摘要:

Primary productivity and herbivory were studied in the Serengeti National Park, Tanzania, and Masai Mara Game Reserve, Kenya, during the annual cycle of 1974—1975, and wet—dry season transitions in 1976—1979. Basic state variables measured were aboveground plant biomass inside permanent and temporary fences, and outside fences. Productivity was calculated as the sum of positive plant biomass increments. Control productivity (cPn) was calculated from biomass dynamics inside permanent fences. Temporary fences were moved in concert with grazing by the region's abundant ungulates to estimate actual aboveground primary productivity (aPn). Primary productivity was highly stochastic with productive periods poorly synchronized even among nearby sites. Short—term productivities could be extremely high, exceeding 30 g°m—2°d—1. Grazing animals adjusted their densities in relation to grassland productivity. The average proportion of annual aPn that was consumed by herbivores was 0.66, with a minimum of 0.15 and a maximum of 0.94. Green forage was available everywhere late in the wet season in May but was available only at high rainfall sites in the northwest late in the dry season in November. By the end of the dry season, the residual plant biomass outside fences averaged only 8% of cPn. Nomadic grazers moved seasonally in response to grassland productivity. The growing season ranged from 76 d in low rainfall areas to virtually continuous in high rainfall areas. Annual cPn was linearly related to rainfall and averaged 357 g°m—2°yr—1over the year and 1.89 g°m—2°d—1during the growing season. Actual aPn was substantially greater than cPn at most sites, averaging 664 g°m—2°yr—1. Growing season aPn averaged 3.78 g°m—2°d—1. Grazing stimulated net primary productivity at most locations, with the maximum stimulation at intermediate grazing intensities. Stimulation was dependent upon soil moisture status at the time of grazing. Rain had a diminishing effect on primary productivity as the wet season progressed and plant biomass accumulated. Part of the stimulation of grassland productivity by grazing was due to maintenance of the vegetation in an immature, rapidly growing state similar to that at the beginning of the rainy season. Since grazers overrode rainfall—determined productivity patterns, aPn was more closely related to grazing intensity than to ranfall. Grazing was heavier on grasslands that were intrinsically more productive. Rate of energy flow per unit of plant biomass was much higher in grazed vegetation. Grazers ate green leaves almost exclusively during the wet season, but species composition of the diets of different grazers differed markedly. Diets of nomadic grazers were very different in the wet and dry seasons. Vegetation dried out rapidly at the onset of the dry season and dry plant tissues made up a substantial proportion of ungulate dry season diets. However, green forage commonly was more abundant in diets than in the vegetation. Grazing increased both forage quality and its rate of production. Zebras supplemented a high—bulk diet by eating the seeds of awnless grasses. The foraging patterns of different grazers were differentiated by several vegetation properties, including productivity, structure, and species composition, in a manner suggesting resource partitioning. The relationship between the stability of vegetation functional properties and community species diversity was positive in five of seven tests. Greater species diversity was associated with greater biomass stability through the seasons, greater resistance to grazing by a single species of ungulate in both the wet and dry seasons, and greater resilience after grazing. Species diversity was not associated with greater resistance to grazing by several ungulate species or to plant species extinction. Specific properties of trophic web members were identified that produced greater functional stability in more diverse communities. Fire does not appear to have important effects upon the functional properties of the grasslands except for a weak stimulation of productivity in the wet season immediately following dry season burning. Fire did have an important effect upon structural properties of the vegetation that would tend to regulate ungulate feeding. The ecology of neither the plants nor the animals in the Serengeti ecosystem can be understood in isolation; many traits of both suggest coevolution among trophic web members. The functional dynamics of the trophic web suggest that the acceleration of energy and nutrient flow rates due to intense herbivory has resulted in the development of an entire consumer food web due to additive fluxes rather than mere quasi—parasitic fluxes from plants to animals.

