Department of Biological Sciences

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On fitting truncated lognormal distribution to species-abundance data using maximum likelihood estimation
Slocomb, J.; Stauffer, B.; Dickson, K. L. (Ecological Society of America, 1977)
The truncated lognormal distribution can be used to graduate certain species-abundance data, provided that estimates of the location and scale parameters are obtained. A computer program has been written which groups the data on a log2 scale and numerically solves the maximum likelihood equations for this type of distribution. Results show that the estimates obtained by this method compared well with those of Hald and Cohen. Examples are presented using the diatom data of Hohn and Hellerman, and it is shown that a better fit is obtained by using the entire data set instead of selectively disregarding the most abundant tail intervals. Other published techniques for this type of analysis are also discussed.
Abundance and Size Distribution of Particulate Matter Fractions Near a Caribbean Bank Barrier Reef
Simmons, George M. Jr. (Inter-Research, 1979)
Recent research indicates that corals have the ability to occupy several trophic levels and that particulate matter (PM) appears to play a major role in meeting their daily energy requirements. A study was undertaken to determine the amount and size distribution of PM near a bank barrier reef off the West Indies Laboratory on St. Croix, U.S.V.I. Size distributions were determined by filtration through selected sieves and filters ranging between 250 and 0.45 μm. Results showed that PM was greater in back-reef than fore-reef areas and greater during day than night hours. The PM collected on the smallest filters (0.45μm) remained constant during both sampling periods and could potentially supply 60 % of the daily energy requirements for corals. Inclusion of the PM on the next size filter (8.0 μm) would more than account for their daily energy requirements. The mean PM abundance found in this study (1.1 mg 1^-1) is the same as that reported at another Caribbean laboratory in the West Indies.
Water relations of the eggs of anolis-auratus and anolis-limifrons
Andrews, Robin M.; Sexton, Owen J. (Ecological Society of America, 1981)
Eggs of Anolis auratus and Anolis limifrons were incubated on soil substrates differing in matric potential of soil water. Eggs of both species hatched after a mean incubation time of 44 d on soils with matric potentials of -1.48 MPa or greater. On soils with water potentials of -2.13 and -2.74 MPa, eggs gained in mass initially but eventually desiccated. The rate of water storage was greater for A. auratus eggs than for A. limifrons eggs on all treatments. Rates of water loss by transpiration from the surface of eggs exposed to the atmosphere was a linear function of egg mass for both species, with A. auratus eggs losing less water per unit time than A. limifrons eggs. Differences in water flux were correlated with eggshell morphology. Eggshells of A. auratus were thicker, had a greater density of fibrils, and had a thicker matrix of calcium carbonate than eggshells of A. limifrons. Thus, the eggs of A. auratus seem to be adapted to the dryness of their grassland habitat and the eggs of A. limifrons seem to be adapted to the more equitable and humid conditions of their rain forest habitat.
Diurnal and seasonal water relations of the desert phreatophyte prosopis-glandulosa (honey mesquite) in the sonoran desert of california
Nilsen, Erik T.; Sharifi, M. Rasoul; Rundel, Philip W.; Jarrell, Wesley M.; Virginia, Ross A. (Ecological Society of America, 1983)
Diurnal and Seasonal water relations were monitored in a population of Prosopis glandulosa var. torreyana in the Sonoran Desert of southern California. Prosopis glandulosa at this research site acquired its water from a ground water source 4-6 m deep. Measurements of diurnal and seasonal cycles of aboveground environmental conditions, soil moisture, and soil water potential (to 6 m depth) were taken to ascertain environmental water availability and water stress. Leaf water potential, leaf conductance, leaf transpiration, relative saturation deficit of leaves, osmotic potential, and turgor potential were measured to evaluate plant adaptations to environmental water stress. Soil water potential was low (-4.0 to -5.0 MPa) in surface soil in relation to deep soil (-0.2) MPa). This difference was due to high surface soil salinity and low surface water content. the climatic conditions at the research site produced extreme water stress conditions in summer months when temperatures reached 50-C, vapor pressure deficit (VPD) reached 8 kPa, and surface soil water potential was below -4.5 MPa. Although considerable plant water stress developed in these trees (midday leaf water potential -4.8 MPa), osmotic adjustment occurred and turgor was maintained on a diurnal and seasonal cycle. Prosopis glandulosa has adapted to avoid water stress by utilizing deep ground water, but this phreatophyte has also evolved physiological adaptations, such as osmotic adjustment and seasonally changing stomatal sensitivity to VPD, which result in greater tolerance of water stress.
