The population ecology of gammarus tigrinus (sexton) in the reed beds of the Tjeukemeer

The population ecology of Gammarus tigrinus (Sexton) was studied in the Tjeukemeer during 1969 and 1970. G. tigrinus reaches very high densities — up to 24,000/m² in parts of the study area. In 1970, the summer densities were 2–21/2 times greater than in 1969. Individuals do not grow to such large sizes in the summer as at other times of the year. Females begin to carry eggs in March or April and reproduction ceases in November. Large females have larger broods than smaller animals and the average size of the brood varies with the time of year. The egg incubation period and growth rate are dependent upon temperature. At summer temperatures females became sexually mature after about four weeks and the egg incubation period is about io days. The entire population is turned over about three times during the year. A combination of rapid growth rate, early onset of sexual maturity and high fecundity are probably responsible for the rapid spread of G. tigrinus throughout much of the Netherlands.     

Habitat segregation in a salt marsh among adult sticklebacks (Gasterosteidae)

Synopsis The spatial distribution, seasonal abundance and diel activity of four sticklebacks,Gasterosteus aculeatus, G. wheatlandi, Pungitius pungitius, andApeltes quadracus. coexisting in a St. Lawrence salt marsh were examined to see how these closely related species share their habitat. While all four species breed in the Riviè des Vases, a tidal creek, only three species are found in the adjacent salt marsh pools,A. quadracus being absent. Results are interpreted in terms of avoidance of interspecific competition for space during the relatively short breeding season at this latitude.     

SEAWEED ANATOMY AND PHOTOSYNTHETIC PERFORMANCE: THE ECOLOGICAL SIGNIFICANCE OF LIGHT GUIGES,HETEROGENEOUS ABSORPTION AND MULTIPLE SCATTER12

Light absorption by two green seaweeds with similar photophysiology but different anatomies are compared: i) Ulva lactuca var. rigida (C. Ag.) Le Jolis, an optically translucent species of two cell layers both bearing chloroplasts; and, ii) Codium fragile subsp. tomentosoides (van Goor) Silva, an optically opaque species with a colorlelss medulla surrounded by a cortex of choloroplast-bearing utriclels. Thallus absorptance (fraction of incident light absorbed) was measured for various pigment contents. Absorptance by U. lactuca was dependent on pigment concentration in an exponential manner and never exceeded 0.6, whereas absorptance by C. fragile was independent of pigment concentration and always approached a balue of 1.0. Water in the medullary tissue of C. fragile is often of the utricles. The utricles appear to be “integrating spheres” enhancing the capture of incident light, aided by the wave-guide function of the thin peripheral layer of cytoplasm and a reflector function at their base. Photosynthitic performance for U. lactuca saturates at high light intensities and attenuates rapidly with decreasing intensities. In contrast, photosynthetic performance for C. fragile saturates at low light intensities and attenuates slowly with diminishing radiation. Extrapolated diel variation in photosynthesis shows that U. lactuca's anatomy is adaptive for high light intensity environments, whereas C. fragile's anatomy is adaptive for low light intensity environments. Both seaweeds fit into the ecological category of “fugitive” species, and compete in the Long Island Sound (Atlantic Ocean) rocky intertidal for free-space. Predictions are presented for relative species abundances along a monotonic gradient of light intensity.     

The diet of two species of Isoperla (Plecoptera: Perlodidae) in relation to season,site, and sympatry

The seasonal change in gut contents of nymphs of Isoperla grammatica and I. difformis from six streams in southern Sweden was analysed. Both species had ingested a variety of benthic prey and vegetable matter, predominantly diatoms. Some seasonality was evident with high percentages of diatoms in spring in I. grammatica, and in autumn in I. difformis. The scope of food was larger in the latter species which contained about equal proportions of vegetable matter, chironomids, mayfly, stonefly, and black fly larvae. In I. grammatica plant matter and chironomids dominated strongly, comprising > 50% of the gut contents on an annual basis, > 90 % in spring. While small nymphs of I. difformis contained a low proportion of animal matter, only gradually increasing with size, the nymphs of I. grammatica were carnivorous from very early instars. Both species switched to a temporarily strong utilization of algae in spring. This differed among sites, and appeared to reflect differences in insolation and thus the availability of algae. There was a significant negative relationship between the mean densities of Isoperla nymphs and the proportion of animal material found in the guts of I. grammatica (R ² = 0.86). Considering the density of I. grammatica alone, the relationship was weaker (R ² = 0.56). A positive correlation between predator and prey size was observed. With chironomid prey the size range increased with predator size. With simuliid prey, however, prey size increased with predator size in such a way that it suggests selection rather than just an expanding prey size range. Correlations were stronger and regression coefficients significantly higher for I. grammatica than for I. difformis. We suggest that I. grammatica, which ingests a much wider size range of prey might choose prey of optimal sizes more readily than the more synchronously developing I. difformis. Although the life cycles of the two species are staggered, overlap in size distribution indicates that competition for food could be important in spring. However, observed differences in diet should facilitate coexistence. Gut content differences might in turn be accomplished through microhabitat segregation.     

Contest and scramble competitions in two bruchid species,Callosobruchus analis andC. phaseoli (Coleoptera: Bruchidae)

Larval competition between contest and scramble strategists was investigated using the two bruchid species, C. analis (contest species) and C. phaseoli (scramble species) with two different sized mung beans (large and small beans). In both sized beans, the adult emergences of each species dependen on total density of the initial larval densities of the two species and the ratio of the two densities. The emergence of one species was suppressed by the existence of the other species when the initial larval density per bean of the former species was less than that of the latter one. There were many cases in which both C. analis and C. phaseoli emerged from one bean in large beans, but such cases were quite rare in small beans. C. analis performed interference behavior only at late larval stages, whereas C. phaseoli was superior in exploitative competition all through their larval stages. These, combined with the niche segregation inside a bean, are throught to be the major factors of observed density- and frequency-dependent competition results. Based on the above experimental results, long-term competition results between the contest and scramble species were predicted.     

