentation is currently the most pervasive anthropogenic disturbance in tropical forests and some species of leaf‐cutting ants of the genus Atta (dominant herbivores in the neotropics) have become hyper‐abundant in forest edges where their nests directly impact up to 6% of the forest area. Yet, their impacts on the regeneration dynamics of fragmented forests remain poorly investigated. Here we examine the potential of Atta cephalotes nests to function as ecological filters impacting tree recruitment. Growth, survival and biomass partitioning of experimentally planted seedlings (six tree species) were examined at eight spatially independent A. cephalotes colonies in a large Atlantic Forest fragment. Seedling performance and fate (leaf numbers and damage) were monitored up to 27 months across three habitats (nest centre, nest edge and forest understorey). Plants at illuminated nest centres showed twice the gross leaf gain as understorey individuals. Simultaneously, seedlings of all species lost many more leaves at nests than in the forest understorey, causing a negative net leaf gain. Net leaf gain in the shaded understorey ranged from zero (Licania and Thyrsodium species) to substantial growth for Copaifera and Virola, and intermediate levels little above zero for Protium and Pouteria. Also seedling survival differed across habitats and species, being typically low in the centre and at the edge of nests where seedlings were often completely defoliated by the ants. Lastly, seedling survival increased strongly with seed size at nest edges while there was no such correlation in the forest. Our results suggest that Atta nests operate as ecological filters by creating a specific disturbance regime that differs from other disturbances in tropical forests. Apparently, Atta nests favour large‐seeded tree species with resprouting abilities and the potential to profit from a moderate, nest‐mediated increase in light availability.     

Leaf litter fungi in a Central Amazonian forest: the influence of rainfall,soil and topography on the distribution of fruiting bodies

Fungi are important components of tropical ecosystems, especially in the recycling of nutrients. However, there is little information on how fungal diversity is structured at scales suitable to plan their conservation. We tested if the distribution of fruiting bodies of litter fungi was random in the landscape (over 25 km²) in a tropical evergreen forest in Central Amazonia. We used linear regressions to evaluate the influence of rainfall, soil characteristics and topography on morphospecies richness and composition. Fungi were collected twice in thirty 0.25 × 250 m plots. Short-term rainfall was represented by the cumulative rainfall in the three days before each plot was surveyed. Plots were classified in two groups based on cumulative rainfall. Clay content in soil and rainfall influenced morphospecies richness, but responses to edaphic factors depended on rainfall. Wetter periods apparently decreased limiting moisture conditions in higher areas, allowing fungal activity and fruiting body production. Morphospecies composition was influenced by clay content, but influence on fungi was probably indirect as clay content was correlated with altitude, plant community and nitrogen availability. Our results suggest that the species of litter fungi are not randomly distributed in the landscape. Furthermore, they indicate that it is viable to conduct mesoscale evaluations of fungal diversity, if the temporal and spatial variation and their interaction are taken into account.     

Variation in pollinator assemblages in a fragmented landscape and its effects on reproductive stages of a self-incompatible treelet, <Emphasis Type="Italic">Psychotria suterella</Emphasis> (Rubiaceae)

Few studies of plant–pollinator interactions in fragmented landscapes evaluate the consequences of floral visitor variation on multiple stages of plant reproduction. Given that fragmentation potentially has positive or negative effects on different organisms, and that self-incompatible plant species depend on pollinators for sexual reproduction, differences in floral visitor assemblages may affect certain plant reproductive stages. We evaluated how pollinator assemblage, availability of floral resources, pollination, reproductive output, and seed and seedling performance of Psychotria suterella Muell. Arg. varied among three fragmentation categories: non-fragmented habitats, fragments connected by corridors, and isolated fragments. Richness and frequency of floral visitors were greater in fragments than in non-fragmented sites, resulting mainly from the addition of species typically found in disturbed areas. Although 24 species visited Psychotria suterella flowers, bumblebees were considered the most important pollinators, because they showed the highest frequency of visits and were present in eight out of ten sites. Additionally, the number of pollen tubes per flower per visit was lower in areas without bumblebees. The increased visitation in fragments seemed to enhance pollination slightly. However, fruit and seed output, germination, and seed and seedling mass were similar in non-fragmented sites, connected sites, and isolated fragments. Our results suggested that, even for a self-incompatible species, responses to habitat fragmentation at different stages of plant reproduction might be decoupled from the responses observed in floral visitors, if fruit set is not pollen limited. If all reproductive stages were considered, variation on the small scale was more important than the variation explained by fragmentation category. In spite of its self-incompatible breeding system, this plant–pollinator system showed resilience to habitat fragmentation, mainly as a result of high availability of potential mates to P. suterella individuals, absence of pollen limitation, and the presence of bumblebees (Bombus spp.) throughout this highly connected landscape.     

Top‐down effects on pelagic heterotrophic nanoflagellates (HNF) in a large river (River Rhine): do losses to the benthos play a role?

1. The abundance, production and control of pelagic heterotrophic nanoflagellates (HNF) in the Lower River Rhine (Germany) were investigated. Field samples (live‐counting technique) were taken at least every 2 weeks at Cologne (km 685) over a period of 20 months. In addition, Lagrangian sampling was carried out 160 km downstream of Cologne (Kalkar–Grieth, km 845) over a period of 12 months. Potential HNF growth rates and loss rates caused by planktonic predators were estimated in the laboratory (size fractionation experiments) and compared with the changes in HNF‐density in a water parcel flowing downstream. 2. Mean abundance (±SD) ranged from 7 ± 6 to 4890 ± 560 individuals mL^–1 and was positively correlated with discharge. Heterotrophic nanoflagellates abundance increased up to 30‐fold during flood events, whereas there were only minor changes in the taxonomic composition. 3. HNF growth rate ranged from 0.16 ± 0.12 to 1.98 ± 0.10 day^–1. Between 0 and 95% (mean: 32%) of the gross production was lost to planktonic predators; a larger portion between 0 and 195% (mean: 64%) of the HNF gross production was lost by other means. 4. There may be an important role for benthic predators in the control of pelagic HNF. First, production of HNF was high and grazing losses to planktonic predators low at times when HNF abundance was low. Secondly, high in situ loss rates (not explicable by planktonic predators), which were positively related to temperature, indicate the importance of biotic interactions. Thirdly, the dependence of HNF abundance on discharge indicated a decrease grazing intensity with rising water levels (increase in water volume/colonised river bed ratio). 5. The impact of discharge on planktonic HNF mediated by the grazing impact of benthic predators was modelled, showing a good fit with the field data.