Patterns of coexistence in synaptomys cooperi and microtus pennsylvanicus
Linzey, A. V. (Ecological Society of America, 1984)
The microtine rodents Synaptomys cooperi and Microtus pennsylvanicus are sympatric, have a similar life styles, and occur in the same general habitats. The object of this study was to determine the importance of competition in a Microtus-Synaptomys system in southwestern Virginia as a test of the general hypothesis that herbivorous rodents compete directly for space and indirectly for food. The following patterns were observed: (1) Undisturbed populations of Microtus and Synaptomys exhibited microhabitat partitioning when coexisting in heterogeneous habitats that were marginal for Microtus. (2) In the presence of Microtus, Synaptomys microhabitats were characterized by significantly higher densities of deciduous trees and shrubs. (3) Disappearance of Microtus from some habitats during a population decline was accompanied by a shift of Synaptomys into areas formerly occupied by Microtus. Recolonization by Microtus resulted in a return to the previous pattern of microhabitat segregation. (4) Removal of Microtus from an experimental grid was followed by immediate colonization by Synaptomys. (5) One year following removal of Synaptomys from an experimental grid, most of the former Synaptomys habitat remained empty even though Microtus numbers were increasing. (6) Fecal analysis indicated that Synaptomys diet consisted largely of vegetation that is low digestible nutrients (Andropogon in summer, moss in winter). Although Microtus ate Andropogon, this species included more dicotyledons and other monocotyledons in its diet. (7) When Synaptomys was living on a grid from which Microtus had been removed, similarity to food habits of Microtus previously living on the removal grid was much greater than on a grid where the two species coexisted in separate microhabitats (72 vs. 37%). These results suggest that in the southern Appalachians, Synaptomys is excluded from preferred habitats by Microtus and, as a result, lives where cover is sparse and food is low in nutritional value. Although these species do compete, this competition is relaxed when Microtus populations decline. When given access to preferred habitats the food habits of Synaptomys change and resemble those of Microtus, suggesting that the ultimate limiting resource is food. The combination of wider habitat tolerances by Synaptomys and temporal variation in intensity of competition allows coexistence of these species on a regional basis.
Comparative water relations of phreatophytes in the sonoran desert of california
Nilsen, Erik T.; Sharifi, M. Rasoul; Rundel, Philip W. (Ecological Society of America, 1984)
The seasonal and diurnal water relations were compared among six desert phreatophytes, two evergreen shrubs, and one deciduous shrub. All species were located in one wash woodland in the Sonoran Desert of southern California. There are several mechanisms by which these Phaenicia have adapted to the desert environment. One group of winter-deciduous phreatophytes (Olneya tesota, Prosopis glandulosa, and Acacia greggii) experienced summer midday leaf water potentials below -4.0 MPa. These phreatophytes had a series of physiological mechanisms for tolerating summer water stress, including seasonal and diurnal osmotic adjustment and the maintenance of high leaf conductance at low leaf water potential. Osmotic adjustment of these three phreatophytes was similar to or greater than that of two evergreen species (Larrea tridentata and Simmondsia chinensis). Dalea spinosa, a stem-photosynthetic phreatophyte, avoided water stress by maintaining a very small leaf area. The summer-deciduous phreatophytes (Hyptis emoryi, and Chilopsis linearis) demonstrated mechanisms of drought avoidance such as change in leaf biomass and low summer leaf conductance. Little osmotic adjustment occurred in the summer-deciduous phreatophytes. The phreatophytic species studied in this investigation have evolved adaptations to water stress that are similar to those of deciduous and evergreen shrubs of the Sonoran Desert. Desert phreatophytes are a complex group of species with varied adaptive mechanisms to tolerate or avoid drought and should not be considered simply as a group of species that avoid desert water stress by utilizing deep ground water unavailable to other desert species of drought tolerance and avoidance.
