The indirect contribution terrestrial isopods make to decomposition processes by stimulating microbial activites has been quantified in laboratory experiments. The extent to which microbial metabolism is enhanced as a result of the passage of Betula pendula leaf litter through the alimentary system of isopods was measured for both freshly fallen and decayed leaves. Faeces derived from 1 g freshly fallen litter lost 75 mg g D.W. more than did intact leaves, as a result of enhanced microbial metabolism. Faeces derived from 1 g of previously decayed leaves, which were shown to be the preferred food of isopods, lost only 17.5 mg g D.W. more than intact decaying leaves. The isopod's direct contribution to soil metabolism was calculated to be 151 mg and 138 mg g litter ingested when fed on freshly fallen and decayed leaves respectively. It is concluded that the physical and chemical changes in the leaf substrate which result from fragmentation and digestion by isopods do not necessarily accelerate the subsequent decomposition of the litter very significantly. Fungal propagule density was 3.2x and 3.6x higher in faeces derived from freshly fallen and decayed leaves respectively than in the intact litter. Numbers of viable bacteria were correspondingly 126x and 34x higher in faeces than in the freshly fallen and the decayed leaves. Levels of microbial inhibitors were lower in the faeces than in the leaves but levels of free amino acids stayed higher for longer in the faeces than they did in intact litter. In the field the physical removal of litter by the soil macrofauna from surface to deeper and moister microsites may be the most important indirect contribution that they make to decomposition processes.

IntroductionIn ectotherms, most physiological processes are strongly influenced by body temperature (Huey 1982;Heinrich 1993). Sensitivity to temperature has the potential to influence the behaviour, ecology and, ultimately, fitness of an individual. The extent to which ectotherms can tolerate changes in their ambient thermal environment is thus critical in determining their distribution and abundance.There are two broad strategies by which an ectotherm can adapt to its thermal environment:1. Be a thermal generalist (eurythermal) and evolve a physiology that is relatively insensitive to temperature change and which confers a broad range of temperature tolerance. 2. Be a temperature specialist (stenothermal), with a physiology adapted to a relatively narrow range of temperatures. There are then two alternative strategies possible, either (a) a low body temperature optimum close to ambient temperatures, or Ectotherms may be thermal generalists, or high-or low-temperature thermal specialists. The thermal strategy of four species of grasshoppers occurring in Britain is determined, where unpredictable variation in the generally cool climate should preclude the low-temperature thermal specialist strategy. It is predicted that temperature sensitivity will determine geographical distribution, with generalist species widespread, and thermally specialized species restricted to warmer habitats. 2.The developmental and reproductive responses to different rearing temperatures of the grasshoppers are examined in a laboratory experiment. Life-history traits are integrated into a fitness model to determine the sensitivity of each species to temperature change.3. Growth and development rates increased with temperature for each species. The frequency with which an additional instar was inserted during nymphal development increased with temperature in Chorthippus brunneus. Adult mass and size increased with temperature. 4. Egg pod production rate increased with temperature. In Omocestus viridulus, Myrmeleotettix maculatus and Stenobothrus lineatus, temperature had no effect on egg mass, eggs per pod or number of pods per female. Number of pods per female increased with temperature in C. brunneus. 5. Fitness of S. lineatus decreased by 88% for a 5°C fall in temperature compared with 58% and 56% for C. brunneus and M. maculatus, respectively. Omocestus viridulus is least sensitive to temperature change with only a 27% reduction in fitness at the lower rearing temperature. 6. It is concluded that all the species are high-temperature thermal specialists, and variation in their sensitivity to temperature is a good predictor of their distribution. The most generalist species, O. viridulus, is the most widespread, while the more specialist species S. lineatus and M. maculatus are restricted to warmer habitats. Chorthippus brunneus is also a high-temperature specialist, but is more widespread as a consequence of developmental and reproductive plasticity and efficient behavioural thermoregulation.

Retaliation against cheaters can prevent the breakdown of cooperation. Here we ask whether the ant-plant Cordia nodosa is able to apply retaliatory sanctions against its ant symbiont Allomerus octoarticulatus, which patrols new shoots to prevent herbivory. We test the hypothesis that the modular design of C. nodosa physiologically ties the growth of housing (stem swellings known as domatia) to the successful development of the attached leaves. We experimentally simulated herbivory by cutting leaves from patrolled shoots and found that the domatia on such 'cheated' shoots suffered higher mortality and lower growth than did controls, evidence for a host sanction. On the other hand, patrolling is costly to the ant, and experiment shows that non-patrollers run a low risk of being sanctioned because most leaves (and the attached domatia) escape heavy herbivory even when patrollers are absent. This suggests that cheaters might enjoy a higher fitness than do mutualists, despite sanctions, but we find that patrolling provides a net fecundity benefit when the colony and plant exceed a minimum size, which requires sustained ant investment in patrolling. These results map directly onto the principal-agent (P-A) game from economics, which we suggest can be used as a framework for studying stability in mutualisms, where high sampling costs and cheating do not allow market effects to select for mutual benefits.

Life history variations among 27 populations of the grasshopper Chorthippus brunneus from around the British Isles were examined under laboratory conditions over three generations. Multiple-regression analysis was used to examine the relationship between grasshopper life histories and the climates of their ancestral sites. Grasshoppers from cooler sites were heavier at hatching. Grasshoppers from northern sites grew faster and developed through fewer instars, attaining adulthood earlier, at the expense of adult size. Depending on the measure of adult size used, adults were larger in warmer, sunnier or more southerly locations. Ecotypic differentiation is probably widespread among animals as it is among plants, though it is more rarely demonstrated by zoological studies, especially over the wide geographical scale covered here. Evidence from regression analysis supports the hypothesis that ecotypic differentiation in C. brunneus is an evolutionary response to climatic variation. The existence of intraspecific genetic diversity for climatic adaptations has implications for biodiversity conservation and the understanding of biotic responses to climatic change. It deserves wider recognition.

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The microclimate of an improved hay meadow was studied using Tinytag dataloggers to record sward temperature after cutting. Temperatures in the sward were then compared to grasshopper abundances to see if mowing created an excessively hot microclimate unfavourable for sustained grasshopper activity in mid summer. The abundance of Chorthippus albomarginatus and Chorthippus parallelus was significantly reduced on the hay plots compared to the unmanaged control swards, which may have been due to high sward temperatures created by the absence of tall, shady vegetation in which grasshoppers may take refuge to avoid overheating. This study suggests that a combination of mortality caused by the physical process of mowing, and high sward temperatures created by removal of the standing crop by cutting may cause the low abundance of grasshoppers in improved grassland in eastern England. This research is particularly important when considering the orthopteran assemblages of Environmental Stewardship Scheme field margins where mowing for hay in July and August may seriously reduce grasshopper populations. If mowing of grassland has to occur during the grasshopper season, we suggest a later cut in September or a system of rotational mowing, leaving areas of uncut grassland as shelter

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