Home About us Contact
|
Home About us Contact | ||
|
|
||
Demographic Mechanisms (demographic + mechanism)
Selected AbstractsBIODIVERSITY RESEARCH: Population expansion in an invasive grass, Microstegium vimineum: a test of the channelled diffusion modelDIVERSITY AND DISTRIBUTIONS, Issue 5 2010Nathaniel P. Miller Abstract Aim, The greatest biodiversity impact of non-native plant species is caused by rapid expansion of colonist populations. Unfortunately, invasion has rarely been documented in real time at a population scale, and demographic mechanisms of invasion remain unclear. Our goal is to describe real-time expansion of populations, using channelled diffusion as a null model. Location, The study examined three populations of the invasive annual grass Microstegium vimineum in mature second-growth forests of south-eastern Ohio and nearby West Virginia, USA. Methods, Distributions were recorded in belt transects perpendicular to population edges over a period of 3 years. A second group of belt transects documented spread along five types of potential movement corridor. Observed changes in distribution were compared with predictions from a diffusion model. A seed-sowing experiment tested seed availability, microsite quality and proximity to potential movement corridors as factors controlling population spread. Results, Population boundaries showed little change over the study period. Colonization was limited by propagule availability over distances as little as 0.25 m, and to a lesser extent by litter cover. Populations did not advance along several potential movement corridors including unpaved roads, off-road vehicle trails and footpaths. Advance was observed along deer trails and stream courses but did not conform to the wave-form distribution predicted by diffusion theory. During the study, seeds were moved out of experimental plots by sheet flow and minor flooding events along small streams. Main conclusion, At a population level, invasion is driven by processes that are episodic in time and non-random in space , probably a common condition in non-native plant species. Spatially realistic models are likely to be more useful than diffusive models in managing invasions at these scales. [source] Rainfall in arid zones: possible effects of climate change on the population ecology of blue cranesFUNCTIONAL ECOLOGY, Issue 5 2009Res Altwegg Summary 1.,Understanding the demographic mechanisms through which climate affects population dynamics is critical for predicting climate change impacts on biodiversity. In arid habitats, rainfall is the most important forcing climatic factor. Rainfall in arid zones is typically variable and unpredictable, and we therefore hypothesise that its seasonality and variability may be as important for the population ecology of arid zone animals as its total amount. 2.,Here we examine the effect of these aspects of rainfall on reproduction and age specific survival of blue cranes (Anthropoides paradiseus Lichtenstein) in the semi-arid eastern Nama Karoo, South Africa. We then use our results to predict the effect of changes in rainfall at the population level. 3.,Using combined capture-mark-resighting and dead-recovery models, we estimated average survival of cranes to be 0·53 in their first year, 0·73 in their second and third year, and 0·96 for older birds. 4.,We distinguished between three seasons, based on the blue cranes' breeding phenology: early breeding season, late breeding season and nonbreeding season. Cranes survived better with increasing rainfall during the late but not early breeding season. Based on road counts and success of monitored nests, reproduction was positively associated with rainfall during the early but not late breeding season. 5.,A matrix population model predicted that population growth rate would increase with increasing rainfall. A stochastic analysis showed that variation in early breeding season rainfall increased population growth slightly due to the nonlinear relationship between rainfall and reproduction. This effect was opposed by the effect of variation in late breeding season rainfall on survival and overall, variation in rainfall had a negligible effect on population growth. 6.,Our results allow predictions to be made for a range of climate-change scenarios. For example, a shift in seasonality with drier springs but wetter summers would likely decrease reproduction but increase survival, with little overall effect on population growth. [source] The recent declines of farmland bird populations in Britain: an appraisal of causal factors and conservation actionsIBIS, Issue 4 2004Ian Newton In this paper, the main aspects of agricultural intensification that have led to population declines in farmland birds over the past 50 years are reviewed, together with the current state of knowledge, and the effects of recent conservation actions. For each of 30 declining species, attention is focused on: (1) the external causes of population declines, (2) the demographic mechanisms and (3) experimental tests of proposed external causal factors, together with the outcome of (4) specific conservation measures and (5) agri-environment schemes. Although each species has responded individually to particular aspects of agricultural change, certain groups of species share common causal factors. For example, declines in the population levels of seed-eating birds have been driven primarily by herbicide use and the switch from spring-sown to autumn-sown cereals, both of which have massively reduced the food supplies of these birds. Their population declines have been associated with reduced survival rates and, in some species, also with reduced reproductive rates. In waders of damp grassland, population declines have been driven mainly by land drainage and the associated intensification of grassland management. This has led to reduced reproductive success, as a result of lowered food availability, together with increased disturbance and trampling by farm stock, and in some localities increased nest predation. The external causal factors of population decline are known (with varying degrees of certainty) for all 30 species considered, and the demographic causal factors are known (again with varying degrees of certainty) for 24 such species. In at least 19 species, proposed causal factors have been tested and confirmed by experiment or by local conservation action, and 12 species have been shown to benefit (in terms of locally increased breeding density) from options available in one or more agri-environment schemes. Four aspects of agricultural change have been the main drivers of bird population declines, each affecting a wide range of species, namely: (1) weed-control, mainly through herbicide use; (2) the change from spring-sown to autumn-sown cereal varieties, and the associated earlier ploughing of stubbles and earlier crop growth; (3) land drainage and associated intensification of grassland management; and (4) increased stocking densities, mainly of cattle in the lowlands and sheep in the uplands. These changes have reduced the amounts of habitat and/or food available to many species. Other changes, such as the removal of hedgerows and ,rough patches', have affected smaller numbers of species, as have changes in the timings of cultivations and harvests. Although at least eight species have shown recent increases in their national population levels, many others seem set to continue declining, or to remain at a much reduced level, unless some relevant aspect of agricultural practice is changed. [source] Numerical fluctuations in the northern short-tailed shrew: evidence of non-linear feedback signatures on population dynamics and demographyJOURNAL OF ANIMAL ECOLOGY, Issue 2 2002Mauricio Lima Summary 1,We studied a fluctuating population of the northern short-tailed shrew (Blarina brevicauda) in the Appalachian Plateau Province of Pennsylvania, USA, spanning 21 years of monitoring. We analysed the pattern of annual temporal variation fitting both time-series models and capture,mark,recapture (CMR) statistical models for survival and recruitment rates. 