Plant-soil processes in eriophorum vaginatum tussock tundra in alaska: a systems modeling approach
Miller, P. C.; Miller, P. M.; Blake-Jacobson, M.; Chapin, F. S.; Everett, K. R.; Hilbert, D. W.; Kummerow, J.; Linkins, A. E.; Marion, G. M.; Oechel, W. C.; Roberts, S. W.; Stuart, L. (Ecological Society of America, 1984)
The Arctic Tundra Simulator (ARTUS) is a computer-based simulation model of Eriophorum vaginatum tussock tundra ecosystems found in north central Alaska. ARTUS simulates the annual patterns of heat and water balance, carbon fixation, plant growth, and nitrogen and phosphorus cycling. ARTUS runs in 1-d time steps for a growing season from 1 May to 17 September and is intended to run for several years. The abiotic section of ARTUS encodes the seasonal input of the environmental driving variables and calculates the resultant thermal and water regimes to define the heat and water environments for the tussock tundra system. The primary driving variables are daily total solar radiation, air temperature, precipitation, surface albedo, wind, and sky conditions. The soil compartment contains three organic horizons, which are recognized by their state of physical and chemical decomposition, and one mineral horizon. Six vascular plant species and four moss species are simulated. The model has seven compartments for each vascular plant species: total nonstructural carbohydrates, total nitrogen, total phosphorus, leaves grown in the current season, leaves grown in previous years, conducting and storage stems plus roots, and absorbing roots. In ARTUS the functional unit of the plant is the shoot system or ramet. Each shoot system consists of leaves, stems, fine roots (which do not have secondary growth and have a limited life-span), and larger roots, which have secondary growth and an extended life-span. Although plant processes are based on individual shoots, the ARTUS model as a whole is based on a square meter of ground. Values per square meter are calculated from the values per shoot by multiplying by the shoot density of each species. The model was validated by comparing calculated and measured peak season biomasses and nutrient contents, and the seasonal progression of environmental processes, biomass, carbohydrate contents, and nutrient contents. ARTUS successfully simulated the seasonality of the physical environment, but simulated thaw depths were deeper than those measured at all sites. The simulated value for total vascular plant production was 77% of the measured value. The simulated values for ecosystem respiration for Eagle Creek were within the range of measured values. Simulations with ARTUS indicated different patterns of growth and different storage carbohydrate levels in deciduous shrubs, evergreen shrubs, and graminoids. The simulated seasonal course of net primary production of vascular plants and mosses was similar to the pattern measured at Eagle Creek. Sensitivity analysis using ARTUS indicated that the tussock tundra is more sensitive to external environmental factors, such as increased temperature, than to internal ecosystem variables. The development of ARTUS was limited by the unavailability of data on whole plant carbon balance including root and stem respiration. More data are also needed on decomposition processes and nitrogen and phosphorus cycling. Adequate climatological data for northern Alaska are needed for extensive validations of the model. While caution should be used in basing managerial decisions on model simulations, ARTUS can be used to identify and quantify the magnitude and direction of plant responses to changes in state variables in the model.
Energy costs of subduing and swallowing prey for a lizard
Pough, F. Harvey; Andrews, Robin M. (Ecological Society of America, 1985)
We measured the oxygen consumption (aerobic energy cost) and lactic acid production (anaerobic energy cost) of scincid lizards, Chalcides ocellatus, eating domestic crickets. Aerobic metabolism accounted for 90% or more of the total energy cost of subduing and swallowing prey. The time required to subdue and swallow a cricket was linearly correlated with oxygen consumption. Oxygen consumption increased as a power function of cricket mass, but the maximum size of crickets swallowed by the lizards was set by morphological rather than by energetic constraints. The energy cost of subduing and swallowing was 0.2-0.4% of the utilizable energy of the cricket eaten. Net energy gain per unit time spent subduing and swallowing prey (e/t) declined monotonically with increasing cricket mass. Because the energy cost of eating is trivial, the shape of the e/t curve is determined by the function relating prey mass to the time required for subduing and swallowing; the energy value of prey was proportional to prey mass, whereas the time required for subduing and swallowing increased faster than prey mass. The energy value of anthropods is so high, relative to the costs for a lizard of pursuring, subduing, and swallowing, that these costs can be ignored for most ecological purposes.
