Resource exploitation of larvae ofGastrophysa atrocyanea Motschulsky andgalerucella vittaticollis Baly (Coleoptera: Chrisomelidae) under a limited resource condition

Summary Resource exploitation by and intraspecific competition in larvae ofGastrophysa atrocyanea andGalerucella vittaticollis were investigated in field and laboratory experiments. Larvae of both species frequently suffered from food shortages in the field. WhenG. atrocyanea larvae suffered from a food shortage, severe intraspecific competition occurred because of lack of predation and parasitism. This exploitive competition was caused by a local food shortage of the host plant. Individuals survived by fast exploitation when food became abundant (contest type competition). TheG. atrocyanea larvae were wasteful of the food resource, and no mechanism by which to economize on the utilization of the resource was acquired because of their exploitation of the abundant resource. In contrast, theG. vittaticollis population probably is regulated by extrinsic factors such as predation and parasitism. Those larvae grew into smaller adults than those ofG. atrocyanea under a food shortage, so that their wasted food consumption was lower than that ofG. atrocyanea. Although intraspecific competition was similar to that forG. atrocyanea, it was not as severe. The food forG. vittaticollis was apt to be appropriated by other wasteful exploitators such asG. atrocyanea, which was superior in resource exploitation; thereforeG. vittaticollis frequently suffered a food shortage. Consequently selection in relation to tolerance to starvation became more acute forG. vittaticollis than forG. atrocyanea, and individuals ofG. vittaticollis that could endure starvation better may have been selected.     

Population dynamics of the Smith's red-backed vole in highlands of Shikoku

Summary and Conclusion Population dynamics of theSmith's red-backed vole predominantly common through uplands of Shikoku have been in some degree disclosed by the use of my own and Government Forest Station's samples collected since 1955 onward. It has proved that the upper-range population reaches its density peak possibly in late summer or early fall, but the lower-range one does probably in October–November as the seasonal trend in usual years. The upper one produced a peak three times at 3–4 year intervals, the first peak being an outbreak followed by a crash, during the last decade. It seems likely that all the populations through the range have, in the gross, fluctuated in phase after 1959 at least. The cyclic fluctuation may readily be explained by the theory of intrinsic mechanism, because no external factors are considered to have exerted a conclusive effect. Except what was concerned in the outbreak, the role of the social stress could be set at naught. The regulation of fecundiy by density was expressed more markedly in litter size and less in active reproductivity rate and incidence of pregnancy. The mean litter size at term as small as 2.00 is contrary to our expectation in view of the supposed prolific potential, nevertheless the observed frequency of litter poduction and intra-uterine survival rate has proved not to be so high as to make up for the small litter size. The problem in the postnatal mortality has remained to be solved.     

Food limitation of population density in the orb-wed spider,Nephila clavata

Summary Field studies were conducted to clarify whether variation in food availability among habitats influences population density, and whether population density has a negative effect on foraging success in the orb-web spider,Nephila clavata. Lifetime food consumption per individual (i.e., foraging success) strongly correlated with mean body size of adult females and mean fecundity in populations. Also, there was a positive correlation between foraging success and population density. Since foraging success reflected potential prey availability in the habitat, food resource appeared to be a limiting factor for populations in this spider. Mean fecundity per individual correlated with population density of the following year, suggesting that decreased reproduction is a major component of food limitation on population density. Consistent defferences in mean body size between particular sites were observed over years, while such difference was less obvious in density. Thus, ranking of food abundance among habitats seems to be predictable between years. A field experiment revealed that an artificial increase in population density had no negative effect on the feeding rate of individuals, suggesting that intraspecific competition for food is not important in this species.     

