Recruitment rates in forest plots: Gf estimates using growth rates and size distributions

1 In censuses of tree populations in permanent plots, short census intervals and small population size lead to uncertainty in the observed recruitment rate of a minimum size. Increasing the census interval, however, underestimates the rate because of unrecorded 'recruit and die' events.
2 We propose a new Gf procedure for estimation of recruitment rates. Recruitment rate per area is obtained by multiplication of the density in the smallest size class (f) and the average size growth rate in that class (G) divided by the width of the class. This procedure is valid when the size distribution of the population examined is continuous with size.
3 When tree size structure is negative‐exponentially distributed, as is often the case in natural rain forest populations, the Gf estimate of the recruitment rate for a given size class was least biased close to the midpoint size of this class.
4 Gf estimates agreed well with census estimates of recruitment rate from permanent plots in rain forests. A tendency for Gf estimates to be larger than census estimates disappeared when census estimates were corrected for mortality after recruitment.
5 The effects of plot size, census interval and variation in growth rate on estimates of recruitment rate were simulated using model populations. Small plot size caused substantially more among‐plot deviation for the census count of recruitment events than for the Gf estimate. The census recruitment rate also showed larger variation among plots for shorter intervals than the Gf estimate, which was independent of census interval. The Gf estimates were therefore more accurate than census counts in many situations. More than several tens of trees were needed in a size class to allow a reliable Gf estimates.     

Dating of graptolite zones by sedimentation rates: implications for rates of evolution

Preliminary determinations of ancient pelagic sedimentation rates agree with modern rates at about 4 meters per million years. By combining data on the thickness of graptolite zones from the North American Cordillera with data from other parts of the world, we have refined the Early Silurian time scale and obtained much better resolution than is possible for radiometric dates. The new Early Silurian time scale allows estimation of true rates of change in graptolite diversity. The Llandoverian diversity explosion is twice as rapid as was previously thought. The brevity of diversity lows and rapidity of speciation support modern theories of quantum evolution.     

Clearance rates of sessile rotifers: in vitro determinations

We measured laboratory clearance rates of 10 rotifer and one unidentified bryozoan species from 3 different lakes using ³²P labeled algae (Chlamydomonas) or yeast (Rhodotorula). Clearance rates for all rotifers fed yeast ranged from < 2.0 to > 260 µl · animal^–1 · h^–1 depending on species. The in vitro clearance rates of two sessile rotifers (Ptygura crystallina and P. pilula) were not significantly different from previously measured in situ rates (Wallace and Starkweather 1983). Clearance rates for 5 rotifers fed algae ranged from < 5.0 to > 90.0 µl · animal^–1 · h^–1. Ptygura beauchampi, P. crystallina, P. pilula, Floscularia conifera, and F. melicerta ingested both cell types but their clearance rates varied substantially among species and between cell types. There was a substantial time-dependent loss of 32P from formalin-fixed animals (Sinantherina socialis) awaiting processing. This loss stabilized at approximately 20 hours and was estimated to be about 40% of the initial ingested label. Clearance rates for the bryozoan fed yeast or algae were highly variable, ranging from < 1.0 to > 3 000 µl · animal^–1 · h^–1.     

Death in amphetamine users: causes and rates.

The world medical literature contains 43 reports of deaths associated with amphetamines in a 35-year period. These included seven cerebrovascular accidents, six sudden cardiac deaths, three cases of hyperpyrexia, eight poisonings of uncertain mechanism and seven cases of medical complications of intravenous injection; the remainder were of uncertain cause. In contrast, in Ontario alone, in 1972 and 1973 there were 26 deaths in amphetamine users, of which 16 were due to accident suicide or homicide. Of the remaining cases, two were cardiac, two hepatic and the rest were mixed drug overdose. Pulmonary granulomata, subacute hepatitis and other lesions resulting from intravenous drug use were common findings at autopsy. On the basis of the estimated number of regular users of intravenous amphetamine in Ontario, the mortality rate in such users is at least four times as high as in the general population of the same age, and is comparable to that in alcoholics and heroin addicts. However, the absolute number of alcohol-related deaths is far greater than the number of deaths in amphetamine or heroin users.     

Variance and covariance of ovipositional rates and developmental rates in the Phytoseidae (Acari: Mesostigmata): a phylogenetic consideration

Variance and covariance of ovipositional rates and developmental rates in the Phytoseiidae were analysed using comparative methods which consider phylogenetic effects. Nested analysis of variance showed that mean ovipositional rates and developmental rates of phytoseiid mites varied significantly between subfamilies, among genera within subfamilies, among subgenera within genera and among species within subgenera. For example, the mean ovipositional rate (eggs per day) was higher in the Amblyseiinae (2.04) than in the Phytoseiinae (1.39) and within the Amblyseiinae, it was higher in the genus Phytoseiulus (2.66) than in Amblyseius (1.80). Regressions using mean values of subgenera or higher taxa to account for phylogenetic effects showed significant correlation between mean ovipositional rates and developmental rates. The implications of this analysis for selecting species for future comparative analysis of phytoseiid life history variation are discussed.     

