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"POPULATION GROWTH"
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Demography of snowshoe hare population cycles
Cyclic fluctuations in abundance exhibited by some mammalian populations in northern habitats (“population cycles”) are key processes in the functioning of many boreal and tundra ecosystems. Understanding population cycles, essentially demographic processes, necessitates discerning the demographic mechanisms that underlie numerical changes. Using mark–recapture data spanning five population cycles (1977–2017), we examined demographic mechanisms underlying the 9–10-yr cycles exhibited by snowshoe hares (Lepus americanus Erxleben) in southwestern Yukon, Canada. Snowshoe hare populations always decreased during winter and increased during summer; the balance between winter declines and summer increases characterized the four, multiyear cyclic phases: increase, peak, decline, and low. Little or no recruitment occurred during winter, but summer recruitment varied markedly across the four phases with the highest and lowest recruitment observed during the increase and decline phase, respectively. Population crashes during the decline were triggered by a substantial decline in winter survival and by a lack of subsequent summer recruitment. In contrast, initiation of the increase phase was triggered by a twofold increase in summer recruitment abetted secondarily by improvements in subsequent winter survival. We show that differences in peak density across cycles are explained by differences in overall population growth rate, amount of time available for population growth to occur, and starting population density. Demographic mechanisms underlying snowshoe hare population cycles were consistent across cycles in our study site but we do not yet know if similar demographic processes underlie population cycles in other northern snowshoe hare populations.
Journal Article
Methods in human growth research
This volume is a review of up-to-date methods used in human growth research.
Book
Causes of urbanisation and counter-urbanisation in Zambia
This article addresses the debate over the causes of urbanisation and counter-urbanisation in Zambia: Are urbanisation and counter-urbanisation caused mostly by net migration or are they caused mostly by the natural growth or decline of the urban population? Using population censuses, we apply the intercensal forward survival ratio method to measure net migration and the natural population growth of urban and rural areas in 1990, 2000 and 2010. The results show that the most important cause of urbanisation and counter-urbanisation was net migration rather than natural urban population growth or decline. Although natural urban population growth was roughly twice that of net migration, this had very little influence on urbanisation because it was matched by the natural growth of the rural population. We also address the causes of migration by examining employment trends. These results indicate that economic decline during the 1990s resulted in decreased urban employment and a dramatic rise in urban unemployment, which in turn caused migration from urban to rural areas. Conversely, during the 2000s, absolute employment grew and unemployment decreased, which corresponded with increased rural–urban migration (resulting in net urbanisation). Our findings also show that even during the period of net out-migration from urban areas and high urban unemployment levels, the resident urban-born workforce continued to grow strongly through natural increase. Thus, these results also show that urban population growth can increase substantially in the absence of urban economic growth, thereby increasing urban unemployment and urban–rural migration.
本文讨论了关于赞比亚城市化和反城市化原因的争论：城市化和反城市化主要是由净移民引起的还是主要由城市人口的自然增长或衰退造成的？基于人口普查资料，我们运用普查间前瞻性生存率法来衡量1990年，2000年和2010年的城市和农村地区的净迁移和人口自然增长。结果表明，城市化和反城市化的最重要原因是净迁移，而不是城市人口的自然增长或下降。尽管城市人口自然增长大约是净移民的两倍，但这对城市化的影响很小，因为它与农村人口的自然增长相当。我们还通过研究就业趋势来探讨移民的原因。结果表明，20世纪90年代的经济衰退导致城市就业率下降，城市失业率急剧上升，从而导致城市向农村地区迁移。相反，在2000年代期间，绝对就业增加，失业率下降，相应地，农村向城市的迁移增加（导致城市化进程）。我们的研究结果还表明，即使在城市地区净流出和高城市失业率的时期，城市出生的常住劳动力仍然通过自然增长保持强劲增长。因此，这些结果还表明，在没有城市经济增长的情况下，城市人口仍然可以大幅增加，从而增加城市失业和城市向乡村的迁移。
Journal Article
Functional traits explain variation in plant life history strategies
Ecologists seek general explanations for the dramatic variation in species abundances in space and time. An increasingly popular solution is to predict species distributions, dynamics, and responses to environmental change based on easily measured anatomical and morphological traits. Trait-based approaches assume that simple functional traits influence fitness and life history evolution, but rigorous tests of this assumption are lacking, because they require quantitative information about the full lifecycles of many species representing different life histories. Here, we link a global traits database with empirical matrix population models for 222 species and report strong relationships between functional traits and plant life histories. Species with large seeds, long-lived leaves, or dense wood have slow life histories, with mean fitness (i.e., population growth rates) more strongly influenced by survival than by growth or fecundity, compared with fast life history species with small seeds, short-lived leaves, or soft wood. In contrast to measures of demographic contributions to fitness based on whole lifecycles, analyses focused on raw demographic rates may underestimate the strength of association between traits and mean fitness. Our results help establish the physiological basis for plant life history evolution and show the potential for trait-based approaches in population dynamics.
