Fecundity Schedules Research Articles

Demographic Mechanisms in the Fluctuating Populations of the Field Vole Microtus agrestis

Demographic mechanisms in the fluctuating populations of the Central-Scandinavian field vole, Microtus agrestis (L.), are described predominantly on the basis of the author's own long-term studies in Southern Finland. The populations monitored and used as examples have shown abundance peaks at 3 to 4 year intervals. The survival schedules, growth patterns, reproduction rates, population structures and habitat dynamics are discussed in relation both to the season and the cyclic phases. The average life span of a field vole varies greatly with the time of birth, sex and cyclic phase, the overwintered peak-year cohort having the highest value, the early decrease-year cohorts the lowest. Nestling mortality is relatively high, ranging from about 20% in spring to more than 50% during the nutritional crisis. Individual growth pattern and attainment of sexual maturity are closely intercon- nected, especially in females. The fecundity rates of the females vary with the season and the cyclic phase. Post-partum oestrus and a pregnancy rate approaching 100% are the rule during the most intensive part of the breeding season in the early summer months. During the midsummer crisis both maturation and pregnancy rates decline. Towards the autumn they usually rise again, especially during the increase phase of the vole cycle. Litter size varies according to the season and the age (size) of the female, but is one of the least variable parameters in the course of the microtine cycle. Age-specific and cyclic differences in the reproductive performance are summarized in the form of mx and lxmx functions. The prevailing population structures reflect these fecundity schedules and the age and sex specific survival and dispersal schedules. The sex ratios at birth are close to the theoretical 1:1 ratio, but they become heavily weighted to the female side in the course of the breeding season, especially at peak abundance. This change in the functional sex ratio is predomi- nantly due to different dispersal of sexes . The chief production habitats of the field vole are permanent grassland patches with a high proportion of the preferred dicotyledon- ous food plants. The interactions between these and the other habitat types are discussed on the basis of two concrete examples, one representing a typical mixed agricultural habitat complex, the other a forestry-dominated taiga landscape.

LIFE TABLE DATA FOR USE IN DETERMINISTIC AND STOCHASTIC SIMULATION MODELS PREDICTING THE GROWTH OF INSECT POPULATIONS UNDER MALTHUSIAN CONDITIONS

AbstractLife table data from a laboratory strain of Tribolium confusum (DuVal) were obtained for use in deterministic and stochastic models of population growth under optimal environmental conditions. Frequency distributions of mortality, developmental time, and age-specific fecundity schedules taken from experiments conducted at five constant temperatures were transposed onto a physiological time scale measured in degree-days. Data of this detail were necessary for use in models that could predict the growth of laboratory populations of the same strain of beetle reared under optimal conditions.

The Adaptive Significance of Sequential Hermaphroditism in Animals

In some cases, sequential hermaphroditism can convey a selective advantage to an individual by increasing its reproductive potential relative to nontransforming members of the population. This is because age-specific fecundity in many populations is not distributed in the same way for males and females. By functioning as that sex which has the higher fecundity in a particular age span, an individual could increase its reproductive potential relative to lifetime males or females. The magnitude of the reward that comes from such sex changes depends on both the demography of the population and its spawning habits. Protandry may be selected for in populations where female fecundity increases with age and where individuals mate at random. Factors which favor the evolution of protogyny are those which tend to depress male fecundity values at early ages, such as inexperience, territoriality, or female mate selection. Selection for protogyny can also exist when female fecundity decreases with age, although this situation seems rare in the field. It appears that the nature of the fecundity schedule for females has a greater effect on the magnitude of the selection pressure for sequential hermaphroditism than does the age structure of the population. If sequential hermaphroditism becomes genetically fixed in some members of a population with differential age-specific fecundities, the frequency of the hermaphrodites should increase over time until they eliminate the gonochoristic individuals. The pattern of sexual transformation eventually reached in a population is predictable through the use of a simulation model developed here. The pattern depends on the age structure, female fecundity schedule, and mating characteristics of the population. For two examples, the predictions appear consistent with what is seen in nature. At equilibrium, the simulated populations consist of individuals which changed sex at one of two ages, and each type has essentially the same reproductive potential. This suggests that we should see a rather sharp break in age-specific sex ratios in protogynous or protandrous populations. Some hermaphroditic populations exhibit such sharp changes, but others do not. The expected pattern would tend to be obscured by differing growth rates among individuals.

