Showing papers in "Oecologia in 2002"

Plant diversity in tropical forests: a review of mechanisms of species coexistence

Abstract: Evidence concerning mechanisms hypothesized to explain species coexistence in hyper-diverse communities is reviewed for tropical forest plants. Three hypotheses receive strong support. Niche differences are evident from non-random spatial distributions along micro-topographic gradients and from a survivorship-growth tradeoff during regeneration. Host-specific pests reduce recruitment near reproductive adults (the Janzen-Connell effect), and, negative density dependence occurs over larger spatial scales among the more abundant species and may regulate their populations. A fourth hypothesis, that suppressed understory plants rarely come into competition with one another, has not been considered before and has profound implications for species coexistence. These hypotheses are mutually compatible. Infrequent competition among suppressed understory plants, niche differences, and Janzen-Connell effects may facilitate the coexistence of the many rare plant species found in tropical forests while negative density dependence regulates the few most successful and abundant species.

Incorporating concentration dependence in stable isotope mixing models

Abstract: Stable isotopes are often used as natural labels to quantify the contributions of multiple sources to a mixture. For example, C and N isotopic signatures can be used to determine the fraction of three food sources in a consumer's diet. The standard dual isotope, three source linear mixing model assumes that the proportional contribution of a source to a mixture is the same for both elements (e.g., C, N). This may be a reasonable assumption if the concentrations are similar among all sources. However, one source is often particularly rich or poor in one element (e.g., N), which logically leads to a proportionate increase or decrease in the contribution of that source to the mixture for that element relative to the other element (e.g., C). We have developed a concentration-weighted linear mixing model, which assumes that for each element, a source's contribution is proportional to the contributed mass times the elemental concentration in that source. The model is outlined for two elements and three sources, but can be generalized to n elements and n+1 sources. Sensitivity analyses for C and N in three sources indicated that varying the N concentration of just one source had large and differing effects on the estimated source contributions of mass, C, and N. The same was true for a case study of bears feeding on salmon, moose, and N-poor plants. In this example, the estimated biomass contribution of salmon from the concentration-weighted model was markedly less than the standard model estimate. Application of the model to a captive feeding study of captive mink fed on salmon, lean beef, and C-rich, N-poor beef fat reproduced very closely the known dietary proportions, whereas the standard model failed to yield a set of positive source proportions. Use of this concentration-weighted model is recommended whenever the elemental concentrations vary substantially among the sources, which may occur in a variety of ecological and geochemical applications of stable isotope analysis. Possible examples besides dietary and food web studies include stable isotope analysis of water sources in soils, plants, or water bodies; geological sources for soils or marine systems; decomposition and soil organic matter dynamics, and tracing animal migration patterns. A spreadsheet for performing the calculations for this model is available at http://www.epa.gov/wed/pages/models.htm.

An ecosystem engineer, the beaver, increases species richness at the landscape scale

Abstract: Ecosystem engineering - the physical modifi- cation of habitats by organisms - has been proposed as an important mechanism for maintaining high species richness at the landscape scale by increasing habitat heterogeneity. Dams built by beaver (Castor canaden- sis) dramatically alter riparian landscapes throughout much of North America. In the central Adirondacks, New York, USA, ecosystem engineering by beaver leads to the formation of extensive wetland habitat capable of supporting herbaceous plant species not found else- where in the riparian zone. We show that by increasing habitat heterogeneity, beaver increase the number of species of herbaceous plants in the riparian zone by over 33% at a scale that encompasses both beaver-modified patches and patches with no history of beaver occupa- tion. We suggest that ecosystem engineers will increase species richness at the landscape scale whenever there are species present in a landscape that are restricted to engineered habitats during at least some stages of their life cycle.