2,We determined that non-linear first-order models explain almost 80% of the variation in annual per capita population growth rates. In particular, a non-linear self-excited threshold autoregressive (SETAR) model describes the time-series data well. Average snowfall showed positive and non-linear effects on population dynamics. 3,The CMR statistical models showed that a non-linear threshold model with strong effects of population density was the best one to describe temporal variation in survival rates. On the other hand, population density or climatic variables did not explain temporal variation in recruitment rates. Survival rates were high during the study period. Weekly changes in population size attributable to new recruits entering in the population fluctuate between 21% and 0%, while the changes in population size related to survival fluctuate between 79% and 100%. 4,Two important results arise from this study. First, non-linear models with first-order feedback appear to capture the essential features of northern short-tailed shrew dynamics and demography. Secondly, climate effects represented by snowfall appear to be small and non-linear on this insectivore. The population dynamics of this shrew in the Appalachian Plateau are determined apparently by a strong non-linear first-order feedback process, which is related to survival rates. 5,This study links population dynamics and demography by detecting the underlying demographic mechanisms driving population dynamics. The feedback structure of this shrew suggests the existence of population dynamics dominated by intraspecific competitive interactions, such as aggression, solitary nesting, non-overlapping home ranges and territoriality. [source] Which demographic traits determine population growth in the invasive brown seaweed Sargassum muticum?JOURNAL OF ECOLOGY, Issue 4 2009Aschwin Engelen Summary 1Life-history traits commonly associated with plant invasiveness are vegetative reproduction or r -selected traits such as short generation times and high rates of reproduction and individual growth. 2We used matrix modelling to assess which demographic traits are important for the population growth of an invasive seaweed lacking vegetative reproduction and whether demographic and life-history strategies shift with increased dominance of the invader. The vital rates of one of the most successful invading seaweeds, Sargassum muticum, were investigated monthly for 2 years in intertidal pools dominated by the native brown seaweed Cystoseira humilis and by S. muticum, respectively. In order to speculate about the demographic mechanisms that determine invasiveness of S. muticum, and as the study sites were recently colonized, we assumed that C. humilis and S. muticum pools are proxies for early and late phases of invasion, respectively. 3Both deterministic and stochastic matrix models showed positive rates of population growth, and rates were significantly higher in the pools dominated by S. muticum than in the ones dominated by C. humilis, indicating demographic changes with invader dominance. The variability of population growth rates and of reproductive and elasticity values of S. muticum was higher in the pools dominated by C. humilis, suggesting invader-driven stabilization of environmental conditions. Generation times of the species increased with invader dominance, supporting invader-stabilized environmental conditions. 4Elasticity analyses revealed that the most important demographic trait for population growth rate at both levels of invader dominance was the persistence of the non-fertile adult fronds rather than reproduction or growth. No major shifts in the life-history strategy of S. muticum between levels of invader dominance were detected. 5Synthesis. This study suggests that the invasiveness of S. muticum, a perennial invader without vegetative reproduction, relies on K - rather than r -selected traits and without drastic changes in life-history strategy between phases of invasion. [source] Limitation of population recovery: a stochastic approach to the case of the emperor penguinOIKOS, Issue 9 2009Stéphanie Jenouvrier Major population crashes due to natural or human-induced environmental changes may be followed by recoveries. There is a growing interest in the factors governing recovery, in hopes that they might guide population conservation and management, as well as population recovery following a re-introduction program. The emperor penguin Aptenodytes forsteri population in Terre Adélie, Antarctica, declined by 50% during a regime shift in the mid-1970s, when abrupt changes in climate and ocean environment regimes affected the entire Southern Ocean ecosystem. Since then the population has remained stable and has not recovered. To determine the factors limiting recovery, we examined the consequences of changes in survival and breeding success after the regime shift. Adult survival recovered to its pre-regime shift level, but the mean breeding success declined and the variance in breeding success increased after the regime shift. Using stochastic matrix population models, we found that if the distribution of breeding success observed prior to the regime shift had been retained, the emperor penguin population would have recovered, with a median time to recovery of 36 years. The observed distribution of breeding success after the regime shift makes recovery very unlikely. This indicates that the pattern of breeding success is sufficient to have prevented emperor penguin population recovery. The population trajectory predicted on the basis of breeding success agrees with the observed trajectory. This suggests that the net effect of any facors other than breeding success must be small. We found that the probability of recovery and the time to recovery depend on both the mean and variance of breeding success. Increased variance in breeding success increases the probability of recovery when mean success is low, but has the opposite effect when the mean is high. This study shows the important role of breeding success in determining population recovery for a long-lived species and demonstrates that demographic mechanisms causing population crash can be different from those preventing population recovery. [source] | ||||||||||