NORTH AMERICAN GRASSLAND BIRDS: AN UNFOLDING CONSERVATION CRISIS?

Abstract: The widespread and ongoing declines of North American bird populations that have affinities for grassland and grass-shrub habitats (hereafter referred to as grassland birds) are on track to become a prominent wildlife conservation crisis of the 21st century. There is no single cause responsible for the declines of grassland birds. Rather, a cumulative set of factors such as afforestation in the eastern United States, fragmentation and replacement of prairie vegetation with a modern agricultural landscape, and large-scale deterioration of western U.S. rangelands are the major causes for these declines. The North American Bird Conservation Initiative (NABCI) is a set of comprehensive and coordinated strategic actions modeled on the Joint Venture initiatives that were used to successfully implement the North American Waterfowl Management Plan. The NABCI is emerging as a potential broad-scale solution for conserving populations of grassland birds. Coordinating grassland bird conservation efforts with initiatives to stabilize and increase upland game birds that have strong affinities for grassland habitats—such as quail and prairie grouse—presents additional opportunities to leverage funding and resources that will positively impact virtually all species of North American grassland birds.     

Individualistic responses of bird species to environmental change

We investigated how the population dynamics of the same bird species varied in different environments, and how the population dynamics of different species varied in the same environment, by calculating long-term population trends for 59 insectivorous songbird species in 22 regions or strata of eastern and central North America using data from the North American Breeding Bird Survey. Of the 47 species that occurred in more than one region 77% increased in some regions and declined in others. Of the 22 regions 91% had some species that increased and others that decreased. There were only slightly more significant correlations between strata in species trends and between species for stratum trends than would be expected by chance. Because of nonlinearities in the data, the actual patterns of population fluctuations of the same species in different regions and of different species in the same region were even more heterogeneous than suggested by our analyses of linear trends. We conclude that these bird species respond to spatial and temporal variation in their environment in a very individualistic fashion. These individualistic responses show that the extrapolation of population trends gained from a few local studies to a larger spatial scale, and the use of a few indicator species to monitor the status of a broader community, are suspect.     

The relationship between species detection probability and local extinction probability

In community-level ecological studies, generally not all species present in sampled areas are detected. Many authors have proposed the use of estimation methods that allow detection probabilities that are <1 and that are heterogeneous among species. These methods can also be used to estimate community-dynamic parameters such as species local extinction probability and turnover rates (Nichols et al. Ecol Appl 8:1213–1225; Conserv Biol 12:1390–1398). Here, we present an ad hoc approach to estimating community-level vital rates in the presence of joint heterogeneity of detection probabilities and vital rates. The method consists of partitioning the number of species into two groups using the detection frequencies and then estimating vital rates (e.g., local extinction probabilities) for each group. Estimators from each group are combined in a weighted estimator of vital rates that accounts for the effect of heterogeneity. Using data from the North American Breeding Bird Survey, we computed such estimates and tested the hypothesis that detection probabilities and local extinction probabilities were negatively related. Our analyses support the hypothesis that species detection probability covaries negatively with local probability of extinction and turnover rates. A simulation study was conducted to assess the performance of vital parameter estimators as well as other estimators relevant to questions about heterogeneity, such as coefficient of variation of detection probabilities and proportion of species in each group. Both the weighted estimator suggested in this paper and the original unweighted estimator for local extinction probability performed fairly well and provided no basis for preferring one to the other.     

Modelling partitioning between structure and storage in sugar beet: Effects of drought and soil nitrogen

In the UK sugar beet is grown on contrasting soils that vary both in their nutritional status and water-holding capacities. Water shortage and low nitrogen reduce canopy growth and dry matter production, which is compensated in part by an increase in the fraction of assimilates partitioned to storage. Conversely, high nitrogen and ample water encourage growth of the canopy, increase assimilation of carbon dioxide but reduce the proportion of assimilates stored as sugar. This paper sets out to examine simple relationships between sugar yield, total dry matter and soil nitrogen in rain-fed and irrigated sugar beet crops (Beta vulgaris L.) from 46 field experiments spanning 12 years and a range of soil types, in order to improve prediction of sugar yields. Two partitioning functions were fitted to the data. The first represents a useful alternative formulation of the allometric growth function that overcomes some of the difficulties in the interpretation of the parameters. This model adequately described the seasonal progress of sugar yield (Y) in relation to total dry matter (W), but it was difficult to postulate biological mechanisms as to how the parameters should vary in relation to varying soil nitrogen or to drought. The second partitioning function, given by Y = W − (1/k) log(1 + kW), also described the data well, but had the more useful parameter, k, the decay rate of the fraction of assimilates partitioned to structural matter. This was shown to be greater in crops which had experienced significant drought and was inversely proportional to the amount of nitrogen taken up by the crops. Relationships between k and amounts of nitrogen fertilizer applied and/or amounts of residual nitrogen in the soil at sowing, however, were more variable. These could be improved by additionally taking account of soil type and rainfall following nitrogen fertilizer application in late spring. The models are a useful extension to yield forecasting models because they provide a simple means of estimating sugar yield from total dry matter in relation to factors that affect partitioning of assimilates such as drought and soil nitrogen availability. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of recreation areas on avian communities in coastal New South Wales' parks