The influence of temperature on development, longevity, and fecundity in the Rutherglen bug, Nysius vinitor (Hemiptera : Lygaeidae)

The mean duration of the egg stage of N. vinitor at constant temperatures ranged from 36.8 days at 15C to 3.8 days at 32C and that of the nymphs from 45 days at 20C to 12.0 days at 32C. Within the range 20-35C egg and nymphal mortalities were low; temperatures of 12 or 40C were lethal to both eggs and nymphs. Mean nymphal development times for males and females were similar. The threshold temperature for egg development was 14.5C and that for nymphs 15C; 70 and 225 day-degrees were required for completing egg and nymphal development respectively. The immature stages developed more rapidly at fluctuating temperatures out of doors than they did indoors at constant temperatures equal to the mean of the fluctuating temperatures. Within the range of screen temperatures 21.5-23.0C the difference between mean development in shade and in sun was 9-10 days. Age-specific fecundity and mortality schedules were determined for N. vinitor at constant temperatures. Temperature and longevity were inversely related and males survived longer than females. Thus at 22OC mean longevity of males was 115 days and of females 90 days, as compared with 31 and 18 days respectively at 30�C. After a maturation period that was longer at lower temperatures, daily egg-laying per female decreased with age from a maximum that occurred before there was significant mortality of females. The rates of the cumulative egg-laying increased with temperature from 22 to 35C. Mean total number of eggs per female was significantly higher at 25 and 30�C than at 22 or 35�C. However, life-table calculations revealed that a temperature of 35�C yielded the maximum rate of population increase. Within the range 22-35C, temperatures and rc values were linearly related. Unmated females laid significantly fewer eggs than those that had mated, but survived longer.

Rate of Increase

We outline the differences between three notions of rate of increase: r, the observed rate of increase; rs, the rate implied by the prevailing schedules of survival and fecundity, and rm, the maximum rate at which a population with a stable age distribution can increase in a specifie,d environment. Several mammalogists recently calculated r8 for natural populations under the misapprehension that they were calculating rm. However, their calculated values are usually de£ective even as r8 essdmates because they have used life tables constructed from age distributions or from distributions of age at death. The estimates are thereby infiltrated by the unrecognized but implicit assumption that rs = O, and the calculated values of ss are therefore assumptions retrieved as conclusions. If rm, the intrinsic rate of increase, is to be detelmined for a natural population of mammals, it is best calculated either by measuring the rate at which a newly established population initially increases or by fitting a curve to the growth of a population after its density has been artificially reduced. Over about the last 10 years mammalogists have become increasingly interested in population dynamics, and they have begun using concepts and analyses hitherto restricted almost entirely to the demegraphy of man and inselets. This is a welcome trend that should be continued7 but at the same time a warning is necessary against uncritically using special methods of analysis that may not be appropriate to field studies of mammals. By this we do not imply that the demagraphy of mammals differs esslentially from that of insects; we suggest only that some parameters, wholse estimation is relevant to questions that entomalogists ask) may be extremely difficult to estimate for natural populations of mammals. In addition, the paramleters may be irrelevant to the problem in hand. The reverse is also true. Mammalogists ask questions on conservation and harvesting, for example, that entomalogists seldom consider, and they must therefore estimate parameters that entomologists ignore. In this paper we examine some of the ways in which rate of increase can be measured and point out that some mammalolgists, using equations that are valid and widely used in insect ecology, are estimating it incorrectly. 658 KINDS OF RATE OF INCREASE Because populations tend to grow geometrically, rate of increase is best expressed in expl(}nential form. A population of 100 animals ffiat increases to 200 over a year has been multiplied by 2 or increased by 100 percent, however o!ne prefers to express it. Its exponential rate of increase, r, is given by

