ISSN:0012-9658    

DOI:10.2307/1936022

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

Monthly recaptures of marked individuals in intertidal populations of the predatory gastropods Shaskyus festivus and Oceanebra poulsoni for 39 months near La Jolla, California, show annual variation in growth, reproduction, and recruitment rates consistent with the conclusion that the snails experienced only 1 successful year in 3. Eggs are deposited in capsules from March through July, and free—swimming veligers are released in 3 to 4 weeks. More than 99% of the mortality occurs in the plankton or during settlement. The major predator on the adults, the crab Cancer antennarius, annually destroys less than 1% of the adults Shaskyus and less than 2.5% of the adult Ocenebra. After settlement, Shaskyus can expect to live for more than 10 years, and Ocenebra for more than 9 years. The adult mortality rate was generally highest during the reproductive season but lowest in 1969, when the greatest reproductive effort was observed. Marked individuals of both species have been found in breeding aggregations in more than one season, suggesting iteroparity. Shell growth rates decreased and mortality rates increased as shell length increased in both species. The sex ratio of Shaskyus from 25.0 mm to 44.9 mm did not differ significantly from 1:1. The proporations of female Ocenebra longer than 14.9 mm increased as shell length increased. The determination of sex in smaller size classes was uncertain because many smaller males were probably immature and lacked penises. Sexual differences in growth and mortality rates were probably responsible for the paucity of males in the larger size classes. No evidence for sex reversal was found in repeated sex determinations of marked snails.

ISSN:0012-9658    

DOI:10.2307/1936023

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

Summer and winter distributional patterns of attached diatoms were investigated in Yaquina Bay and estuary, Oregon. Differences in species composition and diversity of diatom assemblages at selected stations from fresh water just below Elk City, Oregon, to the marine waters of lower Yaquina Bay were released to environmental gradients. A total of 16,475 diatoms from 30 samples was separated into 256 species and varieties, of which 97 were found in only one sample, and 72 were represented by a single individual. The most abundant diatoms in the August samples were Fragilaria striatual var. californica, Melosira moniliformis, M. nummuloides, Navicula mutica, and Synedra fasciculata, while in the February samples Achnanthes no.2 and no.4, Navicula diserta, N. mutica, and Nitzschia frustulum var. perpusilla were dominant. Of the most abundant taxa, Navicula no. 2, N. diserta, N. gregaria, Nitzschia frustulum var. perpusilla, Synedra fasciculata and Thalassionema nitzschioides were the most evely distributed among the stations. The mean species diversity for diatom assemblages sampled in February was slightly higher than for assemblages collected in August. In February the mean specific diversity within a genus was higher and the mean generic diversity slightly lower than in August. In general, differences in assemblages were closely related horizontally to the salinity gradient and vertically to the desiccation and insolation gradients. However, biological factors were more important in accounting for differences among assemblages in the summer than in the winter and these factors were primarily species interactions between diatoms and macro—algae.

ISSN:0012-9658    

DOI:10.2307/1936024

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

Unisexual forms of Poeciliopsis require sperm from males of gonochoristic species. Selection pressures dictate that males discriminate against unisexuals until the latter become extinct. Most Poeciliopsis unisexual—bisexual populations have a unisexual—bisexual female ratio that favors unisexuals, indicating that selection for conspecific mating by males has either not been operative, not persisted for a sufficient period of time, or been disrupted by unknown factors. In mate selection and insemination tests single males show almost absolute preference for conspecific females, but social interactions among males result in unisexual inseminations. Mate selection reduces unisexual fecundity in natural populations, but the all—female forms receive sufficient sperm to sustain themselves.

ISSN:0012-9658    

DOI:10.2307/1936025

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

All—female forms of Poeciliopsis rely on males of closely related bisexual species for sperm. The natural habitat of Poeciliopsis in Sonora Mexico, consists of a variety of small ponds connected by intermittent watercourses. Optimal areas, containing mixed female aggregates, are defended by territorial males. Social structure in natural populations very closely resembles that of laboratory experiments, wherein subordinant males show reduced mate discrimination and inseminate unisexuals. An equation relating male density to unisexual inseminations is used in a computer simulation model of a population. A stable equilibrium is inherent in unisexual—bisexual species complexes but the level of equilibrium is affected by the environment. Coexistence does not require niche separation. The simulations predict the percentage of unisexuals pregnant in natural populations and explain their distribution pattern. The strength of the mechanism is demonstrated by a natural population in which the percentage of unisexuals pregnant responded strongly to a modest change in unisexual—bisexual composition.