Phosphorus spiraling in a woodland stream: seasonal-variations
Mulholland, P. J.; Newbold, J. D.; Elwood, J. W.; Ferren, L. A.; Webster, Jackson R. (Ecological Society of America, 1985)
Four radiotracer releases were performed over an annual period in 1981-1982 to determine seasonal variation in indices and pathways of phosphorus spiralling in Walker Branch, a small woodland stream in eastern Tennessee, USA. Each release consisted of an addition of -370 MBq each of carrier-free 32PO4 and 3H2O over a 1-h period during baseflow. Concentrations of 32P and 3H dissolved in stream water were measured intensitively at several stations downstream for the radiotracer input during and immediately following each release. Activity of 32P in coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), and aufwuchs was measured 2-3 h after each release and at various intervals for 7 wk. Total biomass of CPOM, FPOm, and aufwuchs at the time of each release was also measured. Uptake of 32PO4 from the water was greatest in November and lowest in August, Uptake length (Sw) of phosphorus, defined as the average distance travelled by a PO4 ion dissolved in water, varied from 22 m in November to 97 m in August. Uptake of 32PO4 by CPOM was generally greatest, with -50% of total uptake, while that by aufwuchs was lowest, with <15% of the total. CPOM abundance was the major determinant of whole-steam PO4 uptake rate and Sw. Turnover length (Sp) of phosphorus, defined as the average distance traveled by an atom of P taken up by particulate material, was short compared to Sw, varying from 1 m in November to 3 m in January. Consequently total spiralling length (S) of P varied from 23 in November, just after peak autumn leaf fall, to 99 m in August, and reflected primarily the travel of P in the dissolved form. Our results indicate that the greatest increase in Sw, (and consequently in S) in Walker Branch occurs in late autumn or winter after storms reduce the abundance of CPOM in the lower portions of the stream bed. Although we calculate that Sp may increase by one of two orders of magnitude for short periods during storms, the greatest effect of storms on P spiralling over the long term is their impact on the quality of CPOM and FPOM in the stream bed after the return to baseflow. For most of the year, detrital organic carbon probably influences phosphorus spiralling more than phosphorus spiralling influences the processing of organic carbon in Walker Branch. Only during the fall and early winter periods, when CPOM abundance is high and Sw is short, does phosphorus spiralling exert strong control over biotic processes downstream.
Cellulose digestion and assimilation by three leaf-shredding aquatic insects
Sinsabaugh, R. L.; Linkins, A. E.; Benfield, Ernest F. (Ecological Society of America, 1985)
The capacity of three leaf-shredding aquatic insects, Pteronarcys proteus (Plecoptera: Pteronarcidae), Tipula abdominalis (Diptera: Tipulidae), and Pycnopsyche luculenta (Trichoptera: Limnephilidae), to digest and assimilate cellulose was investigated. Pteronarcys numphs collected from two second-order woodland streams over a 14-mo period exhibited high levels of cellulolytic activity in their alimentary tracts, especially in the anterior gut. Similar though slightly lower activity levels were measured in Pycnopsyche guts. Cellulolytic activity in Tipula larvae collected from the streams during the same period was low to absent, and when present, was concentrated in the hindgut. General proteolytic activity was activity was similar in the alimentary tracts of all three species. Assimilation of uniformly labelled 14C-cellulose was determined by a dual-label technique, and assimilation efficiencies were estimated at 11.2% for Pteronarcys, 18.5% for Tipula, and 12.0% for Pycnopsyche. Confirmation that labelled digestion products passed the gut wall in two species was obtained by in vitro label transport experiments. Ion exchange fractionation of labelled digestion products crossing the gut wall showed >90% of the label was transported as organic acid and amino acids in Tipula, while >40% of the label crossing the gut wall in Pteronarcys was neutral sugar. Based on the label experiments and published information, we hypothesize that Tipula relies mainly on microbial endosymbionts for cellulose hydrolysis, while Pteronarcys accomplishes hydrolysis largely by means of acquired microbial enzymes obtained through ingestion of microbially conditioned detritus. This study demonstrates the potential for certain leaf-shredding stream insect to derive nutritional benefit from plant polysaccharides, although not without microbial mediation.