ISSN:0012-9615    

DOI:10.2307/1942578

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

Avian communities living in a complex environment were studied by censuses of singing males, using several levels of resolution which focussed on different properties of the community. Within each level of resolution, various community parameters in two grassland habitat types were calculated over the entire breeding season, and for census intervals within the breeding season. The community level of resolution focussed on community—wide patterns, such as total density and total biomass, in order to reveal constraints operating on all species in each habitat. An intermediate level of resolution focussed on relative properties of species such as relative abundances, species richness, and dominance concentration. At the individual species level of resolution, densities and identities of species were the properties upon which the analysis focussed. These techniques allowed characterization of responses of the avian communities to complex changes in the system in which they were embedded. Long—term averages in monthly rainfall on the sites used for this study indicated a bimodal distribution of rainfall. The first peak occurred in December and the second in July and August. During the two years of study, summer rainfall was similar, but winter rainfall in 1983 was °2—3 times as great as in 1982. Avian communities in habitats with mesquite trees (mesquite savannah) responded differently than communities in grassland habitats, and these responses could be seen at different levels of resolution. Peak densities and biomass occurred during May—June in mesquite savannah habitats, while grassland communities achieved highest densities and biomass during July and August. Average density per individual showed little seasonal pattern, but was lower for mesquite savannah communities than for grassland communities. Species richness increased in mesquite savannah habitats in 1983, but decreased in grassland habitats that year. Patterns of evenness and dominance concentration suggested that in 1983, rare species of large body size were added to the community in mesquite savannah, while in grassland habitats, species already present during 1982 increased in density. A community ordination revealed that mesquite savannah habitats were dominated numerically by small—bodied insectivorous species during the early part of the breeding season but became similar to grassland communities in July and August. Grassland communities were dominated by large—bodied emberizine finches. These characteristics of the avian communities corresponded closely to the phenology of important primary producers. Mesquite produces leaves high in nutritional value and produces flowers during April—June; thereafter older leaves decline in nutrient content and flowers disappear. These changes in mesquite productivity undoubtedly produce corresponding changes in insect communities exploitable by small—bodied insectivores. Perennial grasses concentrate aboveground productivity to July—August and insect communities associated with grasses increased in biomass during this period, thus increasing resources available for breeding. These results suggest that avian community responses to complex changes in their environments can be identified and related to outside inputs into the environment, such as rainfall. Identification of community responses is facilitated by emphasis on several observational scales. Communities appear individualistic to observers because (1) adaptational units of species are probably much larger than study areas; (2) species ability to adapt to local conditions might be diluted by gene flow among populations; and (3) if most evolutionary change occurs during speciation, then species adaptations will reflect a relatively narrow range of selection pressures that operated briefly on the species during the past.

ISSN:0012-9615    

DOI:10.2307/1942579

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

I examined the spatial and temporal partitioning of variation in settlement and early survival of an intertidal barnacle, Tesseropora rosea. Monthly, for 3 yr, I monitored the density of newly settled barnacles (<30 d old) and new recruits (30—60 d old) on each of three spatial scales referred to as: within sites (i.e., replicate areas within 3 m2), among sites within shores (i.e., six sites arrayed horizontally in the mid—intertidal region, with 20—50 m between adjacent sites), and among shores (seven headlands along the New South Wales coast with 11—555 km between them). Sites and shores were initially chosen to be as similar as possible with respect to height in the intertidal region, density of adult Tesseropora, and wave exposure. All observed variation was, therefore, within—habitat variation and not readily related to any of the factors already known to affect the density of settlers or their subsequent survival, such as exposure or intertidal level. There was significant variation in the densities of settlers and recruits on all spatial scales at any given time. Variation among months and among years was also great at any given spatial scale. There were no simple, consistent trends, however, in the variation within any spatial or temporal scale, i.e., variation at any spatial scale interacted strongly with that for any temporal scale. The shores ° years interaction (the interaction of large—scale spatial with large—scale temporal variation) produced dominant year—classes on some shores in some years and on other shores in other years. Proportional survival of new settlers also varied significantly on all spatial and temporal scales monitored. Thus, much residual patchiness exists in the spatial and temporal distributions of barnacles<60 d old. Previous authors investigating the structure and maintenance of rocky intertidal communities have played down the fact that settlement and recruitment of the component species vary greatly on different scales. Yet, such variation should be explicitly incorporated in the development of theory for marine populations and communities.