ISSN:0012-9658    

DOI:10.2307/1936026

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

The development of bottom depostis and the distribution of the associated fauna were studied over a 2—year period in a drowned river "estuary" of the newly created Lake Kariba. The lake bottom was initially made up of submerged soil, the chemical characteristics of which could be related to the parent rock. However, the soil was found to be rapidly changing under the influence of shoreline erosion, river sediment, and the accumulation of organic debris. These factors also affected the distribution of bottom fauna, in which chironomid larvae predominated. Faunal biomass was positively correlated with the amount of organic carbon in the profundal zone and inversely associated with the quantity of coarse sand in the littoral. Superimposed on this pattern was a response to water depth. In the littoral zone, faunal biomass decreased with increasing depth. In the profundal region, benthos was absent when the lake was thermally stratified during the hot season. The annual regime of water—level fluctuation affected shoreline processes, for example by interrupting the development of barrier beaches and coastal erosion. Since these processes were fundamental to the development of sediment patterns, changes in water level influenced the distribution of bottom fauna as well.

ISSN:0012-9658    

DOI:10.2307/1936027

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

Tenebrinonid beetles of the genus Cardiosis are confined to sand dunes and other sandby substrates of the Namib Desert in South West Africa and Angola. In the northern Namib the breadth of environments occupied by each species is considerably greater in the southern Namib. This may be due to competition resulting in ecological divergence, since there is sympatry in this genus only in the southern Namib. All species of the genus Cardiosis, including the sympatric C. fairmairei and C. hamiltonuli are diurnal. The daily activity rhythm of these species responds to ambient thermal conditions, and fluctuations in ambient conditions result in differing patterns of activity. Substrate and ambient temperatures are the important microclimatic variables influencing the actual timing of activity. This usually results in a concentration of activity in the least humid hours of the day. On cool days or when winds cool the sand surface, activity may extend through the middle hours of the day. On hot or calm days activity becomes bimodal, with activity limited to intervals when body temperatures can be maintained at levels well above 30 degrees C and substantially below lethal limits. The interval of morning activity is less pronounced than the afternoon activity period and on especially hot days morning activity may be omitted altogether. Unconfined populations of Cardiosis extend their activity over considerably greater intervals than confined populations exposed to identical macroclimatic conditions. This is due to the ability of the unconfined population to utilized a broad spectrum of environments, thus locating favorable thermal conditions as they develop in the course of a daily cycle.

ISSN:0012-9658    

DOI:10.2307/1936028

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

During the height of the breeding season there was relatively little or no social facilitation of nest—building when Africa Village Weaverbirds (Ploceus cucullatus cucullatus) kept in large outdoor aviaries in very small and in larger experimental colonies were compared. The sex ratio was kept the same (1:1) irrespective of size of colony. Social stimulation by crowding of the birds early or late in the breeding season led to the earlier onset and later cessation of nest—building, and so extended the breeding season over a longer period of time. The implications of this findings are discussed.

ISSN:0012-9658    

DOI:10.2307/1936029

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

Experimental colonies of African Village Weaverbirds (Ploceus cucullatus cucullatus) in large outdoor aviaries at Los Angeles were observed. When males were caged singly, each with several females, these males expanded their territorial holdings and had more nests accepted by the females than was the case when other males were present. When several additional males were present during the height of the breeding season, the resident males showed little or no change in their rate of nest—building, but with extra males present more fresh nests became available and more females accepted nests. When the breeding season started to decline, adding more males stimulated renewed onset of nest—building by resident males. When the number of females in each colony was increased four times, the average rate at which the males built and tore down their nests about doubled while the frequency with which the males gave the nest—advertisement display to untamed females tripled. But at the same time the males built on the average only half as many nests per female, reflecting an effective reduction in competition between males for mates when an abundance of females was present.

ISSN:0012-9658    

DOI:10.2307/1936030

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

Vegetational and environmental changes are deduced from a pollen diagram at a site located in present—day prairie grassland. Three pollen assemblage zones were delineated by analysis of the 10—m core starting with a spruce—fir assemblage which began about 12,000 years ago and was replaced by a deciduous forest assemblage about 10,500 years ago. During the period 9,000—7,500 years ago there was a progressive increase in nonarboreal pollens. At the end of this time, the prairie flora is believed to have been well established.

ISSN:0012-9658    

DOI:10.2307/1936031

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY