Stem respiration in tropical forests along an elevation gradient in the Amazon and Andes

Autotrophic respiration involves the use of fixed carbon by plants for their own metabolism, resulting in the release of carbon dioxide as a by‐product. Little is known of how autotrophic respiration components vary across environmental gradients, particularly in tropical ecosystems. Here, we present stem CO₂ efflux data measured across an elevation transect spanning ca. 2800 m in the Peruvian Amazon and Andes. Forest plots from five elevations were studied: 194, 210, 1000, 1500, and 3025 m asl Stem CO₂ efflux (R_s) values from each plot were extrapolated to the 1‐ha plot level. Mean R_s per unit stem surface area declined significantly with elevation, from 1.14±0.12 at 210 m elevation to 0.62±0.09 μmol C m⁻² s⁻¹ at 3025 m elevation. When adjusted for changing forest structure with elevation, this is equivalent to 6.45±1.12 Mg C ha⁻¹ yr⁻¹ at 210 m elevation to 2.94±0.19 Mg C ha⁻¹ yr⁻¹ at 3025 m elevation. We attempted to partition stem respiration into growth and maintenance respiration components for each site. Both growth and maintenance respiration rates per unit stem showed similar, moderately significant absolute declines with elevation, but the proportional decline in growth respiration rates was much greater. Stem area index (SAI) showed little trend along the transect, with declining tree stature at higher elevations being offset by an increased number of small trees. This trend in SAI is sensitive to changes in forest stature or size structure. In the context of rapid regional warming over the 21st century, such indirect, ecosystem‐level temperature responses are likely to be as important as the direct effects of temperature on maintenance respiration rates.     

Effects of freshwater shrimp assemblages on benthic communities along an altitudinal gradient of a tropical island stream

1. In tropical island stream ecosystems freshwater shrimps are often the dominant macroconsumers and can play an important role in determining benthic community composition. However, most studies of the ecological role of shrimps are limited to high‐altitude shrimp‐dominated sites where other biota (fishes and snails) are absent or significantly less abundant than at lower altitudes. 2. We examined how effects of different shrimp assemblages on benthic communities changed along an altitudinal gradient in a tropical island stream in Puerto Rico. We used electroshocking and observations to quantify abundance and taxonomic composition of shrimp assemblages at three sites (300, 90 and 10 m a. s. l) along the Río Espíritu Santo. We also experimentally manipulated access of shrimps to the benthic environment simultaneously at each site using electric fences over a 35‐day period. 3. At the high‐altitude site, exclusion of shrimps (predominantly Atya spp. and Xiphocariselongata) resulted in significantly greater accrual of organic and inorganic material, chlorophyll a and algal biovolume. In the absence of shrimps, the algal community was dominated by filamentous green algae (Chlorophyta: Oedogonium and Rhizoclonium). Excluding shrimps did not affect total insect biomass but significantly increased sessile chironomids (Diptera: Chironomidae). We observed similar treatment effects at the mid‐altitude site where shrimps (primarily Macrobrachium spp. and X. elongata) occurred at lower densities. In contrast, at the low‐altitude site there were no treatment differences in organic and inorganic material, chlorophyll a, algal biovolume, algal assemblage composition and insects. 4. The lack of treatment differences at the low‐altitude site was probably because of very high densities of grazing snails (Thiaragranifera and Neritina spp.) which reduced organic and inorganic resources and obscured potential shrimp effects. 5. This study demonstrates that freshwater shrimps can play an important role in determining benthic community composition; however, their effects vary and appear to depend on the presence of other biota. This study suggests that loss of shrimps as a result of anthropogenic disturbances will have different effects on the stream community depending upon location along the altitude gradient.     

Tadpoles enhance microbial activity and leaf decomposition in a neotropical headwater stream

