bluuebunny/crossref_metadata_2025 huggingface dataset:
Seasonal Drought and Leaf Fall in a Tropical Forest
Authors: Wright S. Joseph , Cornejo Fernando H.
Published: May 2006

Peak rates of leaf fall almost always occur during dry seasons in low—latitude, low—elevation tropical forests. The hypothesis that plant water stress is the proximal cue for leaf fall was tested by augmenting water supplies during the 4—mo dry season over two 2.25—ha plots of tropical moist forest on Barro Colorado Island (BCI), Panama. The manipulation maintained soil water potentials at or above field capacity throughout the dry season but did not affect atmospheric conditions in the canopy (i.e., relative humidity, temperature, windspeed, incident radiation). The manipulation ameliorated plant water status; for most species, dry—season leaf water potentials in the manipulated plots were similar to wet—season values and both were consistently greater than dry—season values in the control plots. The manipulation delayed leaf fall for 2 of 9 species of trees for which qualitative data are available and possibly for 2 of 20 species of trees and lianas for which quantitative data are available. The timing of leaf fall was indistinguishable in manipulated and control plots for the remaining 25 species. We conclude that plant water status is rarely the proximal cue for leaf fall on BCI. Atmospheric conditions may be important for some species, but there is no reason to presuppose that a majority of tropical plants are responsive to any single cue.
Prey Selection and Feeding Behavior of the Green Treefrog (Hyla Cinerea)
Authors: Freed Arthur N.
Published: May 2006

In laboratory experiments on prey choice, Hyla cinerae consistently selected the house fly over four mosquito species. There was no correlation between prey size and predator choice. A positive correlation did exist between choice of prey and prey activity as well as with difficulty of prey capture. A test for partial correlation between prey size, prey activity, and predator choice indicated that activity, and not size, was the key factor determining prey selection. The green treefrog selected prey in relation to the proportion of time a prey species remained active, as well as the types of activity most often displayed. Similar motor patterns between species as well as different activity patterns within prey species did not provide equal feeding stimuli for the frog.
Steady State and the Continuous Input Model of Litter Accumulation and Decompostion in Australian Eucalypt Forests
Authors: Birk Elaine M. , Simpson R. W.
Published: May 2006

The continuous input litter accumulation and decomposition model of Olson (1963) is examined in the context of periodically burned eucalypt forests. Eucalypt forests are generally not in steady state since fires regularly deplete the pool of accumulated litter. A decay constant calculated assuming steady state would considerably overestimate the forest floor turnover rate if based on data collected much less than 10 yr after a fire. The approximate magnitude of the error has been determined for jarrah (Eucalyptus marginata Sm.) and karri (E. diversicolor F. Muell.) forests as a function of time since fire. The predicted decay constants could also vary by up to 25% due to annual variations in the rate of litter input.
Nitrogen Nutrition and Salinity Tolerance of Distichlis Spicata and Spartina Alterniflora
Authors: Smart R. Michael , Barko John W.
Published: May 2006

In a greenhouse investigation Distichlis spicata (L.) and Spartina alterniflora (Loisel.) were grown from seed on freshwater, brackish, and marine sediments. These species were also concurrently grown in sand culture under conditions of nitrogen or phosphorus limitation in order to establish critical tissue concentrations of these elements. These experimentally determined values were used to assess the limiting nutrient status of the plants grown on the sediments. Growth of both species was nitrogen limited on most sediments, and phosphorus was consumed luxuriously on all sediments. Aboveground biomass was correlated with interstitial water nitrogen concentration of the sediments. Under low nutrient conditions relatively more biomass was allocated to belowground rather than aboveground portions of the plants. Although the availability of nitrogen ultimately determined biomass accrual, growth rate was affected by the sediment salinity. Analysis of plant tissues and the sediment interstitial water at the end of the investigation revealed selective uptake of potassium and exclusion of sodium. These processes increased the ratio of sodium to potassium in the interstitial waters and also resulted in increased sediment salinities. The effect of salinity stress on plant growth in Spartina marshes is discussed in relation to nitrogen limitation.
Production Dynamics and Resource Utilization of Snag‐Dwelling Mayflies in a Blackwater River
Authors: Benke Arthur C. , Jacobi David I.
Published: May 2006

