Prey switching in four species of carnivorous stoneflies

1. Previous studies compared the functional responses to their prey, and both intraspecific and interspecific interference, in mature larvae of Dinocras cephalotes, Perla bipunctata, Isoperla grammatica and Perlodes microcephalus. The present study examines switching by larvae of the same species presented with different proportions of two contrasting prey types; larvae of Baetis rhodani and Chironomus sp. In each experiment, 200 prey were arranged in nine different combinations of the two prey types (20 : 180, 40 : 160, 60 : 140, 80 : 120, 100 : 100, 120 : 80, 140 : 60, 160 : 40, 180 : 20). Prey were replaced as they were eaten. A model predicted the functional response in the absence of switching and provided a null hypothesis against which any tendency for switching could be tested. 2. No evidence for prey switching by Dinocras and Perla was obtained, both species showing a slight preference for Baetis over Chironomus. Prey switching occurred in Isoperla and Perlodes. As the relative abundance of one prey type increased in relation to the alternative, the proportion eaten of the former prey changed from less to more than expected from its availability, the relationship being described by an S‐shaped curve. Isoperla and Perlodes switched to a preference for Baetis when its percentage of the total available prey exceeded 57 and 42%, respectively. Equivalent values for Chironomus were 43 and 58% for Isoperla and Perlodes, respectively. Switching was strongest in Perlodes. 3. Non‐switching in Dinocras and Perla was related to their feeding strategy, both species being more successful when using a non‐selective ambush strategy at dusk and dawn rather than a search strategy during the night. Both Isoperla and Perlodes used a search strategy. The smaller Isoperla fed chiefly at dusk and dawn, and preferred Chironomus larvae, whereas most of the larger Perlodes fed continuously from dusk to dawn and preferred Baetis larvae.     

Contrasting diel activity and feeding patterns of four species of carnivorous stoneflies

1. The hypothesis under test was that larvae of Dinocras cephalotes (Curtis), Perla bipunctata Pictet, Isoperla grammatica (Poda), and Perlodes microcephalus (Pictet) differed markedly in their diel activity and feeding patterns. Mature larvae collected about 1 month prior to adult emergence were used either for gut analyses or for observations of their activity and feeding patterns in three stream tanks with natural substrata and glass bottoms, so that activity could be observed above and below the substratum. A dull red light was used for observations in the dark. Food (larvae of Ephemeroptera, Simuliidae, and Chironomidae) was provided in excess. 2. Larvae for gut analyses were taken 1 h before dusk or dawn (n = 30 larvae per species for each day or night sample). The only species with food in the gut for the day samples was P. microcephalus. All species fed at night, the mean number of prey per larva being very similar for D. cephalotes, P. bipunctata, and I. grammatica but significantly higher for P. microcephalus. Most prey were insect larvae, especially Simuliidae and Chironomidae. 3. Diel activity patterns of single larvae differed interspecifically but not intraspecifically. Larvae of D. cephalotes and P. bipunctata were rarely active during the day; their activity increased at dusk and decreased at dawn, and was highest during the night; their success at prey capture was highest at dusk and dawn, with an ambush rather than a search strategy. Isoperla grammatica was rarely active during the day, most active at dusk and dawn when prey capture was highest, using a search strategy, and less active for the rest of the night. Perlodes microcephalus was active during the day, but only below the substratum, and very active from dusk to dawn with a high prey‐capture success, using a search strategy. These experiments provided support for the hypothesis under test. The discussion concludes that the results could also help to explain known differences in growth rate and the length of the life‐cycle in these four species.     

Interspecific interference and the functional response of four species of carnivorous stoneflies

1. A previous study compared the functional responses to their prey and intraspecific interference in mature larvae of Perlodes microcephalus, Isoperla grammatica, Dinocras cephalotes and Perla bipunctata. The present study extends this work by assessing interspecific interference between pairs of these species in equal numbers (one, two or three larvae per species) to provide total predator densities of two, four or six larvae. Baetis larvae as prey were replaced as they were eaten, and their density per predator was varied between 20 and 200 larvae. 2. The number of prey eaten by each competing species increased curvilinearly with prey density, the relationship being well described by a Type II model. Of the two constants in the model, handling time varied considerably between species, mean values being shortest for Perlodes, slightly higher for Isoperla, and much higher for Dinocras and Perla. It was not affected significantly either by predator density or the identity of the competing species. 3. Attack rate also varied between species and decreased with predator density. This decrease was slight for Perlodes, and also for Dinocras and Perla in competition with Isoperla. The decrease in Dinocras and Perla was similar to that for intraspecific interference. 4. The decrease in attack rate was described by a convex curve for Perlodes with the other three species and for Dinocras/Perla with Isoperla, but by a concave curve (negative power function) for Isoperla competing with the other three species, and for both Dinocras and Perla in competition with Perlodes. Prey consumption also decreased with predator density, the severity of competition with different species reflecting that for attack rate. 5. A comparison with previous results for intraspecific interference showed that the latter was dominant for Perlodes in all contests and for Dinocras or Perla competing with Isoperla, whilst interspecific interference dominated for Isoperla in all contests and for Dinocras and Perla competing with Perlodes. Both types of interference were applicable to competition between Dinocras and Perla. Isoperla was the least, and Perlodes the most, aggressive of the four species with Dinocras and Perla intermediate.     