How much variance can be explained by ecologists and evolutionary biologists

Abstract: The average amount of variance explained by the main factor of interest in ecological and evolutionary studies is an important quantity because it allows evaluation of the general strength of research findings. It also has important implications for the planning of studies. Theoretically we should be able to explain 100% of the variance in data, but randomness and noise may reduce this amount considerably in biological studies. We performed a meta-analysis using data from 43 published meta-analyses in ecology and evolution with 93 estimates of mean effect size using Pearson's r and 136 estimates using Hedges' d or g. This revealed that (depending on the exact analysis) the mean amount of variance (r 2) explained was 2.51–5.42%. The various 95% confidence intervals fell between 1.99 and 7.05%. There was a strongly positive relationship between the fail-safe number (the number of null results needed to nullify an effect) and the coefficient of determination (r 2) or effect size. Analysis at the level of individual tests of null hypotheses showed that the amount of variance key factors explained differed among fields with the largest amount in physiological ecology, lower amounts in ecology and the lowest in evolutionary studies. In all fields though, the hypothesized relationship (e.g. main effect of a fixed treatment) explained little of the variation in the trait of interest. Our finding has important implications for the interpretation of scientific studies. Across studies, the average effect size reported is between Pearson r=0.180 and 0.193 and Hedges' d=0.631 and 0.721. Thus the average sample sizes needed to conclude that a particular relationship is absent with a power of 80% and α=0.05 (two-tailed) are considerably larger than usually recorded in studies of evolution and ecology. For example, to detect r=0.193, the required sample size is 207.

Marine reserves demonstrate top-down control of community structure on temperate reefs

Abstract: Replicated ecological studies in marine reserves and associated unprotected areas are valuable in examin- ing top-down impacts on communities and the ecosys- tem-level effects of fishing. We carried out experimental studies in two temperate marine reserves to examine these top-down influences on shallow subtidal reef com- munities in northeastern New Zealand. Both reserves examined are known to support high densities of preda- tors and tethering experiments showed that the chance of predation on the dominant sea urchin, Evechinus chlorot- icus, within both reserves was approximately 7 times higher relative to outside. Predation was most intense on the smallest size class (30-40 mm) of tethered urchins, the size at which urchins cease to exhibit cryptic behav- iour. A high proportion of predation on large urchins could be attributed to the spiny lobster, Jasus edwardsii. Predation on the smaller classes was probably by both lobsters and predatory fish, predominantly the sparid Pagrus auratus. The density of adult Evechinus actively grazing the substratum in the urchin barrens habitat was found to be significantly lower at marine reserve sites (2.2±0.3 m -2 ) relative to non-reserve sites (5.5±0.4 m -2 ). There was no difference in the density of cryptic juve- niles between reserve and non-reserve sites. Reserve populations were more bimodal, with urchins between 40 and 55 mm occurring at very low numbers. Experi- mental removal of Evechinus from the urchin barrens habitat over 12 months lead to a change from a crustose coralline algal habitat to a macroalgal dominated habitat. Such macroalgal habitats were found to be more exten- sive in both reserves, where urchin densities were lower, relative to the adjacent unprotected areas that were domi- nated by urchin barrens. The patterns observed provide evidence for a top-down role of predators in structuring shallow reef communities in northeastern New Zealand and demonstrate how marine reserves can reverse the indirect effects of fishing and re-establish community- level trophic cascades.

Leaf size, specific leaf area and microhabitat distribution of chaparral woody plants: contrasting patterns in species level and community level analyses

Abstract: We examined variation in leaf size and specific leaf area (SLA) in relation to the distribution of 22 chaparral shrub species on small-scale gradients of aspect and elevation. Potential incident solar radiation (insolation) was estimated from a geographic information system to quantify microclimate affinities of these species across north- and south-facing slopes. At the community level, leaf size and SLA both declined with increasing insolation, based on average trait values for the species found in plots along the gradient. However, leaf size and SLA were not significantly correlated across species, suggesting that these two traits are decoupled and associated with different aspects of performance along this environmental gradient. For individual species, SLA was negatively correlated with species distributions along the insolation gradient, and was significantly lower in evergreen versus deciduous species. Leaf size exhibited a negative but non-significant trend in relation to insolation distribution of individual species. At the community level, variance in leaf size increased with increasing insolation. For individual species, there was a greater range of leaf size on south-facing slopes, while there was an absence of small-leaved species on north-facing slopes. These results demonstrate that analyses of plant functional traits along environmental gradients based on community level averages may obscure important aspects of trait variation and distribution among the constituent species.