We quantified production dynamics, determined the trophic basis of production, and assessed the overlap in resource utilization within a diverse assemblage of mayfly larvae from the submerged woody (snag) habitat of a Coastal Plain blackwater river (Ogeechee River). Total annual production (as dry mass) was among the highest reported for mayflies, ranging from 20.8 to 42.2 g/m2 of snag surface (7.4 to 12.3 g/m2 of channel bottom) in two consecutive years. Relatively little change occurred between years for four out of six families. Biomass turnover rates (annual production/biomass) were generally high, ranging from 8 (single generation per year) to 96 (multiple generations per year) for individual taxa. Distinct types of temporal production patterns occurred: (1) concentration in winter (Ephemerella argo, E. dorothea, Eurylophella sp., and Isonychia), (2) concentration in summer (Baetis ephippiatus, Tricorythodes sp., Caenis spp.) or (3) spread throughout the year and peaking in summer (Baetis intercalaris, Stenonema modestum, S. integrum, S. exiguum, and Hexagenia sp.). Temporal overlap of production, using the proportional similarity index, was highest among closely related taxa (>0.60), with the least overlap (<0.15) among taxa in different families. Overlap based upon the amount that various food types contribute to production was high among all species, suggesting even less resource partitioning for food than for time. Of the annual food consumption (as dry mass) by mayflies per unit snag surface area (423 g°m—2°yr—1), °87% consisted of amorphous detritus that is rapidly replenished on snags from floodplain—derived seston. This food source accounted for °70% of total mayfly production. We suggest that a fluctuating habitat (snags), a continuously replenished food supply, high biomass turnover, and high drift densities help perpetuate a persistent, resilient, and diverse mayfly assemblage with little resource partitioning.
Effects of Nitrogen Supply on Canopy Structure and Leaf Nitrogen Distribution in Carex Species
Authors: Aerts Rien , de Caluwe Hannie
Published: May 2006

Optimization theory of leaf nitrogen distribution in canopies with respect to photosynthetic carbon gain predicts that vertical leaf nitrogen gradients are steeper in dense canopies than in more open canopies. We tested this prediction at the intraspecific and the interspecific level by studying the effects of two levels of nitrogen supply on canopy structure and vertical leaf nitrogen distribution in experimental populations of three Carex species that dominate in low—productivity fens (Carex diandra and Carex rostrata) and in high—productivity fens (Carex acutiformis), respectively. The high—productivity species had a significantly higher amount of leaf area per unit ground area, a higher absolute amount of nitrogen in the canopy, proportionally more leaf area in upper canopy regions, and steeper gradients of leaf nitrogen in the canopy than the low—productivity species. Similarly, increased nitrogen supply resulted for each species in a higher amount of leaf area per unit ground area and a higher absolute amount of nitrogen in the canopy. However, only in C. acutiformis did an enhanced N supply led to proportionally more leaf area in upper canopy regions and to steeper gradients of leaf nitrogen in the canopy. It is suggested that the observed patterns in Carex acutiformis (a very high canopy with a large amount of leaf area) are not directed towards maximization of photosynthetic carbon gain, but are also directed towards suppression of other species in the competition for light. The interspecific differences in canopy structure and leaf nitrogen distribution are in agreement with the optimization theory. However, at the phenotypic level the low—productivity species did not show proportionally more leaf area in upper canopy regions and steeper gradients of leaf nitrogen in the canopy upon enhanced N supply, despite a strong increase in the total amount of leaf area. This suggests that low—productivity species have a lower phenotypic plasticity with regards to their vertical leaf distribution pattern and their vertical leaf nitrogen distribution than high—productivity species.
A Model of Wind Dispersal of Winged or Plumed Seeds
Authors: Greene D. F. , Johnson E. A.
Published: May 2006

We derived a micrometeorological model for the dispersal of winged or plumed seeds from a point source. The model is based on six measurable parameters: mean release height, mean and standard deviation of the terminal velocities of seeds, standard deviation of vertical wind velocities, and the mean and standard deviation of the natural logarithms of horizontal wind velocities. Predictions of the model include (1) the distribution of the dispersal curve (defined as numbers vs. distance from source) is right skewed; (2) the median and long—range dispersal distances need not be well correlated; and (3) increased variance in flight trajectories for a seed population will place the mode of the dispersal curve closer to the source. Empirical tests of the model showed that it adequately characterizes the observed dispersal curves for experimental releases. It is shown that a simple ballistic provides a good estimate of the median dispersal distance (and mean distance if the skew of the dispersal curve is slight) for experimental releases. Tests of the model using natural releases from isolated trees indicated that winged seeds do not detach from the parent randomly with respect to horizontal wind velocity. The need to understand the relationship between the probability of detachment and the frequency distribution of horizontal wind velocities is stressed.
Strong Effects of Foraging Minnows on a Stream Benthic Invertebrate Community
Authors: Gilliam James F. , Fraser Douglas F. , Sabat Alberto M.
Published: May 2006

Behavioral responses to piscivorous fish are known to alter the spatial distribution of small invertebrate—feeding fish in streams, producing concentrations of small fish in safer areas. We asked whether such variation in the local density of small juvenile creek chubs (Semotilus atromaculatus) would, in turn, produce local alterations in the stream benthic invertebrate community. Replicated treatments of 0, 2, and 6 juvenile chubs per 0.5—m2 enclosure were established in a warm—water, soft—sediment stream. Over a 3—mo period, the presence of fish resulted in reductions of 79—90% in total invertebrate volume relative to the zero—fish treatments. Numbers were reduced 55—61%. The two major taxa, Oligochaeta and Isopoda, showed strong fish effects, and size distributions of these two taxa showed shifts to smaller sizes. Densities of less abundant taxa (e.g., Chironomidae, Sphaeriidae) showed no statistically significant fish effects. Evenness of the invertebrate community increased in the presence of fish. No differences in invertebrate densities were detected between the two— and six—fish treatments; however, the above—sediment activity of Oligochaeta was reduced more in the six—fish treatment than in two—fish treatment, and growth of the fish was slower in the six—fish treatment. Unlike previous experimental studies in stony streams, this experiment shows that foraging fish can significantly alter the behavior, population structure, and community structure of stream benthic invertebrates.
Drosophila on Oranges: Colonization, Competition, and Coexistence
Authors: Nunney Leonard
Published: May 2006

To investigate how two ecologically very similar species can coexist, I studied two species pairs of fruit flies in a California orange grove. These species are members of the guild of “cosmopolitan” Drosophila species. This guild is found almost worldwide, and it shows consistent dynamic patterns. Seasonal changes in the species composition in southern California were consistent over two years and were very similar to those previously recorded in Australia. However, the mechanisms maintaining the guild are poorly understood. In particular, the members of two species pairs, D. melanogaster/D. simulans and D. immigrans/D. hydei, have been classified as ecologically very similar. I investigated the extent to which two factors might be important and maintaining the integrity of this guild. First, Shorrocks and his coworkers have suggested that the spatial distribution of larvae among identical resource items can promote coexistence; and second, the state of decay of an orange might provide an additional resource dimension promoting coexistence. I carried out four fields experiments to record the sequence in which Drosophila colonized decaying oranges. I showed that, within individual oranges, larval competition was acting to reduce adult emergence, but that the spatial distribution of the species among oranges was not consistent with the Shorrocks model. However, the ecologically similar pairs were different in their pattern of colonization: D. simulans adults arrived earlier than D. melanogaster adults and D. immigrans earlier than D. hydei. These differences in adult arrival time were reflected in differences in successful oviposition, and are consistent with the hypothesis that the different species specialize on different parts of the process of decay.
Furanocoumarin Induction in Wild Parsnip: Evidence for an Induced Defense against Herbivores
Authors: Zangerl Arthur R.
Published: May 2006

Induction of furanocoumarins, toxic and antifeedant secondary compounds, in wild parsnip, Pastinaca sativa L., was examined to assess the efficacy of inducible defenses and to assess whether in the case of wild parsnip the induction response is likely to represent an adaptation to herbivory. Leaflets of undamaged plants were experimentally damaged by Trichoplusia ni, a generalist folivore that occasionally feeds on wild parsnip. Intact and damaged leaflets were then measured to obtain constitutive and induce levels of furanocoumarin. In a separate experiment, levels of furanocoumarin in manually damaged leaflets were compared with intact leaflets. Both insect and manual damage induced furanocoumarin production in leaves of wild parsnip. Larvae of T. ni, moreover, grew only a fourth as rapidly on induced leaflets compared to non—induced leaflets from the same leaf. By feeding larvae artificial diets containing two of the furanocoumarins in amounts equivalent to constitutive or induced plant levels, I found that furanocoumarin induction can account for much of the reduced larval growth association with induction in intact plants.
Furanocoumarin Induction in Wild Parsnip: Genetics and Population Variation
Authors: Zangerl Arthur R. , Berenbaum May R.
Published: May 2006

Inducible defenses by their nature are phenotypically variable traits. To assess the degree of genetic control over induction and therefore the availability of these traits for selection, we measured the levels of genetic variation and genetic correlation for induced and constitutive furanocoumarins in two populations of wild parsnip, Pastinaca sativa L., near Urbana, Illinois, USA. Leaflets of greenhouse—grown from families of each population were artificially damaged. After 48 h undamaged and damaged leaflets were analyzed for amounts of furanocoumarins, secondary compounds with known insecticidal properties. The same plants were subsequently used in a transplant experiment to determine the frequency of attack by leaf—feeding herbivores for each population in each habitat. While individuals from both populations experienced similar frequencies of attack within each location, the overall infestation rate was much higher at the Race Street location (41%) than at the Phillips Tract location (5%). The two populations contained significant amounts of genetic variation for constitutive and induced levels and induced increments of certain furanocoumarins, and differed significantly in the constitutive levels of two of the furanocoumarins. The Phillips Tract population, which experiences a low probability of herbivory, contained significantly lower constitutive amounts of furanocoumarin. The results of these studies are consistent with the interpretation that these populations have divergent defense strategies that are in accordance with their probabilities of being attacked. This study provides the first evidence that selection can act on inducibility of plant chemical defenses.
Patterns of Species Diversity in the Temperate and Tropical Litter Mites
Authors: Stanton Nancy L.
Published: May 2006