1. Relationships between biodiversity and ecosystem function are of increasing interest, particularly in freshwater ecosystems where species losses are occurring at unprecedented rates. Amphibian declines have been associated with a loss of ecosystem function in neotropical streams, but little is known of the potential roles of stream‐dwelling tadpoles in leaf decomposition. Leaf litter is an important energy source to streams, and the breakdown of this material to fine particulate organic matter (FPOM) is a key ecosystem function. 2. We used mesocosms in a natural stream setting to quantify the effects of grazing tadpoles, shredding macroinvertebrates and a combination of the two on leaf decomposition and associated microbial activity. We measured respiration rates of decomposing leaves, particulate organic matter (POM) and leaf biofilm biomass and C : N : P ratios, and leaf area loss in 4 treatments: Control, tadpole only (TP), tadpole and shredding macroinvertebrates (TP + INV) and shredding macroinvertebrates only (INV). We hypothesised that tadpoles would enhance leaf decomposition by changing nutrient availability and stimulating microbial activity. 3. Respiration rates ranged from 3.1 to 6.0 mg O₂ dry mass^?1 h^?1 and were significantly higher in the TP and TP + INV treatments than in the control. The TP + INV treatment had significantly higher POM in chambers than the control and INV treatments. The TP treatment had significantly lower leaf biofilm biomass than the control and INV treatments. 4. Tadpoles influenced the elemental balance of C and N in POM and leaf biofilm. In contrast to our prediction, molar C : N ratios were higher in the TP + INV treatment than in the control. Mean molar N : P ratios in POM were higher in the TP + INV treatment than in any other treatment. Leaf biofilm followed a similar pattern, but both TP and TP + INV had significantly higher N : P ratios than the control and INV treatments. Leaf area loss was greatest when tadpoles and invertebrates were together (TP + INV = 0.6% leaf area loss per mg organism) than separate (TP = 0.1%, INV = 3%), indicating facilitation. 5. Tadpoles indirectly affected leaf decomposition by influencing microbial communities and macroinvertebrate feeding. As such, ongoing amphibian declines may adversely affect a critical ecosystem function in freshwater habitats.     

Forest‐fire regimes affect thermoregulatory opportunities for terrestrial ectotherms

Fire‐induced changes in canopy openness may affect sunlight penetration to the forest floor, and thus the operative temperatures available to terrestrial ectotherms. We examined thermal regimes for two types of ectotherms: diurnally active species that utilize sun‐exposed patches to regulate their body temperatures, and nocturnally active species that depend upon solar radiation striking the rocks under which they shelter. We measured canopy openness, shrub height, radiation transmission and operative environmental temperatures in the open and inside reptile retreat‐sites, at 24 study sites in eucalypt forests in two regions (Gosford and Yengo) in south‐eastern Australia. All sites were last burnt in 2000–2001, but had experienced different fire frequencies (1–4 fires over the previous 37 years). In Gosford, higher fire frequencies reduced canopy openness and radiation transmission at ground and shrub level, and thus reduced environmental temperatures and the thermal quality of reptile habitats. Our modelling based on thermal preferenda of an endangered snake species (the broad‐headed snake Hoplocephalus bungaroides) suggests that increased fire frequency at Gosford halved the amount of time an animal could spend within its preferred (set‐point) range, regardless of whether it thermoregulated beneath rocks or basked out in the open. At Yengo, however, fire frequency did not affect the thermal quality of reptile habitats. Thus, the effects of fire frequency on forest structure and the thermal environment at ground level differed between adjacent areas, and relatively small changes in canopy openness translated into major effects on thermoregulatory opportunities for reptiles. Although fire is a useful management tool for creating open habitats, we need to understand more about the effects of fire frequency on vegetation structure and thermal environment before we can use fire to manage habitats for reptiles.     

Comprehensive assessment of carbon productivity, allocation and storage in three Amazonian forests

The allocation and cycling of carbon (C) within forests is an important component of the biospheric C cycle, but is particularly understudied within tropical forests. We synthesise reported and unpublished results from three lowland rainforest sites in Amazonia (in the regions of Manaus, Tapajós and Caxiuanã), all major sites of the Large‐Scale Biosphere–Atmosphere Programme (LBA). We attempt a comprehensive synthesis of the C stocks, nutrient status and, particularly, the allocation and internal C dynamics of all three sites. The calculated net primary productivities (NPP) are 10.1±1.4 Mg C ha⁻¹ yr⁻¹ (Manaus), 14.4±1.3 Mg C ha⁻¹ yr⁻¹ (Tapajós) and 10.0±1.2 Mg C ha⁻¹ yr⁻¹ (Caxiuanã). All errors bars report standard errors. Soil and leaf nutrient analyses indicate that Tapajós has significantly more plant‐available phosphorus and calcium. Autotrophic respiration at all three sites (14.9–21.4 Mg C ha yr⁻¹) is more challenging to measure, with the largest component and greatest source of uncertainty being leaf dark respiration. Comparison of measured soil respiration with that predicted from C cycling measurements provides an independent constraint. It shows general good agreement at all three sites, with perhaps some evidence for measured soil respiration being less than expected. Twenty to thirty percent of fixed C is allocated belowground. Comparison of gross primary productivity (GPP), derived from ecosystem flux measurements with that derived from component studies (NPP plus autotrophic respiration) provides an additional crosscheck. The two approaches are in good agreement, giving increased confidence in both approaches to estimating GPP. The ecosystem carbon‐use efficiency (CUEs), the ratio of NPP to GPP, is similar at Manaus (0.34±0.10) and Caxiuanã (0.32±0.07), but may be higher at Tapajós (0.49±0.16), although the difference is not significant. Old growth or infertile tropical forests may have low CUE compared with recently disturbed and/or fertile forests.     

