Evidence Of Density-dependent Mortality Research Articles

Partial Life Tables From Three Generations of <I>Enaphalodes rufulus</I> (Coleoptera: Cerambycidae)

We used life table analyses to investigate age specific mortality and to better understand the population dynamics of the red oak borer, Enaphalodes rufulus (Haldeman) (Coleoptera: Cerambycidae). We continually sampled populations within 177 trees at primarily two sites in the Ozark National Forest in Arkansas throughout three (2-yr) generations. The first cohort (adults emerged in 2003) was sampled during a severe population outbreak, whereas the second and third (2005 and 2007) were sampled during the population crash that followed. Generation mortality was 94% in 2003 and 99% in both 2005 and 2007. Estimates of apparent mortality indicated that the E. rufulus population crash likely occurred during or before the first overwintering period (2003-2004) of the generation that emerged as adults in 2005. We found limited evidence for density dependent mortality, which suggest that intraspecific competition after the first active feeding period was apparently not an important mortality factor during E. rufulus development. Life tables revealed that E. rufulus larvae generally experienced the greatest apparent mortality during the second summer of active feeding (80-94%) when larvae were feeding in, and moving between phloem and sapwood. The least apparent mortality was incurred during the following spring and early summer (26-67%) when late stage larvae and pupae were deepest and most protected within sapwood or heartwood tunnels. We found very little evidence for mortality from associated species. Scarring of vascular tissue in response to E. rufulus feeding occurred during early life stages and may be an important tree resistance mechanism and E. rufulus mortality factor.

Decadal‐scale dynamics and response to pulse disturbance in the intertidal rockweed Fucus distichus (Phaeophyceae)

AbstractThe rockweed Fucus plays an important role in structuring intertidal communities on northern temperate and sub‐arctic shores. We performed a 12‐year study of Fucus dynamics on a rocky shore in Alaska to (i) describe characteristic dynamics of the Fucus canopy, (ii) test the generality of specific observations that emerged from biological studies following the Exxon Valdez oil spill (EVOS), and (iii) provide a basis for longer‐term change analysis in this sub‐arctic system. We annually sampled 36 permanent plots within a Fucus community in the Gulf of Alaska to describe canopy dynamics. In two consecutive years, we created small, high‐intensity disturbances to test whether synchronous cycles of growth and decline could be detected, as was reported following EVOS. We found only equivocal evidence of synchronous cycles and no long‐term signal of intense experimental disturbance. We found no evidence of recruitment inhibition by an established canopy, nor did we find evidence of density‐dependent mortality among adults. We did find that Fucus abundance tended to converge on a mode of 50–60% cover, and that annual average deviation from the 12‐year mean was 30% or less. We suggest that this system is both dynamic and resilient to natural forcings and intense experimental disturbance events.

Suppression of nematodes in a coastal grassland soil

The nematophagous fungi Arthrobotrys oligospora and Myzocytiopsis glutinospora increase to large numbers (>103 propagules/g of soil) when moth larvae killed by entomopathogenic nematodes are added to soil microcosms. In spite of these increases, it is unclear how effective these nematophagous fungi are in suppressing nematodes. We measured nematode mortality in microcosms with small numbers of assay nematodes, and we examined assay nematodes recovered at the end of the experiment for signs of fungal parasites. Because the microcosms did not have a moat or other refuge, the assay nematodes remained vulnerable for the 3 days that they were in the soil. Mortality in this experiment was not substantially increased compared to a previous experiment, which measured the mortality of a larger number of assay nematodes in microcosms surrounded by a moat. Mortality, however, increased from 34 to 50% when recovered assay nematodes were examined and when those with conidia of the nematophagous fungus Hirsutella rhossiliensis were considered dead. The zoosporic fungus M. glutinospora was not detected, perhaps because the soil water potential was too low. Contrary to our expectations, there was no evidence of negative feedback on nematodes (i.e., no evidence of density-dependent mortality) because the addition of dead moth larvae greatly increased numbers of resident nematodes and A. oligospora but did not greatly affect the probability of nematode mortality.