The effects of food and density of the movement ofArion ater andAriolimax columbianus (Pulmonata: Stylommatophora) between habitats

Summary Experiments were conducted to determine if slug density, or food quantity and quality could cause dispersal by slugs.Arion ater dispersed quickly from overcrowded areas, where high mortality was incurred by this species. Poor food conditions, caused more dispersal than good food conditions, but food had no effect on the mortality ofA. ater. Neither food nor density factors had significant effects on the dispersal ofAriolimax columbianus. However, moreA. columbianus died in the high- than low-density treatments.A. columbianus mortality was unaffected by the food factor. DispersingArion were heavier than non-dispersers, but there was no difference between the weights of dispersing and non-dispersingAriolimax. There were significant seasonal changes in the dispersal rate, mortality and weight of dispersingArion, whereas only the mortality ofAriolimax changed significantly during the experiments. SinceArion ater did not markedly reduce its activity during unsuitable weather it was able to leave areas of poor food and high slug density fairly rapidly.Ariolimax columbianus' large size allowed it to remain inactive during unfavourable weather and permitted this species to be relatively unresponsive to the experimental treatments.     

Two perspectives on sustainable development

This paper applies the Perspectivist conception to the issue of sustainable development in such a way as to make perspicuous two competing views on the issue. These views, here termed theeconomic and theecological, are respectively that of the United Nation's Brundtland Commission, which favours economic growth as a means of overcoming the world's environmental problems, and an alternative view, according to which environmental problems must be dealt with directly, and economic growth may well be a hindrance to their solution. The paper concludes with a theory of human development based on thevicious circle principle, which shows theeconomic perspective on sustainable development to be incoherent, and theecological perspective to remain a viable alternative.This paper, here published with permission, is an expansion of Appendix VI of the author's book,Scientific Progress.Funding for research has been provided by the Swedish Environmental Protection Agency.     

Parental care and EGG size in salamanders: An examination of the safe harbor hypothesis

Summary The safe harbor hypothesis includes the suggestion that parental care causes the embryonic stage to be the safest harbor, and, therefore, egg size will increase in populations with parental care to decrease the duration of subsequent, higher risk stages. Neither the safe habor hypothesis nor r and K theory seem adequate to explain the correlation between egg size and the presence/absence of parental care among salamanders, a group in which there is a further correlation between the larval (hatchling) habitat and egg size/parental care. Pond-breeding salamanders generally have small eggs and lack parental care, and stream-breeding salamanders generally have large eggs and parental care. I argue that the fundamental difference in the food available to hatchling salamanders between lentic (plankton-rich) and lotic (plankton-poor) environments selects for relatively lower parental investment in the lentic environment. From the standpoint of parental fitness, small (more numerous) hatchlings have a greater payoff where the available food is mall and dense (zooplankton in lentic environments), and large hatchlings are selectively advantageous where the food is of large size and less dense (benthic invertebrates in lotic environments). Selection for larger hatchlings in lotic environments results in longer embryonic periods and,ceteris paribus, greater total embryonic mortality. Embryo hiding and guarding have evolved among lotic-breeding salamanders as compensatory mechanisms to reduce the rate of embryonic mortality. In this view, parental care is a consequence of selection for larger egg size and not an umbrella that allows egg size to increase, contrary to the safe harbor hypothesis. The relationship between variance in parental investment and food available to offspring, developed here for salamanders, may be of general significance. YosiakiIt?, a critic of r and K theory, independently arrived at a similar conclusion from a broader data base.     

Competitive exclusion through reproductive interference

Summary A simple differential equation model was developed to describe the competitive interaction that may occur between species through reproductive interference. The model has the form comparable to Volterra's competition equations, and the graphical analysis of the outcome of the two-species interaction based on its zero-growth isoclines proved that: (1) The possible outcome in this model, as in usual models of resource competition, is either stable coexistence of both species or gradual exclusion of one species by the other, depending critically upon the values of the activity overlapping coefficientc _ij ; (2) but, for the samec _ij -values, competitive exclusion is much more ready to occur here than in resource competition; (3) and moreover, the final result of the competition is always dependent on the initial-condition due to its non-linear isoclines, i.e., even under the parameter condition that generally allows both species to coexist, an extreme bias in intial density to one species can readily cause subsequent complete exclusion of its counterparts. Thus, it may follow that the reproductive interference is likely to be working in nature as an efficient mechanism to bring about habitat partitioning in either time or space between some closely related species in insect communities, even though they inhabit heterogeneous habitats where resource competition rarely occurs so that they could otherwise attain steady coexistence.     

