bluuebunny/crossref_metadata_2025 huggingface dataset:
An Experimental Study of Plant Community Invasibility
Authors: Burke M. J. W. , Grime J. P.
Published: May 2006

A long—term field experiment in limestone grassland near Buxton (North Derbyshire, United Kingdom) was designed to identify plant attributes and vegetation characteristics conducive to successful invasion. Plots containing crossed, continuous gradients of fertilizer addition and disturbance intensity were subjected to a single—seed inoculum comprising a wide range of plant functional types and 54 species not originally present at the site. Several disturbance treatments were applied; these included the creation of gaps of contrasting size and the mowing of the vegetation to different heights and at different times of the year. This paper analyzes the factors controlling the initial phase of the resulting invasions within the plots subject to gap creation. The susceptibility of the indigenous community to invasion was strongly related to the availability of bare ground created, but greatest success occurred where disturbance coincided with eutrophication. Disturbance damage to the indigenous dominants (particularly Festuca ovina) was an important determinant of seedling establishment by the sown invaders. Large seed size was identified as an important characteristic allowing certain species to establish relatively evenly across the productivity—disturbance matrix; smaller—seeded species were more dependent on disturbance for establishment. Successful and unsuccessful invaders were also distinguished to some extent by differences in germination requirements and present geographical distribution.
Physiological and Growth Responses of Arctic Plants to a Field Experiment Simulating Climatic Change
Authors: Chapin F. Stuart , Shaver Gaius R.
Published: May 2006

Field manipulations of light, temperature, nutrients, and length of growing season in directions simulating global environmental change altered biomass of the four most abundant vascular plant species in tussock tundra of northern Alaska. These species are Betula nana, Ledum palustre, Vaccinium vitis—idaea, and Eriophorum vaginatum. Biomass response to our treatments reflected changes in both growth and mortality, with growth being stimulated by treatments that enhanced biomass, and mortality being enhanced by all treatments (except in Vaccinium). Those species with highest leaf and stem turnover (the graminoid and deciduous shrub) initially showed large positive responses to nutrient addition. By contrast, slow—turnover evergreen species showed little initial change in production in response to our manipulations, and their long—term biomass responses were in the opposite direction to those of the responsive species. Short—term measurements of leaf expansion, photosynthesis, and phosphate uptake showed little correlation with net production or biomass change in response to manipulations because of compensatory mechanisms at levels of growth and allocation. Changes in nutrient distribution among species accounted for many of the long—term changes in biomass and productivity. Processes that are readily integrated at annual time steps (e.g., shoot growth, shoot mortality, allocation) were more useful than instantaneous physiological measurements in predicting decadal vegetation changes because (1) compensating responses among physiological processes buffer plant responses at progressively longer time scales, (2) species interactions in the community buffer ecosystem processes such as productivity and nutrient cycling from changes in growth of individual species, and (3) different time lags between physiological, demographic, and ecosystem processes complicate modeling of long—term responses from short—term mechanisms
Seed Dispersers as Disease Vectors: Bird Transmission of Mistletoe Seeds to Plant Hosts
Authors: del Rio C. Martinez , Silva A. , Medel R. , Hourdequin M.
Published: May 2006

The relationship between mistletoes and birds has been studied from the perspectives of mutualism and seed dispersal. Here, we emphasize the role that avian dispersers play as agents of mistletoe seed transmission to plant hosts. We describe the patterns of transmission of the seeds of Tristerix aphyllus, an endophytic Chilean mistletoe, on two of its columnar cacti hosts (Eulychnia acida and Echinopsis skottsbergii) by the Chilean Mockingbird Mimus thenca. In north—central Chile, these cacti grow in relatively discrete subpopulations on north—facing slopes. We measured variation in seed transmission within 10 subpopulations varying in species composition, host density, parasite density, parasite prevalence (defined as the percentage of hosts infested in a given population), and disperser abundance. Seed transmission was independent of species, but was strongly dependent on prior parasitism. Parasitized individuals received seeds much more frequently than expected from their relative abundance. We found no correlation between the density of hosts and seed transmission. We found strong positive correlations, however, between parasite prevalence and seed transmission to both parasitized and nonparasitized hosts. Seed transmission of T. aphyllus seeds by M. thenca appeared to be frequency—rather than density—dependent. Seed transmission was also tightly and positively correlated with the abundance of seed—dispersing birds at each site. Because bird abundance and parasite prevalence were correlated, we conducted path analysis to disentangle their relative effect on seed transmission. A model including only the direct effect of bird abundance and the indirect effect of parasite prevalence through bird abundance explained roughly the same variance as a full model including both the direct and indirect effects of bird abundance and prevalence on seed transmission. Apparently, variation in bird abundance was the main determinant of variation in transmission. We suggest that mistletoes, host plants, and the birds that disperse mistletoe seeds are systems well suited for studies of the ecological and evolutionary dynamics of disease transmission.
Ectoparasitic Effects on Host Survival and Reproduction: The Drosophila‐‐ Macrocheles Association
Authors: Polak Michal
Published: May 2006

