摘要:

Every year the Kafue River, one of the main tributaries of the Zambezi River in Zambia, floods a broad level area, the Kafue Flats, to a depth of up to 5 m for several months. The flats are 235 km long and up to 40 km wide. The life of the flats is conditioned primarily by the alternating rainy and dry seasons and by the floods. About 80 cm of rain falls from November to April. The Kafue rises slowly during the rains, is highest in May, and falls during the latter part of the dry season. Vegetation is composed primarily of grasses, especially Oryza barthii. Because of the abundant water the primary productivity of the flats is much greater than that of the surrounding woodlands. Secondary productivity also is high. Every year there is an alternation of aquatic and terrestrial faunas. During the floods fish move onto the flats from the Kafue, and most spawning takes place there. Terrestrial species are driven off, but as the floods recede they reoccupy the flood plain and use what is by far the best grazing in the region. Large mammals find shelter in tall stands of grass, small mammals in the thick mat of vegetation that covers much of the ground or in the deeply cracked soil. There is a gradient in use of the flood plain: some species (hippopotamus, otter) always stay near the river at low water, others (lechwe, zebra, wildebeest) go for varying distances onto it, and more than half of the mammals (kudu, squirrel, vervet, aardvark) go onto it little if at all. Failure to use the flood plain seems to be due to absence of suitable habitats or food, rather than exclusion by the floods. The most abundant large mammal on the flood plain is the lechwe, the only terrestrial ungulate that sometimes feeds in the floodwaters. Several shrews and mice, especially Mastomys natalensis, are common on the food plain and breed there during the rains. During the floods they leave the flood plain or take refuge on natural levees along the Kafue. Large populations of water birds are common. Crocodile and monitor live near the water's edge and move in and out with the floods like the hippopotamus. Some snakes are common on the flats, but turtles and frogs are not. Although ants and termites are abundant in the surrounding region, they are largely excluded from the flats by the floods.

ISSN:0012-9615    

DOI:10.2307/1942365

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

To elucidate the causal factors responsible for diversity gradients in avian communities, avian populations were studied for 12 months in seven lowland tropical areas in the Republic of Panama and compared with populations in structurally similar habitats in Illinois. Resident (breeding) species made up 38—49% of the species on temperate areas but only 20—36% of the species in similar tropical habitats. There were significantly more species of irregular occurrence in mature tropical habitats. Tropical grassland avifaunas were no higher in diversity (number of species and information measures of diversity) than those of temperate grassland areas, but shrub and forest habitats in the tropics had higher diversity as measured by either diversity measure. The number of species was proportionately higher than the information measure because of the relatively small effect of the many rare species on the information measure. Determination of the biomass and energy relations for the bird populations showed that higher total population size in tropical areas is correlated with smaller bird size and reduced individual energy requirements. When only breeding seasons are compared, the reduced energy requirements are a result of the higher temperatures in tropical areas. Twenty—five to fifty per cent of the increased number of tropical breeding species when compared to similar temperate habitats results from the addition of a new food source, i.e., fruits. Additional species are primarily insectivores. The increase in insectivores seems to be due to additional subdivision of the resources and exploitation of a new kind of insect food, especially by species that depend on relatively large insects. Since energy requirements of tropical and temperate avifaunas are about the same, increased productivity is not related causally to the increased tropical diversity. The stratal distribution of species in several tropical (lowland and montane) and temperate avifaunas indicates that avian communities may subdivide the vegetation profile similarly throughout the world despite sharp differences in the number of species in the various areas. Bark species are about equally numerous in Panama and Illinois forest, but ground, low, medium, and high strata contain increased numbers of species in the tropics. The shift away from classical defended territories in mature tropical habitats seems to be correlated with patchy distribution of food resources in frugivores and ant—following species and reduced food abundances in other species. As a result of this food distribution, the frequency of flocking increased in shrub and forest habitats. In grassland habitats, which experience greater seasonal changes in precipitation, the spacing systems of birds are more similar to those in all temperate habitats studied. Degree of territorial defense against conspecifics and amount of interspecific flocking is inversely related to the distribution of food resources; more patchy or unpredictable food distributions, or both, result in fewer species defending exclusive territories. The major factor related to the increased diversity of tropical avifaunas seems to be the relative stability of food resources resulting from environments that are more or less "equitable" throughout the year. Both within— and between—habitat increases in diversity of avian communities are discussed. The stability—time hypothesis is discussed in light of available data on organization of avian communities.

