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1. The effect of weather on the size of British butterfly populations was studied using national weather records and the Butterfly Monitoring Scheme (BMS), a national database that has measured butterfly abundance since 1976. 2. Strong associations between weather and population fluctuations and trends were found in 28 of 31 species studied . The main positive associations were with warm summer (especially June) temperature during the current and previous year, low rainfall in the current year and high rainfall in the previous year . Most bivoltine species benefited from warm June weather in the current year, three spring species and two that overwinter as adults benefited from warm weather in the previous summer, and most species with moist or semi-shaded habitats increased following high rainfall and cooler weather in the previous year. 3. Simple models incorporating weather variables and density effects were constructed for each species using the first 15 years' population data (1976 -90) . These fitted the observed data for that period well (median R 2 = 70%). Models were less good at predicting changes in abundance over the next 7 years (1991-97), although significant predictive success was obtained. 4. Parameter values of models were then adjusted to incorporate the full 22-year data-run . For the eight species whose models had best predicted population changes or fitted the data well ( R 2 > 85%), models were run from 1767 to 1997, using historical weather records, to 'predict' trends in abundance over the past two centuries . For three species it was possible to compare predicted past trends with contemporary accounts of abundance since 1800 . In each case, the match between predictions and these qualitative assessments was good. 5. Models were also used to predict future changes in abundance, using three published scenarios for climate change . Most, but not all, species are predicted to increase in the UK under warmer climates, a few species stayed stable, and only one species -the agricultural pest Pieris brassicae (Cabbage White) -is predicted to decline.

We report a distribution-free approach to the detection of density-dependence in the variation of population abundance, measured by a series of annual censuses. The method uses the correlation coefficient between the observed population changes and population size and proposes a randomization procedure to define a rejection region for the hypothesis of density-independence. It is shown that the use of the proposed statistic under the randomization approach is equivalent to the likelihood ratio test for a particular family of time series models. The randomization test is compared with two other recently proposed tests. Using computer-generated density-independent and density-dependent data, it is shown that, unlike the other tests, the randomization test is effective whether or not there is a marked trend in the observed data. Arguments are presented showing how one of the other two tests can be further improved. Caution is urged in the use and interpretation of any test for detecting density-dependence in census data because (a) the tests depend on assumptions about population processes, (b) errors of measurement may lead to spurious detection of density-dependence.

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In cultures of Escherichia coli 15 (thymine(-), leucine(-)) which were incubated at high hydrostatic pressures, cell division occurred only at pressures below 430 atm but in a somewhat synchronous fashion at around 250 atm. The rate of leucine-(14)C incorporation into a macromolecular fraction of the cells diminished to a zero value at about 580 atm and that of uracil-(14)C incorporation to a zero value at about 770 atm. The rate of thymine-(14)C incorporation at pressures around 330 atm was that to be expected with a culture in which DNA synthesis is somewhat synchronous. At pressures above 500 atm, thymine-(14)C was incorporated only over the initial part of the pressure incubation and further incorporation under pressure was not observed no matter how long the duration of the incubation. We present evidence along several lines that the thymine incorporation kinetics reflect an effect of pressure on a locus at the origin (or termination) of a replication of the bacterial chromosome. The recovery of cell division and of the incorporation rates upon release of pressure were found to depend on the magnitude of the pressure and the duration of the pressure incubation.

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