Age-specific metabolic rates and mortality rates in the genus Drosophila

Early theories of aging suggested that organisms with relatively high metabolic rates would live shorter lives. Despite widespread tests of this 'rate of living' theory of aging, there is little empirical evidence to support the idea. A more fine-grained approach that examined age-related changes in metabolic rate over the life span could provide valuable insight into the relationship between metabolic rate and aging. Here we compare age-related metabolic rate (measured as CO2 production per hour) and age-related mortality rate among five species in the genus Drosophila. We find no evidence that longer-lived species have lower metabolic rates. In all five species, there is no clear evidence of an age-related metabolic decline. Metabolic rates are strikingly constant throughout the life course, with the exception of females of D. hydei, in which metabolic rates show an increase over the first third of the life span and then decline. We argue that some physiological traits may have been shaped by such strong selection over evolutionary time that they are relatively resistant to the decline in the force of selection that occurs within the life time of a single individual. We suggest that comparisons of specific traits that do not show signs of aging with those traits that do decline with age could provide insight into the aging process.     

Exponential growth rates of phytoplankton calculated from 14C uptake rates: a clarification

Phytoplankton growth rates can be estimated from measurementsof light-dependent [¹⁴C]bicarbonate uptake (photosynthesis)rates using the integrated (logarithmic) form of the exponentialgrowth law. A clanfication of this method is presented herein view of confusions in the recent literature.     

Synonymous substitution rates in Drosophila: Mitochondrial versus nuclear genes

Synonymous substitution rates in mitochondrial and nuclear genes of Drosophila were compared. To make accurate comparisons, we considered the following: (1) relative synonymous rates, which do not require divergence time estimates, should be used; (2) methods estimating divergence should take into account base composition; (3) only very closely related species should be used to avoid effects of saturation; (4) the heterogeneity of rates should be examined. We modified the methods estimating synonymous substitution numbers to account for base composition bias. By using these methods, we found that mitochondrial genes have 1.7–3.4 times higher synonymous substitution rates than the fastest nuclear genes or 4.5–9.0 times higher rates than the average nuclear genes. The average rate of synonymous transversions was 2.7 (estimated from the melanogaster species subgroup) or 2.9 (estimated from the obscura group) times higher in mitochondrial genes than in nuclear genes. Synonymous transversions in mitochondrial genes occurred at an approximately equivalent rate to those in the fastest nuclear genes. This last result is not consistent with the hypothesis that the difference in turnover rates between mitochondrial and nuclear genomes is the major factor determining higher synonymous substitution rates in mtDNA. We conclude that the difference in synonymous substitution rates is due to a combination of two factors: a higher transitional mutation rate in mtDNA and constraints on nuclear genes due to selection for codon usage. Received: 27 November 1996 / Accepted: 8 May 1997     

Tooth replacement rates in early chondrichthyans: a qualitative approach

The continuous replacement of teeth throughout their lifetime is a common characteristic of most chondrichthyans. This process was already present in the earliest representatives of the group. It has been well established that different species of extant sharks show rapid tooth replacement rates; however, some authors have suggested that in early chondrichthyans this rate might have been much slower. Here we present a qualitative approach to analyse tooth replacement rates in the Early Devonian shark Leonodus carlsi , the earliest tooth-bearing shark known to date. For this, we have examined 1,103 isolated teeth from Celtiberia, Spain. Our study provides strong evidences of an extremely slow dental replacement in this primitive chondrichthyan based on three independents analyses: (1) statistical analysis of the wear degree, demonstrating that teeth remain functional for a long period of time; (2) analysis of both the histological and the morphological features of the teeth cusps suggests that this chondrichthyan used a maturation process that optimizes its function, thus worn teeth show an efficient working shape that implies their teeth remained functional for a long time after being modelled by use; and (3) estimations of size increments between teeth (Δs) of the same dental family for some recent sharks whose rates of replacement were known prove that Δs is inversely proportional to the rate of replacement ( R ² = 0.8327). The estimated values of tooth replacement rates obtained from Δs for L. carlsi and for some Late Devonian cladoselachian sharks are significatively slower than those observed in current sharks.     