ISSN:0012-9615    

DOI:10.2307/1942580

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

I examined the effects of phenolic compounds in marine brown algae on the feeding behavior of the intertidal gastropod Tegula funebralis and other marine herbivores. The effects of algal nitrogen content and thallus toughness were also investigated. T. funebralis showed consistent feeding preferences in the laboratory and in the field in pairwise tests of 13 species of marine brown algae (Phaeophyta) from central California. Relative levels of phenolic compounds, which are known to deter feeding by herbivorous snails, differed greatly between preferred and nonpreferred species of algae. The total phenolic content of the six most preferred species of algae averaged 0.83% of dry mass; the phenolic content of the seven least preferred species averaged 4.53%. Differences in relative tanning ability were similarly large. Tannic acid, a terrestrial plant tanin, when experimentally added to one kelp species, deterred feeding by T. funebralis. These results and analyses of studies of additional species of brown algae also showed consistent differences in the phenolic content between species in the order Laminariales (typically low) and those in the order Fucales (uniformly high). Feeding preferences of other gastropod, echinoid, and crustacean algivores from the northeast Pacific Ocean are strikingly similar to those of T. funebralis; this supports the contention that high levels of phenolic compounds in brown algae are a generally effective feeding deterrent against many kinds of marine invertebrate herbivores. Organic nitrogen content of the algae was positively correlated with the feeding preferences of T. funebralis. However, this was most likely an indirect effect of the significant negative correlation between algal nitrogen and phenolic contents. Thallus toughness of the algae did not seem important in determining overall feeding preferences of Tegula, but may have affected choices between algae that were low in phenolics. I suggest that feeding preferences of Tegula funebralis for brown algae are primarily due to avoidance of algal defenses; positive aspects of food quality (such as attractants or nutritional content) probably play a lesser role in determining preference.

ISSN:0012-9615    

DOI:10.2307/1942581

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY

摘要:

The carbon, phosphorus, and water cycles of a subtropical floodplain forest, and related ecosystem characteristics, were studied. Located at 750 m elevation in Puerto Rico (latitude 18°N) the forest had 27 tree species, 3059 stems/ha, a basal area of 42.4 m2/ha, maximum height of 17 m, and a leaf area index of 3.3. Palm (Prestoea montana) dominated the forest, and, with two other species, accounted for 68% of the dominance. Throughfall, stemflow, interception, runoff, transpiration, and evapotranspiration accounted for 81.6, 9.8, 8.6, 77.7, 13.7, and 22.3% of annual rainfall (3725 mm) respectively. The stand carbon storage was 28.77 kg/m2, distributed as follows: aboveground 35.4%, vegetation 44.2%, soil to 1 m depth 55%, palms 10.9%. Vegetation biomass was partitioned as follows: leaves 9.8% (75% are palm leaves), wood 68%, and roots 21.8%. Net aboveground primary carbon productivity (NPP) was 876 g°m—2°yr—1. Average litterfall was 2.4 g°m—2°d—1(palm leaves 32%, other leaves 39%, and wood 10%). Half—lives of decaying material were 188, 306, 462, and 576 d, respectively, for palm leaves still attached to the parent tree, dicotyledonous leaves, palm leaves on the ground, and palm trunks. Total organic carbon concentrations in stream water increased with increasing stream discharge (from 2 g/m3to 30 g/m3). Watershed export of carbon was 50 g°m—2°yr—1(including 12 g°m—2°yr—1in the form of leaf litter). Mean P concentration in palm leaves (1.18 mg/g) was twice that in dicotyledonous leaves (0.64 mg/g). Compared to a rainfall phosphorus input to the watershed of 63 mg°m—2°yr—1, leaching from the canopy was high (167 mg°m—2°yr—1), as was the loss of P from the watershed (611 mg°m—2°yr—1). Phosphorus—conserving mechanisms included a high rate of retranslocation in palms (504 mg°m—2°yr—1). In spite of these mechanisms, there was a net P loss from the watershed that ranged from 136 to 544 mg°m—2°yr—1. Periodic flooding, poor soil aeration, intensive year—round rainfall, and low atmospheric saturation vapor pressure deficits are believed to be the main driving forces of the floodplain forest, which exhibits many characteristics typical of lowland rain forests and floodplain wetlands. Rates of NPP, litterfall, and biomass turnover (residence time of 14—17 yr) are faster than expected for the climatic conditions, whereas rates of wood production and storage of organic matter in the vegetation and soil profile are lower than expected for the climate.

ISSN:0012-9615    

DOI:10.2307/1942582

出版商:Ecological Society of America    

年代:1985

数据来源: WILEY