Species richness and diversity of litter mites were measured in 3 vegetative stands in Wyoming and in Costa Rica (a pine forest, a broad—leaved forest and a field) by sampling the natural litter and by experimentally manipulating the type (pine, broad—leaved and field litter) and amount (100 g dry weight/600 cm2 plot) of litter present within each stand. Within sample species richness of the experimental plots was the same in all 4 forests (12—14 species/100 g litter) but the 2 fields supported significantly fewer species (5/100 g). In the natural forest litter, within sample richness ranged from 13 species (Costa Rica forest) to 27/600 cm2 sample (Costa Rica pine); but again both fields were significantly less rich (4 and 6 species). Mite diversity (H') in the fields also was < the forests in both natural and experimental litter. Total within stand diversity and beta diversity were highest in the tropical forests. Also, a higher percentage of temperate species were classified as colonizers as opposed to the tropical species where the majority were noncolonizers; but tendency to colonize appears to be inversely related to the amount of litter present in a habitat.
Vegetational Patterns and Processes in the Balsam Fir Zone, White Mountains New Hampshire
Authors: Reiners William A. , Lang Gerald E.
Published: May 2006

Vegetation structure and dynamics of the upper subalpine or fir zone were studied in the White Mountains of New Hampshire. The fir zone extends from 1220 m, the approximate upper limit of Picea rubens occurrence, to treeline (°1450 m) where it is usually represented by a low krummholz. Live tree density in the part of the zone in which trees are >2 m tall averages 5000 stems/ha. Eighty—four percent of these are Abies balsamea, and most of the rest are Betula papyrifera var. cordifolia. Basal area averages 30 m2/ha; canopy height averages 6.8 m. Stands tend to be dominated by discrete age classes. Tree ages range up to 111 yr (average 55 yr). Fir zone vegetation is subject to a number of processes that lead to a hierarchical set of overlapping patterns. Elevation and wind exposure produce general, or first—order, patterns over the landscape, e.g., canopy height decreases with elevation and exposure. Overlying this general pattern are a series of second—order patterns. Two acute disturbance factors–hurricanes and avalanches–have stamped discrete impressions over the general pattern. Other, more endogenously generated patterns, marks the landscape with more subtle textures. These patterns include fir waves, broken gaps, strips, and glades. A conceptualization of vegetation as a hierarchical series of overlapping patterns is an extension of Watt's view of vegetation dynamics and bears important ecological implications.
Treefalls and the Coexistence of Conifers in Subalpine Forests of the Central Rockies
Authors: Veblen Thomas T.
Published: May 2006

Differences in replacement patterns and frequencies of treefalls were investigated for subalpine fir (Abies lasiocarpa) and Englemann spruce (Picea engelmannii) in the Colorado Front Range. In seven old—growth unlogged stands, data were collected on age and size of trees, frequencies of fallen trees, growth rates of trees, and sizes and abundances of occupants of treefall gaps. Subalpine fir accounted for 74.5% of the potential successors in the 125 treefall gaps sampled. The much greater abundance of subcanopy trees of subalpine fir implies more frequent recruitment into the canopy. However, its greater recruitment rate into the canopy is approximately balanced by its greater loss from the canopy; even though subalpine fir accounted for only 37% of the canopy trees it accounted for 76% of the fallen trees. The consistently lower frequency of Engelmann spruce as treefalls and its great longevity compared to subalpine fir imply a lower adult mortality rate for the spruce. Consequently, the greater proportion of young subalpine fir does not imply that it will gradually replace Engelmann spruce in old—growth stands unaffected by large—scale exogenous disturbance. The results of this study provide empirical support for the coexistence of ecologically similar species by means of different life history strategies.
Experimental Evidence for Defense of Inga (Mimosoideae) Saplings by Ants
Authors: Koptur Suzanne
Published: May 2006

Inga densiflora and I. punctata have foliar nectaries that are visited by a variety of ants. Extrafloral nectar secretion on new leaves is continuous throughout the day and night; ant activity on both old and new leaves is also continuous. Experiments compared the effectiveness of five species of ants in removing caterpillars and three other types of insect herbivores from leaves. All ant species tested could remove insect herbivores to some extent; of these ants, Pheidole biconstricta was the most effective. Leaves from which ants were excluded suffered substantially greater damage than control leaves. Ant—defense is a generalized and effective defense operating in the protection of Inga saplings against herbivores, but the extent of protection may vary according to what species of ants visit the nectaries–a consequence of facultative mutualism.
Ribbed Mussels and Spartina Alterniflora Production in a New England Salt Marsh
Authors: Bertness Mark D.
Published: May 2006

