No abstract

A promising way of predicting how species' populations may respond to climate warming in the Holarctic is to examine how the same organisms function at lower altitudes or further south in their ranges, where current climates are warmer. Here, we compare how the quantity and distribution of the habitats of four species of ectothermic animal differ in space and time across a heathland landscape under the climates that prevail near their northern edges of range and 300-400 km south, where mean spring and summer temperatures are 2-3°C higher. Both these habitat parameters are key determinants of the size and persistence of animal populations in a region (Dempster & McLean 1998). The quantity (and quality) of habitat determines the carrying capacity of a site for a species or the levels at which populations of interacting species equilibrate, while the juxtaposition and continuity of habitat patches of different sizes influence the extent to which species can track the occurrence of new patches of habitat, and can persist as metapopulations in a landscape. Thomas (1991Thomas ( , 1993Thomas ( , 1995 has suggested that small differences in mean temperatures may have large effects on the availability of habitat for ectotherms in temperate regions because coldblooded animals generally need to raise their body temperatures above a threshold before activity or development becomes possible (Logan et al. 1976) and are often unable to influence this by thermoregulation during certain life-stages, or can do so to only a limited extent within the range of microclimates avail- 2.Habitats both at the northern edge of their ranges and 300-400 km further south for the four species were defined qualitatively from existing sources and then expressed quantitatively in terms of the attributes recorded in the Dorset Heathland Survey. The Survey was then used as a GIS to map the occurrence of the habitats of the four species under two climates and a decade apart. 3. The model predicts that an increase of 2-3°C can result in a large increase in the area of habitat available to these north temperate species, that the length of time that individual patches of successional habitat may be occupied increases and that the distance between habitat patches within the biotope decreases. 4. The warmer conditions should result in a more stable metapopulation structure for P. argus, with fewer metapopulations existing in the landscape but each, on average, containing a greater number of larger and more stable constituent populations. 5. These predictions are of significance to ectothermic species which currently live at the northern limits of their ranges in the British Isles. The reverse effect is likely for species at the southern limits of their ranges. Conservationists who wish to maintain the status quo may be able to reduce some effects of these changes by appropriate habitat management.

No abstract

Frequency judgments of words repeated 2, 3, and 5 times in a list with 0, 1, 2, 4, 8, or 16 items intervening between repetitions were examined under different instructional sets. In Experiment 1, separate groups of subjects were given one of three types of instructions prior to list presentation: that a frequency judgment test would follow (intentional instructions), that a memory test would follow (nonspecific instructions), or that they were simply required to rate each word on its connotative strength with no mention being made of the ensuing test (incidental instructions). Both judged frequency and the effect of spacing were highest for the incidental condition and lowest for the nonspecific condition. In Experiment 2, subjects received incidental orienting instructions designed to produce a graphemic, acoustic, or semantic level of processing. Both the frequency and spacing effects were enhanced by the acoustic and semantic instructions. The results were related to a levels-of-processing view of memory, and implications for current theoretical views of frequency estimation and the spacing effect were discussed.

It is a common assumption that species' ranges are limited by their physiological tolerances to climatic factors, Biotic factors, such as competition, are rarely considered. We investigated the distributions of Ulex minor and U. gallii at three spatial scales from geographic ranges to individual heaths -to examine whether the species are negatively associated, as predicted by the hypothesis that ihe ranges of the species are limited by competition with each other. Distribution maps for the British Isles and France (100-400 km^ survey units) show the two species have largely separated, but slightly overlapping ranges. A region of range overlap on the heaths of Dorset, southern England was mapped using 4 ha survey squares. There was strong negative association between the species, and the heaths could be divided into zones where one species was dominant. There was some indication of edaphic differences between the V. m/Ro/--dominated zones and the V. gallii zones. The few heaths where the species co-occurred were surveyed using 4 m^ quadrats placed along transects. Usually one species was widespread over the heath, while the other occurred in patches. The species were strongly negatively associated in all transects. Therefore, the two species showed strong negative associations at three mapping scales. Apparent co-occurrences detected at one spatial scale largely disappeared when species were mapped at finer scales, emphasising the fractal nature of distributions. This provides evidence that the distributions of the two species are not independent and that they cannot coexist, and therefore that their ranges are limited by competition. Over their ranges, competitive superiority is probably determined by the climate. At the range boundaries in the region of overlap, climate is not important, but other physical factors such as edaphic conditions may determine the outcome of competition.J. M. Bullock (jmbul@ceh.ae.uk). R.