Ecology

Stark, N.

doi: 10.2307/1935706pmid: N/A

Wilbur, Henry M.

doi: 10.2307/1935707pmid: N/A

Populations of six species of amphibians were manipulated in field enclosures to study the biological tractability of current concepts of the organization of natural communities. Experimental communities with a known composition of mature eggs were introduced into screen enclosures in a pond to assay the importance of competition and predation to the ecology of amphibian larvae in temporary ponds. The competitive ability of each population was measured by its survivorship, mean length of its larval period, and mean weight at metamorphosis. Three simultaneous experiments (requiring 70 enclosures and 137 populations) were replicated in a randomized complete—block design for variance analysis. The assumptions of the classical Lotka—Volterra model of competition were tested by raising Ambystoma laterale, Ambystoma tremblayi, and Ambystoma maculatum in all combinations of three initial densities (0, 32, and 64). All three measures of competitive ability were affected by competition with other species. Higher—order interactions decreased the variance of the outcomes of the experiments as species were added to the communities. The statistical effects of these higher—order interactions between the densities of competing species often exceeded the simple effects of competition. The increase in community stability with the addition of species to the community is not predicted by the classical models of community ecology. The second experiment tested the effects of adjacent trophic levels on the structure of the three—species community. Eggs of Ambystoma tigrinum, a predator, and Rana sylvatica, an alternate prey of Ambystoma tigrinum, were added singly and together into systems with 16 eggs of species in the Maculatum species—group. Ambystoma tigrinum was a predator if it acquired an initial size advantage by preying on Rana sylvatica tadpoles; otherwise it was principally a competitor. Rana sylvatica adversely affected the Maculatum group by competing with invertebrate prey for periphyton and photoplankton. The three species in the Maculatum group had nearly the same response to the addition of both A. tigrinum and R. sylvatica. Ambystoma texanum, which occurs sporadically in southn Michigan at the northern limit of its range but not on the study area, was introduced as a test for community saturation. Ambystoma texanum was successfully raised alone. When mixed with the Maculatum group, Ambystoma texanum had a low survivorship, a small body size, and a long larval period. The native species were affected equally by the introduction of Ambystoma texanum, demonstrating the complexity of the food web and the ecological pliability of salamander larvae. The uncertainty of the temporary pond environment precludes extreme ecological specialization among these species of salamanders. Coexistence is a consequence of the relative advantages of the species in different years and the long adult life spans. The complexity of the food web and "predator switching" are probably important elements of the density dependent interactions that contribute to the stability of communities within seasons.

Miller, Philip C.

doi: 10.2307/1935708pmid: N/A

A model of primary production and transpiration of forest canopies was developed from the energy—budget equation of individual leaves to clarify some of the physical processes affecting primary production. The model calculates hourly vertical profiles of temperature, transpiration, respiration, and gross and net photosynthesis of both sunlit and shaded leaves; and calculates appropriately weighted totals for the hourly water loss and gross and net photosynthesis for each level in the canopy. The model includes variations in leaf resistances caused by changes in absorbed solar radiation and changes in leaf water deficit and also takes into account the interdependence of the infrared profiles and the leaf—temperature profiles within the canopy. The model was tested with data collected on red mangrove (Rhizophora mangle Roxb.) forests. Canopy structure and the daily courses of solar and infrared radiation, air temperature, humidity, and wind, and ground temperatures were measured and used as input data for the model. The model produced realistic leaf temperatures, leaf resistances, transpiration rates, and primary production rates and was used to indicate the relative importance of environmental variables in influencing leaf temperature transpiration, and primary production. Maximum leaf temperatures occurred at the top of the canopy in June and the overcast day in January, but at the bottom of the canopy on the clear day in January. The model presently calculates water uptake as a constant rate to leaves and does not include a redistribution of water within the plant. The calculated transpiration was about 20% of the total water loss of the stand, the remaining loss coming directly from the moist substrate under the canopy. The inclusion in the model of stomatal movements reduced daily transpiration under conditions of mild water stress by 0.04 cm day‐1. Total evapotranspiration was 0.67 cm day‐1, and transpirational water loss was 0.12 cm day‐1. Levels in the middle of the canopy had the highest transpiration rate because of their high leaf area, but leaves at the top had the highest transpiration rate per unit leaf area. Leaf water deficits great enough to initiate stomatal closure occurred early in the morning at the top of the canopy in June on clear days, later in the morning at the top of the canopy in June on cloudy days, early in the afternoon at the top of the canopy in January on clear days, and not at all at the top of the canopy in January on cloudy days.Leaf water deficits did not develop within the canopy in either June or January. Net photosynthesis calculated with the model was 5.6 g organic matter m‐2 day ‐1 for sunny days and 3.5 for cloudy days in June. Gross photosynthesis per unit leaf area was greater at the top of the canopy than at the bottom, but the middle levels of the canopy had the greatest production. The efficiency of water utilization increased from top to bottom of the canopy. A weighted monthly estimate for production was 3.4 for June and 2.2 for January, giving an average annual net production rate of 2.8 g organic matter m‐2 day‐1. The model predicts that the maximum photosynthesis for mangrove stands will occur with a leaf—area index of about 2.5 if no acclimation to shade within the canopy occurs. A leaf area greater than about 2.5 may decrease production. The environmental variables with the greatest influence on primary production were air temperature and humidity. Production was decreased by increasing air temperature and increasing humidity. Increasing total solar radiation increased production up to a point, then decreased it. Increasing the diffuse fraction of the total solar radiation increased production. Increasing infrared radiation decreased production. Production and transpiration increased with increasing leaf—area index with steeply inclined leaves and leaf absorptance. Production was decreased by increasing leaf—area index with nearly horizontal leaves, leaf width, and the ap value. The canopy distribution of red mangrove appears to be nearly optimum to maximize the efficiency of water utilization rather than production. This indicates that the canopy is adapted to maximize production under conditions of saturated water supply.

Craig, Alan J.

doi: 10.2307/1935709pmid: N/A

A pollen diagram is presented from Lake of the Clouds, Lake County, Minnesota, in the Boundary Waters Canoe Area. Of the 5.6 m of sediment analyzed, almost 5 m are laminated. The 9,349 laminations counted, composed of alternating dark organic and light layers, are annual varves which provide a time scale for study of vegetational history and calculation of pollen influx. The pollen concentration in each sample was determined by a technique involving the addition of microspheres. The pollen influx was calculated from the pollen concentration and varve count. Pollen concentration was very low in the herb pollen zone of late—glacial time. During postglacial time the pollen influx was generally 2—4 x 104 grains cm‐2 years‐1. The pollen diagram has been divided into six zones. Zone 1 records tundra—like vegetations; most of the pollen rain, which was sparse, was probably blown in from a great distance. Zone 2 records spruce forest, with some pine and thermophilous deciduous trees, succeeded in zone 3 about 9,200 years ago by forests dominated by jack or red pine or both. With the start of zone 4, dated about 8,300 years ago, alder migrated into the area, followed about 7,000 years ago by white pine, concurrently with the maximum eastward extension of prairie in the state. In the period represented by zone 5, commencing about 6,500 years ago, white pine was very important, spruce was at a minimum, and the climate was probably relatively dry. Environmental changes brought about increases of spruce and cedar and a decline of pine in zone 6, commencing about 3,000 years ago. The diagram is truncated at 200 years or more before the present.

Cameron, Guy N.

doi: 10.2307/1935710pmid: N/A

This study analyzes tropic relationships of the insect components of two intertidal salt march communities dominated by Salicornia pacifica and Spartina foliosa, respectively. Seasonality of that component is determined and influence of physical microenvironmental factors on trophic diversity is assessed. Adult insect populations were monitored weekly, trophic diversity was computed, and relationships with primary production and litter accumulation were quantified. Temporal diversity trends were similar in both communities although the amplitude was slightly greater in Salicornia. In both communities, herbivore diversity was highest during the spring months while saprovore diversity was highest during midwinter. Predator diversity responded to both herbivore and saprovore diversity, although it was more closely tied to herbivore fluctuations in Salicornia. Standing crop biomass was maximum during October and litter accumulation was highest during Janurary. Two classes of adult insects occurred: peristent species, representing a low percentage of the total species compliment, were present as adults throuhout the year in both marsh communities; seasonal species, on the other hand, were present as adults only during the growing season. Seasonal succession in species of herbivores and saprovores reflected productional and transformational changes in plant matter; predators responded likewise, but more impressive was the numerical response to prey populations by particular predators. Correlations were high between each trophic group and its respective resource. Physical microenvironmental factors, especially temperature and vapor pressure deficit, seemed to be important in cuing larval development, but did not exert a dramtic effect on adult diversity trends. Several strategies of habitat utilization are considered. It is hypothesized that the persistent and seasonal species have evolved as specialists to avoid competitive interactions and maximize resource utilization. During annual expansion of resource states, the salt marsh insect component undergoes speices packing wherein additional species enter the system temporarily to utilize the expanded resource base.