13C content of ecosystem respiration is linked to precipitation and vapor pressure deficit

Abstract: Variation in the carbon isotopic composition of ecosystem respiration (δ 13 C R ) was studied for 3 years along a precipitation gradient in western Oregon, USA, using the Keeling plot approach. Study sites included six coniferous forests, dominated by Picea sitchensis, Tsuga heterophylla, Pseudotsuga menziesii, Pinus ponderosa, and Juniperus occidentalis, and ranged in location from the Pacific coast to the eastern side of the Cascade Mountains (a 250-km transect). Mean annual precipita- tion across these sites ranged from 227 to 2,760 mm. Overall δ 13 C R varied from -23.1 to -33.1‰, and within a single forest, it varied in magnitude by 3.5-8.5‰. Mean annual δ 13 CR differed significantly in the forests and was strongly correlated with mean annual precipitation. The carbon isotope ratio of carbon stocks (leaves, fine roots, litter, and soil organic matter) varied similarly with mean precipitation (more positive at the drier sites). There was a strong link between δ13C R and the vapor saturation deficit of air ( vpd) 5-10 days earlier, both across and within sites. This relationship is consistent with stomatal regulation of gas exchange and associated changes in photosynthetic carbon isotope discrimination. Recent freeze events caused significant deviation from the δ13C R versus vpd relationship, resulting in higher than expected δ 13 CR values.

A rainout shelter design for intercepting different amounts of rainfall

Abstract: Field manipulative experiments represent a straightforward way to explore temporal relationships between annual precipitation and productivity. Water exclusion usually involves the use of rainout shelters, which are in general formed by a complete roof that intercepts 100% of the rainfall and require complicated mechanisms to move the shelter into place. The rainout-shelter design described here is a fixed-location shelter with a roof consisting of bands of transparent acrylic that blocks different amounts of rainfall while minimally affecting other environmental variables. We constructed thirty 3.76-m2 shelters in an arid steppe near Rio Mayo, Argentina (at 45°41′S, 70°16′W), to impose 30%, 55%, and 80% of rainfall interception. We tested the effectiveness of the design by collecting all the intercepted water in storage tanks and we evaluated changes in soil water content with the time domain reflectometry technique. We also measured soil water content in regular grids to assess the magnitude of the edge effect. We analysed the microclimate impact of the shelters by measuring photosynthetically active radiation and air and soil temperature inside and outside shelters. We did not detect significant differences between the observed and the expected rainfall interception for the 30% and 55% interception treatments but the 80% shelters intercepted 71% of incoming rainfall, which was significantly (P<0.05) lower than the expected value. Soil water content was significantly (P<0.05) higher in the control plots than in the plots with rainout shelter at all dates, except in January (summer). Radiation was not affected by the 30% interception treatment, but the roof with the largest number of acrylics bands (80% interception treatment) reduced incident radiation throughout the day by 10%. Air and soil temperatures were lower under than outside the shelters during the period of highest radiation but the opposite occurred with low radiation but with smaller differences. The two characteristics of the shelter, fixed design and low cost, allow for proper replication in space, which is required in ecological field experiments.