Algal periphyton growth on nutrient-diffusing substrates: an in situ bioassay
Fairchild, G. Winfield; Lowe, Rex L.; Richardson, William B. (Ecological Society of America, 1985)
Differences in nutrient limitation for dominant species within an algal periphyton community were determined using additions of N and P supplied by nutrient-diffusing artificial substrates. Sealed clay flowerpots were filled with 2% agar and one of nine nutrient treatments (all combinations of K2HPO4 at 0.0, 0.05, and 0.5 mol/L with NaNO3 at 0.0, 0.05, and 0.5 mol/L). The pots were submerged at 0.5 m depth in Douglas Lake, Michigan, and diffused N and P to their outer surfaces in proportion to internal concentrations. After 51 d the pots were scraped and analyzed for attached algae. Total algal biomass as chlorophyll a on the pots ranged from 0.17 - 0.02 (SE) mg/cm2 for pots without added nutrients to 15.7 - 2.0 mg/cm2 for pots with K2HPO4 at 0.05 mol/L and NaNO3 at 0.5 mol/L. chlorophyll a on pots containing just P (0.05, 0.5 mol/L) increased 6- to 10-fold over controls. The diatoms Epithemia adnata and Rhopalodia gibba and the blue-green alga Anabaena increased significantly on the P-only pots; these species are suspected of N-fixing capability. Chlorophyll a on pots containing just N (0.05 mol/L) increased 1.5- to 2-fold, though this increase was nonsignificant; Achnanthes minutissima, Gomphonema tenellum, and Cocconeis placentula showed enhanced growth on these pots. Combinations of N and P caused heavy growth of the filamentous alga Stigeoclonium tenue. Naviculoid diatoms were also most abundant on the N + P pots. Average nutrient levels in Douglas Lake during the study were: NH3, 2.02 _mol/L; NO3, 0.44 _mol/L; and PO4, 0.06 _mol/L. The low ambient concentrations of both N and P, together with results of the periphyton bioassay, indicate that the two nutrients may jointly limit overall growth, and that the form of growth limitation differs by species within the periphyton community.
The ecophysiological significance of leaf movements in rhododendron-maximum
Bao, Yija; Nilsen, Erik T. (Ecological Society of America, 1988-10)
The patterns of leaf movements and their environmental stimuli have been documented for over a century. Although there have been several studies on the physiological significance of heliotropisms, thermotropic movements have been largely ignored. The objective of this study was to evaluate the ecophysiological significance of thermotropic leaf movements in a subcanopy plant through controlled experiments. Rhododendron maximum L. was selected because of its strong winter season, thermotropic leaf movements, and the ease of identifying even-aged leaf cohorts. A series of leaf manipulations was established to restrict curling, changes in leaf angle, or both. The effects of these treatments on leaf energy budget, chlorophyll content, photosynthesis, and leaf survival were evaluated. Leaves that were maintained in a horizontal position (whether they were allowed to curl or not) had the greatest decrease in chlorophyll content and the most inhibited photosynthetic rate. Reduced photosynthetic potential was due to both lower quantum yield and lower light-saturated nthetic rates. Optimal temperature for photosynthesis was several degrees above air temperature, corresponding to a horizontal leaf temperature in a sunfleck. Leaf angle influenced leaf temperature during daylight and night time; but the influence was small and most likely had no ecophysiological significance. Leaf curling had little to no influence on the ecophysiological parameters measured in this study.