The Influence of Temperature on Longevity and Fecundity in the Bug Lygus hesperus (Hemiptera: Miridae)

Age-specific fecundity and mortality schedules were determined for Lygus hesperus Knight adults at constant, alternating, and variable temperatures. Constant temperatures ranged between 55 and 100°F. In the alternating temperatures, the bugs were alternated on a 3-day schedule between a constant high and a constant low temperature, while under the variable temperature regimes, the bugs were exposed to daily fluctuations of 55–95°F or 55–100°F. At 55°F, L. hesperus produced 1 egg each 3rd day and the mean female longevity was 120 days and maximum longevity was 200 days. Temperatures above 90°F were detrimental; no eggs were produced at 100°F. Life table calculations revealed that constant 90°F or alternating temperatures of 80–90°F yielded the maximum potential rate of population increase. Egg hatch was inversely related to temperature; only 6% of eggs laid at 95°F hatched. The rate of egg production at either of the alternating temperatures was the same as that which occurred when the bugs were exposed to similar constant temperatures.

A Demographic Analysis of Fenced Populations of the Whiptail Lizard, Cnemidophorus tigris, in Southern Nevada

Between 1964 and 1967, spring densities of Cnemidophorus tigris in four study areas in southern Nevada ranged from 3-8 per acre. Estimated biomass ranged from around 43 to 114 g/acre. The sex ratio was 1:1. Minimal annual survival of adults was 54-60%, and life-spans of at least 7 years are postulated. An undetermined proportion of mature females laid two clutches of eggs in 1965, but the general pattern was one clutch of 2-4 eggs per year. Large females produced more eggs than small ones. Occasionally females 9-10 months of age laid eggs, but sexual maturity normally was attained at about 21 months. By assuming that all mature females laid two egg clutches in 1965 our fecundity estimates could be approximately reconciled with the observed size and age composition of populations in the spring of 1966. Possible compensatory errors in this analysis are discussed. One of the challenging problems of contemporary ecology is the analysis of ecosystems and the development of mathematical models capable of simulating the trophic transfers of energy and materials and the interactions between species which collectively constitute ecosystem function. This endeavor requires not only an understanding of the processes of primary production, but also the roles of primary and secondary consumers. The net production and material transfers of a consumer population can be estimated only if one understands the time changes in the structure and size of such populations, i.e., the rates of age-specific mortality and the schedule of recruitment by which the population is sustained. In another paper we have discussed the demography of Uta stansburiana and related annual changes in densities of this species to changes in schedules of female survival and fecundity (Turner et al., 1 Present address: PO Box 236, Zion National Park, Springdale, Utah 84767

Productivity of Glossina austeni Newst. maintained on lop-eared rabbits

A life-table and age-specific fecundity schedule are given for females of Glossina austeni Newst. maintained on lop-eared rabbits. From these the innate capacity for increase in numbers (τm) is calculated, from two sets of data, as 0.0166 and 0.0155 per day, respectively. These values are better than previously obtained for Glossina and it is concluded that as such they were mainly due to the excellent nutrition of the flies. A colony in which all female offspring produced are added to the breeding stock is referred to as being in the ‘expanding phase’. The stable age distribution that would be established in a rabbit-fed colony of G. austeni in the expanding phase and the expected rate of increase in numbers of the colony when the stable age distribution exists are calculated from the value of τm.The characteristics of a colony of G. austeni in the ‘stationary phase’, when only those offspring required to maintain the colony at a constant size are retained, are described. It is concludef that a stationary colony of y adult flies would have a disposable output per week of 0.277y young adult males and 0.233y young adult females. Some implications of these findings in relation to the sterible male techinque for tsetse control are briefly discussed.