Density-dependent Patterns Research Articles

Does the pattern of density dependence in organizational mortality rates vary across levels of analysis? evidence from the German brewing industry

Using life-history data on all Bavarian breweries over the period 1900–1981, we find that state-level density dependence in mortality rates conforms to the pattern found at the national level. The effects of state-level and national organizational density primarily reflect the “competition” phase of industry evolution. Brewery mortality rates in the largest cities do not exhibit any city-level density dependence. However, breweries located in the five largest Bavarian cities exert competitive pressures on the entire organizational population whereas rural/small-town breweries generate mutualistic effects. The results emphasize the importance of identifying the levels of analysis at which Hannan's (1986) model of density-dependent evolution in organizational populations applies.

Density-Dependent Parasitism and the Evolution of Aggregated Nesting in the Solitary Hymenoptera

Aggregated nesting is widespread among the solitary Hymenoptera and may represent an important step toward the evolution of eusociality via the semisocial pathway. Five hypotheses have been proposed to explain the evolution of nesting aggregations: first, aggregation may occur in response to a restricted distribution of some key resource; second, aggregation may increase foraging efficiency; third, aggregation may confer improved protection from natural enemies; fourth, aggregation may decrease the cost of nest site search; and fifth, aggregation may increase opportunities to reuse old nests. Proximate explanations for the formation of nesting aggregations, including the philopatry hypothesis, do not compete with these ultimate explanations but, rather, complement them. The role of parasite pressure in favoring or disfavoring aggregated nesting has become an area of active research and debate. This paper reviews field studies of density-dependent foraging by nest parasites and resulting patterns of nest exploitation to test generalizations concerning the effects of parasites. Nest parasites foraged either independently of host nest density (n = 2) or they concentrated in areas of high nest density (n = 4). Nest parasitism was directly density dependent (n = 8), density independent (n = 4), or inversely density dependent (n = 2). Thus, parasite pressure may favor either aggregated or dispersed nesting. Diversity in patterns of density dependence appears to result from the interaction of variable parasite foraging strategies and host defensive strategies. Inversely density-dependent parasitism may result from parasites being limited by egg supply or handling time, or from improved defense capabilities of grouped hosts. Improved defense may be a product of increased parasite confusion, active group defense, selfish herding, or improved parasite detection. Data supporting these hypotheses are critically reviewed and further research needs are identified.

Temporal and spatial patterns in pheromone-trap catches of Helicoverpa spp. (Lepidoptera: Noctuidae) in cotton-growing areas of Australia

AbstractHelicoverpa armigera (Hübner) and H. punctigera (Wallengren) are major pests of cotton and other field crops in Australia. Using data on then abundance of males in pheromone traps at many sites over three seasons, the spatial and temporal variation in trap catch of both species were examined using Taylor's power law and spatial autocorrelation. The distribution of both species was highly clumped, both temporally and spatially. Regression Coefficients for the relationship of spatial mean to variance (bs) were similar to those for noctuids in general, while similarly derived temporal values (bt) for H. armigera fell towards the upper end of the noctuid range and those for H. punctigera well above the range given by Taylor & Woiwood (1980). Taylor and colleagues suggested that patterns of dispersion are species-specific and that they reflect density-dependent patterns of movement towards and away from centres of abundance. Although the relationship between variance and mean abundance for both spatial and temporal aspects of trap catches of Helicoverpa is well fitted by Taylor's power law, it is argued that these patterns of dispersion are a consequence of demographic and environmental stochasticity. Little need is to invoke specific density-dependent behaviours as the major factor responsible in this case. In addition, while Taylor's power law indicated both species had a similar clumped distribution, limited autocorrelation analysis suggested a random dispersion of pheromone-trap catches for H. armigera and small scale patchiness (over distances of 1–2 km) in trap catches of H. punctigera.

Inverse Density‐Dependent Parasitism of Cornus Amomum Fruit by Rhagoletis Cornivora

We tested the prediction that decreased fruit density would lead to decreased parasitism of Cornus amomum fruits by Rhagoletis cornivora flies in a field manipulation. Fruit density following manipulation ranged from no reduction to 88% reduction. We found no evidence of parasitism decreasing with fruit density. Instead, the relationship was inversely density dependent. Although individual tephritid fruit flies concentrate attacks on fruits in areas of high fruit densities, the searching behavior of individual flies apparently does not result in a density—dependent pattern of parasitism at the population level.