The ribbed mussel, Geukensia demissa, is commonly found associated with the salt marsh cordgrass, Spartina alterniflora. Mussels attach to the basal portion of S. alterniflora stems with strong proteinaceous byssal threads and deposit fecal material on the surrounding sediment as a byproduct of their filter—feeding activity. Here I demonstrate by manipulating mussel densities in the field that the presence of G. demissa stimulates S. alterniflora growth, and examine experimentally a number of potential mechanisms of this facilitation. In the natural habitat, mussel density is positively correlated with increased grass height, biomass, and flowering, and experimental removal and addition of mussels in these habitats demonstrates that mussels stimulate both aboveground and belowground S. alterniflora production. In tall—form S. alterniflora habitat, net primary production is positively correlated with mussel density and soil nitrogen levels. Experiments in this habitat show that mussels increase soil nitrogen, and this increase in nutrients would appear to be responsible for stimulating S. alterniflora growth. On the seaward edge of the marsh, net primary production is strongly correlated with mussel density, but not soil nutrients. On the marsh edge, mussels are shown to bind sediments and prevent erosion and physical disturbance. The relationship between S. alterniflora and G. demissa appears to represent a facultative mutualism that leads to increased marsh net primary production and stability. While previous research has shown that S. alterniflora production increases G. demissa growth and reproductive success, G. demissa also has strong stimulating effects on S. alterniflora. Nonconsumer plant—animal interactions such as the G. demissa—S. alterniflora association are potentially important determinants of marsh growth, stability, and community structure that have not been previously appreciated.
Adaptation of Some Large North American Mammals for Survival In Snow
Authors: Telfer Edmund S. , Kelsall John P.
Published: May 2006

Chest heights, foot loading, and behavior of moose (Alces alces), caribou (Rangifer tarandus), wapiti (Cervus canadensis), white—tailed deer (Odocoileus virginianus), Dall sheep (Ovis dalli), bighorn sheep (Ovis canadensis), pronghorn antelope (Antilocapra americana), and bis (Bison bison) were used to calculate indices of snow—coping ability for each species. When species were grouped into regional faunas, those with higher indices occurred in more snowy regions. Within a local area, a variety of species can survive in winter by living and feeding in different habitats defined on the basis of snow conditions. Bison, deer, and wapiti have considerable sexual dimorphism in chest height and foot loading. These differences between the sexes have potential value for individual survival in snow by permitting use of food resources over a greater proportion of the species' home range. Wolves (Canis lupus), coyotes (C. latrans), and a wolverine (Gulo luscus) showed similar levels of morphological adapation to snow, higher than all but two of the ungulates studied (caribou and moose); predation appears to have been a factor influencing evolution of ungulate behavior in snow.
Stream Periphyton and Insect Herbivores: An Experimental Study of Grazing by a Caddisfly Population
Authors: Lamberti Gary A. , Resh Vincent H.
Published: May 2006

The effects of grazing by the herbivorous caddisfly Helicopsyche borealis on benthic algae and bacteria were experimentally studied in a northern California stream, Big Sulphur Creek. By elevating artificial substrate tiles above the stream bottom, larvae of Helicopsyche, but not other herbivorous insects, were effectively excluded. In three replicate experiments, grazing by Helicopsyche resulted in low amounts of algae (as chlorophyll a, 0.3—1.4 mg/cm2) and bacteria (0.3—0.7 ° 108 cells/cm2) but a high algal turnover rate (O2 evolved per unit chlorophyll a = 34 mg°mg—1 h—1). When larvae were excluded, higher amounts of algae (as chlorophyll a, 8.1—15.1 mg/cm2) and bacteria (1.9—4.8 ° 108 cells/cm2) were present, but the algal turnover rate (O2 evolved per unit chlorophyll a ° 7 μ°μ—1 h—1) was lower. In a second set of field experiments, the effects of periphyton on the density and spatial distribution of grazers were assessed by varying the standing crop of periphyton. Nearest—neighbor analysis indicated that Helicopsyche larvae were aggregated on periphyton with higher standing crop than that which was present as background levels on surrounding substrate. Such larvae became randomly distributed after their grazing reduced the experimentally enhanced periphyton to the background levels. Helicopsyche's grazing resulted in a diatom—dominated algal community with a low standing crop that, because of a high turnover rate, was able to support a high biomass of consumers. This high rate of production probably occurred because competition for light and nutrients was less severe in the grazed than in the ungrazed periphyton.
Interactions of Snowshoe Hare and Feltleaf Willow in Alaska
Authors: Bryant John P. , Wieland Gregory D. , Clausen Thomas , Kuropat Peggy
Published: May 2006

Evidence from experimental feeding trials and chemical analysis showed that, when severe winter browsing of adult—form Alaska feltleaf willow (Salix alaxensis) causes it to revert to juvenile—form stump sprouts, the twigs are unpalatable and of lower nutritional quality to the hares than are twigs of adult—form S. alaxensis plants. Further feeding choice trials demonstrated that the palatability of winter—dormant S. alaxensis to snowshoe hares is a function of the age of the sprout and its chemistry rather than the accessibility (height) or morphology of the twigs. The low nutritional quality of juvenile sprouts is partially caused by decreased dry matter and nitrogen digestibility, which may be related to increased lignin and phenolic content of the twigs. This browsing—induced resistance to future winter browsing by hares relaxes within 3 yr and within the height range of twigs normally available to snowshoe hares in winter. These results are compatible with hypothesis that the inducation and relaxation of resistance to browsing in the preferred winter food of snowshoe hares is a cause of the well—known 10—yr snowshoe hare cycle.
Cooperative Enhancement Of Reproductive Success in Tropical Wren Societies
Authors: Rabenold Kerry N.
Published: May 2006

