Thresholds in the long-term responses of breeding birds to forest cover and fragmentation

Mitigating the effects of habitat loss requires estimating the minimum amount of habitat necessary for the persistence of wildlife populations in a changing landscape. Assessing minimum habitat amounts, however, relies on identifying ecological thresholds in species’ responses to landscape change. Using two repeated state-wide atlases, our objective was to investigate the responses of 25 forest birds to a range of forest cover and fragmentation. Repeat atlases allow for the analysis of four population dynamics including: (1) colonization, (2) persistence, (3) extinction, and (4) absence. Our objective was to test the hypothesis that forest birds demonstrated thresholds in these four basic dynamics to varying amounts of forest cover and fragmentation.We found thresholds to be a common, though not pervasive, characteristic of how forest birds respond to forest cover and pattern. We found that the probability of persistence was positively correlated with forest cover and 22 species demonstrated threshold responses. In addition, 15 of 25 birds demonstrated discrete thresholds in extinction dynamics. The existence of a colonization threshold has received significantly less attention in ecology. We also found that 17 out of 25 species demonstrated thresholds in their colonization response to a greater amount of forest cover. The effects of forest fragmentation, independent of forest amount, were less clear. We found support for incorporating the effects of fragmentation, but this fragmentation effect was found both below and above threshold points. We conclude that incorporating ecological thresholds in environmental planning should be species-specific and focus on populations on the verge of rapid ecological change.     

Epiphytic lichen diversity in late-successional Pinus sylvestris forests along local and regional forest utilization gradients in eastern boreal Fennoscandia

Pinus sylvestris-dominated forests have been heavily utilized across all of boreal Fennoscandia and the remaining natural forests are generally highly fragmented. However, there are considerable local and regional differences in the intensity and duration of past forest utilization. We studied the impact of human forest use on the diversity of epiphytic and epixylic lichens in late-successional Pinus sylvestris-dominated forests by assessing species richness and composition along both local and regional gradients in forest utilization. The effects of local logging intensity were analysed by comparing three types of stands: (i) near-natural, (ii) selectively logged (in the early 20th century) and (iii) managed stands. The effects of regional differences in duration and intensity of past forest use were analysed by comparing stands in two contrasting regions (Häme and Kuhmo–Viena). The species richness of selectively logged stands was as high as that of near-natural stands and significantly higher in these two stand categories than in managed stands. Species richness increased with the density of small understorey Picea, which correlated strongly with decreasing intensity of local forest use and increasing structural complexity of selectively logged and near-natural stands. Stands in the Häme region hosted a lower number of species, and were less likely to host many old-growth indicator species than the Kuhmo–Viena region, suggesting that species have been lost from stands in the Häme region due to a longer history of intensive forest use. We conclude that selectively logged stands, along with near-natural stands, are valuable lichen habitats particularly for species confined to old-growth structures such as coarse trees and deadwood. In landscapes where natural forests have become fragmented, the management or restoration of the remaining late-successional Pinus-dominated forests, e.g. through the use of fire, should be carefully planned to avoid adverse effects on lichen species richness.     

萃取光度法测定树叶上的铅含量

[目的]用萃取光度法测定树叶上铅的含量。[方法]将树叶浸在硝酸溶液中,将附着于表面的铅溶解。在pH为8.0~9.0的条件下,使铅与双硫腙形成红色配合物,用二氯甲烷萃取,然后测定该配合物的吸光度,通过标准曲线找出试液的含铅量。[结果]试验测得公路附近树叶样品含铅量为2.10×10-3g/L,离公路较远的树叶样品含铅量为1.26×10-3g/L。[结论]该研究方法灵敏,操作简单,专一性强,符合大气污染分析的技术要求。     

The roles of carrying capacity, immigration, and population synchrony on persistence of stream-resident cutthroat trout

Like many other species, cutthroat trout (Oncorhynchus clarki) have declined substantially relative to historic conditions, and many existing populations are isolated. To inform conservation and restoration efforts for stream-resident cutthroat trout, I built a stage-structured stochastic simulation model to explore the dynamics of populations in isolation or connected by immigration. Relatively small increases in carrying capacity substantially reduced extinction risk in small, isolated populations. In the sensitivity matrix, the two most important elements affecting population growth rate were the survival of subadults that became reproductively mature the next year and the survival of young-of-the-year fish. Increasing either of these should allow populations to recover more rapidly from disturbances and reduce extinction risk. When populations were allowed to interact with one another, relatively low immigration rates (=7 individuals/year) between populations substantially reduced extinction risk to those populations without sufficient space to maintain persistence when isolated. However, higher immigration rates were required to maintain persistence the more synchronous the dynamics of interacting populations became. The results highlight the importance of maintaining adequate space or increasing habitat capacity for isolated populations and the importance of connectivity and large population cores if remnant populations are to be maintained.     

Resource use and rarity among frugivorous birds in a tropical rain forest on Sulawesi

Relatively few studies have focussed on the relationship between resource use and rarity. Rare species could differ from common species in two ways: rare species may utilise resources that are themselves rarer or rare species may utilise a narrower range of resources than common species (i.e., they are more specialised). I investigated the relationship between local abundance and patterns of food resource use for an assemblage of 19 frugivorous birds in the tropical lowland rain forest of Sulawesi, Indonesia. Over a 10-month period I collected data on two measures of rarity, the local abundance and monthly variability in local abundance of each bird species, together with data on their feeding ecology including dietary specialisation, resource availability, resource overlap and habitat use. These measures produced 18 ecological variables, 17 of which I correlated with the two measures of rarity using univariate correlations and minimum adequate multiple regression models. These were carried out using the original data and independent contrasts that controlled for potential phylogenetic bias. Both measures of rarity were most strongly correlated with measures of resource availability. In both the original data and independent contrasts regression models of the temporal variability in bird abundance, only temporal fruit availability was included as a significant predictor. In the regression models of bird abundance, the strongest significant predictor was the amount of rare fruit in the diet (a negative relationship). Both models also included measures of resource specialisation, however, these were not significant in the absence of the measure of resource availability. On Sulawesi, rare frugivorous birds utilise rare resources. Specialisation, however, might be an important factor in species persistence once they become rare. The generality of the findings need to be tested.     