Summary Information on the impacts of outdoor recreation on wildlife within national parks and reserves can be useful to natural area managers. This study aimed to (i) investigate the density, diversity and species composition of avian communities in recreation areas in bushland settings in comparison to surrounding natural habitats, and (ii) determine the influence of the presence of people on avian assemblages in such recreation areas. Avian density, species richness and community composition were compared between six high‐visitation bushland camping and picnic areas (recreation areas) and surrounding undisturbed habitats to examine the effect of recreation areas on avian assemblages. While total numbers of birds detected was found to be higher in recreation areas, species richness trends indicated that a greater diversity of birds was associated with the surrounding natural habitats, which were found to support a taxonomically different avian assemblage to the recreation areas. Interestingly, species previously shown to distinguish urban avian communities were commonly present and often more abundant in the recreation areas than the surrounding natural habitats. We investigated the effect of the intermittent presence of people (rather than clearing alone) and addressed changes in the diurnal distribution of species, by comparing avian assemblages at 22 picnic areas in the morning, at midday and in the afternoon between days of high and low human visitation. Observations of the availability of anthropogenic food resources and subsequent utilization by avifauna were recorded. Generally, avian assemblages appeared to be independent of the presence of people, although the provision of anthropogenic foods is thought to have the potential to adversely affect individual birds. The findings of this study suggest that even small‐scale habitat clearance to create picnic areas adversely affects the avian assemblage present, with temporal changes in visitation levels of humans in these areas appearing to have had no additional influence unless through indirect impacts from anthropogenic foods.     

Population density of North American elk: effects on plant diversity

Large, herbivorous mammals have profound effects on ecosystem structure and function and often act as keystone species in ecosystems they inhabit. Density-dependent processes associated with population structure of large mammals may interact with ecosystem functioning to increase or decrease biodiversity, depending on the relationship of herbivore populations relative to the carrying capacity (K) of the ecosystem. We tested for indirect effects of population density of large herbivores on plant species richness and diversity in a montane ecosystem, where increased net aboveground primary productivity (NAPP) in response to low levels of herbivory has been reported. We documented a positive, linear relationship between plant-species diversity and richness with NAPP. Structural equation modeling revealed significant indirect relationships between population density of herbivores, NAPP, and species diversity. We observed an indirect effect of density-dependent processes in large, herbivorous mammals and species diversity of plants through changes in NAPP in this montane ecosystem. Changes in species diversity of plants in response to herbivory may be more indirect in ecosystems with long histories of herbivory. Those subtle or indirect effects of herbivory may have strong effects on ecosystem functioning, but may be overlooked in plant communities that are relatively resilient to herbivory.     

Modelling Spatio-Temporal Effects of Environment on Atlantic Herring, <Emphasis Type="Italic">Clupea harengus</Emphasis>

Trends in mean abundance of North Sea Atlantic herring, Clupea harengus, over the period of 1992–1995, were modelled as a function of spatial location and ocean environmental conditions using generalized additive models (GAM). In all four years, the average herring abundance was found to be highest in latitudes around 60.5°N, and decreased with increasing latitude. The thermocline depth had a significant effect on prespawning herring abundance both directly, as a main effect, and indirectly, through its interactive effect with the temperature at 60m. Average herring abundance was highest in areas having deeper thermocline depths (up to 45m) and temperatures at 60m between 9 and 11°C. Prespawning herring abundance was greater in areas of cooler surface waters in the south than in the north. Well-mixed waters and transition zones between frontal and stratified areas having sea surface temperatures mainly between 11 and 12°C and to a lesser extent between 13 and 14°C were associated with the highest herring abundance. Herring appeared to avoid the cold bottom waters in summer. Multiyear GAM analysis revealed consistent environmental preferenda of herring and affirmed further a significant decrease in herring abundance. As herring numbers declined, the population aggregated in the most preferred areas. The inter-relationships of herring and environmental factors across the study period, were similar in their structure and significance, suggesting that preferred areas for location of herring can be reasonably predicted.     