Relationships of latitude, altitude, and body size to litter size and mean annual production of offspring inPeromyscus

Summary Litter size was positively correlated with latitude and altitude but not with production of offspring or with body size inPeromyscus. Increased litter size in northern populations probably reflects shortening the breeding season by climate and not a greater mortality rate at northern latitudes compared to southern latitudes. Production of offspring was negatively correlated with body size but not with latitude, altitude, or litter size. This is probably due to larger species living longer and taking longer to mature.     

Process generating the ditribution pattern of eggs of the common cabbage butterfly,Pieris rapae crucivora

Summary The process generating the negative binomial in the distribution pattern of eggs of the common cabbage butterfly,Pieris rapae crucivora, was investigated by releasing the female adults in a net house where cabbages were planted. The distribution of butterflies visited and laid an egg or more per plant followed thePoisson series under the uniform light condition, while that of eggs laid per visit conformed to the logarithmic distribution. From these results, it may be concluded that the negative binomial arises from compounding of thePoisson and the logarithmic distribution. The observed frequency of eggs found per plant fitted to the negative binomial with parameter thus computed theoretically. The change in the degree of aggregation with the increase of the parental density was considered in connection with the above results. Aided by a grant from Scientific Research Expenditure of the Ministry of Education.     

The role of parasitoids in evolution of habitat and larval food plant preference by three Pieris butterflies

This article attempts to explain that parasitoids provide the evolutionary pressure responsible for relationships between habitat use and larval food plant use in herbivorous insects. Three species of butterflies of the genus Pieris, P. rapae, P. melete, and P. napi use different sets of cruciferous plants. They prefer different habitats composed of similar sets of cruciferous plants. In our study, P. rapae used temporary habitats with ephemeral plants, P. melete used permanent habitat with persistent plants, although they also used temporary habitats, and P. napi used only permanent habitat. The choice experiment in the field cages indicated that each of the three butterfly species avoided oviposition on plants usually unused in its own habitat, but accepted the unused plants which grew outside its own habitat. Their habitat use and plant use were not explained by intrinsic plant quality examined in terms of larval performance. Pieris larvae collected from persistent plants or more long lasting habitats were more heavily parasitized by two specialist parasitoids, the braconid wasp Cotesia glomerata and the tachinid fly Epicampocera succincta. The results suggest that Pieris habitat and larval food plant use patterns can be explained by two principles. The evolution of habitat preference may have been driven by various factors including escape from parasitism. Once habitat preference has evolved, selection favors the evolution of larval food plant preferences by discriminating against unsuitable plants, including those which are associated with high parasitism pressures. Received: December 3, 1998 / Accepted: January 20, 1999     

Effects of interspecific competition on development and reproduction in two giant water bugs,Diplonychus japonicus vuillefroy andDiplonychus major esaki (Hemiptera: Belostomatidae)

Summary We studied the interpecific competition between 2 species of predatory aquatic bugs,Diplonychus japonicus andD. major by conducting a field experiment. We set up 3 types of experimental plots in the paddy fields whereD. major predominated. The two plots contained single species of eitherD. japonicus orD. major, respectively, and one plot had both species in equal number. We compared the development and the reproductive performance between plots in each species. InD. japonicus, the number of eggs and early instar nymphs were significantly smaller in the plots containing both species than in the monospecific plots. However, the numbers of late instar nymphs and newly emerged adults were not significantly different between plots. The proportions of starved nymphs in both plots were larger than those in theD. japonicus's natural habitats. The final densities of adults in both plots were lower than those in the natural habitats. These results suggest that lower density ofD. japonicus in these paddy fields is due to the lack of available food for nymphs rather than the effects of interspecific competition withD. major. InD. major, significant differences were not found in the number of eggs, each instar nymphs and adults. These results suggest that the effects of interspecific competition did not affect the reproductive performance ofD. major.     