Numerous insects, including Drosophila nigrospiracula, that inhabit necrotic cacti of the Sonoran desert are host to Macrocheles subbadius, a cosmopolitan mesostigmatid mite. This paper shows that the mite pierces Drosophila integument and ingests haemolymph while attached to the abdomen of its host. Among field—caught flies, there was a strong negative relationship between mite load and wet mass of adult flies of both sexes. Thus, M. subbadius is an ectoparasite of D. nigrospiracula, contrary to the widespread belief that macrochelids form only phoretic associations with adult flies. The parasite harms its host in many ways. Under laboratory conditions, there was a significant negative effect of mite load on survivorship. In the field, there was a significant negative relationship between mite load and number of eggs carried by sexually mature females, suggesting that infested females resorb their oocytes to compensate for nutritional stress imposed by feeding mites. Likewise, in the laboratory, infestation lengthened somewhat the period prior to onset of oviposition, and decreased the number of eggs laid by females over their life. However, mite—infested females maintained on a yeast—supplemented diet overcame the potentially debilitating effects of mites, and were actually slightly, but not significantly, more fertile than uninfested females. Since the dietary conditions of flies influenced the degree to which fecundity was affected, the debilitating effect of mites is comparable to that of starvation. This conclusion derives support from the observation that laboratory females infested for 4 d, but whose mites were subsequently removed, produced fewer progeny than uninfested females. Although this effect was significant only among old females, it nevertheless indicates that the observed effects of mites on fecundity are not the result of mechanical constraints on oviposition behavior. Since intensities of infestation used in laboratory experiments were similar to those encountered in nature, observed ectoparasitic effects on fly fitness suggest that M. subbadius can reduce fly numbers in nature.
Proximate Causes of Sexual Size Dimorphism in the Iguanian Lizard Microlophus Occipitalis
Authors: Watkins Graham G.
Published: May 2006

Among iguanian lizards, males are often larger than females. As a result, much attention has been directed toward understanding the evolution of sexual size dimorphism (SSD) in this group. SSD can result from a number of developmental, physiological, and demographic processes, which may cause sex differences in growth trajectories, timing of maturity, and adult age distributions. Knowledge of these proximate mechanisms can improve our understanding of the evolution of SSD. Only recently, however, have efforts been directed toward understanding proximate mechanisms. I examined the proximate causes of SSD in a population of a South American iguanian lizard, Microlophus occipitalis. The majority of the SSD in this population resulted from continued postmaturity growth in males and reduced postmaturity growth in females. Additionally, adult males tended to live longer than females, which affected adult age and size distributions. The proportion of young individuals in samples had strong effects on temporal fluctuations in SSD in the population. I discuss the importance of studying proximate mechanisms for evolutionary analyses of SSD in organisms that continue to grow after reaching maturity.
Experimental Prevention of a Population Cycle in Red Grouse
Authors: Moss R. , Watson A. , Parr R.
Published: May 2006

Red Grouse (Lagopus lagopus scoticus) show unstable population dynamics. The number shot for sport at Rickarton moor in northeast Scotland, for example, has cycled with 10—11—yr periodicity since 1946. Here, demographic and other causes of a population cycle were documented from 1979—1989, and an experiment tested the prediction that removing some cocks during the increase phase would prevent a subsequent cyclic decline. Throughout the study, sport shooting was stopped on the area where the main work was done. During 1979—1982, before the experimental removal of cocks began, numbers over the whole moor rose from a trough at the start of the study. On the control area, the cyclic peak in 1983 was followed by a decline until 1988, as predicted in advance from models derived from a previous study elsewhere. On the experimental area, enough territorial cocks were removed each spring from 1982 to 1986 to prevent the population from attaining peak densities for five successive years, and no cyclic decline occurred. The removal of cocks resulted in similar numbers of hens being lost from the breeding population. The main demographic cause of population change on control (cycling) and experimental (cycle broken) areas was variation in the recruitment of young cocks to the spring population. On the control area, recruitment was related to cycle phase and breeding success. Changes in food, nitrogen metabolism, and parasite burdens could not explain the cycle. Demographic patterns were consistent with a model in which changes in age structure affected recruitment. These and previous results refute four hypotheses as necessary causes of population cycles in Red Grouse: (1) maternal nutrition, (2) a version of Chitty's genetic hypothesis, (3) host—parasite (caecal threadworm), and (4) predator—prey relationships. The hypothesis that age structure changes and associated behavior cause cycles by affecting recruitment and, thus, population change, remains unrefuted.
Top‐Down Impacts on Creosotebush Herbivores in a Spatially and Temporally Complex Environment
Authors: Floyd Ted
Published: May 2006