稻草还田对水稻土固氮基因(nifH)组成结构和多样性的影响

以中国科学院桃源农业生态试验站长期定位施肥试验为平台,选取稻草还田(C)、氮磷钾(NPK)、氮磷钾加稻草还田(NPK+C)和不施肥对照(CK)4个处理,在晚稻的分蘖期、孕穗期和成熟期分别采集土样,利用实时定量PCR (Q-PCR)和末端限制性片段多态性(TRFLP)等分子生物学方法研究长期稻草还田对水稻土含nifH基因固氮微生物群落丰度、组成和多样性的影响.结果表明:与对照相比,稻草还田和单施化肥处理均显著增加nifH基因的丰度(分蘖期除外),NPK+C处理中含nifH基因的微生物数量最高;nifH基因组成也受到长期施肥的影响,其中CK处理nifH基因组成与各施肥处理明显不同,C与NPK处理间nifH基因组成存在一定差异,而NPK与NPK+C处理间无显著差异.长期施肥不会引起含nifH基因微生物群落多样性的显著改变.可见,稻草还田不仅引起nifH基因群落的组成发生变化,而且导致其数量显著增加,因而可增加土壤的固氮能力.     

Effects of coupled natural and anthropogenic factors on the community structure of diadromous fish and shrimp species in tropical island streams

1. Overlapping river and road networks provide a framework for studying the complex interactions between natural and human systems, with river‐road intersections as focal areas of study. Roads can alter the morphology of stream channels, pose barriers to freshwater fauna, provide easy access to streams for humans and non‐native species and accelerate the expansion of urban development. 2. We determined what variables control the structure of diadromous fish and shrimp communities and assessed whether particular road crossings altered community structure in north‐eastern Puerto Rico. We identified 24 sites that represented a range of river and road sizes across two catchments that drain El Yunque National Forest in Puerto Rico. 3. The location of natural barriers and the size of stream pools were the most important variables for predicting six of fifteen fish and shrimp distributions. Predatory fishes were predicted to be limited to areas in the river network below large, steep waterfalls, whereas adult shrimp Atya lanipes (Atyidae) were predicted to be present above these waterfalls. The fish Awaous banana was predicted to be present in pools >11.6 m wide, whereas the shrimp Xiphocaris elongata was predicted to be present in pools <10.4 m wide. The distributions of nine species were predicted poorly, but three of these species were common and three were rare. 4. Although urban and agricultural land covers were among the top three predictors of five species distributions, they were probably good predictors because they were correlated with the natural gradient. Further study is necessary to disentangle natural and anthropogenic gradients. 5. Road crossings, 10 of which were culverts, were not dispersal barriers for fishes or shrimps. On average, species were present both upstream and downstream from road crossings at 68% of sites where they occurred. Absences upstream or downstream from road crossings occurred at 16% of sites each and likely resulted from a failure to detect species. 6. Several existing features of these catchments and taxa may aid in fish and shrimp conservation. The headwaters are protected by management practices of El Yunque National Forest, connectivity within the river network has been maintained, and the diadromous life history of these organisms makes them resilient to pulsed disturbances.     

Effects of Anisops sardea (Hemiptera: Notonectidae) on oviposition habitat selection by mosquitoes and other dipterans and on community structure in artificial pools

We investigated the effects of the predatory backswimmer, Anisops sardea, on oviposition habitat selection of mosquitoes and other dipterans, and on community structure in experimental pools. We predicted that those dipteran species whose larvae were shown to be highly vulnerable to predation by Anisops would avoid Anisops pools when choosing an oviposition site. We established the following treatments in plastic tubs: (1) Control (without Anisops); (2) Free Anisops (ten Anisops within pool but not caged); (3) Caged Anisops (ten Anisops in cage). The pools were open to colonization by insects. We added resting stages of crustaceans and first instar larvae of the mosquitoes Culiseta longiareolata and Culex laticinctus. Among the dipteran species, Culiseta longiareolata, Culex laticinctus, Chironomus riparius (Chironomidae) and Forcipomyia sp. (Ceratopogonidae), only Culisetalarvae were highly vulnerable to predation. As predicted, based on larval vulnerability, Culiseta, but not the other species, avoided Anisops pools when ovipositing. Free Anisops reduced taxon richness. This reduction resulted largely from the elimination of the cladoceran Ceriodaphniasp. and Culiseta in most free Anisops pools. Thus, Anisops sardea structures the community, both by a behavioral response of prey to its presence and by consumption of prey.