An individual-based model for competingDrosophila populations

An individual-based model forDrosophila is formulated, based on competition amongst larvae consuming the same batch of food. The predictions of the model are supported by data for single speciesDrosophila populations reared in the laboratory. The model is used to build a simple discrete model for the dynamics ofDrosophila populations that are kept over a number of generations. The dynamics of a single species is shown to give either a stable equilibrium or fluctuations which can be periodic or chaotic. When the dynamics of a species in the absence of the other is periodic or chaotic, we found coexistence or two alternative states, on neither of which the species can coexist.     

Speciation and evolutionary dynamics of asymmetric mating preference

Asymmetric mating preferences occur in two closely related species, if females of one species are highly selective against males of the second, while females of the second show less selection against males of the first species. It has been suggested that such asymmetry is an indicator of common ancestry between the two species, but actual observations are contradictory and inconclusive. We developed a scenario of speciation history and asymmetric mating preference, incorporating invasion dynamicsvia frequency-dependent interspecific sexual competition. A newly isolated (derived) species may form at the periphery of the ancestral species’ distribution by invading a new range. Only a few closely related species would be expected in the new area, while many related species are expected to coexist with the ancestral species. In a peripherally derived species, female mating preferences should be relaxed through sexual character release, owing to a lack of sympatric species and a scarcity of intraspecific mating opportunities. Secondary contacts may then happen as: 1. repeated invasions, i.e. subsequent invasion by the ancestral species into the new range or, 2. backward invasions, i.e. derived species incursions into the ancestral range. Repeated invasions could lead to the coexistence of both the derived species and the newly invading ancestor. Backward invasions by the derived species can succeed only when the derived females develop a strict mating discrimination against the ancestral males. We then expect strong character displacement in the derived species. Thus, peripheral isolation and repeated invasions lead to the relaxed female mating preferences in the derived species and backward invasions lead to stronger female mating preferences in the derived species. This agrees withDrosophila data from Hawaii and the continents. Experimental data of theDrosophila arizonaemojavensis species cluster also support the hypothesis.     

Seasonal host alternation by the andromeda lace bug,Stephanitis takeyai (Heteroptera: Tingidae) between its two main host-plant species

I studied the seasonal occurrence of the andromeda lace bug,Stephanitis takeyai, on its two main host-plant species. In a secondary forest in Kyoto, this bug altered its hosts seasonally, i.e., from an evergreen shrub,Pieris japonica, in winter to a deciduous shrub,Lyonia elliptica, in summer. In contrast, in Nara park where fewL. elliptica were available, the bug exploited onlyP. japonica. Thus, seasonal host alternation by this bug is not obligate. A comparison of adult longevity and fecundity on the two host-plant species demonstrated the higher quality ofL. elliptica as a food resource. Corresponding to this difference in host quality, there was a dramatic difference in the seasonal population growth in the two study sites. In Nara, the population size at the beginning of the 2nd generation was almost the same as in the overwintered generation, whereas in Kyoto the population size in the 2nd generation was approximately one hundred times as large as in the overwintered generation. Thus seasonal host alternation is adaptive for the bug. In a previous study, I reported that overwintering as eggs in living leaves of their hosts is likely to be common among all the related species of this bug. Thus, this trait can be considered to be a phylogenetic constraint to the group. I speculate that host alternation by this bug has been derived because it is more adaptive from autoecy on an evergreen plant, similar to the pattern currently found in Nara, and that this bug can not only exploit deciduous host due to a phylogenetic constraint.     

