Races of the Barley Root-Knot Nematode,Meloidogyne naasi. III. Reproduction and Pathogenicity on Creeping Bentgrass

Reproduction and pathogenicity of the five known races of Meloidogyne naasi on two selections of creeping bentgrass were compared. Toronto C-15 was a host for Races 3, 4 and 5, whereas Northmoor 9 supported reproduction of all five races. Differences in susceptibility and population increase demonstrated that the races could be separated by degree of reproduction on the two selections. Root weights generally were unaffected. Based on cumulative dipping weights, all but Race 1 were pathogenic on at least one of the selections; Race 3 stunted top growth of both. Slight differences in degree of pathogenicity, associated with final populations, were not broad enough to be useful in race separation.     

How training and testing histories affect generalization: a test of simple neural networks

We show that a simple network model of associative learning can reproduce three findings that arise from particular training and testing procedures in generalization experiments: the effect of (i) 'errorless learning', (ii) extinction testing on peak shift, and (iii) the central tendency effect. These findings provide a true test of the network model which was developed to account for other phenomena, and highlight the potential of neural networks to study the phenomena that depend on sequences of experiences with many stimuli. Our results suggest that at least some such phenomena, e.g. stimulus range effects, may derive from basic mechanisms of associative memory rather than from more complex memory processes.     

Dimensions of global population projections: what do we know about future population trends and structures?

The total size of the world population is likely to increase from its current 7 billion to 8–10 billion by 2050. This uncertainty is because of unknown future fertility and mortality trends in different parts of the world. But the young age structure of the population and the fact that in much of Africa and Western Asia, fertility is still very high makes an increase by at least one more billion almost certain. Virtually, all the increase will happen in the developing world. For the second half of the century, population stabilization and the onset of a decline are likely. In addition to the future size of the population, its distribution by age, sex, level of educational attainment and place of residence are of specific importance for studying future food security. The paper provides a detailed discussion of different relevant dimensions in population projections and an evaluation of the methods and assumptions used in current global population projections and in particular those produced by the United Nations and by IIASA.     

Changes in the Cell Volume of Didinium nasutum During Population Increase*

SYNOPSIS. In 3 species of carnivorous protozoa, the rate of individual food intake per generation declines with an increase in the density of the population. In all, the rate of division remains constant. Three hypotheses may be proposed to explain these phenomena: the individual size decreases, thus bringing about a decline in food need; the individuals in the earlier stages of population growth consume excess food which is passed on to later generations to supplement their food intake; an increase in density within the carnivore population decreases the activity rate, thus reducing the food requirement. Experiments using Didinium nasutum as the carnivore and Paramecium aurelia, syngen 4, as the food source were conducted to test these 3 hypotheses. The results contradicted the 1st hypothesis and part of the 2nd. A new working hypothesis based on the remaining hypotheses is proposed.     

Survival of Svalbard pink-footed geese Anser brachyrhynchus in relation to winter climate, density and land-use

1. Global change may strongly affect population dynamics, but mechanisms remain elusive. Several Arctic goose species have increased considerably during the last decades. Climate, and land-use changes outside the breeding area have been invoked as causes but have not been tested. We analysed the relationships between conditions on wintering and migration staging areas, and survival in Svalbard pink-footed geese Anser brachyrhynchus. Using mark-recapture data from 14 winters (1989-2002) we estimated survival rates and tested for time trends, and effects of climate, goose density and land-use. 2. Resighting rates differed for males and females, were higher for birds recorded during the previous winter and changed smoothly over time. Survival rates did not differ between sexes, varied over time with a nonsignificant negative trend, and were higher for the first interval after marking (0.88-0.97) than afterwards (0.74-0.93). Average survival estimates were 0.967 (SE 0.026) for the first and 0.861 (SE 0.023) for all later survival intervals. 3. We combined 16 winter and spring climate covariates into two principal components axes. F1 was related to warm/wet winters and an early spring on the Norwegian staging areas and F2 to dry/cold winters. We expected that F1 would be positively related to survival and F2 negatively. F1 explained 23% of survival variation (F1,10=3.24; one-sided P=0.051) when alone in a model and 28% (F1,9=4.50; one-sided P=0.031) in a model that assumed a trend for survival. In contrast, neither F2 nor density, land-use, or scaring practices on important Norwegian spring staging areas had discernible effects on survival. 4. Climate change may thus affect goose population dynamics, with warmer winters and earlier springs enhancing survival and fecundity. A possible mechanism is increased food availability on Danish wintering and Norwegian staging areas. As geese are among the main herbivores in Arctic ecosystems, climate change, by increasing goose populations, may have important indirect effects on Arctic vegetation. Our study also highlights the importance of events outside the breeding area for the population dynamics of migrant species.     