I studied top—down effects on creosotebush (Larrea tridentata) insect herbivores by protecting them from bird and arthropod predation. The purpose of my study was to investigate experimentally the differential and/or interactive effects of two predator guilds on herbivore densities. I performed my experiments on 24 creosotebushes at each of three sites within the Jornada Long—Term Ecological Research Site in the Chihuahuan Desert of southwestern New Mexico, USA. At each site the experiment consisted of: six creosotebushes from which birds were excluded with nylon mesh cages; six creosotebushes from which arthropod predators were removed by hand or aspirator; six creosotebushes from which both birds and arthropod predators were removed; and six control creosotebushes from which neither birds nor arthropod predators were removed. I conducted nondestructive nocturnal visual censuses of herbivores on each creosotebush at the beginning of the experiments in mid—May, 6 wk after the start of the experiment in late June, and 12 wk after the start of the experiment in early August. I conducted these experiments and censuses in 1993 and again in 1994. In both years the herbivore densities became significantly higher in experimental than in control creosotebushes. The effects of bird and arthropod predation on herbivore densities were additive in 1993, but they were compensatory in 1994. In 1994 arthropod predator densities became lower in creosotebushes from which birds had been removed than in creosotebushes from which birds had not been removed, but this result did not obtain in 1993. These results may be due to a combination of factors including: avian and arthropod predation on herbivores, “intraguild” predation of birds on arthropod predators, and competition within the herbivore community. The relative numerical impacts of the predator—removal experiments varied among seasons and among sites within either year, but temporal and spatial variation in predator impacts did not correlate strongly with known gradients of climatic or bottom—up heterogeneity in this system. The results of this study confirm the important direct and cumulative effects of multiple predator guilds, even against a complex background of temporal and spatial heterogeneity.
Disease in Metapopulation Models: Implications for Conservation
Authors: Hess George
Published: May 2006

Several conservation measures are intended to enhance the movement of individuals among populations. These include the establishment of wildlife corridors, captive breeding and release programs, and translocation of individuals among populations. Many metapopulation models show that increasing movement among populations. Many metapopulation models show that increasing movement among populations reduces the chance of metapopulation extinction. However, epidemiological models indicate that increased contact among populations enhances the spread of disease and can trigger epidemics. I have synthesized elements of mathematical epidemiology with metapopulation models. An analytic model showed that highly contagious diseases of moderate severity spread widely, increasing the probability of metapopulation extinction. I also used a simulating model to examine four spatial arrangements of populations: island, necklace, loop, and spider. When infected individuals were allowed to move freely among populations, all of the arrangements exhibited qualitative behavior similar to that exhibited by the analytic models. The most dangerous diseases were those for which infected populations grew large enough to produce dispersers that infected other populations, but which also reduced the geometric rate of increase for infected populations to near unity. Under those conditions, random demographic and environmental events caused metapopulation extinction. Major differences among the spatial arrangements emerged when a quarantine population was established. A centralized quarantine in the spider and necklace arrangement yielded the most dramatic reductions in metapopulation extinction probability. A single quarantine patch was of little value in an island arrangement. These results have several implications for managing metapopulations. Most notably, some spatial arrangements of populations are more amenable to disease control than others, and establishing a quarantine population can increase the probability of detecting new diseases and reduce the impact of diseases that do appear.
Disease, Endocrine‐‐Immune Interactions, and Sexual Selection
Authors: Zuk Marlene
Published: May 2006

Parasites and disease have been increasingly recognized as agents of ecological and evolutionary importance. Because the endocrine and immune systems are related, it is also likely that hormonally controlled traits, such as secondary sexual characters, may be influenced by responses to disease. Testosterone appears to be associated with a suppressed immune system in many vertebrates, and evidence from captive Red Jungle Fowl suggests that the sexes differ in blood parameters involved in immunity and that comb length is negatively correlated with lymphocyte number in males. Immune system parameters may provide a useful gauge of condition in captive and field—caught animals.
On the Spatial Pattern of Soil Nutrients in Desert Ecosystems
Authors: Schlesinger William H. , Raikes Jane A. , Hartley Anne E. , Cross Anne F.
Published: May 2006

We examined the spatial distribution of soil nutrients in desert ecosystems of the southwestern United States to test the hypothesis that the invasion of semiarid grasslands by desert shrubs is associated with the development of "islands of fertility" under shrubs. In grasslands of the Chihuahuan Desert of New Mexico, 35—76% of the variation in soil N was found at distances <20 cm, which may be due to local accumulations of soil N under Bouteloua eriopoda, a perennial bunchgrass. The remaining variance is found over distances extending to 7 m, which is unlikely to be related to nutrient cycling by grasses. In adjacent shrublands, in which Larrea tridentata has replaced these grasses over the last century, soil N is more concentrated under shrubs and autocorrelated over distances extending 1.0—3.0 m, similar to mean shrub size and reflecting local nutrient cycling by shrubs. A similar pattern was seen in the shrublands of the Mojave Desert of California. Soil PO_4, Cl, SO_4, and K also accumulate under desert shrubs, whereas Rb, Na, Li, Ca, Mg, and Sr are usually more concentrated in the intershrub spaces. Changes in the distribution of soil properties may be a useful index of desertification in arid and semiarid grasslands worldwide.
Tropical Tree Communities: A Test of the Nonequilibrium Hypothesis
Authors: Terborgh John , Foster Robin B. , Percy Nunez V.
Published: May 2006