Biological integrity: a long-neglected aspect of water resource management
Karr, J. R. (Ecological Society of America, 1991-02)
Water of sufficient quality and quantity is critical to all life. Increasing human population and growth of technology require human society to devote more and more attention to protection of adequate supplies of water. Although perception of biological degradation stimulated current state and federal legislation on the quality of water resources, that biological focus was lost in the search for easily measured physical and chemical surrogates. The "fishable and swimmable" goal of the Water Pollution Control Act of 1972 (PL 92-500) and its charge to "restore and maintain" biotic integrity illustrate that law's biological underpinning. Further, the need for operational definitions of terms like "biological integrity" and "unreasonable degradation" and for ecologically sound tools to measure divergence from societal goals have increased interest in biological monitoring. Assessment of water resource quality by sampling biological communities in the field (ambient biological monitoring) is a promising approach that requires expanded use of ecological expertise. One such approach, the Index of Biotic Integrity (IBI), provides a broadly based, multiparameter tool for the assessment of biotic integrity in running waters. IBI based on fish community attributes has now been applied widely in North America. The success of IBI has stimulated the development of similar approaches using other aquatic taxa. Expanded use of ecological expertise in ambient biological monitoring is essential to the protection of water resources. Ecologists have the expertise to contribute significantly to those programs.
Population stability of a tropical lizard
Andrews, Robin M. (Ecological Society of America, 1991-08)
Populations of the lizard Anolis limifrons were censused for 19 yr at the Lutz site on Barro Colorado Island (BCI) and for 4 yr at 20 additional sites in central Panama. Census data provided estimates of population density at the end of the wet season (December) and indices of per-capita food intake and annual recruitment. Year-to-year changes in density of as much as 5- to 8-fold were observed at Lutz over 19 yr and of 2- to 3-fold were observed over 4 yr at other sites. Changes in population density at 11 BCI sites were synchronous over 1983-1986, while population density fluctuated independently at non-BCI sites. Populations of A. limifrons exhibited significantly greater generation-to-generation variability than populations of West Indian Anolis and of lizards in temperate-zone habitats (mostly arid lands in North America). An index of per-capita food intake was negatively related to density both for temporal comparisons at Lutz and for spatial comparisons among sites. The relative number of young individuals at the end of the wet season was negatively related to population density and positively related to an estimate of food intake. Partial correlations indicated that the positive association between the relative number of young individuals in the population and food intake was independent of the negative association between food intake and population density. Recruitment was thus depressed when density was high and enhanced when food was relatively abundant. Despite the potential for density-dependent regulation, Anolis limifrons populations are highly unstable, presumably because of random perturbations of the environment. Rainfall is implicated causally although population density was only weakly associated with rainfall - density at BCI was positively associated with rainfall during the dry season and negatively associated with rainfall during the wet season and with total annual rainfall. Population attributes that contribute to instability are rapid population turnover and a middle-level trophic position in a complex food web.
Importance of Submarine Groundwater Discharge (SGWD) and Seawater Cycling to Material Flux across Sediment Water Interfaces in Marine Environments
Simmons, George M. Jr. (Inter-Research, 1992)
The movement of water across sediment/water interfaces is very important to the ecology of aquatic habitats. Submarine groundwater discharge (SGWD) occurs primarily by advective flow and may be due to a variety of factors. In nearshore marine environments, the major factor is probably groundwater flow from upland regions. In oceanic environments, SGWD is probably influenced more by tides and surge action. SGWD, therefore, represents various mixtures of fresh groundwater and seawater. This paper summarizes the first regional study of SGWD and its attendant solutes in shallow estuarine, continental shelf and coral reef habitats, and calls attention to the magnitude of water volumes which appear to be circulating through marine sediments. Data on SGWD from sites in the Florida Keys and on the southeastern continental shelf of the U.S. indicate that water movement across sediment/water interfaces is a common occurrence at least to water depths of 30 to 35 m. Discharge values from the Florida Keys were 8.9 l m-2 d-1 (N congruent-to 344) for depths < 27 m and 5.4 l m-2 d-1 (N congruent-to 261) for depths of ca 27 to 39 m. On the southeastern continental shelf, discharge ranged between ca 6 and 20 l m-2 d-1. One site was found in 20 m depth where there was a persistent negative hydraulic head and a mean influx of seawater to the sediments (ca 10.8 l m-2 d-1). Even though geohydrological models would predict coupling of SGWD with landbased hydraulic heads, definitive lower salinity SGWD could not be detected. The driving force seems to be subtidal pumping and much of the discharge measured was probably recycled seawater. This research also demonstrated that SGWD serves to move dissolved solutes into the water column, and could be an important link in benthic-pelagic coupling in continental shelf ecosystems.