Xylem hydraulic properties of roots and stems of nine Mediterranean woody species

Abstract: We studied the hydraulic architecture and water relations of nine co-occurring woody species in a Spanish evergreen oak forest over the course of a dry season. Our main objectives were to: (1) test the existence of a trade-off between hydraulic conductivity and security in the xylem, and (2) establish the safety margins at which the species operated in relation to hydraulic failure, and compare these safety margins between species and tissues (roots vs. stems). Our results showed that the relationship between specific hydraulic conductivity (Ks) and resistance to cavitation followed a power function with exponent ≈–2, consistent with the existence of a trade-off between conductivity and security in the xylem, and also consistent with a linear relationship between vessel diameter and the size of inter-vessel pores. The diameter of xylem conduits, Ks and vulnerability to xylem embolism were always higher in roots than in stems of the same species. Safety margins from hydraulic failure were narrower in roots than in stems. Among species, the water potential (Ψ) at which 50% of conductivity was lost due to embolism ranged between –0.9 and Cistus albidus=Ilex aquifolium>Phillyrea latifolia>Juniperus oxycedrus. Gas exchange and seasonal Ψ minima were in general correlated with resistance to xylem embolism. Hydraulic safety margins differed markedly among species, with some of them (J. oxycedrus, I. aquifolium, P. latifolia) showing a xylem overly resistant to cavitation. We hypothesize that this overly resistant xylem may be related to the shape of the relationship between Ks and security we have found.

Complex life cycles and density dependence: assessing the contribution of egg mortality to amphibian declines.

Abstract: In the last decade there has been increasing evidence of amphibian declines from relatively pristine areas. Some declines are hypothesized to be the result of egg mortality caused by factors such as elevated solar UV-B irradiation, chemical pollutants, pathogenic fungi, and climate change. However, the population-level consequences of egg mortality have not been examined explicitly, and may be complicated by density dependence in intervening life-history stages. Here we develop a demographic model for two amphibians with contrasting life-history strategies, Bufo boreas and Ambystoma macrodactylum. We then use the complementary approaches of elasticity and limitation to examine the relationships among stage-specific survival rates, larval-stage density dependence and amphibian population dynamics. Elasticity analyses showed that for a range of density dependence scenarios both species were more sensitive to changes in post-embryonic survival parameters, particularly juvenile survival, than to egg survival, suggesting that mortality of later stages may play an important role in driving declines. Limitation analyses revealed that larval density dependence can dramatically alter the consequences of early mortality, reducing or even reversing the expected population-level effects of egg mortality. Thus, greater focus on later life stages and density dependence is called for to accurately assess how stressors are likely to affect amphibian populations of conservation concern.

Blowing in the wind: a field test of overland dispersal and colonization by aquatic invertebrates

Abstract: Despite the importance of dispersal to ecology, accurate estimates of dispersal rates are often difficult to obtain, especially for organisms that rely on passive dispersal of propagules to colonize new sites. To investigate potential dispersal vectors and relative colonization rates of zooplankton, we conducted a field experiment in which we restricted potential dispersal vectors (insects, birds, amphibians) from transporting zooplankton to mesocosms. Twenty-six non-insect invertebrate taxa invaded our array during 2 years. Colonization rates of organisms varied considerably, with some species appearing several weeks after the experiment began and others appearing after a year. We observed no difference in colonization rates among treatments, suggesting that species were transported to our experiment primarily by wind or rain, rather than by animal vectors. The absence of an additional 13 zooplankton species common in ponds immediately adjacent to the array either occurred because of dispersal limitation or an inability to invade the existing communities. Ecologists generally assume that all zooplankton are rapidly dispersed hence the potential for dispersal limitation is generally ignored. Our results suggest that zooplankton vary in their dispersal and colonization ability. Hence, increased attention should be focused on the potential role of dispersal limitation and its importance for understanding the structure and function of aquatic communities.

The relationship between tree height and leaf area: sapwood area ratio.