We have conducted a test of Hubbell's nonequilibrium model of tropical forest dynamics and found that it fails to account for high levels of compositional similarity in disjunct samples of floodplain forest in the Manu River basin of southeastern Peru. In describing how the composition of a tropical forest would change over many tree generations in the absence of density dependence, Hubbell's model, as presented, is untestable. However, by substituting space for time, it can be tested. Here we take advantage of the meandering of the Manu River to generate independently replicated, primary—successional chronosequences which culminate in tall, "mature—phase" forests possessing a characteristic complex vertical structure. By the logic of the nonequilibrium hypothesis, spatially uncorrelated forests should show uncorrelated species abundances. We tested this prediction by examining the compositional similarity of 1000—tree samples in four types of comparisons: (1) between nearby (potentially spatially correlated) and remote (presumably uncorrelated) plots of mature floodplain forest, (2) between real and synthetic forests generated by scrambling the relative abundances of the species, (3) between successional stages and the "mature phase" of floodplain forest, and (4) between mature floodplain forest and adjacent upland forest on a different soil type. In all comparisons, five mature floodplain—forest plots showed higher levels of similarity inter se than to any of the other real or synthetic forests. The 10 most abundant species in the three potentially spatially correlated plots appeared prominently in the 10 top ranks of two remote plots located 30 and 39 km to the east. The probability of obtaining the observed result under the expectation of the nonequilibrium hypothesis was $\approx 10‐7. We conclude with a plea for the application of biological models to the problem of tropical forest tree diversity.
Irradiance and Spectral Quality Affect Asian Tropical Rain Forest Tree Seedling Development
Authors: Lee David W. , Baskaran Krishnapillay , Mansor Marzalina , Mohamad Haris , Yap Son Kheong
Published: May 2006

Plant developmental responses to shade are the combination of reductions in photosynthetic photon flux density (PPFD) and changes in spectral quality (reductions in the quantum ratio of red to far—red band widths, R:FR). We studied the seedling development of six Asian tropical rain forest trees, Dryobalanops aromatica, Endospermum malaccense, Hopea wightiana, Parkia javanica, Shorea singkawang, and Sindora echinocalyx under varying PPFD and R:FR. Seedlings were grown in replicated shadehouse treatments: (1) 40% solar PPFD and 1.25 R:FR; (2) 11% PPFD and 1.25 R:FR; (3) 11% PPFD and 0.24 R:FR; (4) 3% PPFD and 1.25 R:FR; and (5) 3% PPFD and 0.23 R:FR. Species differed in the influence of light variables on seedling (1) total height; (2) internode distance; (3) branch to trunk internodes; (4) stem length/mass; (5) leaf area/stem length; (6) percent allocation to leaf, stem and root mass; (7) specific leaf mass; (8) mean leaf area; (9) leaf thickness; (10) petiole length; and (11) stomatal density. The simple factorial design of treatments 2–5 allowed a two—way ANOVA and the calculation of coefficients of determination of the treatment effects. The characters in most taxa were primarily influenced by light intensity, but spectral quality also influenced characters in many cases. The taxa that responded most strongly to the light treatments were the most shade—intolerant: E. malaccense and P. javanica; the former species responded strongly to R:FR, particularly in stem mass allocation and leaf area/stem length. The four taxa with moderate—to—extreme shade tolerance varied considerably in responses of individual characters to R:FR and PPFD. The patterns of morphological responses to reduced PPFD and R:FR help explain how the shade tolerances of the seedlings of rain forest trees vary in a continuous manner. Recommendations concerning seedling shade tolerance for sylviculture or nursery practice may need revision if they were based on shade trials using spectrally neutral shade fabrics or slat houses. Future research on the effects of shading on tree seedling development and ecology must consider the potential influence of changes in spectral quality under canopy shade.
Photosynthesis and Leaf Nitrogen in Five Amazonian Tree Species During Early Secondary Succession
Authors: Ellsworth D. S. , Reich P. B.
Published: May 2006