A LONG-TERM STUDY OF A SHORT-LIVED FISH: THE DEMOGRAPHY OF GASTEROSTEUS ACULEATUS

Abstract(1) We present the results of the first long-term investigation on the demography of a population of three-spined sticklebacks. This study describes the changes in the abundance of a population from a backwater of the Afon Rheidol in mid-Wales, based on mark-recapture data collected in October of each year from 1972 until 1998. (2) The population showed quasi-cyclical changes in abundance around a trend line with a period of approximately 6 years. Over the duration of the study there was a tendency for the population to decline. During the unusually dry summers of 1976, 1983, 1984 and 1989 the population was either at a minimum size or was below the estimated trend line. Estimates of stickleback densities varied between 7-28 m-2. (3) Analysis of length-frequency data showed there were two cohorts present. These were assumed to represent two age classes, 0+ and 1+ , with the population consisting largely of the 0+ class. Thus the population was essentially annual. (4) There was evidence of density-dependent mortality and growth of 0+ sticklebacks during and shortly after the spawning period.

Spatial Patterns of Trees and Structuring Factors in a Trachypogon Savanna of the Orinoco Llanos

Tree dispersion was evaluated in a Trachypogon savanna by measuring the distance from individual trees to their nearest conspecific neighbor. The vegetation is a closed bush island savanna with coexistent isolated trees and small patches of semideciduous forest. The isolated trees were in clumps similar to trees growing in savannas and tropical forests. In our study, adults were less clumped compared to juveniles. Environmental and biotic differences related to edaphic heterogeneity, life history, and reproductive strategy could account for the clumped pattern. Different spatial patterns emerged for stems in groves. Thus, uniform spatial distribution was evident in the individuals of the fire resistant species (Curatella americana, Byrsonima crassifolia and Bowdichia virgilioides) and a gap filling species (Casearia decandra). Changes in the demographic components between different age groups provide evidence for density-dependent mortality. A hypothesis mainly based on two scales of disturbance (human impact and gap formation) acting synergetically with a large scale factor (substrate patchiness) is put forward to explain the dynamics of tree invasion and the maintenance of the Trachypogon savanna.

A quantitative study of mortality at emergence in the damselfly Pyrrhosoma nymphula (Sulzer) (Zygoptera: Coenagrionidae)

SUMMARY. Emergence of the damselfly Pyrrhosoma nymphula was studied over three seasons in two ponds in northern England. Numbers emerging were significantly negatively correlated with temperature and atmospheric pressure. Overall mortality at emergence was 28%. of which predators (largely birds, ants and spiders) accounted for 22% and climatic factors 6%. Daily mortality estimates were significantly positively correlated with precipitation. There was no evidence of density dependent mortality at emergence.

Factors Affecting Recruitment and Mortality Rates of Rainbow Smelt ( Osmerus Mordax) in Lake Erie, 1963–85

Recruitment of young-of-the-year (YOY)rainbow smelt ( Osmerus mordax) per parent was nqt dependent upon water temperature (rate of warming and average temperature) during spawning, Glugea infestation, or parental stock-size. Only in the eastern basin of Lake Erie was recruitment to age 1 + years-old positively correlated with the YOY index in the preceding summer. Recruitment of age 1 + year-old smelt was not related to parental stock-size. Recruitment (age 1 + years-old) per spawner was inversely related to the abundance of age 1 year-old smelt present during the year of hatch in both the eastern and central basins of Lake Erie. This could be taken as evidence to support the hypothesis that the alternating dominance of year-classes (1963–74) was caused by cannibalism of YOY by age 1 year-old smelt. After 1974, alternating year-class-dominance was not evident; we considered three possible explanations for the absence of strong year-classes. First, the size at age declined in both basins during the study so that more age classes could have fed optimally on young-of-year (YOY), resulting in a poorer survival of YOY smelt. Second, the harvest of smelt increased greatly after 1974, particularly for yearling smelt, so that year-class strength measured by the abundance of yearlings was reduced by fishing. Third, predation on smelt by stocked salmonids probably increased after 1974 as the stocking rates increased. Adult total mortality rates (ages 2 and 3 years-old) were positively related to stock density providing some evidence for density dependent mortality of smelt. There was no evidence that the variations in adult mortality were associated with the incidence of Glugea.

A 13-Year Study of the Population Dynamics and Growth of the European Eel Anguilla anguilla in a Norwegian River: Evidence for Density-Dependent Mortality, and Development of a Model for Predicting Yield

A 13-Year Study of the Population Dynamics and Growth of the European Eel Anguilla anguilla in a Norwegian River: Evidence for Density-Dependent Mortality, and Development of a Model for Predicting Yield