关键词： Sorbus aucuparia   autumnal senescence   pigments   co-evolution   photoinhibtion   handicap signal  

摘要： We explored two mutually nonexclusive hypotheses on autumnal leaf colouration. The co-evolutionary hypothesis states that autumnal leaf colouration functions as a handicap signal to herbivorous insects, whereas the photoprotection hypothesis posits that plant pigments primarily protect the plant against cold-induced photoinhibition and enhance nutrient transfer. To contrast both hypotheses, we compared yellow and red leaf colouration in three groups of mountain ash (Sorbus aucuparia L.). Two montane groups of different age were characterised by low aphid numbers and low temperature, and a lowland group by high aphid numbers and high temperature. There were no consistent altitudinal differences in leaf colouration. Compared to young trees, adult trees developed fewer red but more yellow leaves at high altitude. In the lowland population, the development of red leaf colour was related to decreasing daytime temperature, whereas the appearance of yellow leaf colouration corresponded to the decreasing photoperiod. This is consistent with the photoprotection hypothesis. Individual differences in red and yellow leaf colouration were inversely correlated to the number of fruits, which might be interpreted as a trade-off between reproductive and protective commitment. Temperature effects explained variation in aphid numbers over time and leaf colouration explained aphid distribution on a given day. As predicted by the co-evolutionary hypothesis, strongly coloured individuals harboured fewer aphids than green or dull-coloured ones. Since decreasing temperature reduced the number of migrating aphids but induced red leaf colouration, these processes are not mutually fine-tuned, which likely restricts the potential for co-evolution between mountain ash and aphids.

关键词： Austrocedrus   climate change   disturbances   fire   livestock   Nothofagus   Patagonia   seedling survival   tree regeneration  

摘要： 1. Wildfires are predicted to increase in many ecosystems in relation to globally increasing temperatures but future patterns of post-fire vegetation change are largely unknown, particularly when there are synergistic effects from introduced biota. In the late 1990s northern Patagonia, Argentina, experienced extreme droughts which led to severe wildfires affecting a range of Andean ecosystems. 2. We experimentally examined how variations in moisture, temperature and herbivory by livestock affect post-1999 fire patterns of the three main tree species. Over two years we monitored, in three forest types, the survival and growth of tree seedlings in a factorial warming (+2 degrees C)/livestock exclosure/watering experiment. 3. Seedling survival in the warmed treatments and in the controls was nil for the evergreen Nothofagus dombeyi and the conifer Austrocedrus chilensis at the two low-elevation experimental sites. Survival of the subalpine Nothofagus pumilio in the warmed treatments at high elevation tended to be lower than in the control;for all treatments of warming alone there were no significant differences compared with the controls. 4. In all three forest types, increased water availability was essential for higher rates of tree seedling survival. Doubling water availability during the growing season resulted in up to fourfold increases in seedling survival and up to threefold increases in seedling biomass. 5. In the subalpine forest, livestock reduced seedling survival by c. 30% in non-watered treatments compared with watered treatments, probably due mainly to soil desiccation and to consumption of or damage to facilitating plants. In contrast, at lower elevation, where livestock pressure was lower, seedling survival of N. dombeyi and A. chilensis tended to be higher in unfenced sites, possibly due to reduced competition from highly palatable shrub species. 6. General circulation models predict a warming-drying trend in northern Patagonia during the twenty-first

关键词： wildlife   rainfall   predation   competition   apparent competition   non-protected area  

摘要： Conserving African wildlife in human-occupied landscapes requires management intervention that is guided by a mechanistic understanding of how anthropogenic factors influence large-scale ecological processes. In Laikipia District, a dry savanna region in northern Kenya where wildlife share the landscape with humans and livestock, we examined why five of nine wild ungulate species suffered protracted declines on properties receiving the greatest conservation investment. Of 10 alternative causes examined, only an increase in predation, interacting with brief periods of high and low rainfall, was consistent with the timing, synchrony, duration and species composition of observed ungulate declines. The principal factor causing predation to increase was a shift in land use from cattle ranching, under which predators and plains zebras were severely suppressed, to wildlife conservation and ecotourism. This prompted a 5-fold increase in plains zebra abundance, and created a demand for living predators. Plains zebras ultimately comprised more than half the available prey biomass, and supported a substantial predator community, but were not limited by predators. We infer that increasing predation pressure caused predator-susceptible prey species to decline, via mechanisms that included apparent competition. Herbivore dynamics in Laikipia shared features with previously reported responses by prey communities to predator manipulation in Kruger and Serengeti National Parks. All featured one or a few numerically dominant herbivore species, which were primarily limited by rainfall and density, supporting a predator community that in turn limited the abundance of other prey species. In each case, predation had a profound effect, but on only a subset of prey species, reducing the evenness component of prey diversity. The presence of cattle in the landscape may affect predator-prey dynamics in both direct and indirect ways, depending on rainfall. In extreme years (floods or drought),