Field measurements of maximum net photosynthesis (Pmax), leaf nitrogen (N) content (leaf N per area and percent N), and specific leaf area (SLA) were made for Amazonian tree species within and across early successional sites of known ages after abandonment from slash—and—burn agriculture. We examined five species across a successional sere near San Carlos de Rio Negro, Venezuela, to test whether plasticity was associated with successional status and to determine whether changes in foliar properties during secondary succession can be attributed to shifts in species composition, in resourceavailability, or both. Average leaf N concentration was high (nearly 3%) for a pioneer species (Cecropia ficifolia) early in succession (1—3 yr after abandonment) but was always lower for the other early and mid—to—late succession species, especially later in succession (1—2% at 5—10 yr after abandonment). Net photosynthetic capacity (Pmax/area and Pmax/mass) varied as much as sixfold, being higher in pioneer species such as Cecropia and Vismia on recently abandonedsites and lower in late successional species such as Miconia and Licania on 10—yr abandoned agricultural sites. Total daily light availability also varied widely (14—fold) from its peak 1 yr after farm abandonment to low levels 9 yr into succession. During the first 5 yr of secondary succession, there were significant (P < 0.05) differences in Pmax and leaf N concentration among species in any given year. In most species, Pmax values declined with increasing time since abandonment within any given site.There were important differences in photosynthetic plasticity among species: Pmax tended to be much greater in earlier than later successional species soon after abandonment. Also, the difference in Pmax among species narrowed (or reversed) over time since abandonment, largely because of decreasing Pmax in pioneer species. The results suggest that changes in both species composition and in resource availability combine toproduce the common pattern of decreasing leaf N concentration and photosynthetic rates during early rain forest succession after agriculture. Early successional species showed strong (r2 ≥ 0.57, P = 0.0001) mass—based photosynthesis—N relationships but weak (r2 = 0.40 or lower, P = 0.0001) area—based relationships both across the secondary successional sere after agriculture and acrosssites varying in types of disturbance. Both mass— and area—based photosynthesis—N relationships were poorer or not significant (P > 0.05) for mid— to late—successional species. Higher instantaneous Pmax/N and greater slopes of the photosynthesis—N relationships in early than late successional species suggest that pioneer species may show greater carbon assimilation capacity withelevated leaf N concentration on early successional sites than co—occurring species. The data suggest that early and late successional species may differ in the mode and degree of leaf—level physiological plasticity across succession.
Density‐Dependent Fecundity by Habitat Heterogeneity in an Increasing Population of Spanish Imperial Eagles
Authors: Ferrer Miguel , Donazar Jose A.
Published: May 2006

We report on a 32—yr study of a population of Spanish Imperial Eagles, Aquila adalberti, which increased during the first 16 yr of study but remained stable during the last 16 yr. We analyzed changes in the mean and variance of fecundity in relation to population density to test predictions of two hypotheses of density—dependent fecundity. According to the interference" hypothesis, as density increases, frequency of agonistic encounters increases, resulting in a relatively uniform decrease in habitat quality. Consequently, mean fecundity decreases, and no relationship is expected between density and variance in fecundity. For the habitat heterogeneity hypothesis, however, as density increases, a greater proportion of individuals are forced to occupy lower quality habitats. Thus, mean fecundity decreases and fecundity variance must increase. Additionally, for this hypothesis, fecundities in good sites are expected to be equal in both low— and high—density situations. An inverse relationship between fecundity and population size was found in this eagle population. Annual variance in productivity showed significant increases over the study period. This trend was inversely related to mean productivity. Variance in productivity was related to the year of pair establishment, being higher in recently occupied territories. Mean and variance of the longer term territories remained constant during the study period. These results are in accordance with the habitat heterogeneity hypothesis.
Habitat Heterogeneity and Survival in a Bush Cricket Metapopulation
Authors: Kindvall Oskar
Published: May 2006

The bush cricket, Metrioptera bicolor Philippi (Orthoptera, Tettigoniidae) lives in a restricted area with grassland patches separated by mainly pine forest. The regional population dynamics can be described as a metapopulation with local extinctions and colonizations. I used inventory data in order to test the following hypotheses: (1) temporal variability of local population size should be negatively correlated with habitat heterogeneity, when habitat quality changes in relation to fluctuating weather conditions; (2) extinction risk should be higher on homogeneous habitat patches than on heterogeneous patches. Both hypotheses are convincingly corroborated and I find a compensatory effect between patch area and heterogeneity. Local extinctions may occur on large, but homogeneous patches, and local populations can survive quite well on small patches that contain a mosaic of several types of grassland vegetation with different humid conditions. I discuss the implications of these findings for the preservation of endangered species that are susceptible to regional stochasticity.
Assessing Habitat Selection when Availability Changes
Authors: Arthur Stephen M. , Manly Bryan F. J. , McDonald Lyman L. , Garner Gerald W.
Published: May 2006

We present a method of comparing data on habitat use and availability that allows availability to differ among observations. This method is applicable when habitats change over time and when animals are unable to move throughout a predetermined study area between observations. We used maximum—likelihood techniques to derive an index that estimates the probability that each habitat type would be used if all were equally available. We also demonstrate how these indices can be used to compare relative use of available habitats, assign them ranks, and assess statistical differences between pairs of indices. The set of these indices for all habitats can be compared between groups of animals that represent different seasons, sex or age classes, or experimental treatments. This method allows quantitative comparisons among types and is not affected by arbitrary decisions about which habitats to include in the study. We provide an example by comparing the availability of four categories of sea ice concentration to their use by adult female polar bears (Ursus maritimus), whose movements were monitored by satellite radio tracking in the Bering and Chukchi Seas during 1990. Use of ice categories by bears was nonrandom, and the pattern of use differed between spring and late summer seasons.
Fossil Carbon/Nitrogen Ratios as a Measure of Peat Decomposition
Authors: Kuhry Peter , Vitt Dale H.
Published: May 2006

Net accumulation rates of carbon in a 9000 calendar year record of Sphagnum fuscum peat in western boreal Canada range from 13.6 to 34.9 g · m‐2·yr‐1. The depth vs. age curve is slightly convex due to generally declining net productivity at the site. Fossil carbon/nitrogen ratios of bulk Sphagnum fuscum peat and its components are used to calculate the rate and total amount of decay in the deeper anoxic peat deposits. The proportional rate of decay in the catotelm of the peatland declines linearly over time. Carbon loss from the catotelm is estimated at 50% after 1700 calendar years and 65% after 7500 calendar years. Carbon has been added to the catotelm at an average rate of 28.0 g·m‐2·yr‐1 over the last 1174 years, whereas, at present, the cumulative loss of carbon over the entire catotelm is 19.4 g·m‐2·yr‐1. The peatland continues to represent a sink for carbon.
Facultative Mutualism Between Red Mangroves and Root‐Fouling Sponges in Belizean Mangal
Authors: Ellison Aaron M. , Farnsworth Elizabeth J. , Twilley Robert R.
Published: May 2006

We report results of transplant experiments that examined direct interactions between red mangrove (Rhizophora mangle) roots and two common root—fouling sponges (Tedania ignis and Haliclona implexiformis) on carbonate—based, oligotrophic mangrove cays in Belize, Central America. On these cays, subtidal prop roots of mangroves at water's edge often extend 1—2 m below lowest low water before anchoring in the substrate and host a community of algal and invertebrate epibionts dominated by massive sponges. Live sponges transplanted onto otherwise bare roots increased root growth rate two— to fourfold relative to controls. Roots fouled naturally by these and other massive sponges produce adventitious fine rootlets that ramify throughout sponge tissue; these rootlets structurally resemble underground rootlets that function in nutrient uptake. Sponges transplanted onto bare mangrove roots induced rootlet proliferation within 4 wk. Only live sponges elicited this response, indicating that adventitious rootlet production is not simply a by—product of anoxia or darkness. Sponges transplanted onto bare roots grew 1.4—10 times faster than did sponges grown on polyvinyl chloride (PVC) tubes at identical depths and locations. Relative abundances of ${15}$N (expressed as $\delta{15}$N per mil) and ${13}$C ($\delta{13}$C per mil) in Tedania, Haliclona, an additional sponge, Ulosa ruetzleri, and rootlets, roots, stems, twigs, and leaves of mangrove hosts suggest that mangrove roots obtain dissolved inorganic nitrogen from sponges, and that sponges obtain carbon from mangrove roots. No transfer of N or C was observed in similar analyses of roots fouled by the red alga Acanthophora spicifera. We conclude that where they co—occur, massive sponges and mangroves are facultative mutualists. In mangrove forests, as in other marginal habitats, facilitations may enable increased growth and production of component species.
Variability in Recruitment of Coral Reef Fishes: The Importance of Habitat at Two Spatial Scales
Authors: Caselle Jennifer E. , Warner Robert R.
Published: May 2006

We investigated spatial and temporal variability in recruitment of coral reef fishes to ST. Croix, U.S. Virgin Islands, with the goal of assessing the importance of habitat and physical oceanogrpahic processes in determining patterns of distribution at two spatial scales. Recruitment was monitored visually each month, using SCUBA, at sites spaced equally around the island and on transects within each site. Spatial patterns of distribution were consistent within seasons and between years at both spatial scales. Although recruitment showed a seasonal pattern, there were few inter—annual differences at any spatial scale. At the largest scale, recruitment varied significantly among sites around the island, and a unique pattern of distribution was observed. On the windward shore, recruitment ranged from hig on the eastern (upcurrent) end to low on western end of the island. Recruitment showed the opposite pattern on the leeward shore, ranging from low on the eastern end to high on the western end. At the transect scale, certain features of the habitat significantly predicted recruitment density. None of the microhabitat features that prerdicted recruitment at the small scale could explain patterns of recruitment observed at the large scale. In contrast, physical process affecting larval availability or transport were more important than habitat in determining the spatial patterns of recruitment at this scale. Thus, physical oceanographic processes appeared to be responsible for patterns of recruitment to the two shores of St. Croix, and to sites along those shores. Despite consistent patterns of recruitment to sites, habitat selection did not appear to be important at the site scale. Once delivered to a site, however, settlers or new recruits distributed themselves based on habitat preferences.
Analysis of Frequency Count Data Using the Negative Binomial Distribution
Authors: White Gary C. , Bennetts Robert E.
Published: May 2006

The statistical distributions of the counts of organisms are generally skewed, and hence not normally distributed, nor are variances constant across treatments. We present a likelihood—ratio testing framework based on the negative binomial distribution that tests for the goodness of fit of this distribution to the observed counts, and then tests for differences in the mean and/or aggregation of the counts among treatments. Inferences about differences in means among treatments as well as the dispersion of the counts are possible. Simulations demonstrated that the statistical power of ANOVA is about the same as the likelihood—ratio testing procedure for testing equality of means, but our proposed testing procedure also provides information on dispersion. Type I error rates of Poisson regression exceeded the expected 5%, even when corrected for overdispersion. Count data on Orange—crowned Warblers (Vermivora celata) are used to demonstrate the procedure.
High Foliar Nitrogen in Desert Shrubs: An Important Ecosystem Trait or Defective Desert Doctrine?
Authors: Killingbeck Keith T. , Whitford Walter G.
Published: May 2006

