摘要:

Many pterygote (winged) insects have secondarily become flightless; why has this occurred? In this paper I test for an association between the frequency of flightlessness and (1) environmental heterogeneity, (2) geographic variables, (3) gender, (4) alternate modes of migration, and (5) taxonomic variation. Various authors have predicted that decreased environmental heterogeneity will favor the evolution of flightlessness. This hypothesis is consistent with a number of studies on the variation of wing dimorphism and flightlessness and with data analyzed in the present paper on the relationship between habitat type and wing morph in the North American Orthoptera. The incidence of flightlessness is also exceptionally high in woodlands, deserts, on the ocean surface, in specific habitats on the seashore (dunes and rock crevices but not the foreshore), in aquatic habitats, in the winter months, in hymenopteran and termite nests, and among ectoparasites of endotherms and parasites of arthropods. It is low in habitats bordering rivers, streams, ponds, etc. and in arboreal habitats. Some of these habitats can be classified as persistent, but others cannot be so designated or are too vaguely defined. The incidence of flightlessness increases with altitude and latitude but, contrary to "conventional" wisdom, it is not exceptionally high on oceanic islands compared to mainland areas. Several authors have hypothesized that the clinal variation in flightlessness is a consequence of clinal variation in habitat persistence. Though the available data are consistent with this hypothesis, other factors such as impairment of flight by low temperatures cannot be rejected, though considered unlikely. Flightlessness is more frequent than expected by chance among parthenogenetic species and more frequent among females than males. Furthermore, there is a significant negative association between female mobility and flightlessness in males. I suggest that loss of flight is favored in females because it permits greater allocation of resources to egg production, but that flight is retained in males because it increases the probability of finding a mate. In some species alternate modes of migration, viz phoresy and ballooning, have evolved. Phoretic transport (i.e., via other animal species) may be important in those species in which it occurs, but it appears to have evolved relatively rarely. Among the Insecta ballooning occurs only in the larvae of Lepidoptera. Aptery (winglessness) is unusually frequent among Lepidoptera that eclose in the fall and winter months and live in woodlands and forests. I hypothesize that larval migration by ballooning, the large—scale spatiotemporal stability of woodlands, and the small—scale unpredictability of spring bud burst are primary factors favoring the evolution of flightlessness in these Lepidoptera. The mode of metamorphosis is correlated with the frequency of flightlessness; hemimetabolous and holometabolous insects are rarely flightless, while it is common among paurometabolous insects. Two factors that may favor such an association are differences in relative mobility of the larvae and niche shifts between the adult and larval stage. In the holometabolous insects taxonomic families with at least one flightless species have more species than those with only winged species. A possible explanation for this is that the occurrence of the appropriate ecological conditions favoring flightlessness in holometabolous insects and the requisite mutations are both very rare events.

ISSN:0012-9615    

DOI:10.2307/1943013

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Myxoma virus was released into Australia to control the introduced European rabbit, Oryctolagus cuniculus. Within a few years after introduction, the virulence of the virus had declined to an intermediate level, while the resistance of field rabbits and increased sharply. In the nearly 40 yr since the disease was introduced, host resistance has continued to increase, while viral virulence has only recently begun to show signs of counter—increases in some areas. The two questions of interest are thus: Is this system in a coevolutionary arms race (Dawkins and Krebs 1979); that is, will both host and pathogen continue to evolve antagonistically? Will the virus continue to control the rabbit in the future? We present a simulation model based loosely on previous host–pathogen models (Anderson and May 1979), but with detailed accounting of the virus titer in infected hosts, and using realistic estimates of the demographic parameters of the rabbit, including age structure and seasonally varying reproduction. For a single virulence grade, by varying the non—disease (or "natural") mortality of the rabbit, the age at first reproduction of the rabbit, and the virulence grade of the virus, we explored the parameter range for which the rabbit population is controlled. For the most prevalent grades of the virus, grades IIIB and IV, the virus can control the rabbit for most realistic values of natural mortality and age at first reproduction. However, control is dependent on both natural mortality and virus virulence. Since natural mortality varies both geographically and seasonally, the usefulness of the virus may vary geographically and seasonally, and management policies must be sensitive to this variation. When competing against several virus strains that together encompass the complete range of virulence seen in the field, a strain of grade IV virulence competitively excludes strains of all other grades. This competitively dominant grade is close to the most prevalent virulence grades seen in the field. We discuss possible mechanisms of coexistence, including local competitive exclusion with global persistence, variability in host resistance, high mutation rates, and trade—offs between within—host and between—host competitive ability. By examining the effects of flea transmission efficiency, we are able to show that, contrary to commonly held belief, whatever effect fleas have upon the outcome of selection on virulence cannot be due to differences in transmission efficiency between fleas and mosquitoes. Finally, by including host resistance, we improve our prediction of the most prevalent grade of virulence. We conclude that control of the rabbit by the virus is likely for the near future, but that until we understand the genetics of resistance in the rabbit and the relationship between resistance and virulence for different grades of virulence, for different grades of virulence, we cannot make a useful prediction of the long—term state of this system.