Stripe—backed Wrens, Campylorhynchus nuchalis, live in cooperative groups of up to 14 adults in which all members share in defense of the common territory, in construction and defense of roosting and breeding nests, and in the care of young produced by a single breeding pair. A 5—yr study of a completely marked population of 25—30 groups in the Venezuelan savanna shows that breeders (principals) assisted by two or more nonreproductive auxiliaries have much greater reproductive success than those with less help. Per capita production of independent juveniles in higher, as well. Only groups with at least two auxiliaries successfully rear two clutches in a season. The proportion of nesting attempts successful in producing fledglings is much higher for groups of four adults or more than for smaller groups, probably because of lesser predation on the nests of large groups. The success of large groups is not likely due to differences in territory quality of breeder experience. Rates of food delivery to nestlings and number of fledglings per successful nest are as high for pairs and trios as for larger groups. However, feeding of nestlings and fledglings by auxiliaries frees the principals from these tasks and probably makes multiple successful clutches possible. Auxiliaries directly defend the breeding nest from predators and competitors, although the principle male is most active in defense. Feeding efforts of auxiliaries are probably linked to increased nesting success primarily through enabling the principal male to defend the nest better. Aid—giving by auxiliaries is explained (1) by its effectiveness in improving the reproductive success of close kin through reducing risk of predation for nestlings, (2) by the probability of reciprocation of such effective help, and (3) by the low productivity of breeding outside a large established group. Data from this population of wrens constitute some of the strongest evidence to data that group defense against predators can provide the impetus for cooperative breeding.
Modeling Terrestrial Ecosystems in the Global Carbon Cycle With Shifts in Carbon Storage Capacity by Land‐Use Change
Authors: Emanuel William R. , Killough George G.
Published: May 2006

A five—compartment model for carbon cycling in the world's terrestrial ecosystems, which includes a concise treatment of the releases of carbon and shifts in carbon storage due to forest clearing, is presented. The dynamics of the less abundant isotopes, 13C and 14C, are included in the model to allow interpretation of available isotopic time series. The sensitivity of the net carbon flux between the terrestrial component of the model and the atmosphere to 10% variability in terrestrial rate coefficients and grown parameters is examined; for the particular case considered here, the variability in model response is °10%. Response of the model agrees reasonably well with observations of historical changes in the specific activity of 14C in the atmosphere. The model—calculated Seuss effect in 1952 is 2%, and the time constant of the expotential decrease in atmospheric 14C following the weapons test ban is 14 yr. By adjusting the releases of carbon due to forest clearing, a fit of model response to 13C/12C tree—ring time series is derived. The resulting forest—clearing carbon release function rises to 2.5 Pg/yr by 1910 and remains constant to the present. Due to establishment of ground vegetation following clearing, the net carbon flux from the terrestrial biotic system to the atmosphere is less than the release due to clearing in some instances. To accommodate this net input to the atmosphere in addition to that due to fossil fuel combustion, the pre—industrial CO2 concentration must be assumed to have been lower than is implied by extrapolating the modern instrument records backward in time.
Long‐Distance Dispersal and Colonization in the Bay Checkerspot Butterfly, Euphydryas Editha Bayensis
Authors: Harrison Susan
Published: May 2006

Dispersal, patch finding, and colonization abilities of the Bay checkerspot butterfly (Euphydryas editha bayensis) were measured in field experiments. Butterflies released in non—habitat were capable of moving several kilometres to reach a distant habitat patch. However, their interhabitat movements appeared to be non—oriented, unless they were within 50 m of a suitable patch. To quantify colonization ability, I transplanted propagules of 100 postdiapause larvae each to 38 vacant habitats. Twenty—four transplants (63%) produced adults in the initial year, and 6(25%) of these initially successful transplants persisted to produce adults one year later. Thus the chances of successful population establishment, given the presence of gravid female on a vacant habitat, appear to be low (i.e., at most a 6.25% chance of persisting 2 yr). The present distribution of the butterfly suggests that hilly intervening terrain inhibits the colonization of distant habitat patches. These results shed additional light upon the dynamics of a metapopulation of the Bay checkerspot butterfly.
Costs of Flower and Fruit Production in Tipularia Discolor (Orchidaceae)
Authors: Snow Allison A. , Whigham Dennis F.
Published: May 2006