Abstract: The leaf area to sapwood area ratio ( Al:As) of trees has been hypothesized to decrease as trees become older and taller. Theory suggests that A l :A s must decrease to maintain leaf-specific hydraulic sufficiency as path length, gravity, and tortuosity constrain whole-plant hy- draulic conductance. We tested the hypothesis that A l :A s declines with tree height. Whole-tree Al:As was measured on 15 individuals of Douglas-fir ( Pseudotsuga menziesii var. menziesii) ranging in height from 13 to 62 m (aged 20-450 years). A l :A s declined substantially as height in- creased (P=0.02). Our test of the hypothesis that A l :A s declines with tree height was extended using a combina- tion of original and published data on nine species across a range of maximum heights and climates. Meta-analysis of 13 whole-tree studies revealed a consistent and signif- icant reduction in A l :A s with increasing height (P<0.05). However, two species (Picea abies and Abies balsamea) exhibited an increase in Al:As with height, although the reason for this is not clear. The slope of the relationship between A l :A s and tree height (∆ A l :A s /∆ h) was unrelated to mean annual precipitation. Maximum potential height was positively correlated with ∆ A l :A s /∆ h. The decrease in A l :A s with increasing tree size that we observed in the majority of species may be a homeostatic mechanism that partially compensates for decreased hydraulic con- ductance as trees grow in height.

Light gradient partitioning by tropical tree seedlings in the absence of canopy gaps.

Abstract: To explore the importance of light availability for seedling growth in low light environments, we examined light-dependent growth, biomass allocation and mortality of tree seedlings growing in sites with 0.2-6.5% full sun, the range of light commonly encountered in the understory of closed canopy, lowland tropical forests. We transplanted seedlings of the canopy tree species, Dipteryx panamensis, Virola koschnyii, and Brosimum alicastrum into second-growth forest and native tree plantations at La Selva Biological Station, Costa Rica. We assessed seedling survival, growth, and seedling light environments bimonthly for 14 months. Plants were harvested at the end of the study to assess leaf area, total biomass, biomass partitioning and root growth. Survivorship of all species exceeded 60% across all microsites, although both D. panamensis and B. alicastrum had lower probabilities of survival in the darkest microsites. All species showed a strong positive relationship between light availability and growth, increasing in total biomass as light increased. However, the strength of the growth response differed among species causing a change in the rank order of species growth rates as light availability increased. Although D. panamensis showed the lowest growth rates in the darkest microsites, a strong response to increasing light led to a cross-over in performance, such that D. panamensis had the highest growth rate at the highest light levels studied. These data suggest that resource gradient partitioning could occur even in low light environments (0.2-6.5%). Given the limited range of light regimes sampled (i.e., non-gap microsites), our data demonstrate that growth of tropical tree seedlings beneath closed canopies is highly sensitive to light availability and that shade-tolerant species vary in these responses. Our results show that understory light heterogeneity, in the absence of canopy gaps, can significantly affect recruitment processes for shade-tolerant tree species.

Seasonal patterns of reflectance indices, carotenoid pigments and photosynthesis of evergreen chaparral species

Abstract: This study examined the ability of the Photochemical Reflectance Index (PRI) to track seasonal variations in carotenoid pigments and photosynthetic activity of mature evergreen chaparral shrubs. Our results confirm that PRI scales with photosystem two (PSII) photochemical efficiency across species and seasons, as demonstrated by PRI's strong correlation with de-epoxidized (photoprotective) xanthophyll cycle pigment levels (normalized to chlorophyll) and with the chlorophyll fluorescence index, ΔF/Fm'. PRI and carotenoid pigment levels (de-epoxidized xanthophyll cycle pigments normalized to chlorophyll or total carotenoid pigments normalized to chlorophyll) were correlated with seasonal fluctuations in midday net CO2 uptake of top-canopy leaves. By contrast, chlorophyll levels (as measured by the Chlorophyll Index) were not as strongly linked to photosynthetic activity, particularly when all species were considered together. Likewise, the Normalized Difference Vegetation Index (NDVI, an index of canopy greenness) did not correlate with net CO2 uptake. Canopy NDVI also did not correlate with canopy PRI, demonstrating that these indices were largely independent over the temporal and spatial scales of this study. Together, these patterns provide evidence for coordinated regulation of carotenoid pigments, PSII electron transport, and carboxylation across seasons and indicate that physiological adjustments are more important than structural ones in modifying CO2-fixation capacity during periods of photosynthetic down-regulation for these evergreen species. The strong correlation between PRI of whole canopies and PRI of top-canopy leaves suggests that the canopy can be treated as a "big leaf" in terms of this reflectance index and that PRI can be used in "scalable" models. This along with the links between carotenoid pigments, PSII photochemical efficiency and carboxylation across species and seasons supports the use of optical assays of pigment levels and PSII activity in CO2 flux models to derive photosynthetic rates.

