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1. The use of live trapping in Britain to monitor small mammal distribution and population fluctuations is reviewed. Four small mammal monitoring programmes from Britain, the results of which are either partially or wholly unpublished, were examined to determine the nature and extent of information that can be gathered from long‐term and/or large‐scale live‐trapping programmes, the degree to which such information can be extrapolated across habitats, and the likely precision with which long‐term changes in numbers can be quantified. The most extensive of the surveys examined were The Mammal Society National Woodland Small Rodent Survey and the ADAS Hedgerow Survey. These monitored wood mouse Apodemus sylvaticus, bank vole Clethrionomys glareolus and, in the ADAS survey only, common shrew Sorex araneus population fluctuations on a broad spatial scale. 2. The studies provided evidence of population synchrony and demonstrated quantitatively the effects of mast crops and local food supplies on rodent dynamics. Evidence of population synchrony between sites for wood mice and shrews suggests that it may be possible to extrapolate population changes from site‐specific to national scenarios. However, this may be problematic for some species and across habitats, although comparison of the data from the woodland and hedgerow surveys suggests that there may be a degree of long‐term synchrony in bank vole populations between the two habitats. 3. Analysis of the capture data from the two surveys indicated that annual changes of 3–11% from the population mean would be detectable with 85% power if monitoring is carried out for 10 years on 50 sites; increasing site replication further is unlikely to result in a significant increase in precision. 4. The nature of the information provided by live trapping with grids, as carried out in The Mammal Society and the ADAS surveys, is compared with that obtained from line trapping and from a range of indirect techniques. It is suggested that future monitoring will need to take an integrated approach in which information from direct and indirect monitoring of small mammal populations is linked with broad‐scale data on changes in habitat availability.

Abstract. Methods for monitoring and survey of plant species abundance which do not account for variation in scale are often insensitive and imprecise. In monitoring, repeated observations are usually made within a fixed unit. Counts of species' presence must be made in a range of subunit sizes to accommodate the range of scales at which different species occur. A method comprising a rectangular fixed unit containing an 8 × 4 grid of square subunits has been developed and tested. Each subunit comprises a series of nested cells of increasing size, within which species are recorded cumulatively. Using this method, the concept of optimum scale is introduced. The optimum scale for a species is that for which its frequency count is closest to the midpoint. Two characteristics, sensitivity (absolute change detected) and blindness (failure to detect change in a species) were calculated for 24 plots from lowland grazing marsh which had been partially flooded between the time of two surveys. Optimum scale had greater sensitivity and lower blindness than any single scale in the majority of plots. Combining sensitivity and blindness, optimum scale was always superior. For the whole sample, optimum scale was less likely to cause Type I or Type II errors. The method is recommended for monitoring grasslands and allied plant communities in large geographical areas.

The major histocompatibility complex (MHC) plays an important role in infectious disease resistance. The presence of certain MHC alleles and functionally similar groups of MHC alleles (i.e., supertypes) has been associated with resistance to particular parasite species. Farmed and domesticated fish stocks are often depleted in their MHC alleles and supertype diversity, possibly as a consequence of artificial selection for desirable traits, inbreeding (loss of heterozygosity), genetic drift (loss of allelic diversity) and/or reduced parasite biodiversity. Here we quantify the effects of depletion of MHC class II genotype and supertype variation on resistance to the parasite Gyrodactylus turnbulli in guppies (Poecilia reticulata). Compared to the descendants of wild‐caught guppies, ornamental fish had a significantly reduced MHC variation (i.e., the numbers of MHC alleles and supertypes per individual, and per population). In addition, ornamental fish were significantly more susceptible to G. turnbulli infections, accumulating peak intensity 10 times higher than that of their wildtype counterparts. Four out of 13 supertypes were associated with a significantly reduced parasite load, and the presence of some supertypes had a dramatic effect on the intensity of infection. Remarkably, the ornamental and wildtype fish differed in the supertypes that were associated with parasite resistance. Analysis with a genetic algorithm showed that resistance‐conferring supertypes of the wildtype and ornamental fish shared two unique amino acids in the peptide‐binding region of the MHC that were not found in any other alleles. These data show that the supertype demarcation captures some, but not all, of the variation in the immune function of the alleles. This study highlights the importance of managing functional MHC diversity in livestock, and suggests there might be some immunological redundancy among MHC supertypes.