The relationship between size, budding rate, and growth efficiency in three species of hydra

Summary Three species of the fresh water carnivore hydra,H. littoralis, H. pseudoligactis, andC. viridissima present a graduation in size with the first species the largest and albinoChlorohydra the smallest. When presented with a daily overabundance of food (artemia), considerable variation in food intake and gross efficiency of growth (proportion of food energy consumed that is turned into new protoplasm or buds) existed among the species. The degree of association between size of species and food intake was highly significant. However, budding efficiency among the species was found to be independent of food intake (when the effects of species size were eliminated) and of species size (when the effects of food intake were removed). However, species with high (low) efficiencies have significantly higher (lower) reproductive rates. A lowering of the temperature from 25° to 15° C. increased the size of the species, increased food intake, but decreased reproductive rate. In all species exceptH. pseudoligactis a corresponding increase in the production of bud energy with no change in efficiency also occurred. On the other hand, lowering of the temperature forH. pseudoligactis significantly lowered reproductive efficiency but had no effect on the total calorific output of buds. This species, in constrast to the others, appears to have a compensatory ability to adjust its efficiency to maintain a high calorific output when temperature increases. It was also found that albinoChlorohydra have budding efficiencies of around 35 percent which are not influenced by changes in food intake or light. Normal green hydras, however, have efficiencies which range from 40 to 62 percent above their albino counterparts when fed once a day and once every two days in light respectively. It it concluded first, that the symbiotic algae in the gastrodermals cells of green hydra contribute quantitatively in the order of the above amounts to the growth process in this species, and second, that green hydras have the ablity to increase their growth efficiency when food intake is reduced thus reducing the drop in calorific but output that normally occurs in the albino (control) form. This study was supported by a National Science Foundation Grant (GB-912)     

The effects and limits of territoriality on population regulation in grey red-backed voles,Clethrionomys rufocanus bedfordiae

Summary The effects of breeding territoriality on the stability of grey red-backed vole (Clethrionomys rufocanus bedfordiae) populations were investigated on a control grid and a grid on which the voles were fed, in an outdoor enclosure in Hokkaido, Japan. Vole populations were monitored by live trapping from 1984 to 1986: (1) Population density was 2–7 times greater on the experimental grid to which food was added than on the control grid. Reproductive output was more closely associated with the difference in density between grids than survival or dispersal (immigration and emigration) rates. (2) The number of adult females and pregnancy rate of the experimental population were significantly greater than those of the control one. The difference in the number of adult females between the populations was greater than that in pregnancy rate. (3) The proportion of successful litters and the number of weanlings per litter were not significantly different between the control and experimental population. (4) Adult females held territories on both the control and experimental grid; they were spaced out more than would be expected from random occupation. The territories overlapped more on the experimental grid than on the control grid. (5) Mean territory size of adult females on the experimental grid was about half of that on the control grid. The territory size was correlated negatively with population density. (6) The proportion of trap sites that were used by adult females was significantly greater on the experimental grid than on the control grid. This suggests that adult females on the experimental grid used the area more extensively. This factor, in association with territory size and overlapping of territory, was also important in causing the difference in the number of adult females between the grids. (7) These results call into question the hypothesis that territoriality stabilizes the density in populations ofClethrionomys.     

Space partitioning among the wolf spiderPardosa amentata species group in Hokkaido, Japan

Summary There are 4 species belonging to the wolf spiderPardosa amentata species group in Hokkaido. The 4 species have separately occurred in Hokkaido in general, but a few species have sometimes occurred together at the same sites. Their biology at the coexisting sites, with special reference to partitioning of space was studied.P. agraria mainly inhabited fields and occurred widely in Hokkaido.P. hokkaido, P. brebivulva andP. lugubris inhabited the vicinity of forests and occurred in southern Hokkaido and on the side of the Sea of Japan in Hokkaido, all over Hokkaido excent the side of the Sea of Japan in central Hokkaido, and the lowlands and highlands in northern and eastern Hokkaido and mainly the highlands in other regions, respectively. In eastern Hokkaido whereP. brevivulva andP. lugubris have coexisted, as the number of spiders detected and the number of localities of the former were less than those of the latter, it was concluded that the former occurred mainly in southern Hokkaido and the Hidaka region. In southern Hokkaido whereP. hokkaido andP. brevivulva have coexisted, few individuals of both species were collected together at the same sites, and they were distributed allotopically. In short, the partitioning of space among theP. amentata species group occurring in Hokkaido was clear. From investigations on the coexisting sites and some other reports, it was considered that food, life cycle and diel activity periodicity amongPardosa occurring in Hokkaido were similar to one another. This may be one of the causes of the clear space partitioning amongPardosa. SincePardosa is a raptorial predator that does not weave but wanders on the ground and uses space horizontally, available prey resource forPardosa would be limited. It was therefore conjectured that the spider would be prone to space segregation, even if the spider could segregate the other niche dimensions. The partitioning of space betweenP. agraria andP. hokkaido inhabiting distinct habitats was loose when it was compared with that between species inhabiting similar habitats. As most of the spiders observed in the overlapping areas were females with egg cocoons, which required little food, the reduced feeding was considered to have allowed the loose partitioning.     