ISSN:0012-9615    

DOI:10.2307/1942366

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

The stream limpet Ferrissia rivularis, which occurs extensively in North America and is ubiquitous in streams of upper New York State, was studied from two distinct populations, one located in the Canandaigua Outlet at Alloway, N. Y. (AL) and the other in Black Creek at Cleveland, N. Y. (BC). These environments differ substantially, that at AL being the more eutrophic. At BC there is one generation per year, but at AL there are two generations per year and more rapid growth rates. Thirteen months is the adult life span for BC limpets, whereas at AL 11 and 3 months are normal for spring and summer adults respectively. At BC 8.4 eggs per limpet are laid, and 34.8 eggs per limpet are laid by spring adults at AL. Biomass and egg production are assessed as total carbon (equivalent to calorific measures) and expressed as biomass—turnover rates (growth and egg production). Egg—production rates can be double those for growth at AL; at BC egg—production rates are usually less than those for growth. This is partly a consequence of greater primary productivity at AL, but may also reflect implied genetic differences. Limpets at BC build up carbon during late summer and early fall; an increase in the C:N ratio (to 4.8) indicates an accumulation of storage compounds. During the winter (lower temperature, solar input, and primary productivity) the C:N ratio decreases, and during spring breeding there is a more marked loss of carbon associated with egg laying. In field—acclimated BC animals, respiration is lowest during the winter; during the spring, respiratory rate is closely correlated with temperature and egg—laying intensity. Oxygen—consumption measurements at 10° and 20°C for all seasons reveal that these limpets show reverse acclimation at low temperature (qualitative and quantitative downward adjustment of metabolism). This acclimation pattern is accompanied by a decrease in Q10(3.3 to 2.1) which can act to dampen the effects of winter temperature fluctuations. Energy balance (respiration, growth, egg production) is expressed as rates of carbon turnover for the population at BC. Respiration rates are greatest during the summer and spring and reflect increased assimilation due to growth or reproduction or both. The total carbon turnover per year is approximately 1.25 g/m2. Data on energy balance emphasize the adaptive importance of reverse acclimation at times of low energy input.

ISSN:0012-9615    

DOI:10.2307/1942367

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY

摘要:

In Newfoundland many sites occupied by Picea mariana forests and Kalmia angustifolia heathland formerly supported an Abies balsamea forest. Stands of each cover type were studied to determine whether these vegetation changes affect the fertility of the sites. They were selected on the basis of similarity of soil conditions; all occurred on well—drained, sandy glaciofluvial soils and had developed after a forest fire about 65 years ago. In the Kalmia heath the raw humus horizon weighed 293 tons/ha and contained 3,070 kg N, 436 kg Ca, 118 kg P, and 99 kg K per hectare. In contrast, the raw humus of the Picea forest weighed 87 tons/ha and that of the Abies forest only 65 tons/ha; nutrient contents were proportionally lower with N showing the largest, and K the smallest, differences between cover types. The annual return of organic matter and nutrients is highest in the Abies forest and lowest in the Kalmia heath. Litter fall is the single most important source in the forests, but annual root mortality contributes most in the Kalmia heath. Additions from the moss layer amounted to about one—tenth of the annual return in the forests. The rate of organic—matter decomposition decreases from Abies to Picea to Kalmia cover type, with accumulations in the raw humus representing 14, 21, and 78 times the annual supply of the present stands, respectively. Mineralization of N and P show the same trend, but Ca and K mineralization are slower in the Kalmia heath only. A comparison of the nutrients in the Abies and Kalmia cover types showed that actual losses of K, and possibly Ca, had occurred from the Kalmia ecosystem during the 65—year period since the fire, and that the quantity of N in the Kalmia ecosystem was about double that in the Abies cover type. A net input of more than 23 kg/ha per year is required to account for this increase. No reliable comparison could be made between the Picea and Abies ecosystems, but the data available do not indicate any obvious fertility changes beyond a greater immobilization of nutrients in the raw humus of the Picea forest.

ISSN:0012-9615    

DOI:10.2307/1942368

出版商:Ecological Society of America    

年代:1971

数据来源: WILEY