Genomic diversity,population structure,and migration following rapid range expansion in the Balsam Poplar,Populus balsamifera

Rapid range expansions can cause pervasive changes in the genetic diversity and structure of populations. The postglacial history of the Balsam Poplar, Populus balsamifera, involved the colonization of most of northern North America, an area largely covered by continental ice sheets during the last glacial maximum. To characterize how this expansion shaped genomic diversity within and among populations, we developed 412 SNP markers that we assayed for a range‐wide sample of 474 individuals sampled from 34 populations. We complemented the SNP data set with DNA sequence data from 11 nuclear loci from 94 individuals, and used coalescent analyses to estimate historical population size, demographic growth, and patterns of migration. Bayesian clustering identified three geographically separated demes found in the Northern, Central, and Eastern portions of the species’ range. These demes varied significantly in nucleotide diversity, the abundance of private polymorphisms, and population substructure. Most measures supported the Central deme as descended from the primary refuge of diversity. Both SNPs and sequence data suggested recent population growth, and coalescent analyses of historical migration suggested a massive expansion from the Centre to the North and East. Collectively, these data demonstrate the strong influence that range expansions exert on genomic diversity, both within local populations and across the range. Our results suggest that an in‐depth knowledge of nucleotide diversity following expansion requires sampling within multiple populations, and highlight the utility of combining insights from different data types in population genomic studies.     

Significance of spring migration and flexibility in flight-muscle histolysis in waterstriders (Heteroptera, Gerridae)

Abstract. 1. Individuals of long-winged waterstrider (Gerridae) species were found in spring far from their breeding habitats, which indicates that they fly before reproduction.
2. Field samples and laboratory studies show that once they return to their breeding sites, many individuals of three waterstrider species ( Gerris odontogaster (Zett.), Gerris lacustris (L.) and Limnoporus rufoscutellatus (Lat.)) histolyse wing muscles and lose flight ability during their reproductive period.
3. The extent of flight-muscle histolysis varies with environmental factors. Food scarcity affects flight-muscle histolysis in G.odontogaster females. In G.Lacustris , flight-muscle histolysis was more common in the laboratory than in the field samples. Proportionately more females than males lost their flight ability by the end of the reproductive period.
4. Flight ability had direct costs in reproductive potential with (non-flyer) females, which histolysed their flight muscles, laying more eggs than (flyer) females, which maintained flight ability. This was also the case during food scarcity. Non-flyer males of G.odontogaster survived longer than flyer males.
5. Spring migration was distinguished from dispersal during the reproductive period, because these flights serve different functions. Flight-muscle histolysis of females during reproduction is a qualitative reproductive option, with a trade-off between dispersal ability and reproductive potential. Ability to change reproductive behaviour depending on environmental conditions increases an individual's ability to cope with a large variety of habitats.     

Potential decoupling of trends in distribution area and population size of species with climate change

Global climates are changing rapidly and biological responses are becoming increasingly apparent. Here, we use empirical abundance patterns across an altitudinal gradient and predicted altitudinal range shifts to estimate change in total population size relative to distribution area in response to climate warming. Adopting this approach we predict that, for nine out of 12 species of regionally endemic birds, total population size will decline more rapidly than distribution area with increasing temperature. Two species showed comparable loss and one species exhibited a slower decline in population size with change in distribution area. Population size change relative to distribution area was greatest for those species that occurred at highest density in the middle of the gradient. The disproportional loss in population size reported here suggests that extinction risk associated with climate change can be more severe than that expected from decline in distribution area alone. Therefore, if we are to make accurate predictions of the impacts of climate change on the conservation status of individual species, it is crucial that we consider the spatial patterns of abundance within the distribution and not just the overall range of the species.     

全球变化与野生物种:观测和预测

由于人类活动的影响,世界正在变暖。迅速的全球变化很可能对野生物种产生巨大影响,同时伴随着城市化、农垦和造林实践所引起的自然生境的丧失和破坏。观测和预测的气候变化对野生物种的影响着眼于4个方面：生活史的时间、物种分布与种群格局、迁移对策以及重要地点。许多物种可能因气候的变化而灭绝,而气候变化所造成的野生物种分布的变化很可能对人类产生长期久远的影响。     

Evolutionary age and risk of extinction in the global avifauna

Species at high risk of extinction are not distributed at random among higher taxa. Here we demonstrate that there is a positive relationship between the proportion of species in a taxon which are considered to be threatened and the evolutionary age of that taxon, both for the global avifauna and the avifauna of the New World. The potential mechanisms and consequences of the relationship are examined.