Some beneficial effects of aggregation in young larvae ofPryeria sinica moore (Lepidoptera: Zygaenidae)

Summary The effects of group size on the survival and development of young larvae ofPryeria sinica Moore were investigated by laboratory and field experiments. Under laboratory conditions, about 20% of isolated larvae died of unsuccessful feeding in the first instar, however, larvae survived successfully in aggregations of four or more individuals. In the field, larvae emerge in early spring and wait for new leaves to open before feeding. In this period, the larger the group size of hatchlings the survival rate became higher. The nest-web spun by hatchlings was considered to play an important role in protecting them from desiccation. In the period that larvae began to feed on leaves, more than 36 larvae are necessary to aggregate for the successful establishment of feeding groups. The nest-web played an important role also in the establishment of feeding group. However, the natural group size of the first instar larvae was larger than the minimum group size to spin a sound nest-web in the field experiment. On the other hand, in later stage, larvae in a large group did not have an excess advantage in survival or developmental rate over larvae in a small group. It was found that the experiments on survival and developmental rates could not explain the reason that this species maintain large compact groups in the most part of larval period. The study was partially granted by the Ministry of Education (No. 374205).     

Inter- and intraspecific interactions among larvae of specialist and generalist parasitoids

Intra- and interspecific larval interactions that take place in a host body were investigated for two tachnid fliesEpicampocera succincta andCompsilura concinnata (Diptera: Tachinidae) parasitizingPieris butterfly larvae.E. succincta, a specialist onPieris butterflies, showed contest-type intraspecific competition, eliminating all the other conspecific larvae. On the other hand, an extreme generalist parasitoidC. concinnata exhibited scramble-type competition, sharing the host with other conspecifics and suffering reduced body size as a result. However, when these two species occurred together in a single host,C. concinnata had a much higher chance of survival. Moreover,C. concinnata could often survive in the presence of a parasitoid waspCotesia glomerata (Hymenoptera: Braconidae) whileE. succincta could not. The high tolerance ofC. concinnata could be attributable to its being an extreme generalist: To attack and survive on many different hosts, one has to be able to deal with various competitors. The competitive inferiority of the specialistE. succincta, on the other hand, may be a result of relatively recent encounter with, those competitors.     

Age structure of six populations of old-field mice,Peromyscus polionotus

Summary An accurate age-estimating technique, based on biochemical changes in eye lens protein, was used to study age structures of six populations of the old-field mouse,Peromyscus polionotus. A new mathematical procedure permitted quantitative comparisons of these populations. Four inland populations had essentially the same median ages (75–84 days), maximum ages (248–297 days) and relative production rates (56–58% of the surviving population had been born in the 100 days prior to sampling). Approximately 50% of the females were old enough to have weaned a litter. One inland population had a lower median age (49 days), a lower maximum age (181 days) and a higher relative production rate (73%). Relatively fewer females (38%) could have weaned a litter. The single beach population was also different, with higher median age (180 days), and lower relative production rate (2%). All females were old enough to have weaned a litter. Maximum age was essentially the same as for the four similar, inland populations. Factors which may have contributed to observed differences in these populations are discussed.