关键词： carbon uptake   ecosystem productivity   herbivory   modelling   photosynthesis  

摘要： 1. Through its effect on plant productivity and fitness, herbivory can regulate forest diversity and ecosystem productivity. We used the interaction between phloem-feeding scale insects [Ultracoelostoma assimile (Homoptera: Coelostomidiidae)] and the beech trees [Nothofagus solandri var. solandri (Hook f.) Oerst.] they feed on to estimate directly the carbon costs of herbivory for a forest tree species over two consecutive years. The scale insects live on the bark of tree trunks and branches, and excrete readily collected droplets of carbon-rich honeydew - this allows us to make an estimate of the carbon cost of herbivory that is not complicated by feeding damage to leaves and buds. 2. Canopy photosynthesis (A) was estimated with an environmentally driven process-based model, developed with parameters derived from measurements of photosynthesis on trees with (HD+) and without (HD-) honeydew scale insects during year 1. Honeydew dry matter production (P-HD) was estimated from values for air temperature and humidity, and accounting for variation in honeydew production due to host-tree diameter. The carbon cost of scale insect herbivory, C-HD, was estimated as P-HD/A(HD+) x 100%, where A(HD+) is canopy photosynthesis for trees with scale insects. 3. Over the two years annual canopy photosynthesis at the site was 1720 g C m(-2) ground area. In spring-summer, values for parameters describing photosynthesis (V-cmax, the maximum rate of rubisco-catalysed RuBP carboxylation, and J(max), the rate of electron transport when irradiance is saturating) were 44% and 66% higher, respectively, in infested, as opposed to uninfested, trees. Consequently, for equivalent basal area, annual canopy photosynthesis was 4% greater for infested trees. 4. Assuming that honeydew dry matter was 42% carbon by mass, annual honeydew production was 59 g C m(-2) ground area, which represents 4% of infested-tree gross photosynthesis (c. 8% of net photosynthesis). Daily values of honeydew production w

关键词： Senna racemosa   Fabaceae   dihydroxyanthraquinones   cassine   antigiardial   antiamoebic  

摘要： Methanol extracts of leaves, roots and bark of Senna racemosa (Mill.) H.S. Irwin & Barneby (syn. Cassia racemosa Mill.) were tested for antiprotozooal activity against Giardia intestinalis and Entainoeba histolytica. All of the tested extracts showed good activity against both protozoa species. Extracts from stem bark and leaves were most active, with an IC50 of 2.10 mu g/mL for Giardia intestinalis and 3.87 mu g/mL for Entainoeba histolytica. Of the previously isolated compounds from Senna racemosa, the piperidine alkaloid cassine had greater activity against Giardia intestinalis with an IC50 of 3.28 mu g/mL and chrysophanol, a 1,8-dihydroxy-anthraquinone, was the most active agent against Entamoeba histolytica, with an IC50 of 6.21 mu g/mL. (c) 2007 Elsevier Ireland Ltd. All rights reserved.

关键词： MALE MATING SUCCESS   SEXUAL SELECTION   TREE FROGS   HYLA-VERSICOLOR   BODY-SIZE   BEHAVIOR   ANURAN   TOADS   RISK   BUFO  

摘要： Despite the large amount of work on frog mating systems, the potential role of predators as an agent of selection on breeding adults has received very little study. Here, I use data from multiple populations of Pacific Treefrogs (Pseudacris regilla) to demonstrate that sexual selection from mating success favors larger males, but natural selection from predation by giant water bugs favors smaller males. Additionally, I found no relationship between male body condition and mating success or predation risk. This result demonstrates that predation is a potentially important agent of selection that counteracts sexual selection in anurans.