Population Structure of the Lower Keys Marsh Rabbit as Determined by Mitochondrial DNA Analysis

ABSTRACT We used nucleotide sequence data from a mitochondrial DNA fragment to characterize variation within the endangered Lower Keys marsh rabbit (Sylvilagus palustris hefneri). We observed 5 unique mitochondrial haplotypes across different sampling sites in the Lower Florida Keys, USA. Based on the frequency of these haplotypes at different geographic locations and relationships among haplotypes, we observed 2 distinct clades or groups of sampling sites (western and eastern clades). These 2 groups showed low levels of gene flow. Regardless of their origin, marsh rabbits from the Lower Florida Keys can be separated into 2 genetically distinct management units, which should be considered prior to implementation of translocations as a means of offsetting recent population declines.     

Phylogeny and taxonomic revision of Heteropachylinae (Opiliones: Laniatores: Gonyleptidae)

Heteropachylinae Kury, 1994 is currently considered an early lineage of Gonyleptidae Sundevall, 1833 and includes small harvestmen that occur in the Atlantic Forests, mainly in the Northeast Region of Brazil. The species of Heteropachylinae were spread in different subfamilies until the establishment of the group. As a result of that, until now there were no comparative works on the subfamily in the literature. In this study, the first systematic review for Heteropachylinae was carried out and the first phylogenetic hypothesis for their taxa is proposed, by means of a cladistic approach. A total of 102 morphological characters was coded for 21 terminal taxa: 11 species of Heteropachylinae, two species from the genus Mangaratiba Mello‐Leitão, 1940 (considered as Heteropachylinae before this paper), seven other gonyleptids, and one cosmetid. The data were analysed using heuristic search algorithms, resulting in one most parsimonious tree (280 steps, consistency index = 0.47, retention index = 0.63). A parsimony analysis under implied weights was also performed, resulting in three trees. According to the results, Heteropachylinae is a monophyletic and well‐supported group if Mangaratiba is excluded. This genus should be returned to where it was originally described – Pachylinae. The subfamily distribution is restricted from Ceará to Espírito Santo Brazilian states. Pseudopucrolia Roewer, 1912 is considered a senior synonym of Melloa Roewer, 1930, Thaumatoleptes Roewer, 1930, and Tribunosoma Roewer, 1943. Two species of Heteropachylus Roewer, 1913, Heteropachylus ramphonotus sp. nov. and Heteropachylus gracilis sp. nov. , from Bahia and Espírito Santo Brazilian states, are newly described. Heteropachylus is considered a senior synonym of Chavesincola Soares & Soares, 1946 and Aesotrinoma H. Soares, 1977. A new genus is also described for a new species from Bahia and Espírito Santo Brazilian states, Magnispina neptunus gen. et sp. nov. Lectotypes are designated for the following species: Pseudopucrolia incerta (Mello‐Leitão, 1928) comb. nov. , Pseudopucrolia discrepans (Roewer, 1943) comb.nov. , Pseudopucrolia rugosa (Roewer, 1930) comb. nov. , Heteropachylus inexpectabilis (Soares & Soares, 1946) comb. nov. and Heteropachylus spiniger Roewer, 1913. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 163 , 437–483.     

Variation in singing behaviour among morphs of the sand field cricket,Gryllus firmus

1. Trade‐offs play a fundamental role in the evolution of many traits. 2. In wing‐polymorphic field crickets, the long‐winged morph can disperse from unfavourable environments, but has lower reproductive success than the short‐winged morph, because of costs associated with flight capability. 3. However, long‐winged individuals may minimise costs in favourable environments by histolysing their flight muscles and becoming flightless. 4. Few studies have examined how flight‐muscle histolysis affects male signalling and mate attraction. 5. We examined differences in singing activity and song characteristics among the flightless (short‐winged and histolysed long‐winged) and the flight‐capable male morphs, and female preferences for male song, in the sand field cricket. 6. We found: (i) both flightless morphs sang more than the flight‐capable morph, (ii) song characteristics varied among the three morphs, and (iii) females preferred songs characteristic of the long‐winged morphs. 7. Histolysis should increase mating success of long‐winged males because it increases singing activity. 8. Histolysed long‐winged males may have higher mating success than short‐winged males as they sing as frequently but produce more attractive songs. 9. Therefore, plasticity within the long‐winged morph may reduce costs of maturing in environments from which dispersal is not advantageous; non‐flying morphs may be pursuing different reproductive tactics.