Mark Hassall

Climate warming has changed the spatial distributions and temporal behaviours of several taxa. A key question that remains is how behaviours that alter microclimatic conditions might be used to buffer the effects of climate change. In this study, we focus on the aggregation behaviour of four species of terrestrial isopods, in which groups of individuals clump together to prevent moisture-loss, and use a laboratory protocol to investigate the effects of changes in relative humidity (RH) and temperature on aggregation. We find that three species aggregate more at lower RH and at higher temperatures, and that species that are better adapted to reducing moisture loss aggregate less than do species with fewer such adaptations. These results suggest that behavioural variation can buffer the effects of changes in microclimate. We finish by suggesting that it is critical to determine how such behavioural shifts will affect isopod densities and competitive hierarchies.

The hypotheses that the sheltering behavior of four species of terrestrial isopods varies in relation to differences in their morphological, physiological, and behavioral adaptations to the terrestrial environment were tested using artificial refugia together with independent estimates of density to derive an index of sheltering activity. (1)Porcellio scabersheltered significantly more thanPlatyarthrus hoffmannseggi, Armadillidium vulgare, orPhiloscia muscorum, which sheltered the least. (2) There was a decline in the sheltering index (SI) for all four species after the breeding season, continuing through to the autumn and remaining low throughout the winter. (3) Changes in the sheltering behavior of each species in relation to changes in environmental conditions were used to interpret known differences in the position and breadth of their resource utilization curves along a gradient of rabbit grazing intensity. (4)Porcellio scabersheltered more where the soil was more calcareous,P. muscorummore under the shade of trees, and bothP. muscorumandA. vulgaremore in grazed than in ungrazed swards. (5) Sheltering behavior was found to be positively correlated to both rainfall and soil temperature the day before sampling forA. vulgarebut negatively to rainfall forP. muscorum.There was a positive relationship between the SI forP. scaberand daily air temperature range. (6) Variations in the sheltering behavior of these four species of terrestrial isopod are discussed in the context of their foraging and digestive strategies and in relation to their morphological, physiological, and behavioral adaptations to the terrestrial environment.

Analysis of size of offspring reared through three laboratory generations from populations of the field grasshopper Chorthippus brunneus from 27 sites around the British Isles showed that offspring were larger towards the cooler‐wetter conditions in the western and northern limits of the range. This variation had a significant genetic component. There was a trade‐off between clutch size and offspring size between and within populations. Under favourable thermal and feeding conditions maternal fitness was optimal when individuals produced the largest clutches of the smallest eggs, but under poor conditions maternal fitness was optimal when individuals produced small clutches of very large offspring. Calculation of geometric mean fitness over time indicated that having larger offspring near to the edge of the range could be advantageous as a conservative risk‐spreading strategy. As well as geographic variation in egg size, significant environment–genotype interactions in egg size in relation to temperature were observed.

Armadillidium vulgare differed in growth and survivorship on two field sites. Growth rates were higher at a site with consistently higher quality food than at the other site where less high-quality food was produced and which was less predictably accessible. Survivorship was higher at the second site where temperature fluctuations were consistently smaller. Individuals from the two populations were kept for 6 months under the same food and temperature conditions and patterns of resource allocation to reproductive traits analysed. Members of the population from the site with good growth conditions had significantly higher reproductive allocation, by 13.5%, and larger broods, by 9.1%, than those from the site with poor growth conditions. Contrary to theoretical predictions, they also had significantly larger offspring, by 7.5% dry mass. Larger offspring survived better than small ones. This differential survivorship, by 20% for a 3.4% difference in live mass, was much more pronounced under conditions of moisture stress and under fluctuating temperature regimes. Larger offspring would therefore be at a selective advantage on the site with more severe temperature fluctuations. Phenotypic plasticity in reproductive traits in response to experimental changes in food quality, temperature and crowding were monitored. Reproductive allocation was increased by 20.8% under conditions of higher food quality, by 14.7% at higher temperatures, and by 12.5% under less crowded conditions. Brood size, but not offspring dry mass, increased when food quality increased. When crowding increased by 25.0%, the size of broods remained the same but the dry mass of individual offspring decreased by 11.2%. Members of the population from the site with more variable access to high-quality food showed more plasticity in reproductive traits in response to changes in food supply than members of the population from the site with the more predictable food supply. Members of the population from the site with more stable temperatures showed less plasticity to temperature changes than members of the population from the site with greater temperature fluctuations. It is concluded that the observed microevolutionary processes and phenotypic plasticity have adaptative value as responses to spatial and temporal heterogeneity in environmental conditions.

The hypothesis that the balance of trade-offs between survivorship, growth and reproductive allocation in the terrestrial isopod Armadillidium vulgare will change when resource input is increased has been investigated experimentally. When the quality of food available was increased, by adding a mixture of litter from herbaceous dicotyledonous plants to a background low-quality food of dead grasses, survivorship was found to be the most phenotypically plastic trait, increasing by 168%. Growth rates increased by 99% but reproductive allocation by only 21%. In the field, members of a population from a site with more high-quality food grew more than twice as fast as those from a site where less high-quality food was available. The population from the site with higher food availability, contrary to predictions from the laboratory study, did not survive as well as that from the site with less available high-quality food. This may be because the site that is more favourable for growth has a more stressful physical environment due to much bigger temperature fluctuations, which are known to be an important cause of mortality in this species. When individuals from both populations were reared under controlled laboratory conditions, both the parental and F1 generations from the poor growth environment survived better than those from the good growth habitat. However, even when given an excess of high-quality food those from the poor growth environment continued to grow more slowly and had a lower reproductive allocation than those from the site with higher food availability. We conclude that microevolutionary changes may have occurred in the balance of resource allocation between survivorship, growth and reproductive allocation, to favour higher survivorship during the longer prereproductive period at the site where growth to the threshold size for reproduction takes longer.

Feeding behaviour and preferences of brent geese, Branta b. bernicla were observed on pastures of different sward lengths and nitrogen contents. On swards of 2.0–6.0 cm captive geese took larger bites and had a higher intake rate when feeding on 6-cm swards than when feeding on shorter ones. In the field wild geese chose 6-cm swards in preference to both shorter and longer ones. For unfertilized pastures there was a strong negative relationship between nitrogen content and sward height: 11-cm swards contained 2.8% nitrogen, 3.5-cm swards 4.2% nitrogen. Application of 75 kg N ha^–1 organically based fertilizer at the end of October eliminated this relationship between nitrogen content and sward height, swards of all heights then having a mean content of 4.1% N. On fertilized plots the geese preferred swards longer than 6 cm with no indication of a decrease in preference up to the maximum height investigated, 16 cm. Breaking strain of grass laminae was measured to give an indication of the proportion of strengthening tissues in the leaves and hence their digestibility. Apical laminae from longer swards had a higher breaking strain than those from shorter swards. Unfertilized swards had a higher breaking strain than fertilized swards but the difference in breaking strain between long and short swards was the same on fertilized and control treatments. These results are discussed in relation to the forage maturation hypothesis and are interpreted as indicating that the primary constraint on maximizing energy intake rates is not the reduced energy digestibility of older foliage but the reduced nitrogen content in the higher-biomass swards. We conclude that it is the balance between maximizing energy intake and nitrogen absorption rates which results in the observed preference for intermediate-height swards.