A Consumer-Resource Approach to Community Structure

Because all species are consumers and all, eventually, are consumedby other species, consumer-resource interaction is one of themost fundamental processes of ecology. Simple models that includethe direct mechanisms of consumer-resource interactions maythus be the fundamental building-block for models of communitystructure. These models are easily extended to include suchcomplexity as the effects of physical limiting factors, spatialheterogeneity in resource supply, fluctuating resource supply,and multiple trophic levels. Each such modification places constraintson the traits of species that can persist. Consumer-resourcemodels make predictions about many aspects of community structure,including species richness, species composition, species dominance,population dynamics, morphological or physiological traits ofspecies, and patterns of phenotypic variation within species.Thus, each model affords numerous opportunities to test andmodify or reject it. A review of a variety of communities suggeststhat much of the structure of each community can be explainedby a relatively simple consumer-resource model, but that differentelements of complexity may be important in different communities.     

A revision of the genus Paradoxostoma Fischer (Crustacea; Ostracoda) in British waters

Species of the genus Paradoxostoma Fischer are an important component of marine and estuarine ostracod faunas throughout the world. This revision of the genus in British waters deals with 17 species of which six are described as new. Each species is diagnosed and illustrated in detail, and comments are made on their ecology and distribution. The specialized mouthparts which characterize the genus are illustrated by means of the scanning electron microscope for the first time. A key to the genus is given.     

Parapsylla, a Gondwanan element in the psyllid fauna of southern Africa (Homoptera)

The genus Parapsylla (Homoptera: Psylloidea) is redescribed and placed in the Diaphorininae of the Family Aphalaridae. Its relationships within this subfamily are discussed. The genus Agmapsylla syn. nov. is placed into the synonymy of Parapsylla , and Pennavena syn. nov. and Eudiaphorina syn. nov. are placed into the synonymy of Diaphorina. The eight known species of Parapsylla are distributed in southern Africa and upland areas of East Africa. Host plant relationships are not clearly established but the genus appears to be associated with species of Maytenus (Celastraceae), Olinia (Oliniaceae) and possibly Olea (Oleaceae) and Syzygium (Myrtaceae). Parapsylla relicta syn. nov, and A. aureus syn. nov. are placed into the synonymy of Arytaina capensis. Parapsylla capensis comb. nov. is transferred from Arytaina, Parapsylla valens comb. nov. is transferred from Diaphorina , and the following new species are described: P. angolensis sp. nov., P. eafra sp. nov., P. huila sp. nov., P. marginipennis sp. nov., P. rufa sp. nov. and P. theroni sp. nov.     

Macrophyte-gastropod associations: observations and experiments on macrophyte choice by gastropods

SUMMARY.

• 1 The abundance and microdistribution of thirteen gastropod and ten macrophyte species were monitored over 2 years in a 1 ha shallow eutrophic pond near Oxford, England. While areas of allochthonous leaf litter supported a depauperate and nondistinctive gastropod fauna, three different macrophyte habitats supported more diverse and distinctive assemblages of gastropods.
• 2 Most gastropod species were clearly more abundant (number m^-2pond bottom) on one macrophyte type than on other substrates. One exception, the limpet Acroloxus lacustris (Linn.) was abundantion both the water lily Nymphaea alba L. and on emergent macrophyles. Planorbis vortex (Linn.) was associated with graminoid emergent macropytes, especially Glyceria maxima (Hartm.) Holmberg, while Lymnaea peregra (Mull.) was associated with submersed macrophytes, perhaps more particularly with Elodea canadensis Michx.
• 3 The association of, 4. lacuxtris with N. alba and emergent macrophytes is perhaps explained by constraints of the morphology of the limpet requiring a relatively broad smooth substrate for attachment and locomotion.
• 4 The reason for the associations of P. vortex with G. maxima, and of L. peregra with E.canadensis are less obvious. Experiments giving each snail species a choice between the two macrophytes showed that P. vortex regularly exhibited a preference for G. maxima but failed to reveal substrate selection by L. peregra. Neither snail species affected the macrophyte choice of the other. The results suggested that L. peregra did not behave naturally in experiments.
• 5 The preference for G, maxima by P. vortex was not affected by prior conditioning of the plant by the snail, but was affected by the emergent nature of G. maxima and by the presence/absence of a natural periphyton assemblage and associated detritus.
• 6 The distribution of A. lacustris is probably determined by the physical structure of the substrate, while that of P. vortex is determined by the presence and quality of epiphytic periphyton-detritus. The observations and experiments with L. peregra reported here leave the reasons for its distribution in doubt.