Numbers and size of sperm storage tubules and the duration of sperm storage in birds: a comparative study

Estimates of the numbers of sperm storage tubules (SSTs) in the utero-vaginal junction of 11 bird species are presented. Numbers of SSTs varied by a factor of 40 between species, and ranged from 500 to 20000. Body mass accounted for over 50% of the variation in SST mumbers. SST length was positively correlated with the length of spermatozoa across species. The duration of sperm storage was not correlated with the number of SSTs or the volume of sperm storage tissue. However, the number of 'active' SSTs appears to vary between species and it was not possible to make allowance for this. Sperm storage duration was weakly, positively correlated wth clutch size, but showed a significant positive relationship with the number of days over which laying occurred. The number of SSTs was also positively correlated with the number of sperm per ejaculate. The best predictor of sperm storage duration was a multiple regression equation using the spread of laying and the length of sperm storage tubules. The duration of sperm storage in birds which remain together during the pre-laying period is such that a single insemination immediately before the start of laying could fertilize the entire clutch.     

Physiological and growth responses of Centaurea maculosa (Asteraceae) to root herbivory under varying levels of interspecific plant competition and soil nitrogen availability

Summary Centaurea maculosa seedlings were grown in pots to study the effects of root herbivory by Agapeta zoegana L. (Lep.: Cochylidae) and Cyphocleonus achates Fahr. (Col.: Curculionidae), grass competition and nitrogen shortage (each present or absent), using a full factorial design. The aims of the study were to analyse the impact of root herbivory on plant growth, resource allocation and physiological processes, and to test if these plant responses to herbivory were influenced by plant competition and nitrogen availability. The two root herbivores differed markedly in their impact on plant growth. While feeding by the moth A. zoegana in the root cortex had no effect on shoot and root mass, feeding by the weevil C. achates in the central vascular tissue greatly reduced shoot mass, but not root mass, leading to a reduced shoot/root ratio. The absence of significant effects of the two herbivores on root biomass, despite considerable consumption, indicates that compensatory root growth occurred. Competition with grass affected plant growth more than herbivory and nutrient status, resulting in reduced shoot and root growth, and number of leaves. Nitrogen shortage did not affect plant growth directly but greatly influenced the compensatory capacity of Centaurea maculosa to root herbivory. Under high nitrogen conditions, shoot biomass of plants infested by the weevil was reduced by 30% compared with uninfested plants. However, under poor nitrogen conditions a 63% reduction was observed compared with corresponding controls. Root herbivory was the most important stress factor affecting plant physiology. Besides a relative increase in biomass allocation to the roots, infested plants also showed a significant increase in nitrogen concentration in the roots and a concomitant reduction in leaf nitrogen concentration, reflecting a redirection of the nitrogen to the stronger sink. The level of fructans was greatly reduced in the roots after herbivore feeding. This is thought to be a consequence of their mobilisation to support compensatory root growth. A preliminary model linking the effects of these root herbivores to the physiological processes of C. maculosa is presented.     

The impact of limb autotomy on mate competition in blue crabs Callinectes sapidus Rathbun

Summary This study is the first to demonstrate experimentally that autotomy (self-amputation of a body part) adversely affects competition for mates. Experiments were conducted using blue crabs Callinectes sapidus Rathbun to examine the consequences of limb loss and pairing precedence on mate acquisition by males. Two adult males of equivalent size were introduced sequentially into pools containing a sexually-receptive female and observed after 24 h and 48 h. One male in each pair was left intact, while the other experienced: (1) no autotomy, (2) autotomy of one cheliped, or (3) autotomy of both chelipeds, one walking leg, and one swimming leg. In the absence of a competitor (first 24 h), both intact and injured males established precopulatory embraces with females. Intact males were highly successful (84–95%) in defending females from intact or injured intruders in the second 24 h period. Both autotomy treatments, however, significantly reduced the ability of males to defend females from intact intruders. Females in experiments suffered greater frequency of limb loss than did males. In the field, paired blue crabs showed significantly higher incidence of limb loss than unpaired crabs. Limb loss frequency increases with body size, and field observations indicated that larger males may be more successful than smaller males in obtaining females. Both experimental manipulations and field studies provide strong evidence for mate competition in this ecologically and commercially important portunid species.     

Current status of Agrilus hyperici (Coleoptera: Buprestidae) released in Australia in 1940 for the control of St John's wort: Lessons for insect introductions

The root‐borer Agrilus hyperici was established successfully in Australia in 1940 for the biological control of the weed Hypericum perforatum. It failed to spread widely, however, and surveys in 1980 found that it survived in only one area; near Mudgee in New South Wales. Two isolated populations were found here, both restricted to sheltered creek systems. The distribution of the larger population, at Piambong, was confined to an area of about 10 km². In 1981–82, 76.8% of plants in the core of this area showed damage by larvae to their roots. Although the level of attack was lower than that known from southern Europe, survivorship patterns from larva to adult were similar, confirming A. hyperici to be a ’K‐selected’ insect. The population declined significantly following a severe drought in 1982. Consideration of the early history of their introductions coupled with knowledge of their biologies suggest that strong competition by Chrysolina spp., strongly ‘r‐selected’ insects introduced at the same time, may have restricted the development of A. hyperici. The danger of co‐releasing insects with different life‐history strategies is discussed.