Desiccation and thermal tolerance of eggs and the coexistence of competing mosquitoes

Abstract: We tested the hypothesis that differences in temperature and desiccation tolerances of eggs of the container-dwelling mosquitoes Aedes albopictus and Aedes aegypti influence whether invading A. albopictus coexist with or exclude A. aegypti in Florida. In the laboratory, egg mortality through 30 days for A. albopictus was strongly temperature and humidity dependent, with low humidity and high temperature producing greatest mortality. In contrast, mortality through 30 days and through 60 days for A. aegypti was very low and independent of temperature and humidity. Mortality through 90 days for A. aegypti showed significant effects of both temperature and humidity. In the field, the proportion of vases occupied by A. albopictus was significantly lower at four of six sites at the start of the wet season (after a dry period) versus well into the wet season (after containers had held water for weeks). The proportion of vases occupied by A. aegypti was independent of when during the wet season vases were sampled. These results imply that dry periods cause disproportionately greater mortality of A. albopictus eggs compared to A. aegypti eggs. Container occupancy at tire and cemetery sites was significantly related to two principal components derived from long-term average climate data. Occupancy of containers by A. albopictus was greatest at cool sites with little or no dry season, and decreased significantly with increasing mean temperature and increasing number of dry months. In contrast, occupancy of containers by A. aegypti was lowest at cool sites with little or no dry season, and increased significantly with increasing mean temperature and increasing dry season length, and decreased significantly with total precipitation and number of wet months. We suggest that local coexistence of these species is possible because warm, dry climates favor A. aegypti and alleviate effects of competition from A. albopictus via differential mortality of A. albopictus eggs.

Influence of groundwater depth on the seasonal sources of water accessed by Banksia tree species on a shallow, sandy coastal aquifer

Abstract: In Mediterranean ecosystems vegetation overlying shallow, transient aquifers is often dominated by woody phreatophytes, trees and shrubs that have been shown to be dependent on groundwater for their water requirements. Natural and anthropogenic alterations of groundwater tables (abstraction) are of clear importance to phreatophytic vegetation as reduction of water tables may sever these plants from their natural water sources. Seasonal water sources were determined for species growing on a coastal dune system that overlies a shallow sandy aquifer in south-western Australia. The plants studied grew over groundwater that ranged in depth from 2.5 to 30 m. The naturally occurring stable isotope of hydrogen (deuterium, δ2H) was used to distinguish potential water sources. Isotopic ratios from vascular water of the dominant species of the study area (Banksia ilicifolia R. Br. and Banksia attenuata R. Br. trees) were compared with those of potential sources of precipitation, soil moisture and groundwater. A relatively shallow-rooted perennial shrub, Hibbertia hypericoides Benth., was also included as an isotopic reference. The results suggest that both B. attenuata and B. ilicifolia are phreatophytic as they derived some of their water from groundwater throughout the dry-wet cycle, with the exception of B. attenuata at the site of greatest depth to groundwater (30 m) which did not use groundwater. A high proportion (>50%) of groundwater use was not maintained throughout all seasons. With the onset of the hot Mediterranean summer, progressive drying of the surface soils resulted in increased use of groundwater and deep soil moisture. During the wet winter plants used proportionately more water from the upper layers of the soil profile. The degree to which groundwater was utilised by the study species was dependent on the proximity of groundwater, availability of moisture in shallower horizons of the soil profile, root system distribution and maximum root depth.