Emergence,maturation and upstream oviposition flights of Plecoptera from the Breitenbach,with notes on the adult phase as a possible control of stream insect populations

Upstream oviposition flights of several Plecoptera species of the genera Leuctra, Nemoura, Isoperla and Siphonoperla along the Breitenbach, Hesse, Germany are inferred from 1. oviposition occuring upstream from emergence areas; 2. size differences and size-shifts between emerging and egg-laying females; 3. concentration of oviposition in upstream sections of the emergence area. The last point relies on direct catches of egg-laying females with sticky traps, and on over-representation of old, mature females in upstream emergence traps towards the end of, or even after actual emergence periods. Criteria for age class discrimination of the respective taxa are described. The validity of the present and of other indirect evidence for directed adult flights and the significance of adult upstream displacement are discussed. Optical orientation cues are suggested. Females of Leuctra prima and Siphonoperla torrentium are shown to gain 25–50% dry weight by feeding in the terrestrial environment; general information indicates many other Plecoptera may be similar in this respect. Terrestrial adult life is shown to last for 3–8 weeks. This long time, evidence of important flight activity and feeding together suggest that the terrestrial phase is spent as an active adult, the inevitable risks of which may form an important control of population dynamics.     

Tree-hugging koalas demonstrate a novel thermoregulatory mechanism for arboreal mammals

How climate impacts organisms depends not only on their physiology, but also whether they can buffer themselves against climate variability via their behaviour. One of the way species can withstand hot temperatures is by seeking out cool microclimates, but only if their habitat provides such refugia. Here, we describe a novel thermoregulatory strategy in an arboreal mammal, the koala Phascolarctos cinereus. During hot weather, koalas enhanced conductive heat loss by seeking out and resting against tree trunks that were substantially cooler than ambient air temperature. Using a biophysical model of heat exchange, we show that this behaviour greatly reduces the amount of heat that must be lost via evaporative cooling, potentially increasing koala survival during extreme heat events. While it has long been known that internal temperatures of trees differ from ambient air temperatures, the relevance of this for arboreal and semi-arboreal mammals has not previously been explored. Our results highlight the important role of tree trunks as aboveground ‘heat sinks’, providing cool local microenvironments not only for koalas, but also for all tree-dwelling species.     

The role of weather-related habitat use on the impact of the European speckled wood Pararge aegeria on the endemic Pararge xiphia on the island of Madeira

Habitat use and microclimatic constraints on the activity of the endemic Madeiran speckled wood butterfly and European speckled wood were studied in September 1989 and April 1990. The endemic species is the most closely associated with laurel forest and the recently established European speckled wood with pine and eucalyptus forest. The relative abundances of the two species in different sites changes with season, with the endemic species being relatively more common in low level sites in April at the end of the cool season, than in September at the end of the warm summer period. It is suggested that changes of abundance in different locations are related to the thermal biology of the two species. The endemic speckled wood is active at lower air temperature than the European species, and the cool winter period may facilitate occupation of open sunny sites. The activity of the endemic speckled wood is less constrained by cool and dull conditions than is that of the European species, which requires higher temperatures for activity. It is proposed that the activity and behavioural repertoire of the endemic is most suited to climatic conditions in, and structural features of, laurel forest. The European species is most suited for activity in open woodland and agricultural habitats. Interactions between adults of the two species do not indicate direct competition. Changes in the distribution of the two species can be linked to probable changes of habitat on the island of Madeira.     

Small bees overheat in sunlit flowers: do they make cooling flights?

1. Although thermoregulation by large bees in cool climates has been well studied, less is known about the very different thermoregulatory strategies of small bees, especially those subjected to heat stress. 2. Studies were carried out on small (< 20 mg fresh weight), dark‐coloured, solitary bees (mostly halictids and hylaeine colletids) experiencing an extreme radiative heat load, enhanced by the high‐altitude location and by reflection of incident radiation by the high‐albedo petals of the flowers of Potentilla lancinata. 3. When foraging in the flowers, such bees experienced peak operative temperatures exceeding 44 °C. In these conditions, males largely stopped foraging but females continued, usually limiting their flower visits to a few seconds and making frequent short flights. These flights would cool the bees down, because bees suspended in air were cooler than bees in sunlit flowers, and convective cooling during flight would further enhance the cooling effect of departure from the flower. 4. As far as is known, cooling flights in small bees have not been proposed before, providing a new avenue for exploration of bee thermoregulatory strategies.