Effects of Changing Density and Food Level on Metamorphosis of a Desert Amphibian, Scaphiopus Couchii
Newman, R. A. (Ecological Society of America, 1994-06)
Amphibians that breed in temporary ponds provide a good opportunity to study the ecological and evolutionary consequences of environmental variability. Ephemeral aquatic habitats provide larval amphibians a transient and highly variable opportunity for growth. In the desert ponds used by Couch's spadefoot toad (Scaphiopus couchii), tadpole density varies considerably among ponds and often increases within a pond as it dries. Models of optimal size and timing of metamorphosis predict that, relative to constant high resource environments, metamorphosis should occur at a smaller size in constant low resource environments, and smaller and earlier in declining resource environments, assuming all else is equal. Considerable evidence supports the first prediction, but few studies have examined the effect of decreasing resource availability. I conducted an experiment to examine the effects of increasing density and decreasing food level on metamorphosis in Couch's spadefoot toads. Tadpoles at constant high per capita food metamorphosed at the largest sizes. At low or decreasing per capita food, tadpoles metamorphosed at a uniformly small size, but varied in the time required to reach that size. Tadpoles from both increasing density and decreasing food treatments metamorphosed earlier than tadpoles from constant density, constant food treatments. These results support the idea that tadpoles can respond adaptively to resource variation.
Sediment-Water Column Oxygen and Nutrient Fluxes in Nearshore Environments of the Lower Delmarva Peninsula, USA
Reay, William G.; Gallagher, Daniel L.; Simmons, George M. Jr. (Inter-Research, 1995)
Sediment-water column exchanges of oxygen, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) were measured in situ over an annual cycle for sandy and silt-clay sediment types in a shallow Chesapeake Bay (USA) inlet. Benthic oxygen and inorganic nutrient fluxes differed between sediment types. Based on metabolic rate estimates and photosynthetic pigment concentrations, nearshore sandy sediments were more productive than silt-clay sediments. Overall benthic community respiration rates were 872 mu mol m(-2) h(-1) for sandy sediments and 2220 mu mol m(-2) h(-1) for silt-clay sediments. Elevated ammonium and DIP sediment fluxes were associated with silt-clay sediments. Sandy and silt-clay sediment ammonium fluxes ranged from -44 to 358 and -30 to 615 mu mol m(-2) h(-1) respectively, with DIP fluxes ranging from -5.3 to 42.0 and -3.3 to 35.7 mu mol m(-2) h(-1). Negative nutrient flux values denote sediment uptake. Sediment ammonium and DIP fluxes were dependent on benthic aerobic respiration rates for silt-clay sediments. In contrast, sandy sediment ammonium fluxes were less dependent and DIP fluxes showed no relationship to benthic aerobic respiration rates. Ammonium and DIP flux rates were significantly reduced in transparent chambers as compared to opaque chambers indicating the importance of the benthic microalgal community. On an annual basis, sandy sediments could supply 11% of the phosphorus and 6% of the phytoplankton nitrogen requirements based on gross productivity estimates, whereas silt-clay sediments could supply 11 and 14%, respectively. Positive correlations between sandy and silt-clay sediment DIN fluxes and phytoplankton DIN assimilatory demands emphasize the importance and interdependence of sediment heterotrophic and water column autotrophic processes. Short water column DIN and DIP turnover times, on the order of hours, were characteristic of summer conditions when water column nutrient concentrations were low and silt-clay sediment nutrient fluxes high. Conversely, nutrient turnover times on the order of days were characteristic of winter conditions when water column nutrient concentrations were high and sediment nutrient fluxes low.