Seasonal changes in birch leaf chemistry: are there trade-offs between leaf growth and accumulation of phenolics?

Abstract: Several plant-herbivore hypotheses are based on the assumption that plants cannot simultaneously allocate resources to growth and defence. We studied seasonal patterns in allocation to growth and putatively defensive compounds by monitoring several chemical and physical traits in the leaves of mountain birch from early June (budburst) to late September (leaf senescence). We found significant seasonal changes in all measured characteristics, both in terms of concentrations (mg g–1) and amounts (mg leaf–1). Changes were very rapid in the spring, slow in the middle of the season, and there was another period of fast changes in the senescing leaves. Co-occurring changes in physical leaf traits and concentrations of several compounds indicated a seasonal decline in foliage suitability for herbivores. Concentrations of protein and free amino acids declined through the growing season whereas individual sugars showed variable seasonal patterns. The seasonal trends of phenolic groups differed drastically: concentrations of soluble proanthocyanidins increased through the season, whereas cell wall-bound proanthocyanidins, gallotannins and flavonoid glycosides declined after an initial increase in young leaves. We failed to find proof that the seasonal accumulation of phenolics would have been seriously compromised by leaf or shoot growth, as assumed by the growth/differentiation balance hypothesis and the protein competition model hypothesis. On the contrary, there was a steady increase in the total amount of phenolics per leaf even during the most active leaf growth.

Forest carbon balance under elevated CO2

Abstract: Free-air CO2 enrichment (FACE) technology was used to expose a loblolly pine (Pinus taeda L.) forest to elevated atmospheric CO2 (ambient + 200 µl l–1). After 4 years, basal area of pine trees was 9.2% larger in elevated than in ambient CO2 plots. During the first 3 years the growth rate of pine was stimulated by ~26%. In the fourth year this stimulation declined to 23%. The average net ecosystem production (NEP) in the ambient plots was 428 gC m–2 year–1, indicating that the forest was a net sink for atmospheric CO2. Elevated atmospheric CO2 stimulated NEP by 41%. This increase was primarily an increase in plant biomass increment (57%), and secondarily increased accumulation of carbon in the forest floor (35%) and fine root increment (8%). Net primary production (NPP) was stimulated by 27%, driven primarily by increases in the growth rate of the pines. Total heterotrophic respiration (Rh) increased by 165%, but total autotrophic respiration (Ra) was unaffected. Gross primary production was increased by 18%. The largest uncertainties in the carbon budget remain in separating belowground heterotrophic (soil microbes) and autotrophic (root) respiration. If applied to temperate forests globally, the increase in NEP that we measured would fix less than 10% of the anthropogenic CO2 projected to be released into the atmosphere in the year 2050. This may represent an upper limit because rising global temperatures, land disturbance, and heterotrophic decomposition of woody tissues will ultimately cause an increased flux of carbon back to the atmosphere.

Primary producers sustaining macro-invertebrate communities in intertidal mangrove forests

Abstract: In contrast to the large number of studies on the trophic significance of mangrove primary production to the aquatic foodweb, there have been few attempts to provide an overview of the relative importance of different primary carbon sources to invertebrates in the intertidal mangrove habitats. We determined carbon and nitrogen stable isotope ratios (δ13C, δ15N) in sediments, primary producers, and 22 invertebrate species from an intertidal mangrove forest located along the southeast coast of India in order to determine the contribution of mangrove leaf litter and other carbon sources to the invertebrate community. Organic matter in sediments under the mangrove vegetation was characterized by relatively high δ13C values and low C:N ratios, indicating that mangrove-derived organic matter was not the principal source and that imported phytodetritus from the mangrove creeks and adjacent bay significantly contributed to the sediment organic matter pool. Invertebrates were found to display a wide range of δ13C values, most being 3-11‰ enriched relative to the average mangrove leaf signal. The pulmonate gastropod Onchidium sp. showed unusually low δ15N values (-5.6±0.9‰), but further work is needed to adequately explain these data. A compilation of stable isotope data from various sources indicates that significant assimilation of mangrove-derived carbon is only detectable in a limited number of species, and suggests that local and imported algal sources are a major source of carbon for benthic invertebrate communities in intertidal mangrove forests. These results provide new insights into carbon utilization patterns in vegetated tropical intertidal habitats and show a striking similarity with results from temperate salt marsh ecosystems where local plant production has often been found to contribute little to intertidal foodwebs.

