Evidence For Population Regulation Research Articles

Is There Evidence for Population Regulation in Amphibians and Reptiles?

Abstract From the 1950s to the present, many researchers have tested time series data for density dependence. All kinds of organisms have been studied, from microorganisms to insects and vertebrates to plants. A variety of techniques and population growth models were developed, and the conceptual framework to study populations has been improved. We searched for long time series data on amphibians and reptiles in the literature. From 102 population time series, and after filtering the dataset, we tested for density dependence in time series data for 69 populations (52 species) of amphibians (anurans and caudatans), serpents, lacertilians, chelonians, rhynchocephalians, and crocodilians. We used the exponential growth state-space model and the Ornstein-Uhlembeck state-space model as proxy models for density-independent and density-dependent population growth models, selecting between them with the parametric bootstrap likelihood ratio test. The hypothesis of density independence was rejected for 2 amphibian...

Dreissenids in Lake St. Clair in 2001: Evidence for Population Regulation

Zebra mussel ( Dreissena polymorpha [Pallas]) density was surveyed at 12 stations in Lake St. Clair in September 2001. Lake-wide mean density was 1,824 individuals/m 2; whole wet biomass was 148 g/m 2; and dry tissue biomass was 1.23 g/m 2. Compared to historical data, density did not change significantly, whereas biomass showed a significant downward trend. Our data support the assertion that the zebra mussel population in Lake St. Clair has undergone important changes since the mid-1990s. Some areas of the lake are now juvenile-dominated, others are adult-dominated, and some have a balanced size distribution. These data are consistent with the hypothesis that zebra mussels have changed the lake ecosystem in two ways that have contributed to their own population limits in a density-dependent manner. First is the reduction of adult microhabitat due to the elimination of native mussels from the lake proper. Second is the massive redirection of larval settlement onto a greatly expanded aquatic macrophyte community which senesces and dies at the end of each season, thus decreasing survivorship of juvenile D. polymorpha. If sustained, these recent changes, especially biomass reduction, suggest that the impact of dreissenids on the Lake St. Clair ecosystem will be more moderate in the future.

Population decline in the Delta caribou herd with reference to other Alaskan herds

After growing continuously for nearly 15 years, the Delta caribou herd began to decline in 1989. Most other Interior Alaskan herds also began declining. In the Delta herd, and in other herds, the declines were caused primarily by high summer mortality of calves and increased natural mortality of adult females. Other minor causes included increased winter mortality of calves, and reduced parturition rates of 3-year-old and older females. The decline in the Delta herd also coincided with increased wolf (Canis lupus) numbers, winters with deeper than normal snow, and warm summers. Mean body weight of annual samples of 10-month-old female calves was consistently low during the decline. Except in some of the smallest Interior Alaskan herds, we conclude that evidence for population regulation in Alaskan caribou is weak, and that herds are likely to fluctuate within a wide range of densities due to complex interactions of predation and weather. Unless wolf numbers are influenced by man, the size of a caribou herd in a given year is likely to be largely a function of its size during the previous population low and the number of years of favorable weather in the interim.

Demography of the George River Caribou Herd: Evidence of Population Regulation by Forage Exploitation and Range Expansion

The George River caribou herd in northern Quebec/Labrador increased from about 5000 animals in 1954 to 472,200 (or 1.1 caribou/km) prior to the 1984 calving season. The range used by the herd expanded from 160,000 to 442,000 sq km for the period 1971-84. The exponential rate of increase (r) was estimated at 0.11 in the 1970s. Calf:female ratio in autumn was relatively constant (x=0.52) from 1973 to 1983, but decreased to about 0.39 in 1984-86. The harvest rate was relatively low in the 1970s (about 3%/y), but seemingly increased in the mid-1980s to 5-7% as a result of more liberal regulations and a greater impetus to exploit caribou for subsistence. The cumulative impact of lower calf recruitment and more intensive hunting may have appreciably depressed the growth rate of the herd in 1984-86. A greater year-round competition for food resources and a greater energy expenditure associated with range expansion are presented as probable regulatory factors for the George River herd. It is argued that the nature of caribou-habitat interactions in continental regions generate long-term fluctuations in caribou numbers if human exploitation remains low. At present, wolf predation does not appear to be an important mortality factor capable of regulating the George River herd.Key words: caribou, population regulation, food limitation, northem Quebec, Labrador, wolf predation

Colonization patterns of the leaf‐cutting ant, Atta bisphaerica Forel: evidence for population regulation

AbstractFollowing the nuptial flight, colonization patterns of queens of the leaf‐cutting ant, Atta bisphaerica, were studied by means of belt transects. Queen density on the ground following swarming varied from 500 to 13500 queens/ha. Colonization attempts were greater in areas occupied by mature colonies of conspecifics, probably due to disturbance caused by large colonies. However, queen execution by conspecifics was also greater in areas of mature colonies. Approximately 89% of all founding queens were executed, and another 3% were initially lost to predators on the ground in areas with mature colonies. Losses to predators were 59% during the pre‐claustral phase in queens in areas of no mature colonies.ZusammenfassungKoloniebildungsmuster der Blattschneider‐Ameise, Atta bisphaerica Forel: Ein Nachweis der PopulationsregulationDie dem Hochzeitsflug folgenden Koloniebildungsversuche der Atta bisphaerica‐Königinnen wurden mit Hilfe von streifenförmigen Transekten untersucht. Die Königinnendichte auf dem Boden nach dem Schwärmflug variierte zwischen 500 und 13 500 Königinnen/ha. Versuche, eine Kolonie in Gebieten zu gründen, in denen bereits Kolonien der gleichen Art vorhanden waren, waren häufiger als in bisher unbesiedelten Gebieten. Jedoch wurden Königinnen in solchen Gebieten auch häufiger durch Artgenossen getötet. In bereits durch Blattschneider‐Ameisen besiedelten Gebieten wurden ungefähr 89 % aller gefundenen Königinnen getötet und weitere 3 % geräubert. In Gebieten, in welchen bisher keine Kolonien vorhanden waren, betrug der Verlust durch Räuber in der Vor‐Einschließungsphase 59 % der Königinnen.

Environment, Population Size and Vital Statistics: An Analysis of Demographic Data from 18th Century Villages in the Province of Reggio Emilia (Italy)

We have analyzed a set of crude vital statistics from six of the years between 1690 and 1793 for a number of villages in the province of Reggio Emilia (Italy). These statistics were computed from population counts and registered baptisms and burials. We investigated the relationship between these vital statistics and population size for evidence of population regulation by density—dependent mechanisms. Since the villages were located in three quite distinct environmental zones (mountains, hills and plains), we also investigated the existence of environmental effects. The statistical results indicate that: (1) birth rates do not depend on population size but increase significantly from mountains to hills and plains; (2) death rates increase significantly with population size, also increasing by environmental zone in the same order as birth rates; (3) consequently, growth rates depend significantly on both environment and population size. However, the last relation does not emerge so strongly as for death rates alone because growth rates include the statistical variations in both the birth and death rates.