Changes in nestedness in experimental communities of soil fauna undergoing extinction

Nestedness is the degree to which a set of communities represent different-sized subsets from the same ordered composition of species. Strong nestedness has been associated with communities presumably ordered by extinction – landbridge islands, habitat fragments – but development of nestedness in such systems due to extinction over time has rarely been followed. We examined the effects of local extinction on the nested structure of experimentally isolated nematode and microarthropod communities. Experiments were conducted in Georgia, USA, intact soil and litter microcosms, and Derbyshire, UK, in situ moss microecosystems. Nestedness increased initially, as predicted, but there was evidence of a decline in nestedness later in both experiments. Increasing nestedness was consistent with predictable losses of species, such as loss of less common species, predators, or species of large body size. We hypothesize that the later phase of declining nestedness was due to increasing disorder in species’ chance of extinction as the experiments progressed, with the resulting stochastic differentiation of communities reducing nestedness.     

Patterns and causes of non-natural mortality in the Iberian lynx during a 40-year period of range contraction

We analyse the spatial and temporal variation in non-natural mortality during a 40-year period of strong contraction of the geographic range of the Iberian lynx (Lynx pardinus), which shrank from 40,600 to 22,300 km². We recorded 1258 lynx deaths, an average of 31.5 losses per year over the study period. Given the reduced lynx population size, especially later in the period (around 1100 individuals), this level of non-natural mortality may have contributed significantly to the quick decline of the Iberian lynx. Non-natural mortality was not spatially correlated with, and probably did not shape the pattern of, relative abundance of lynx across its core range, but may have reduced its absolute density. Lynx losses were caused mainly by traps set not only for predator control but also for rabbits (Oryctolagus cuniculus), the lynx's staple food. We did not find evidence that non-natural mortality was higher in small lynx populations through edge effects. The highest mortality levels were recorded in regions where small game was a valuable economic resource compared with other activities. Mortality decreased throughout the period because of changes in the prevailing game regimes rather than because of legal protection. The Iberian lynx is now critically endangered and effective protection should be urgently enforced, especially in small game estates, which are environmentally favourable for rabbits but risky for lynx due to predator control. Lynx reintroductions would be better attempted in traditional rabbit hunting areas. Some big game estates where small game is not exploited and predators are not controlled may be good candidates for lynx reintroduction too, provided that habitat is managed towards a suitable interspersion of woody cover and grassland.     

Modelling demography and extinction risk in the endangered Balearic shearwater

Several demographic parameters of Balearic shearwater (Puffinus mauretanicus) were estimated and used to model extinction probabilities for the species, which is an endemic seabird confined to a Mediterranean archipelago. Adult survival was analyzed through capture-recapture models at two colonies free of adult predators during 1997-2002. Extinction probabilities for the world breeding population were modelled using Monte Carlo simulations of population trajectories, introducing stochasticity both demographic and environmental. Adult survival estimate (0.780, SE=0.020, 95% Confidence Intervals: 0.739-0.816) was unusually low for a Procellariiform, suggesting that sources of mortality other than predators (e.g. fishing gears) also occur. A deterministic model showed a declining trend for the world breeding population (λ=0.952), and significantly less than 1. Population viability analysis showed that in the presence of environmental and demographic stochasticities, mean extinction time for the world population was estimated at 40.4 years (SE=0.2), and mean growth rate showed a 7.4% decrease per year. Demographic stochasticity played a more important role than environmental stochasticity in population growth rate.     

Endangered species management requires a new look at the benefit of fire: The Cape Sable seaside sparrow in the Everglades ecosystem

Although disturbance processes play important roles in maintaining habitat heterogeneity, the potential effects of such processes on rare or endangered species is virtually unknown and difficult to test. We use an unplanned fire, which burned half of a long-term study plot, as a natural experiment to test the effects of fire on the federally endangered Cape Sable seaside sparrow in Everglades National Park. By implementing a before-after-control-impact study design we determine the mechanistic link between fire and demography of this endangered sparrow. Our results show that while the sparrow tolerates fire, neither sparrow density nor nesting success are enhanced by fire, which runs contrary to the current paradigm in which sparrows are expected to benefit and therefore require fire for persistence. Our results caution against the assumption that occupancy of disturbance-prone habitat automatically suggests dependence on disturbance. Land managers must prevent large and frequent fires from burning occupied sparrow habitat to best manage for the species. Moreover, it is imperative that more studies focus on the effects of disturbance processes on rare and endangered species in order to prevent further loss of biodiversity.     

高产棉花光合特性的初步研究

光照充足，而且照射时间长，是南疆棉区气候生态条件的一个重要特点。棉花光合速率较高，光合时间较长，光合产物形成与积累较多，可能是其高产的主要生理原因。棉花群体内部透光性有很大的差别，叶面积增形分布较伞形分布合理。通过棉田涓光系数的测定，在高密度栽培条件下，叶面积仍以3．0－3．5为宜。能否将叶面积系数3．5（封行期）作为南疆高产棉田群体高限的参考指标，尚须进一步研究。