The cost of reproduction may be an important constraint on the evolution of life history traits, yet it has seldom been adequately measured in plants. To demonstrate a net cost of reproductive structures one must show that their production has a negative effect on future growth. Using a summer—deciduous orchid, we compared subsequent growth of plants with no inflorescence, no fruits, few fruits (<10), or many fruits (>10). Greater investment in reproduction led to increasingly negative effects on future corm size and leaf area. The cost of an inflorescence without fruits was about one—half the cost of an inflorescence with few fruits. Each fruit cost the plant °2% of its total leaf area the next year. Natural pollination resulted in a mean of seven fruits per plant. Hand—pollination led to a dramatic increase in fruit set (up to 25 fruits per plant), and a large decrease in subsequent growth. Plants with many fruits were less likely to propagate vegetatively than those with few fruits. Also, plants with many fruits were less likely to flower the following year, probably because they were smaller than those with few fruits. The potential benefits of greater pollinator visitation are unclear because larger "clutch size" may not result in greater lifetime fecundity.
Vegetational Changes in Western Illinois During the Onset of Late Wisconsinan Glaciation
Authors: Baker R. G. , Sullivan A. E. , Hallberg G. R. , Horton D. G.
Published: May 2006

Peats and silts near Biggsville, western Illinois, have yielded the most detailed paleoecological record available in the midcontinent for the time when Late—Wisconsinan glaciers began to advance. These deposits were analyzed for pollen, vascular—plant macrofossils, bryophytes, insects, and stratigraphy, and date from °28 000 to 22 000 BP. Upland vegetation was a Picea—Pinus banksiana forest from 27 900 to 26 600 BP. From 26 600 to °22 700 BP, Picea and Larix laricina were the dominant trees, and Pinus had all but disappeared. This change probably reflects a climatic cooling when glaciers began to advance into the Midwest. A decrease in Picea pollen percentages and total pollen accumulation rate, an increase in Cyperaceae and other non—arboreal pollen, and the appearance of Selaginella selaginoides after 22 700 BP suggest a further cooling, and indicate an open environment as Late—Wisconsinan glaciers advanced into Illinois. Locally, a small pond, which accumulated organic silts and supported a rich aquatic flora, was present from 27 900 to 27 000 BP. Peatlands surrounding the pond were dominated by rich fen bryophytes and a diverse wetland herbaceous flora. A Picea mariana muskeg replaced the pond and fen between 27 000 and 23 500 BP, the peat became increasingly woody, and the wetland became dominated by P. mariana, Larix laricina, and Betula glandulosa. Peat deposition ceased by 23 500 BP when losses accumulation began; small ponds again were present in the lowland, but they supported only a sparse aquatic flora. By 21 400 BP, the basal loess and paleosol had capped the peats, and subsequently rapid loess deposition buried all previous deposits.
Clonal Diversity in Populations of Polysphondylium Pallidum, A Cellular Slime Mold
Authors: Ketcham Robert B. , Eisenberg Robert M.
Published: May 2006

Populations of the cellular slime mold Polysphondylium pallidum contain clones that differ in their abilities to use bacterial resources. We found abundant diversity among isolates obtained from plots of several square metres and from single soil cores 1.1 cm in diameter. The variation we were measuring was not due to genetic changes induced by our manipulation. Previously, isolation of different species of cellular slime molds from sample areas, metres to hectares in extent, has prompted studies of mechanisms of species coexistence in the face of assumed competition for bacterial resources. We conclude that competition has not only failed to eliminate different species from the community but has failed to eliminate clonal diversity from within species. Previous explanations of species coexistence need reevaluation.
Space‐Limited Recruitment in Open Systems: The Importance of Time Delays
Authors: Bence J. R. , Nisbet R. M.
Published: May 2006

We study the dynamic behavior of open systems where older or larger individuals can inhibit the recruitment of juveniles into the population. Our approach is to examine the predictions of simple dynamic models. In these models, settlement rate into open space is decoupled from local population density. Our more elaborate models are special cases of Roughgarden et al.'s (1985) model for an open marine population with space—limited recruitment. We found, as did Roughgarden et al., that two qualitatively different dynamic outcomes were possible: a stable steady state, and cyclic fluctuation in population density and space occupied. The crucial factor needed to produce cyclic fluctuations is a time delay between settlement and recruitment into the adult population. The introduction of density—dependent mortality in adults serves to stabilize the limit cycle. The effect of growth or development rate on stability depends upon the relationship between these rates and individual size or age. This is because increased growth may act to increase the area occupied at equilibrium (a destabilizing factor), but can cause developmental time lags to either increase or decrease in length. As a result, an unstable system can be stabilized by either an increase or a decrease in juvenile growth rate when adults do not grow, or grow more slowly than juveniles.
Dispersal of Seeds as Nest Material by Birds in Semiarid Karoo Shrubland
Authors: Dean W. R. J. , Milton Suzanne J. , Siegfried W. Roy
Published: May 2006

