Demographic Vital Rates Research Articles

Demographic analysis of the Port Jackson shark Heterodontus portusjacksoni in the coastal waters of eastern Australia

Demographic analyses are used to assess the status and vulnerability of elasmobranchs but their accuracy is often affected by limited basic biological information. Although the Port Jackson shark Heterodontus portusjacksoni (Meyer) is currently not considered at threat, there is insufficient data for eastern Australia to assess this rigorously. The present study determined vital demographic rates of adult and juvenile H. portusjacksoni at four locations on the central and southern coast of New South Wales, Australia from January 2002 to December 2005 using underwater visual census, tag–recapture and samples obtained from a commercial fishery. Natural mortality was low in adults (0.063–0.074 year–1) and juveniles (0.225 year–1), but substantial at the embryonic stage (0.783–0.896 year–1). Adult growth rates (31.4–32.7 mm year–1) were slightly less than that of juveniles (36.8–37.5 mm year–1). Males at both stages grew slightly faster than females. However, H. portusjacksoni had slower growth rates than many other elasmobranch species. Having a low intrinsic rate of increase (r = 0.069 year–1), long generation times (μ1 = 22.5 year) and a low rebound potential, adults are the stage with the greatest impact on population growth. Hence, their life history strategy makes them susceptible to serious decline under exploitation, and management should strive to maintain the adult reproductive stock as a priority.

Low-level parasitic worm burdens may reduce body condition in free-ranging red deer (Cervus elaphus)

Regulation of ungulate populations by parasites relies on establishing a density-dependent relationship between infection and vital demographic rates which may act through the effect of parasites on body condition. We examine evidence for parasite impacts in 285 red deer (Cervus elaphus) harvested during 1991 and 1992 on the Isle of Rum. In the abomasa, prevalence of nematodes was 100% and the most abundant genus observed were Ostertagia species, however, mean intensity of infection was low (less than 1000) relative to other studies. Additional species, also present in low numbers, included Nematodirus spp., Capillaria spp., Cooperia spp., Monieza expanza, Oesophagostomum venulosum and Trichuris ovis. Lungworm (Dictyocaulus spp.) and tissue worm (Elaphostronygylus cervi) larvae were also observed in faecal samples. There was no evidence for acquired immunity to abomasal nematodes. Despite low levels of infection, both adult male and female deer showed significant negative correlation between indices of condition (kidney fat index, dressed carcass weight and larder weight) and intensity of Ostertagia spp. infection. However, there was no evidence that pregnancy rate in females was related to intensity of infection. For calves, there was no relationship between body condition and intensity of infection. The apparent subclinical effects of low-level parasite infection on red deer performance could alternatively be due to animals in poorer nutritional state being more susceptible to infection. Either way the results suggest that further studies of wild populations are justified, in particular where high local host densities exist or alternative ungulate hosts are present, and, where experimental treatments are tractable.

Demographic vital rates determine the performance advantage of crop–wild hybrids in lettuce

Summary Hybridization seems possible for many crop species after pollen transfer from crops to wild relatives in the surrounding vegetation. Subsequent introgression of crop‐specific traits into wild relatives could lead to invasive introgressants. This process has become a public concern following the introduction of genetically modified (GM) crops. Until now, few studies have used demographic vital rates to compare the performance of hybrids with their wild relatives. We created second‐generation (S1 and BC1) hybrids between the non‐transgenic crop Lactuca sativa and its entirely cross‐fertile wild relative Lactuca serriola. Seeds of parents and hybrids were individually sown in field plots at three different locations. Next to germination and survival, we measured a range of single fitness components and morphological traits. We also compared observed phenotypes to phenotypes theoretically expected, according to different inheritance scenarios. Phenotypes of both hybrid classes resembled L. serriola closely, and more than theoretically expected. However, demographic vital rates, i.e. germination and survival of hybrids were much higher than in L. serriola. Our results indicate that hybrids between crop and wild Lactuca are phenotypically indistinguishable from the wild relative and thus will largely remain unnoticed when they occur. However, these hybrids could potentially become invasive because of substantial differences in vital rates and seeds returned per seed sown. Synthesis and applications. A comparative study on single fitness components, such as seed production, alone would not have revealed the performance advantage of crop–wild hybrids in Lactuca. Therefore, studying demographic vital rates of hybrids and back‐crosses to test for long‐term consequences of hybridization should be part of any risk assessment of GM crops. Demographic vital rates are also important for the development of predictive modelling tools that can be employed to test the individual‐ and population‐level consequences of new‐to‐add traits.

Differences in vital demographic rates in three populations of the epiphytic bromeliad, Werauhia sanguinolenta

Abstract Since the response to differences in resource availability is most pronounced in smaller individuals of vascular epiphytes such as Werauhia sanguinolenta Cogn. et Marchal (Bromeliaceae), I expected variation in growth and survival of small individuals to play an important role in the dynamics of entire populations. Four annual censuses (2002–2005) of three study populations, which were located across the isthmus of Panama, allowed me to construct stage transition matrices, and to conduct growth analysis and elasticity analysis. Differences between populations were highly consistent through time, but, contrary to expectations, hardly related to the comportment of smaller plants. For example, although average mortality rates were highest at the driest site, close to the Pacific, small plants were not predominantly affected. Similarly, although the highest relative growth rates (RGR) of individuals and the highest population growth rates (λ) were found in the population with the highest moisture input, which was located close to the Atlantic coast, this was not due to a particularly strong stimulation of RGR in small plants. Elasticity analysis indicated rather small differences in the importance of the three demographic processes growth, survival, and reproduction for population growth in the three populations, but invariably identified the survival of large tanks as the single most important process determining λ.

