Mark Hassall

The temperature-size rule in ectotherms : May a general explanation exist after all?

The majority of ectotherms mature at a larger size at lower rearing temperatures. Although this temperature‐size rule is well established, a general explanation for this phenomenon has remained elusive. In this article, we address the problem by exploring the proximate and ultimate reasons for why a temperate grasshopper, Chorthippus brunneus, is an exception to the temperature‐size rule. Using a complete set of life‐history data to parameterize an established life‐history model, we show that it is optimal for this species to mature at a larger size at higher temperatures. We also show that plasticity in adult size is determined by the relative difference between the minimum temperature thresholds for growth and development rates. The mechanism relates to aspects of the biophysical model of van der Have and de Jong. Ectotherms that obey the temperature‐size rule are identified as having a higher temperature threshold for development rate than for growth rate; exceptions are identified as having a lower temperature threshold for development rate than for growth rate. The latter scenario may arise broadly in two ways. These are discussed in reference to the thermal biology of temperate grasshoppers and ectotherms in general.

Effects of terrestrial isopods on the decomposition of woodland leaf litter

SummaryThe 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-1 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-1 D.W. more than intact decaying leaves. The isopods direct contribution to soil metabolism was calculated to be 151 mg and 138 mg g-1 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.

Ecotypic differentiation in the grasshopper Chorthippus brunneus : life history varies in relation to climate

Abstract 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.

Selection for protection in an ant-plant mutualism: Host sanctions, host modularity, and the principal-agent game

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.

Does microclimate affect grasshopper populations after cutting of hay in improved grassland

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.

The role of coprophagy in the feeding strategies of terrestrial isopods

SummaryThe hypothesis that faeces recycling in isopods evolved as an adaption to facilitate maintenance of an adequate copper balance in terrestrial environments is examined. Experimental observations on the consumption, absorption and growth rates of Porcellio scaber fed Betula pendula leaf litter varying in copper content and extent of microbial decay are reported. Preventing the isopods from reingesting their faeces caused a reduction in the growth rates of experimental animals fed their natural low copper diet but also of those fed copper enriched diets. When the availability of copper in the primary food was increased consumption of the litter decreased and growth rates were significantly reduced. These results suggest that copper is not normally a critically limiting nutrient for terrestrial isopods.When the primary diet was supplemented with shredded carrot, faeces deprivation did not cause a decrease in growth rates. These experimental animals gained weight significantly faster than controls fed decaying leaf litter alone.Faeces formed a significantly greater proportion of the diet when the animals were fed freshly fallen rather than decayed litter.We conclude a) that enhanced microbial activity in the faeces increases their nutrient status in such a way that some coprophagy is necessary in order to optimize overall nutrient uptake, and b) that theability to vary the extent to which faeces are recycled in response to differences in food quality is important in that it introduces greater flexibility into the feeding strategies of these generalist macro-decomposers.

The effects of food quality on the life history parameters of the terrestrial isopod (Armadillidium vulgare (Latreille))

SummaryGrowth, reproduction, and survivorship of Armadillidium vulgare have been observed when laboratory populations were fed experimental diets consisting of leaves from different species of plant in different states of decay.Growth and fecundity were highest when the foods consisted of standing dead dicotyledonous plant leaves. Feeding on monocotyledonous material caused increased mortality as well as reducing both growth rates and reproductive output to very low levels. When the dicotyledonous material was extensively decayed before feeding, growth rates and survivorship decreased but this treatment did not significantly alter the results for monocotyledonous foods.Growth and survivorship rates were not significantly correlated with the total nitrogen content of the food but appeared to be related to the extent to which it could be digested and absorbed across the gut wall.Some of the implications of these results for the dynamics of field populations of terrestrial isopods are discussed.

Behavioural responses of the aphid parasitoid diaeretiella rapae to volatiles from Arabidopsis thaliana induced by Myzus persicae

In response to herbivory by insects, several plant species have been shown to produce volatiles that attract the natural enemies of those herbivores. Using a Y‐tube olfactometer, we investigated responses of the aphid parasitoid Diaeretiella rapae MacIntosh (Hymenoptera: Aphidiidae) to volatiles from Arabidopsis thaliana Columbia (Brassicaceae) plants that were either undamaged, infested by the peach‐potato aphid, Myzus persicae Sulzer (Homoptera: Aphididae), or mechanically damaged, as well as to volatiles from just the aphid or its honeydew. In dual‐choice experiments, female D. rapae given oviposition experience on A. thaliana infested with M. persicae were significantly attracted to volatiles from A. thaliana infested with M. persicae over volatiles from undamaged A. thaliana and similarly were significantly attracted to plants that had been previously infested by M. persicae, but from which the aphids were removed, over undamaged plants. Diaeretiella rapae did not respond to volatiles from M. persicae alone, their honeydew, or plants mechanically damaged with either a pin or scissors. We conclude that an interaction between the plant and the aphid induces A. thaliana to produce volatiles, which D. rapae can learn and respond to. Poor responses of D. rapae to volatiles from an A. thaliana plant that had two leaves infested with M. persicae, with the two infested leaves being removed before testing, suggested the possibility that, at this stage of infestation, the majority of volatile production induced by M. persicae may be localized to the infested tissues of the plant. We conclude that this tritrophic interaction is a suitable model system for future investigations of the biochemical pathways involved in the production of aphid‐induced volatiles attractive to natural enemies.

Food and feeding rates of the terrestrial isopod Armadillidium vulgare (Latreille)

SummaryFood preferences and feeding rates of Armadillidium vulgare were measured with leaf litter from nine species of plant from a grassland heath. Litter from dicotyledonous plants was significantly preferred to that from monocotyledonous and bryophyte species but preferences for monocotyledonous litter increased as this decayed.For a given state of decay consumption and absorption rates were highest for dicotyledonous foods. Decayed monocotyledonous foods were consumed and absorbed at a greater rate than standing dead foods. The decayed dicotyledonous material was also consumed more rapidly than the standing dead food but the absorption rate was not significantly different because the decayed food was absorbed less efficiently.It is suggested that A. vulgare maintains high absorption rates on some low quality foods by increasing consumption rate but that its ability to do this depends upon which defences were used by the plants when alive.

Effects of headland management on invertebrate communities in cereal fields

Abstract Two of the management prescriptions for the Breckland Environmentally Sensitive Area in East Anglia, Great Britain, concern the outer 6 m of cropped fields: ‘Uncropped Wildlife Strips’ (no crop sown and restricted pesticide applications) and ‘Conservation Headlands’ (restricted pesticide applications). The impact of these two management regimes on communities of spiders, carabid beetles and Heteroptera in cereal field headlands was evaluated in 1988. The total abundance of each group was highest in uncropped strips, but also higher in Conservation Headlands than in fully sprayed headlands. Species diversity for spiders and Heteroptera was significantly higher in uncropped strips and Conservation Headlands than in fully sprayed headlands. Species richness for both these groups and carabids was higher in all fields in uncropped headlands and Conservation Headlands than in the fully sprayed controls. The community structure of carabids and spiders was altered by headland management; it also changed with the age of the uncropped strips. Movement of Heteroptera from field boundaries into the crop was affected by headland treatment. The implications of these results are discussed with reference to gamebird survival, biological control of cereal pests and wildlife conservation issues.