Journal Article
East Asia's changing urban landscape : measuring a decade of spatial growth
This volume explores the results from analysis that measured the expansion and population change in urban agglomerations across East Asia between 2000 and 2010. Key findings show an overall rapid pace of urban growth, both in terms of land and population, dominated mostly by China.
Book
compadre Plant Matrix Database: an open online repository for plant demography
Schedules of survival, growth and reproduction are key life‐history traits. Data on how these traits vary among species and populations are fundamental to our understanding of the ecological conditions that have shaped plant evolution. Because these demographic schedules determine population growth or decline, such data help us understand how different biomes shape plant ecology, how plant populations and communities respond to global change and how to develop successful management tools for endangered or invasive species. Matrix population models summarize the life cycle components of survival, growth and reproduction, while explicitly acknowledging heterogeneity among classes of individuals in the population. Matrix models have comparable structures, and their emergent measures of population dynamics, such as population growth rate or mean life expectancy, have direct biological interpretations, facilitating comparisons among populations and species. Thousands of plant matrix population models have been parameterized from empirical data, but they are largely dispersed through peer‐reviewed and grey literature, and thus remain inaccessible for synthetic analysis. Here, we introduce the compadre Plant Matrix Database version 3.0, an open‐source online repository containing 468 studies from 598 species world‐wide (672 species hits, when accounting for species studied in more than one source), with a total of 5621 matrices. compadre also contains relevant ancillary information (e.g. ecoregion, growth form, taxonomy, phylogeny) that facilitates interpretation of the numerous demographic metrics that can be derived from the matrices. Synthesis. Large collections of data allow broad questions to be addressed at the global scale, for example, in genetics (genbank), functional plant ecology (try, bien, d3) and grassland community ecology (nutnet). Here, we present compadre, a similarly data‐rich and ecologically relevant resource for plant demography. Open access to this information, its frequent updates and its integration with other online resources will allow researchers to address timely and important ecological and evolutionary questions.
Journal Article
Biotic and anthropogenic forces rival climatic/abiotic factors in determining global plant population growth and fitness
Multiple, simultaneous environmental changes, in climatic/abiotic factors, interacting species, and direct human influences, are impacting natural populations and thus biodiversity, ecosystem services, and evolutionary trajectories. Determining whether the magnitudes of the population impacts of abiotic, biotic, and anthropogenic drivers differ, accounting for their direct effects and effects mediated through other drivers, would allow us to better predict population fates and design mitigation strategies. We compiled 644 paired values of the population growth rate (λ) from high and low levels of an identified driver from demographic studies of terrestrial plants. Among abiotic drivers, natural disturbance (not climate), and among biotic drivers, interactions with neighboring plants had the strongest effects on λ. However, when drivers were combined into the 3 main types, their average effects on λ did not differ. For the subset of studies that measured both the average and variability of the driver, λ was marginally more sensitive to 1 SD of change in abiotic drivers relative to biotic drivers, but sensitivity to biotic drivers was still substantial. Similar impact magnitudes for abiotic/biotic/anthropogenic drivers hold for plants of different growth forms, for different latitudinal zones, and for biomes characterized by harsher or milder abiotic conditions, suggesting that all 3 drivers have equivalent impacts across a variety of contexts. Thus, the best available information about the integrated effects of drivers on all demographic rates provides no justification for ignoring drivers of any of these 3 types when projecting ecological and evolutionary responses of populations and of biodiversity to environmental changes.
Journal Article
great human expansion
Genetic and paleoanthropological evidence is in accord that today’s human population is the result of a great demic (demographic and geographic) expansion that began approximately 45,000 to 60,000 y ago in Africa and rapidly resulted in human occupation of almost all of the Earth’s habitable regions. Genomic data from contemporary humans suggest that this expansion was accompanied by a continuous loss of genetic diversity, a result of what is called the “serial founder effect.” In addition to genomic data, the serial founder effect model is now supported by the genetics of human parasites, morphology, and linguistics. This particular population history gave rise to the two defining features of genetic variation in humans: genomes from the substructured populations of Africa retain an exceptional number of unique variants, and there is a dramatic reduction in genetic diversity within populations living outside of Africa. These two patterns are relevant for medical genetic studies mapping genotypes to phenotypes and for inferring the power of natural selection in human history. It should be appreciated that the initial expansion and subsequent serial founder effect were determined by demographic and sociocultural factors associated with hunter-gatherer populations. How do we reconcile this major demic expansion with the population stability that followed for thousands years until the inventions of agriculture? We review advances in understanding the genetic diversity within Africa and the great human expansion out of Africa and offer hypotheses that can help to establish a more synthetic view of modern human evolution.
Journal Article