Nitrogen concentrations in green and senesced leaves of perennial desert shrubs were compiled from a worldwide literature search to test the validity of the doctrine that desert shrubs produce foliage and leaf litter much richer in nitrogen than that in the foliage of plants from more mesic environments. Mean nitrogen concentration in the green leaves of 78 species of shrubs growing in 11 deserts on five continents (2.2%) was not different from that in 67 species of trees and shrubs growing in deciduous, and mixed deciduous forests (2.2%), and only slightly higher than that in overstory (2.0%) and understory (2.1%) plants growing in tropical wet forest. Mean nitrogen concentration in the green leaves of a ubiquitous shrub that dominates large areas of desert in the United States (Larrea tridentata, 2.1%), and in the green—stem tissues of leafless desert shrubs (2.1%) were also similar to that in plants from mesic environments. Mean green—leaf nitrogen was similar in shrubs growing in different deserts. Mean nitrogen concentration in leaf litter was 1.1% for 11 species of desert shrubs, and 1.0% for the 10 species of this group that were not capable of symbiotic nitrogen fixation. Both concentrations were lower than those routinely provided to describe nitrogen in the litter of desert shrubs (1.5—1.7%), and only slightly higher than the mean nitrogen concentration in 77 species of woody perennials growing in a wide variety of environments worldwide (0.9%). Nitrogen in the leaf litter of one desert shrub (0.4%, Brickellia laciniata) was nearly as low as the lowest leaf—litter nitrogen concentration known for any woody species (0.3%). Because nitrogen concentrations in the foliage of desert shrubs are not higher than those in plants growing in more mesic environments, the tenet that desert shrubs support extraordinarily nitrogen—rich foliage can no longer be supported.
Persistence of an Extinction‐Prone Predator‐Prey Interaction Through Metapopulation Dynamics
Authors: Holyoak Marcel , Lawler Sharon P.
Published: May 2006

In theory, predator—prey pairs with extinction—prone local populations can persist through metapopulation dynamics, wherein local populations fluctuate asynchronously, occasionally providing dispersers that prevent permanent extinction in all patches. A few studies have shown that spatial structure can extend predator—prey persistence. However, no studies have unequivocally demonstrated the asynchrony among patches, low dispersal rates, and rescue effects that prove metapopulation dynamics extend persistence. We used a protist predator—prey pair to show that spatial subdivision lengthens persistence through metapopulation dynamics. The pair comprised the predaceous ciliate, Didinium nasutum, feeding on the bacterivorous ciliate, Colpidium cf. striatum. A replicated experiment assessed how habitat subdivision affects persistence. Undivided habitats were of four volumes: 30, 180, 270, and 750 mL. Subdivided microcosms, or arrays, were groups of nine or 25 linked 30—mL bottles (270 or 750 mL total volume). In arrays, predators and prey persisted for 130 d (602 prey and 437 predator generations), at which point the experiment ended. Predators went extinct in undivided microcosms of equivalent volumes within a mean of only 70 d. Predators persisted for a mean of just 19 d in isolated 30—mL bottles (equivalent to isolated patches of arrays). In a separate experiment, prey were driven extinct in four of 15 isolated 30—mL bottles, and persistence times of predators were broadly similar. We documented the following hallmarks of metapopulation dynamics: (1) asynchronous fluctuations in different subpopulations; (2) frequent local prey extinctions and recolonizations; (3) persistence of protists in arrays, despite extinction of isolated local populations; and (4) rescue effects in predator populations. Other experiments measured dispersal rates and the effects on local dynamics of immigrant predators and prey, and initial predator: prey ratios. Only a small fraction of protists dispersed within a generation, consistent with metapopulation dynamics. Immigration of predators increased the frequency of local extinctions of prey, and immigration of prey increased the persistence of both predators and prey. Higher initial predator: prey ratios decreased the persistence of prey in undivided volumes. Although the pair persisted regionally in arrays, data indicated that local extinctions of prey were common. In array patches, predator: prey ratios were higher and predator—prey cycles were shorter than in undivided volumes. Dispersal made local dynamics more prone to extinction, yet promoted regional persistence because the risk of extinction of distant subpopulations became independent.
Local Communities of Arboreal Herbivores in Papua New Guinea: Predictors of Insect Variables
Authors: Basset Yves
Published: May 2006