Spatial analysis of density dependent pattern in coniferous forest stands

An investigation of spatial pattern in relatively sparse Pinus ponderosa-P. Jeffreyi stands showed that a simple Poisson model of random distribution described the pattern at 5 to 50 m scales in the denser stands examined when allowance is made for inhibition between nearest neighbors. There is evidence for a clumped distribution in large quadrats for the sparsest stands, which concurs with prior work where a mixed Poisson model was fit to the data. The technique used was innovative in that it involved digitally recording tree locations from high resolution aerial photos, which allowed for the automatic application of several statistical techniques in order to determine how pattern varies with plot density and scale. Point locations were recorded for six 11.3 ha plots in three density regions of a 340 ha study area in northeastern California, USA. The inter-event distance distribution, and one- and two-dimensional power spectra were calculated, and variable quadrat analysis was performed for the data sets. The second order and spectral analyses showed no evidence of a distinctive clumped pattern at any scale, and all analyses showed that the pattern was regular at the scale of the average inter-plant distance in the denser stands. For the sparser stands, the counts in large quadrats did not fit a Poisson distribution, but were better fit by a mixed Poisson model describing aggregated pattern.

Increased Reproductive Success at High Densities and Predator Satiation For Periodical Cicadas

The reproductive success of periodical cicadas, measured as the number of offspring produced per adult, is found to increase as adult density increases. The inverse density—dependent pattern at the adult stage occurred over the entire range of adult densities encountered. This result could have been caused by (1) predator satiation, (2) increased fecundity at higher densities, (3) more efficient mating at higher densities, and (4) movement of adults from sites of low initial densities to sites of high densities. The hypothesis of predator satiation was explored in depth. Predation at each site was crudely estimated by collecting cicada wings, which are discarded by avian predators. If predator satiation accounts for the inverse density—dependent mortality, then each cicada's risk of capture will decrease as cicada density increases. The data suggest that this prediction is true; predation did not increase as cicada density increased so that the probability of escaping predation is greater at high—density sites. Estimated predator densities are independent of cicada densities, which indicates that predators cannot respond numerically to the cicada emergence. As cicada density increases, the number of cicada wings recovered per bird does not significantly increase, suggesting that birds are satiated. Significantly more cicada wings were recovered at sites with more birds. Total cicada predation was independent of cicada density but dependent on the density of predators. Some evolutionary consequences of these results are discussed.

Loss of cell-population-density-dependent incorporation of fucose into the lipid fraction of cultured human tumour cells.

The incorporation of radioactively labelled fucose into the lipid fraction of cultured normal human cells and several human tumour-cell lines was examined as a function of the cell population density. Normal cells exhibited a density-dependent pattern of incorporation, whereas in tumour cells the radioactivity incorporated was independent of the cell population density. An exception was found among the tumour cells, which suggests a possible correlation between the loss of this marker and the ability to produce tumours.

A quantitative histological assessment of cellular death, in relation to mitosis, in rat thymus during growth and age involution.

The rates of cellular proliferation and cellular death were investigated in the rat thymus during growth and involution. It was found that the percentage of dead cells gradually increased with age, whilst the rate of cell proliferation declined, indicating that a substantial proportion of lymphocytes that divide in the thymus also die there. It was also found that there exists a density-dependent distribution pattern of mitotic and dead cells and that this distribution pattern changed with age. It is suggested that this mechanism of density-dependent control of the thymus may be an important factor in the age involution of the thymus.

A quantitative analysis of social interaction of the guppy, Poecilia reticulata (pisces: Poeciliidae) as a function of population density

Various controlled densities of guppies, Poecilia reticulata, wer observed in order to determine the effects of population density on frequency of courtship and agonistic interactions. Populations with single male exhibit a low level of courtship activity; the presence of a second male increases this level of activity markedly. Populations with a 1:1 sex ratio in densities greater than one pair exhibit a constant mean number of courtship interactions but, with increased crowding, an increasing variance in level of courtship activity. This results in a frequency profile of courtship interaction characteristic of each observed density. Each population density exhibits a density-dependent pattern of social interaction defined quantitatively by frequencies of courtship aggressive interactions at the population level. The contribution of the behaviour of individual males and to this patterning of social interaction in each population was partially determined.