Codon equilibrium II: Its use in estimating silent-substitution rates

Summary We study the equilibrium in the use of synonymous codons by eukaryotic organisms and find five equations involving substitution rates that we believe embody the important implications of equilibrium for the process of silent substitution. We then combine these five equations with additional criteria to determine sets of substitution rates applicable to eukaryotic organisms. One method employs the equilibrium equations and a principle of maximum entropy to find the most uniform set of rates consistent with equilibrium. In a second method we combine the equilibrium equations with data on the man-mouse divergence to determine that set of rates that is most neutral yet consistent with both types of data (i.e., equilibrium and divergence data). Simulations show this second method to be quite reliable in spite of significant saturation in the substitution process. We find that when divergence data are included in the calculation of rates, even though these rates are chosen to be as neutral as possible, the strength of selection inferred from the nonuniformity of the rates is approximately doubled. Both sets of rates are applied to estimate the human-mouse divergence time based on several independent subsets of the divergence data consisting of the quartet, C- or T-ending duet, and A- or G-ending duet codon sets. Both rate sets produce patterns of divergence times that are shortest for the quartet data, intermediate for the CT-ending duets, and longest for the AG-ending duets. This indicates that rates of transitions in the duet-codon sets are significantly higher than those in the quartet-codon sets; this effect is especially marked for AG, the rate of which in duets must be about double that in quartets.     

Age-specific mortality rates in reef fishes: Evidence and implications

Abstract Mortality is a fundamental demographic rate, the nature of which has profound consequences for both the dynamics of populations and the life-history evolution of species. For example, if per capita mortality rates are age- or stage-specific, life-history traits should evolve in response to age- and stage-specific differences in selection arising from these temporally variable rates. Similarly, variation in the average mortality rate across ages and/or stages can also select for shifts in life history. Mortality rates of recently settled reef fishes can be very high and per capita mortality is commonly assumed to decrease with increasing age. A review of evidence for age-specific per capita mortality rates in reef fishes from early postsettlement up to 13 months postsettlement suggests that during this period these rates are often age invariant. The data on which these interpretations are based, however, are extremely limited both in terms of the proportion of the life cycle over which mortality rates have been sampled and the quality of these data. Nonetheless, these data do suggest that selective pressures associated with patterns of mortality may vary among species of reef fishes and that these species therefore could be more effectively used in the study of life-history evolution. At present, reef fishes are under-represented in the study of life-history evolution compared with other vertebrate taxa.     

27-hydroxycholesterol: production rates in normal human subjects.

We attempted to quantitate production of bile acid via the 27-hydroxylation pathway in six human subjects. After bolus intravenous injection of known amounts of [24-14C]cholic acid and [24-14C]chenodeoxycholic acid, each subject underwent a constant intravenous infusion of a mixture of [22, 23-3H]-27-hydroxycholesterol and [2H]-27-hydroxycholesterol for 6;-10 h. Production rate of 27-hydroxycholesterol was calculated from the infusion rate of [2H]-27-hydroxycholesterol and the serum ratio of deuterated/protium 27-hydroxycholesterol, which reached a plateau level by 4 h of infusion. Conversion of 27-hydroxycholesterol to cholic and chenodeoxycholic acids was determined from the 3H/14C ratio of these two bile acids in bile samples obtained the day after infusion. In five of the six subjects, independent measurement of bile acid synthesis by fecal acidic sterol output was available from previous studies. Endogenous production of 27-hydroxycholesterol averaged 17.6 mg/day and ranged from 5.0 to 28.2 mg/day, which amounted to 8.7% (range 3.0;-17.9%) of total bile acid synthesis. On average 66% of infused 27-hydroxycholesterol was converted to bile acid, of which 72.6% was chenodeoxycholic acid.These data suggest that relatively little bile acid synthesis takes place via the 27-hydroxylation pathway in healthy humans. Nevertheless, even this amount, occurring predominantly in vascular endothelium and macrophages, could represent an important means for removal of cholesterol deposited in endothelium.     

Codon equilibrium II: Its use in estimating silent-substitution rates

We study the equilibrium in the use of synonymous codons by eukaryotic organisms and find five equations involving substitution rates that we believe embody the important implications of equilibrium for the process of silent substitution. We then combine these five equations with additional criteria to determine sets of substitution rates applicable to eukaryotic organisms. One method employs the equilibrium equations and a principle of maximum entropy to find the most uniform set of rates consistent with equilibrium. In a second method we combine the equilibrium equations with data on the man-mouse divergence to determine that set of rates that is most neutral yet consistent with both types of data (i.e., equilibrium and divergence data). Simulations show this second method to be quite reliable in spite of significant saturation in the substitution process. We find that when divergence data are included in the calculation of rates, even though these rates are chosen to be as neutral as possible, the strength of selection inferred from the nonuniformity of the rates is approximately doubled. Both sets of rates are applied to estimate the human-mouse divergence time based on several independent subsets of the divergence data consisting of the quartet, C- or T-ending duet, and A- or G-ending duet codon sets. Both rate sets produce patterns of divergence times that are shortest for the quartet data, intermediate for the CT-ending duets, and longest for the AG-ending duets. This indicates that rates of transitions in the duet-codon sets are significantly higher than those in the quartet-codon sets; this effect is especially marked for A----G, the rate of which in duets must be about double that in quartets.