Clayton, James L.

doi: 10.2307/1935711pmid: N/A

Windblown salt aerosols are a significant input to the mineral cycle along coastal areas. Cationic ratios of aerosol salts differ markedly from the same ratios in sea water. Resulting salt inputs to inland ecosystems reflect initial ion separation during sea—water bubble bursting and subsequent fallout of particulate salts until a base—level aerosol concentration is achieved. Inputs of aerosols to Baccharis brushland ecosystems are concentrated in plants and soil. Cations are trapped on leaf surfaces where they are available for foliar absorption and in some cases, translocation. Tracer studies with 22Na indicated that the greatest accumulation of retained 22Na was in the roots of Baccharis plants. Of the total applied dose of 22Na, 12% was either exuded or leached from roots of hydrophonically grown Baccharis over a 25—day period. This phenomenon may be considered a potential pathway for inputs of large amount of Na to soil near shorelines. The surface ionic content of soils reflects nutrient release from litter fall. At 1— to 2—m depths ionic losses from leaching occur in approximately the same ratios as inputs of salt spray. This suggests a steady state with regard to cycling of Na, K, Mg, and Ca.

Smith, Christopher C.; Follmer, David

doi: 10.2307/1935712pmid: N/A

Gray and fox squirrels show similar preferences for various types of natural foods. Their preferences are based on a combination of two factors; the speed with which they can ingest food energy and the digestibility of the food eaten. The two species of squirrels have essentially the same ability to digest their natural foods. However, five species of nuts differ in their digestibility, ranging in percentage of energy assimilated from about 78% for white oak to 95% in shagbark hickory. The squirrels' efficiency in digesting the kernels of different species of nuts is correlated with the lipid content of the kernels. The niche differences between gray and fox squirrels are not based on food preference or feeding efficiency. Instead, they are probably related to differences in foraging behavior and predator escape behavior which adapt fox squirrels to open forests and forest edges and gray squirrels to dense forests. The annual activity pattern of gray and fox squirrels make the hard—shelled nuts of hickory and walnut the most efficient food in fall and spring and acorns the most efficient food in winter. This change in the relative efficiency of foods results in the squirrels acting as dispersing agents for the seeds of both oaks and hickories in mixed stands of trees.

Helms, John A.

doi: 10.2307/1935713pmid: N/A

Carbon dioxide exchange by intact foliage was monitored in the middle—crown region of 30—m tall ponderosa nine trees growing in their natural environment at 1,300 m in the Sierra Nevada of California. Characteristic diurnal patterns of net photosynthesis as a function of temperature are presented for summer conditions of low, medium, and high environmental stress. Changes in relationship between rates of net photosynthesis and factors of the environment were determined by examining the data within different time periods on days of markedly different stress conditions. Highest correlations were obtained under conditions of low stress when 80—90% of the variation in rates of net photosynthesis could be accounted for by changes in temperature and radiation alone. As general stress conditions increased, ability to predict rates of net photosynthesis decreased, with saturation vapor pressure deficit becoming the dominant factor accounting for variation. General prediction equations using data pooled from a wide range of summer conditions accounted for, at best, 65% of the variation in net photosynthesis.

Allen, L. H.; Lemon, Edgar; Müller, Ludwig

doi: 10.2307/1935714pmid: N/A

Wind speed and turbulence were measured at five heights simultaneously in a Costa Rican forest with non—stalling, heated—thermocouple anemometers. A persistent daytime stable thermal stratification of the air beneath the top canopy decreased small—scale turbulence, which may increase boundary—layer resistance of leaf—to—air transport of water vapor and CO2. Three CO2 source layers (ground level, trunk space, and above the top canopy) and two CO2 sink layers (top canopy and bottom canopy) were found in this forest. Low nocturnal wind speed allowed CO2 from respiration to accumulate at night.

Heatwole, Harold; Levins, Richard

doi: 10.2307/1935715pmid: N/A

Of 59 pieces of flotsam picked up at sea 0.5—16 km from its nearest probable origin, 25% contained at least one live terrestrial animal and 6% had three or more species; one green plant contained 12 species. In many cases a number of conspecific individuals occurred in the same drift item. The animals represented included 19 families of insects, as well as psuedoscorpions, snails, spiders, mites, millipedes, isopods, and worms. Some families were represented by several (up to five) species. Eight drift items recovered approximately 120 km from the nearest land lacked terrestrial animals. A piece of bamboo which washed up on the beach of Cayo Ahogado contained a dipteran and a beetle. Flotsam transport is clearly a significant biogeographic factor among the islands of the Puerto Rican Bank.