SPATIAL AND TEMPORAL DEMOGRAPHIC VARIABILITY IN THE ENDEMIC PLANT SPECIES CENTAUREA CORYMBOSA (ASTERACEAE)

Centaurea corymbosa is an endemic plant species restricted to a 3-km2 area in southern France. This species is known from only six small populations that are highly differentiated genetically. Matrix models based on eight years of data (1994–2001) were used to assess the pattern of variation in the demographic vital rates of this species, and to investigate the causes of their variation. Asymptotic growth rates λ varied widely between years and populations (0.613–1.424). Randomization tests were developed to test for spatial and temporal variation in the asymptotic growth rates. These tests rely on individual data on both survival and fecundity. As our demographic survey only allowed us to estimate average fecundities, additional fecundity data collected from 1994 to 1996 were used to assess the distribution of individual fecundity expected within populations under demographic stochasticity or sampling error. Randomization tests showed that asymptotic growth rates were significantly different between popu...

Alternative causes of edge‐abundance relationships in birds and small mammals of California coastal sage scrub

Changes in the distribution and abundance of bird and small mammal species at urban‐wildland edges can be caused by different factors. Edges can affect populations directly if animals respond behaviorally to the edge itself or if proximity to edge directly affects demographic vital rates (an “ecotonal” effect). Alternatively, urban edges can indirectly affect populations if edges alter the characteristics of the adjacent wildland vegetation, which in turn prompts a response to the altered habitat (a “matrix” or “habitat” effect). We studied edge effects of birds and small mammals in southern Californian coastal sage scrub, and assessed whether edge effects were attributable to direct behavioral responses to edges or to animal responses to changes in habitat at edges. Vegetation species composition and structure varied with distance from edge, but the differences varied among study sites. Because vegetation characteristics were correlated with distance from edge, responses to habitat were explored by using independently‐derived models of habitat associations to calibrate vegetation measurements to the habitat affinities of each animal species. Of sixteen species examined, five bird and one small mammal species responded to edge independently of habitat features, and thus habitat restoration at edges is expected to be an ineffective conservation measure for these species. Two additional species of birds and one small mammal responded to habitat gradients that coincided with distance from edge, such that the effect of edge on these species was expressed via potentially reversible habitat degradation.

Within-Population Variation in Demography of Taraxacum Officinale: Season- and Size-Dependent Survival, Growth and Reproduction

1 Lefkovitch transition matrices were used to determine vital demographic rates of a natural population of Taraxacum officinale in Morgantown, WV, USA. Separate size transition matrices were calculated for each of four seasons, October-January, January-April, April-July, and July-October, to test if demographic rates vary as a function of season and if size-specific rates vary differentially among seasons. Seasondependent demography was also compared for four phenotype classes segregated by cluster analysis of leaf morphology. 2 The finite rate of increase for the entire population was largest in autumn (OctoberJanuary) and declined throughout the rest of the year. Overall, there was a small reduction in the population size. Size-specific probabilities of survival, growth and fertility varied dramatically among seasons. Sensitivity analyses showed that small individuals were particularly important to population growth from autumn to spring. Larger individuals were more important during summer. 3 Highly season-dependent demographic rates have large implications for population distribution and persistence since increased vulnerability to perturbation during particular seasons may constrain population growth and stability. Although T. officinale is a long-lived perennial, annual censuses may mask the importance of certain individuals or life history traits for maintenance of genetic variability and population viability. 4 Seasonal and annual finite rates of increase also varied as a function of phenotype class. Of two phenotype classes which had identical annual growth rates, one grew better in cool seasons while the second performed better in warm seasons. Direct competition for resources should be reduced by such inverse patterns of demography across seasons. 5 If phenotype classes are to some degree genetically determined, the differential responses observed here suggest that temporal variation in the environment could explain the maintenance of genetic diversity within populations.

Effects of harvesting onAscophyllum nodosum (L.) Le Jol. (Fucales, Phaeophyta): a demographic approach

Although populations ofAscophyllum nodosum are harvested commercially, little is known about the effects on demographic vital rates (growth, reproduction, survival). This study examines the effects of harvesting season and harvesting intensity on growth, reproduction and mortality of intact fronds in four size classes and in fronds truncated by the harvest. Knowledge of size-specific vital rates was used to evaluate the response of the population to harvesting.

Estimation and cluster analysis of net migration age profiles for urban areas in England and Wales

Bracken I. J. (1976) Estimation and cluster anlysis of net migration age profiles for urban areas in England and Wales, Reg. Studies. 10, 53–69. This paper describes a development of the residual method for the estimation of net migration data in disaggregated form by age and sex. This is in the form of a model which has been specifically designed to use British census data. Its application to population data for one hundred and twenty four urban areas in England and Wales is described. The paper deals with the particular calibration requirements of the model, the calculation of seven vital demographic rates and the difficulties imposed by the nature of the source data. The resulting net migration profiles by age are subject to a form of cluster analysis which is used to determine a classification based on similarities of the profiles. The paper concludes with some preliminary findings and suggests possible developments of the method.