关键词： bird   corridor   exotic pasture grass   grazing   miner   remnant  

摘要： This study reports on the responses of bird assemblages to woodland clearance, fragmentation and habitat disturbance in central Queensland Australia, a region exposed to very high rates of vegetation clearance over the last two to three decades. Many previous studies of clearing impacts have considered situations where there is a very sharp management contrast between uncleared lands and cleared areas: in this situation, the contrast is more muted, because both cleared lands and uncleared savanna woodlands are exposed to cattle grazing, invasion by the exotic grass Cenchrus ciliaris and similar fire management. Bird species richness (at the scale of a 1-ha quadrat) was least in cleared areas (8.1 species), then regrowth areas (14.6 species), then uncleared woodlands (19.9 species). Richness at this scale was unrelated to woodland fragment size, connectivity or habitat condition;but declined significantly with increasing abundance of miners (interspecifically aggressive colonial honeyeaters). At whole of patch scale, richness increased with fragment size and decreased with abundance of miners. This study demonstrates complex responses of individual bird species to a regional management cocktail of disturbance elements. Of 71 individual bird species modelled for woodland fragment sites, the quadrat-level abundance of 40 species was significantly related to at least one variable representing environmental position (across a rainfall gradient), fragment condition, fragment size and/or connectivity. This study suggests that priorities for conservation management include: cessation of broad-scale clearing;increased protection for regrowth (particularly where this may bolster connectivity and/or size of woodland fragments);control of miners;maintenance of fallen woody debris in woodlands;increase in fire frequency;and reduction in the incidence of grazing and exotic pasture grass.

关键词： artificial waterholes   surface water   water   dependency   grazing heterogeneity   piosphere   Kruger National Park  

摘要： It has been argued that the wide-scale provision of artificial surface water in semi-arid savannas may result in homogenisation of foraging regimes, compromising biodiversity and ecosystem resilience. This is the first landscape-scale study investigating to what degree artificial waterholes and natural rivers influence the distribution of large herbivores, and hence foraging patterns, in the Kruger National Park, South Africa. This is explored by examining consistencies within and differences between distribution patterns of herbivore feeding groups on different geologies. Seven years of dry season aerial census data of thirteen large herbivores were analysed in a GIS. It was found that different herbivores exhibit different distribution patterns around rivers and waterholes. it was found that most grazer species were associated with artificial waterholes, whereas browsers and mixed feeders were indifferent to waterholes and were associated with the main rivers. This has important management implications for artificial water provision in the KNP and potentially for other semi-arid African savanna conservation areas. Man-made waterholes are therefore features in the landscape that can change the distribution of herbivores, even in a landscape where natural water is available, transforming patterns of landscape use. Furthermore, it was also found that the underlying geology interacts with water distribution in shaping herbivore distributions, with artificial waterholes and rivers acting as stronger nodes of herbivore activity on nutrient-rich basaltic soils than nutrient-poor granitic soils. The results from this paper may be useful for designing surface water policies and management plans for Kruger and other savanna reserves in Africa in order to maximise biodiversity conservation. (C) 2006 Elsevier Ltd. All rights reserved.

关键词： corolla length   hummingbird assemblages   hummingbird bill length   Nicotiana glauca   pollination  

摘要： Nicotiana glauca, a hummingbird pollinated plant, exhibits geographical variation in several floral traits. We examined whether geographical differentiation occurred for different flower characters and if this differentiation could be explained, at least in part, by the existence or abundance of different hummingbird species in the respective pollinator assemblages. The comparison between five populations showed significant variation in six floral traits and two female fitness measures. The traits that better discriminated between populations were corolla length and corolla width. There were metric correlations between corolla length and style length in all populations studied and, in four of the five populations, both corolla length and width were also correlated. Among plants in each population, seed weight was correlated positively and significantly with style exertion, suggesting that fruit quality is dependent on the degree of cross pollination. Assemblages of hummingbirds differed between populations in species composition, visitation frequencies, and bill length. Linear regression involving bill length of the more frequent hummingbird pollinators and corolla length yielded positive and significant relationships. Thus, there appears to be an adjustment between pollinators and flowers traits that have high incidence in the among population variation. (c) 2007 The Linnean Society of London.