As a contribution to enlarging and widening the array of studies available for examination of insect—plant relations, this study contrasts local and regional predictors of species richness and host specificity of leaf—chewing insects feeding on New Guinea trees. I tested the degree to which the variance in local insect variables (species richness, ratio of specialist to generalist chewers, abundance, and biomass) for leaf chewers associated with different species of tropical trees was accounted for by local and regional host—related variables, and by local abundance of potential enemies. Leaf—chewing insects feeding externally on ten species of forest trees were collected over a 1—yr field study in a submontane area in Papua New Guinea. The tree species were taxonomically distant, and the list included species of different successional status, different patterns of leaf production, and different heights. The affinity of chewing insects with particular tree species was ascertained by feeding trials in the laboratory. Local insect species richness varied greatly among tree species, by more than four—fold between the poorest (18 species) and the richest (94 species) tree species. The proportion of specialist insect species associated with particular tree species was significantly different among tree species and ranged from 16 to 72%. Trees supporting a rich fauna usually also supported a high proportion of specialists. The abundance and biomass of leaf—chewing insects per host species was difficult to predict. However, path analyses showed that most of the causal variance in insect species richness and in the ratio of specialist to generalist chewers was accounted for by five properties of the tree species: numbers of young leaves available throughout the year, ant (i.e., enemy) abundance, leaf palatability, leaf water content, and altitudinal range. Most of the variation in local species richness could be predicted from local processes (i.e., food resources and abundance of enemies), not from regional processes. This study and others suggest that in complex environments, such as tropical rain forests, local processes may be comparatively more important in maintaining the local species richness of insect herbivores than in less complex ones, such as temperate woodlands. The implications of these findings are evident: the effects of moderate habitat fragmentation and ecological stress may be more pronounced in tropical systems than in temperate systems and may result in a greater proportional loss of local biodiversity in the former.
Seasonal Decline in Reproductive Success of the Great Tit: Variation in Time or Quality?
Authors: Verhulst S. , van Balen J. H. , Tinbergen J. M.
Published: May 2006

Seasonal variation in reproductive success is a common feature of most organisms. To understand the evolution of breeding seasons and reproductive strategies of individual animals, it is necessary to assess the extent to which seasonal variation in reproductive success is causally related to seasonal variation in the environment ('timing' hypothesis), to differences in quality between early and late breeders or their territories ('quality' hypothesis), or to a combination of both. We manipulated timing of breeding in the Great Tit Parus major, a small passerine, to test these hypotheses. A group of experimentally delayed birds was created by removing first clutches, inducing birds to lay a replacement clutch. Reproductive success of delayed pairs was compared with control pairs that bred early and with pairs that bred late. We conclude that seasonal declines in reproductive success at the nestling stage and survival of adult females were caused by differences in quality between early and late breeders. Recruitment of fledglings into the breeding population and the occurrence of second clutches were causally related to the timing of breeding. The seasonal decline in clutch size was caused by a combination of timing and quality effects. We attempted to assess the relative importance of timing and quality in the seasonal decline in reproductive success, expressed as lifetime production of recruits. We tentatively conclude that 87% of the seasonal decline in lifetime reproductive success could be attributed to a timing effect per se, whereas quality differences between early and late breeders accounted for the remaining 13%.
Use of Amazonian Forest Fragments by Understory Insectivorous Birds
Authors: Stouffer Philip C. , Bierregaard Richard O.
Published: May 2006

We sampled understory insectivorous birds in Amazonian forest fragments from before isolation through 9 yr after isolation. We accumulated 3658 mist net captures of 84 insectivorous species in five 1—ha fragments and four 10—ha fragments. Abundance and species richness declined dramatically after isolation, even though fragments were separated from continuous forest by only 70—650 m. Three species of obligate army ant followers disappeared within the first 2 yr after isolation. Mixed—species flocks containing 13 commonly netted species disintegrated within 2—3 yr after isolation, although three species that dropped out of flocks persisted in fragments. Among insectivores not associated with flocks or army ants, only two species of edge specialists were unaffected by fragmentation. Overall, loss of forest insectivores was not compensated for by an increase in nonforest or previously uncommon species. Secondary vegetation surrounding fragments strongly affected use of fragments after isolation. Fragments surrounded by Vismia, the dominant regrowth where felled forest was burned and temporarily used as cattle pasture, remained depauperate. In contrast, many species returned to fragments by moving through regenerating forest dominated by Cecropia, which occurred in areas where the felled forest was not burned. Both 1— and 10—ha fragments surrounded by Cecropia were used by ant followers by 5 yr after isolation. Mixed—species flocks reassembled in 10—ha fragments surrounded by Cecropia by 7—9 yr after isolation, and augmented their group territories by foraging in secondary forest outside fragments. Solitary species were more variable in their responses, although several species returned to 10—ha fragments surrounded by Cecropia. Terrestrial insectivores, such as Sclerurus leafscrapers and various antbirds, did not return to any fragments, and appear to be the group most vulnerable to fragmentation. Ordination of the insectivore community showed that 1—ha fragments diverged from their pre—isolation communities more than did 10—ha fragments. Communities in 10—ha fragments surrounded by Cecropia were more closely associated with pre—isolation communities than those in fragments surrounded by Vismia. Over time, communities in 10—ha fragments surrounded by Cecropia became more like pre—isolation communities, although communities in other fragments generally continued to diverge.