ISSN:0012-9615    

DOI:10.2307/1943014

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

Processes important in the development of subtidal seagrass beds composed of Thalassia testudinum, Syringodium filiforme, Halodule wrightii, and many rhizophytic algal species were examined in situ for 52 mo in a coral reef lagoon on St. Croix, United States Virgin Islands. The study emphasized the early stages of development of the seagrass beds and the role played by colonizing rhizophytic algae. I tested the hypotheses that nutrient accumulation in the sediments limits seagrass recolonization, and that rhizophytic algae facilitate sediment nutrient accumulation by stabilizing the sediments and adding organic matter from rapidly decomposing thalli. Vegetation was removed from 0.25— and 1—m2plots in 3 m of water. Plot treatments consisted of: (1) no further manipulation, (2) adding nitrogen plus phosphorus fertilizer to the sediments, (3) removing colonizing rhizophytic algae to minimize algal effects (e.g., sediment stabilization, organic input), and (4) removing colonizing algae and adding "plastic algae" to stabilize sediments without organic input. Plant densities, sediment grain size, redox potential, inorganic nitrogen concentrations in porewaters, and ammonium production rates were measured over time in all plots, including undisturbed controls. All recolonization occurred through vegetative propagation. The sequence of plant recolonization was unaffected by the treatments, corresponding instead to life history characteristics and nutrient requirements of the species involved. Rhizophytic algae invaded the plots within a few months, followed by the seagrass Syringodium, then Thalassia. The seagrass Halodule was insignificant in the recolonization. Densities of rhizophytic algae and Syringodium declined when the density of Thalassia reached 200 leaf shoots/m2. The rates of increase in seagrass leaf shoot densities and biomass were greatest in the fertilized plots, supporting the nutrient limitation hypothesis. Rhizophytic algae facilitated seagrass recolonization; seagrass densities, biomass, and porewater ammonium concentrations were lowest in plots where algae were removed. Sediment ammonium concentrations decreased when Thalassia became dominant. Ammonium production in the sediments increased as the plant community developed. The nitrogen required for Syringodium productivity was met easily by ammonium production, assuming no competition from Thalassia. In contrast, Thalassia accounted for>93% of the nitrogen required for total seagrass productivity, and ammonium production could supply up to 45% of this requirement. At the end of the experiment (52 mo), Thalassia density and ammonium production rates in the sediments were lower than in the surrounding undisturbed seagrass bed. The experiment provides evidence for a resource—ratio model where the rate of succession is controlled by a sediment nutrient supply that increases over time. The sequence of colonization is determined by relative rates of vegetative propagation by stolons and rhizomes across the sediment surface, which are inversely correlated with whole plant productivity and thus with requirements for nutrients. Algal colonizers tolerate low nutrients by having low productivities. The climax species Thalassia is a competitive species effective at exploiting the sediment nutrient resource. Co—existence, rather than replacement, of species occurs, despite a relatively benign disturbance regime.

ISSN:0012-9615    

DOI:10.2307/1943015

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

ISSN:0012-9615    

DOI:10.2307/1943016

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY

摘要:

The distribution of137Cs,90Sr,238Pu,239,240Pu,241Am and244Cm was studied in the biotic and abiotic components of an abandoned reactor cooling impoundment, Pond B. The impoundment is located at the United States Department of Energy's Savannah River Plant in South Carolina, USA. It received radioactive contaminants via cooling water discharges from R Reactor from September 1961 to June 1964. The radionuclide inventories were estimated in water, seston, sediments, and biotic components after 20 yr of equilibration. Chemical, physical, and biological relationships to the radionuclide distribution patterns were investigated. Biotic components contained some of the highest radionuclide concentration ratios observed to date. However, most of the radioactivity resides in sediments. The principal mechanisms of loss from the system are radioactive decay and periodic outflow of water and suspended materials; biotic export and seepage appear to be inconsequential. Strontium—90 was much more mobile in the system than the other radionuclides. Aquatic macrophytes dominated the biotic component radionuclide inventories and their dynamics exert a strong influence on the spatial distribution and turnover of radioactivity in the ecosystem. Pond B supports a diverse and productive flora and fauna. Cleanup of the system is not indicated. Use of Pond B for recreation is feasible with adequate attention to monitoring and radiological health guidelines.

ISSN:0012-9615    

DOI:10.2307/1943017

出版商:Ecological Society of America    

年代:1990

数据来源: WILEY