Seeds of 55 species of plants were incorporated into the nests of 31 common bird species in the semiarid shrubland of the southern Karoo, South Africa. However, only three plant species were used abundantly by a variety of birds. Nest lining and structural materials included many viable seeds. Our findings suggest that birds facilitated directed dispersal of the creeper Galiurn tornentosurn to sites near trees, and that they moved shrub seeds away from conspecifics. We postulate that plant species with cottony seed coverings (e.g., Eriocephalus) or indehiscent fruits on woolly or branched peduncles, such as Galiurn tornentosurn, are adapted for directed dispersal by birds as nest material.
The Self‐Thinning Rule: Dead or Alive?
Authors: Lonsdale W. M.
Published: May 2006

The self—thinning rule predicts that for a crowded, even—aged plant population, a log—log plot of total plant mass against plant density will give a straight line of slope —1/2. It has been described as one of the more general principles of plant population biology, but the evidence supporting it has recently come under close scrutiny. Recent reevaluations have concluded that the slope is much more variable than previous authors have claimed, that straight lines are the exception rather than the rule, and that the slope varies with aspects of the biology of the plant. Using a range of statistical techniques, I examined the published evidence to test the strength of these conclusions. Much of the recently reported variability in slope has resulted from the inclusion of inappropriate data sets: not all populations for which biomass and density data are available are undergoing self—thinning. I also found that there was no evidence for relationships between shade tolerance or taxonomic groups and the slope, and that the reported relationships between the slope and various allometric growth constants, though real, were weak. The combined data for all populations are not consistent with an interspecific relationship of slope —1/2, the slope being somewhat shallower, at —0.379, possibly because only the stem mass of trees is generally measured. I conclude that there is no evidence at present for a —1/2 power rule of self—thinning, but that final rejection of the idea that there is an ideal slope (which may or may not be —1/2), awaits experiments in which resource levels are carefully controlled.
Phytoplankton in an Oligotrophic Ocean: Species Structure and Interannual Variability
Authors: Venrick E. L.
Published: May 2006

I have examined the interannual variability of the rank order of abundance of phytoplankton species in the North Pacific central environment. Discrete data were integrated within the shallow association (0 to °100 m) and the deep association (°100 to 200 m) of each of 10 stations occupied during six summers between 1973 and 1985 (12.2 yr). Two hundred and forty—five and 231 species were observed in the shallow and deep zones, respectively. In both zones, 21 species accounted for 90% of the individuals. Each zone contained a large number of rare species that, in spite of their low abundances, were frequently present. To reduce analytical bias, the evaluation of interannual variability was based on a reduced list of 194 species. The period between 1973 and 1985 represented @>1000 generation times. During this time there was a slow change in the structure of both phytoplankton assemblages. Change was greater among the species of the deeper association and particularly among the diatoms of the layer. The correlations between the rank orders of abundance never approached zero, even for samples separated by 12.2 yr. Samples from within in the same zone were more similar to each other than were deep and shallow samples collected at the same instant of time, even though the vertical separation of these latter samples was 20 m or less. This study supports earlier conclusions that the epiplanktonic populations of the North Pacific central environment are more stable than those of most other planktonic ecosystems. Most of the variance in abundance is concentrated in the smaller spatio—temporal scales. It is possible that whatever mechanisms moderate species coexistence are operating on these scales.
Invertebrate Predation and Lotic Prey Communities: Evaluation of In Situ Enclosure/Exclosure Experiments
Authors: Walde Sandra J. , Davies Ronald W.
Published: May 2006

The influence of the stonefly Kogotus nonus on prey communities was assessed by varying the predator density within small enclosures in a first—order stream in southwestern Alberta. Experimental containers with natural densities of prey, standardized substrate and detritus, and zero, one, two, or three Kogotus were buried in a riffle for 10 during three periods: July 1981, June 1982, and July 1982. Kogotus depressed the densities of Thienemaniella and some other Orthocladiinae, but had no effect on the densities of Corynoneura (Orthocladiinae), Stempellinella (Tanytarsini), and Baetis tricaudatus (Ephemeroptera). Significant reduction of the densities of Thienemaniella and other Orthocladiinae occurred only when these prey items were present at high densities. The extent to which prey densities were depressed was found to be dependent on predator density within the enclosures: increased predator densities resulted in lower prey densities. However, when predator density in the containers exceeded that commonly observed in the riffle, predator efficiency dropped, probably because of interference between predators, the existence of prey refuges, or changes in predator search effort. An experimental design in which containers that excluded predators were compared with controls accessible to predators was used to determine if results would be similar to results from the enclosure experiments. Using this design, observed results could not clearly be attributed to the presence or absence of the predator, and were more likely due to differences in abiotic conditions between the experimental and control containers. It was concluded that observation of predator effects in field manipulative experiments may be strongly dependent on the selection of an experimental design in which predator effects are not confounded with container effects.