Developing a field of landscape ecotoxicology
Cairns, John Jr.; Niederlehner, Barbara R. (Ecological Society of America, 1996-08)
Since toxicants are spread over ecological landscapes, it seems likely that they have effects at that level of ecological organization. Landscape ecotoxicology examines the effects of toxic chemicals on larger scales than traditional environmental toxicology. This approach is characterized by the use of endpoints appropriate to the spatial scale across which a toxicant is dispersed, attention to interactions between physical and temporal patterns and the process of ecological impairment, and integration of multiple lines of evidence for toxicity at various scales. In addition, landscape ecotoxicology seeks predictive models in order to influence human actions before environmental damage occurs. Integrating information from damaged systems, toxicity tests, simulation models, and biomonitoring of healthy systems provides the best basis for decisions. Rapid progress in landscape ecotoxicology is expected as scientists incorporate tools, such as remote sensing and spatially explicit simulation models, and then calibrate these models using data from longterm biomonitoring of large areas. Further integration into combined socio-economic-ecological models is also possible.
Do similar communities develop in similar sites? A test with zooplankton structure and function
Jenkins, D. G.; Buikema, Arthur L. Jr. (Ecological Society of America, 1998-08)
McCune and Alien (1985) asked the question "Will similar forests develop on similar sites?" and concluded that dissimilar old-growth forests had developed on similar sites due to historical factors (colonization, disturbance, etc.). We asked "Do similar zooplankton communities develop in similar ponds?" We compared zooplankton community structure and function in 12 newly constructed experimental ponds during 1 yr of natural colonization and analyzed a suite of physical-chemical variables to evaluate the assumption of environmental similarity among ponds. Ponds were similar for the measured environmental variables. However, zooplankton communities were structurally different, as indicated by analyses of species presence/absence, colonization and species accrual curves, and taxa (rotifer, copepod, cladoceran, and Chaoborus) density and biomass. Species varied widely in their colonization abilities. Zooplankton communities also differed in productivity of some taxa and community-level respiration rates. Scale was important in detecting structure and function differences among zooplankton communities. Species- and taxa-level analyses showed clear differences among communities, but community-level analyses of structure (species richness, total density and biomass) and function (productivity, respiration, and ammonia regeneration rates) could not identify distinct sets of communities. Community structure and function may be comparable in sensitivity for detecting change but need to be compared at equivalent scales. Dispersal (as evidenced by colonization history) was a regulator of new zooplankton communities, because it did not occur rapidly or uniformly among similar ponds. All zooplankton do not disperse readily. The extent to which dispersal limits older zooplankton communities is unknown, but genetic studies indicate low dispersal rates among established populations. Dispersal also regulates assemblages of organisms expected to be less vagile than zooplankton and in various ecosystems, indicating that "supply-side" and metapopulation concepts are valuable for community ecology. Priority effects may have lasting influence on subsequent community structure, depending on colonization rates and sequences. We propose explicit recognition (and careful examination) of a commonly assumed but rarely tested "quorum effect": local abiotic and biotic processes regulate communities and arrival processes do not, because potential members have already arrived. Given either priority or quorum effects, dispersal may be an important, often-overlooked process regulating community structure and function, especially when it is not rapid.
Interaction of substrate and nutrient availability on wood biofilm processes in streams
Tank, J. L.; Webster, Jackson R. (Ecological Society of America, 1998-09)
We examined the effect of decomposing leaf litter and dissolved inorganic nutrients on the heterotrophic biofilm of submerged wood in streams with and without leaves. Leaf litter was excluded from one headwater stream in August 1993 at Coweeta Hydrologic Laboratory in the southern Appalachian Mountains. We compared microbial processes on wood in the litter-excluded stream to a reference stream using microbial respiration, fungal biomass, and extracellular enzyme activity. Exclusion of leaf litter enhanced microbial respiration and extracellular enzyme activity, and fungal biomass was seven times higher than in the reference stream. Nutrient-releasing substrates placed beneath wood veneers indicated colimitation by nitrogen and phosphorus on biofilms in the reference stream. Our conclusion is that, in the absence of nutrient immobilization by leaves, nutrients are more available for other heterotrophic processes. Nutrient limitation may have been responsible for low microbial respiration, fungal biomass, and extracellular enzyme activity on wood in the reference stream containing leaves. Our results suggest that competition for nutrients may regulate heterotrophic microbial processes in these streams.

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