Oecologia

Fenchel, Tom

doi: 10.1007/BF00364318pmid: 28309294

The distribution patterns of four species of mud snails (Hydrobiidae) in a complex, estuarine environment are described. The species show habitat selection with respect to salinity but the tolerance ranges of the species overlap and it is shown that this factor alone cannot predict the distribution in the field. The found patterns show that the species are subdivided into many, more or less isolated sub-populations. It is conjectured that the distribution of the species results from the interaction of habitat selection, dispersal rates, colonizations, interspecific competition and extinctions. This interpretation explains features of the distribution patterns; e.g. , the boundaries between two species in salinity gradients occur at different salinities in different areas, the species with an intermediate position with respect to salinity preference is the least frequent one, and that coexistence between two, and sometimes three species may occur under certain conditions. The possibility that the species may sustain populations within some areas as “fugitive” species is also discussed. The interpretations are compared with the theory of island biogeography and many parallels are found. The findings are also contrasted to the concept of communities as “natural units”. Finally, in an appendix, a simple mathematical model is described. It treats the possibility of coexistence between competing species as a result of migrations between different habitats.

Fenchel, Tom

doi: 10.1007/BF00364319pmid: 28309295

Populations of coexisting and of allopatrically occurring species of hydrobiid snails ( Hydrobia ulvae, H. neglecta, H. ventrosa and Potamopyrgus jenkinsi ) have been studied in 90 localities within three different areas. When H. ventrosa coexists with H. ulvae they show character displacement, i.e. , the average body size of the former is smaller and that of the latter is larger. When these species live alone they are of approximately the same size. It is shown here that the size ratio between the coexisting species usually found (1.3–1.5) allows stable coexistence based on food praticles size selectivity alone. Variation in the degree of character displacement from locality to locality is explained by different degrees of genetical isolation of the populations. Coexisting H. ventrosa and H. ulvae have shorter, more well-defined periods of reproduction than they do when they occur alone. H. neglecta is larger than coexisting H. ventrosa and smaller than coexisting H. ulvae . Due to the patchy distribution and the fluctuating populations of this species, and due to the fact that pure H. neglecta populations are rare, data on this species are difficult to interpret. Potamopyrgus jenkinsi shows a different food particle size selection than the Hydrobia spp. of identical sizes. Competitive interactions between P. jenkinsi and coexisting Hydrobia spp. are therefore probably weak. In accordance with this, P. jenkinsi does not show character displacement when coexisting with, e.g., H. ventrosa . The fact that the major study area, the Limfjord, is only 150 years old as a marine habitat as well as various more recent man-made changes of the coast line allow estimates of the time scale of the microevolutionary changes which lead to character displacement.

Calow, P.

doi: 10.1007/BF00364320pmid: 28309296

Radiotracer and gravimetric techniques were used to investigate the effects of starvation, temperature, body size and food quality on both the ingestion rates and absorption efficiencies of A. fluviatilis (an algal grazer) and P. contortus (a detrivore which utilises the bacterial fraction of its food). In the face of food supply disturbance snails showed a considerable potential for adaptation. Both intestion rates and absorption efficiencies increased with starvation, and ingestion rate increased with reductions in food quality. Absorption efficiencies were independent of temperature and Q 10 values for absorption rates suggested that snails showed some acclimation to temperature disturbance. Food absorption was linearly related to body surface area but absorption efficiencies were to a large extent independent of age and size. The above homeostases are discussed in terms of their contribution to fitness and also in terms of the possible underlying causal mechanisms. The implications of physiological homeostases for ecological efficiencies are also discussed.

Calow, P.

doi: 10.1007/BF00364321pmid: 28309297

A radiotracer technique involving 51 Cr was used to monitor the egestion rates of A. fluviatilis and P. contortus . When snails were starved, either before or after exposure to labelled food, gut emptying rate reduced. Examination of faecal components suggested that this was due to reductions in the rate of passage of food through the hepatopancreas. Since the latter is the major site of digestion and absorption in the Gastropoda this strategy was explained as an attempt by the snails to extract more nutrient from the disturbed food supply. Other factors affecting egestion rate were, temperature, food quality and snail size. Weight for weight egestion was more rapid in P. contortus than in A. fluviatilis and this difference is typical between detrivores and herbivores. Information on defaecation strategy enabled an estimation of field absorption and ingestion rates in A. fluviatilis . There was good agreement between the latter and estimates predicted on the basis of laboratory observation. However, field determined absorption efficiencies were more constant and generally higher than those predicted on the basis of laboratory measurements.