     

WATER-REGULATORY BEHAVIOUR IN TERRESTRIAL GASTROPODS

1. Terrestrial snails and slugs are exceedingly susceptible to dehydration due to evaporative water loss from the integument and lung surface, and the deposition of a dilute mucous trail. Active slugs can lose 30–40% of their initial body weight (IBW) within 2 h. 2. Both field and laboratory studies have revealed that habitat selection by snails and slugs is well correlated with the availability of water. In addition, numerous species display homing behaviour, returning directly to their moist secluded daytime resting sites at dawn. 3. Several aspects of locomotor activity are affected by body hydration and environmental conditions such as relative humidity. Moist conditions result in termination of aestivation in snails and a generally higher level of activity in both snails and slugs. In contrast, severe dehydration initiates aestivation in snails and an increase in the intensity and duration of circadian locomotor activity in slugs. 4. Huddling behaviour is a specialized example of the general preference of slugs for moist habitats. When groups of slugs are exposed to dry environmental conditions, they form closely packed aggregations. This response results in a decrease in the rate of dehydration of the individual slugs. 5. When slugs have been dehydrated to about 90% IBW, rhythmic closures of the pneumostome are initiated. As dehydration progresses there is a reduction in the open diameter of the pneumostome. These responses reduce the total exposure of the lung surface and thereby evaporative water loss. In slugs dehydrated to about 80 % IBW, these responses can result in a 7 % reduction in water loss. 6. When slugs have been dehydrated to about 65% IBW (67·6 ± 4·3% IBW) they move on to a moist surface, assume a characteristic flattened posture and remain quiescent while water is absorbed through the surface of the foot. Once they are rehydrated (to 93·5 ± 12·4% IBW) they move off the moist surface. Thus there exists a specific dehydration threshold for the initiation of contact-rehydration and a rehydration set-point at which the response is terminated. 7. Both initiation and termination of contact-rehydration are controlled by variations in haemolymph osmotic pressure. The behaviour can be experimentally initiated by injection of hyperosmotic mannitol solution and terminated by injection of dilute saline. 8. Contact-rehydration involves bulk flow of water through an epithelial paracellular pathway in the integument of the foot. The rate of absorption of [¹⁴C]inulin during contact-rehydration is similar to that of water. The efficacy of water movement through the pathway is modulated by body hydration, the pathway being opened only in dehydrated slugs. 9. By means of the dual-limit control of contact-rehydration slugs can behaviourally regulate their body hydration and haemolymph osmolality within the tolerable hydration range described by the upper and lower limits.     

Kin selection and frequency dependence: a game theoretic approach

Game theory models show that the evolution of interactions between relatives is determined by two kinds of fitness effects: Hamilton's inclusive fitness effect, and a frequency-dependent synergistic effect. The latter arises when an individual's behaviour has different effects on the fitness of interactants, depending on whether or not they perform the same behaviour. Knowing the sign of the synergistic effect is sufficient to understand most of the qualitative features of genetic models that show departures from Hamilton's rule. Since this synergistic effect does not depend on the interactants being related, it is best viewed as something distinct from kin selection. In this view, Hamilton's rule is basically correct for describing kin selection, and most deviations from it are due to the distinct process of synergistic selection.     

In Vitro Excystation of Cryptosporidium baileyi from Chickens1

Release of sporozoites from the oocysts of Cryptosporidium baileyi is described from Nomarski interference-contrast microscopy. Just prior to excystation, the four sporozoites became motile and rearranged themselves within the oocyst. The sporozoites were then rapidly expelled through an opening that formed in the oocyst wall, and the residuum was either released or retained within the oocyst. Excysted sporozoites were crescent shaped and measured 5.0–9.0 μm × 1.0–1.6 μm (x?= 6.8 × 1.1 μm). Excystation occurred when sodium taurocholate or a mixture of trypsin and sodium taurocholate was present in the incubation medium. High levels of excystation occurred at 37° or 40°C, but excystation did not occur at 4°C. The ability of biles from two avian and two mammalian hosts to produce excystation of C. baileyi was also studied. After a 2-h incubation at 40°C, the percentages of excystation were 69.5% in goat bile, 45.0% in pig bile, 33.0% in chicken bile, and 34.5% in turkey bile.