Background Mortality (background + mortality)

Distribution by Scientific Domains

Selected Abstracts

Population dynamics of an endangered heathland shrub, Epacris stuartii (Epacridaceae): Recruitment, establishment and survival

David A. Keith
Abstract The only known population of the endangered shrub Epacris stuartii Stapf was studied from 1994 to 2001 using demographic census techniques. The effects of substrate, a fire and a storm on the emergence and survival of seedlings and the survival of established plants of different sizes were examined using failure-time analyses and logit-linear models. Ninety-five per cent of seedling emergence was delayed until the second post-fire spring, an unusual response among species with persistent soil seed banks. Mortality of seedlings was extreme compared with larger-seeded species, but diminished significantly with age. Seedling mortality varied significantly between substrates: 40% of seedlings persisted for more than 5 years in mineral soil, whereas less than 10% lived more than a year on rock and intermediate substrates. However, seedling numbers and local densities were lower on soils than other substrates. Background mortality of established plants was lower on soil and intermediate substrates (0.5% per year) than on rock (3% per year). Small plants may be more susceptible than large plants on rock, but not on soil. Both the fire and the storm resulted in elevated mortality of established plants. The population exhibited a variable response to fire, with plants on rock and intermediate substrates behaving as obligate seeders, whereas plants in soil resprouted. This appears to be the first report of microhabitat variation in fire response at sympatric scales. The effects of the storm were apparently independent of substrate and plant size. The essentially independent disturbance regimes comprising recurring fires and storms are likely to have a profound effect on the long-term population dynamics of E. stuartii. Over the 7-year census period, recruitment has failed to compensate for mortality, resulting in a 30% net decline in the population. The demographic census has proved to be crucial in the detection and diagnosis of this decline. [source]

The influence of greenhouse chrysanthemum on the interaction between the beet armyworm, Spodoptera exigua, and the baculovirus SeMNPV: parameter quantification for a process-based simulation model

F. J. J. A. Bianchi
During the building of a process-based simulation model for the epidemiology of the multicapsid nucleopolyhedrovirus of S. exigua (SeMNPV) in populations of Spodoptera exigua (Hübner) in greenhouse chrysanthemum, it was found that the effect of host plants had been under-rated. ,Missing links' included (i) the ,natural' background mortality of larvae of S. exigua in practical cropping conditions; (ii) the developmental rate of larvae of S. exigua on plant substrate in a glasshouse as compared to artificial medium in the laboratory; (iii) the validity of the results of dose-mortality and time-mortality bioassays conducted on artificial medium as compared to natural plant substrate; (iv) the distribution of inoculum released from deceased caterpillars over chrysanthemum leaves; and (v) the leaf visit rate of healthy caterpillars (as it affects horizontal transmission). Experiments were carried out to quantify these processes. Developmental rates of S. exigua larvae on greenhouse chrysanthemum were 36% lower than on an artificial diet. The fraction survival during the first, second, third and fourth instar S. exigua larvae in greenhouse chrysanthemum was 0.60, 0.80, 0.88 and 0.95, respectively. Forty percent of the first instar larvae reached the fifth larval stage. Second instar S. exigua larvae reared on chrysanthemum were significantly more susceptible to SeMNPV than larvae reared on an artificial diet. The food source had no effect on the time to kill S. exigua larvae. Cadavers of second, third and fourth instar S. exigua larvae contaminated on average 1.4, 2.5 and 3.3 chrysanthemum leaves. Second to fourth instar S. exigua larvae visited 2,3 leaves per day and spent 15,55% of the time on the underside of leaves. The above information is of critical importance for a trustworthy simulation of the epidemiology of SeMNPV in chrysanthemum. [source]

Tracking the decline of the once-common butterfly: delayed oviposition, demography and population genetics in the hermit Chazara briseis

T. Kadlec
Abstract Large populations, seemingly not at risk of extinction, can decline rapidly due to alteration of habitat. This appears to be the case of the butterfly Chazara briseis, which is declining in all of Central and Eastern Europe, even from apparently large areas of its steppe grassland habitats. We combined mark,recapture, allozyme electrophoresis and adult behaviour observation to study the last remaining metapopulation of this once-widespread butterfly in the Czech Republic. The total population estimate was 1300 males and 1050 females in 10 colonies within a 70 km2 landscape. Adults were long-lived, and inseminated females required several weeks before they started ovipositing. Models using realistic lengths of the preoviposition period estimated that due to background mortality, only 25,55% of the female census population lived long enough to contribute to the next generation. This demographic load was unlikely to be balanced by an increased fecundity. Allozyme electrophoresis of 22 loci revealed much higher allelic variation than in most other studies of butterflies living in small populations (mean heterozygosity: 20.7%). If expressed as per individual colony, the genetic variation did not correlate with population density, survival or longevity. This was probably due to frequent movements among colonies; during 8 weeks of adult flight, 5.1% of recaptured males and 3.6% of recaptured females moved between colonies. The high preoviposition mortality indicates that populations of this species must contain more individuals compared with populations not suffering this additional demographic load. The high allelic diversity of each single colony suggests that the population as a whole has not undergone genetic bottlenecks, but now may be facing risks of inbreeding depression due to allele frequency shifts and the possible increase of weakly deleterious alleles. In the past, high effective population sizes were maintained by frequent dispersal in dense networks of steppic grasslands. Generous habitat restoration is necessary to safeguard populations of this specialized, yet formerly common species. [source]

Ectoparasite load is linked to ontogeny and cell-mediated immunity in an avian host system with pronounced hatching asynchrony

Several contrasting hypotheses have been proposed to account for host age-biased parasite distribution, with some of them suggesting a key role of ectoparasites in the evolution and maintenance of weight hierarchies within broods. We examined parasite distribution among individual hosts across the whole period of host exposure to the parasite in a host system that shows distinct within-brood differences in age and age-related mortality. By contrast to previous hypotheses, we found that the abundance of a haematophagous, mobile ectoparasite Carnus haemapterus on nestling European rollers (Coracias garrulus) was highest approximately during the mid-nestling stage of their host, coinciding with the inflection point of the host growth phase. Parasite load increased neither with absolute resource availability (i.e. body size), nor body condition index. By contrast to previous evidence, higher parasite load under natural conditions was associated with a stronger cell-mediated immune response. However, this association was moderated by low parasite densities, as well as a better brood body condition index. Overall, although we revealed remarkable host ontogenetic effects on parasite distribution, the present study suggests that a highly mobile ectoparasite generally prefers healthier hosts. We propose that, in host systems with a marked asynchrony of hatching and background mortality within the brood, parasites favour persistence rather than nutritional attractiveness of the host. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 463,473. [source]