Maschwitz, U.; Mühlenberg, M.

doi: 10.1007/BF00364322pmid: N/A

The sociobiology of Leptogenys ocellifera especially its predatory behavior and food recruitment was investigated. Leptogenys binghami and Leptogenys diminuta were observed comparatively. L. ocellifera : Colony size, nesting sites, and nest emigration. L. ocellifera lives in large colonies of several tens of thousands workers. It nests in existing soil cavities. The nesting site is changed in irregular intervals. According to our results the nest emigration is released by environmental factors. Trail system. There are several elaborated permanent routes leading from the nest into the surroundings. Those routes can be used continuously day and night over a period of several months. Guard ants are posting along the trails. Mainly in the evening, males were observed on the trails running back and forth. This behavior is interpreted as mating running”. In the peripheral surroundings of the nest the routes may change within few hours or even minutes according to the food supply. Recruitment trails. The workers leave the nest singly or in groups. When an ant meets a prey animal it returns to the closest frequented trail rubbing its gaster tip on the ground while the sting is intermittendly extruded. The alarm secretion is released from the venom gland. It works as both orientation and recruiting signal and attracts new workers from the trail. No tactile signals are involved in food recruitment. The new workers in their turn intesify the alarm trail. Prey carrying workers lay trails to the nest which recruit further ants. Once the food source is exploited the alarm effect, even on strong trails, would decrease within half an hour. Recruitment trails which have been used over some time may become permanent routes. Along the permanent routes in addition to the recruiting substance an orientation component could be detected which lasts at least over 5 hrs. Its source is still unknown Predatory behavior. During foraging the workers frequently show digging behavior especially on places with fresh soil. Alarm secretion increases the digging activity. By this method L. ocellifera captures mainly animals which live in the soil, especially termites and earth worms. Other arthropods and snails are also predated. Larger prey objects are hunted and cut into pieces cooperatively. The pieces are transported into the nest by single workers. Killed termites are layed down near or on the trails by the foragers. Such depots are emptied by other workers. L. binghami and L. diminuta: L. binghami lives in natural earth cavities. It forages singly as a rule. Its main prey animals are termites. The trail secretion is mainly used during nest emigration. This is performed in a fast and well organized way. L. diminuta nests near the surface of the soil. It is a typical group prdator. The successful scouts lay recruiting trails while returning into the nest. The observed raids contained three to more than hundred workers led only sometimes by the scout. Prey objects (mainly larger arthropods) are hunted and retrieved cooperatively.

Safriel, U.

doi: 10.1007/BF00364323pmid: 28309299

Vermetids significantly contribute to the features of certain intertidal reefs. In the eastern Mediterranean, notably along exposed shores of Israel, these reefs are large eolianite platforms divided into rimmed flat basins. Their surfaces are incrusted by a dense cover of vermetid shells cemented by coralline algae—the rims by a crust of Dendropoma petraeum , 10–15 cm thick, and the surrounded basins by Vermetus triquetrus , usually less than 5 cm thick. These two species live in very different environments—the rims are incessantly surfswept and often exposed to air while the basins are usually watercovered and relatively calm. Accordingly, Dendropoma differs from Vermetus in certain morphological and anatomical characters, as well as in feeding mechanisms. There is circumstantial evidence that their crust has a protective effect on the underlying rock and is the controlling factor in the development of these platforms. Less common and smaller vermetid-covered reefs of the Israeli shore are circular limestone formations, in which an outer rim incrusted by D. petraeum surrounds a single shallow basin incrusted by V. triquetrus . These small reefs, resembling miniature atolls, show superficial similarity to intertidal reefs in Bermuda, where certain environmental conditions are rather similar to those prevailing in the eastern Mediterranean. As in Israel, a species of Dendropoma, D. irregulare , is an important component in the composition of the reefs, though there is no Vermetus . Also, coralline algae do not function just as cement but are a significant component. Both in Israel and in Bermuda such reefs develop when erosion, brought about by the encroaching sea, is hindered by the protective biogenetic crust. In Israel this crust never increases in thickness beyond a certain level because by the time the rising sea-level permits further upward growth, the reef has already collapsed under the combined action of boring organisms and waves. In Bermuda, voids and borings within the reef are rapidly filled-in and cemented, mainly by transported calcareous debris. Instead of borers and waves weakening it, the Bermudan reef is consolidated and does not readily collapse, and the thickness of its organic crust is remarkable. Thus, the Bermudan “microatolls” are genuine wave-resistant reefs. Their growth is initiated and perpetuated by vermetids and coralline algae. Though a large portion of their framework is removed, it is replaced by a secondary deposition and cementation of other lime-secreting organisms and debris.

Gamboa, George

doi: 10.1007/BF00364324pmid: 28309300

Four colonies of the desert leaf-cutter ant Acromyrmex versicolor versicolor (Pergande) located 30 miles N.E. of Tempe, Arizona were observed over a 7 month period. The ants utilized trails in foraging, a characteristic of higher attines, as well as foraging singly, a typical pattern among the more primitive gardeners. The ants cut and collected both dry and green vegetation with dry grasses comprising the bulk of the forage. The ants increased their cutting of green vegetation after significant rainfall but collected dry grasses almost exclusively during dry periods. Detailed macro-motion picture analysis of leaf-cutting revealed that the desert gardener utilized a unique technique for cutting compound desert leaves.