Nitrogen mineralization and phenol accumulation along a fire chronosequence in northern Sweden

Abstract: Scots pine (Pinus sylvestris L.) forests of northern Sweden are often considered to be N limited. This limitation may have been exacerbated by the elimination of wildfire as a natural disturbance factor in these boreal forests. Phenolic inhibition of N mineralization and nitrification (due to litter and exudates of ericaceous shrubs) has been proposed as a mechanism for N limitation of these forests, but this hypothesis remains largely untested. N mineralization rates, nitrification rates, and sorption of free phenolic compounds were assessed along a fire-induced chronosequence in northern Sweden. A total of 34 forest stands varying in age since the last fire were identified and characterized. Overstorey and understorey vegetative composition and depth of humus were analysed in replicated plots at all 34 sites. Eight of the forest stands aged 3-352 years since the last fire were selected for intensive investigation in which ten replicate ionic resin capsules (used to assess net N mineralization and nitrification) and non-ionic carbonaceous resin capsules (used to assess free phenolic compounds) were installed at the interface of humus and mineral soil. A highly significant correlation was observed between site age and net sorption of inorganic N to resin capsules. Net accumulation of NH4+ and NO3- on resin capsules followed a linear decrease (R 2=0.61, P<0.01) with time perhaps as a result of increased N immobilization with successional C loading. NO3- sorption to resin capsules followed a logarithmic decrease (R 2=0.80, P<0.01) that may be related to a logarithmic increase in dwarf shrub cover and decreased soil charcoal sorption potential along this chronosequence. A replicated field study was conducted at one of the late successional field sites to assess the influence of charcoal and an added labile N source on N turnover. Three rates of charcoal (0, 100, and 1,000 g M-2) and two rates of glycine (0 and 50 g N as glycine M-2) were applied in a factorial design to microplots in a randomized complete block pattern. Net ammonification (as assessed by NH4+ sorption to resins) was readily increased by the addition of a labile N source, but this increase in NH4+ did not stimulate nitrification. Nitrification was stimulated slightly by the addition of charcoal resulting in similar levels of resin-sorbed NO3- as those found in early successional sites. Resin-sorbed polyphenol concentrations were decreased with charcoal amendments, but were actually increased with N amendments (likely due to decomposition of polyphenols). Net N mineralization appears to be limited by rapid NH4+ immobilization whereas nitrification is limited by the lack of an appropriate environment or by the presence of inhibitory compounds in late successional forests of northern Sweden.

Resource availability dominates and alters the relationship between species diversity and ecosystem productivity in experimental plant communities.

Abstract: Experimental evidence that plant species diversity has positive effects on biomass production appears to conflict with correlations of species diversity and standing biomass in natural communities. This may be due to the confounding effects of a third variable, resource availability, which has strong control over both diversity and productivity in natural systems and may conceal any positive effects of diversity on productivity. To test this hypothesis, I independently manipulated resource availability (soil fertility) and sown species diversity in a field experiment and measured their individual and interactive effects on productivity. Although fertility was a far stronger predictor of productivity than diversity, the effect of diversity on productivity significantly increased with fertility. Relative yield analyses indicated that plant mixtures of high fertility treatments significantly "overyielded," or were more productive than expected based on monoculture yields of component species. In contrast, plant mixtures of low fertility treatments had significantly lower-than-expected yields. The effect of diversity on productivity was also driven by sampling effects, where more species-rich mixtures were more likely to include particularly productive species. Unexpectedly, the strength of sampling effects was largely insensitive to fertility, although the particular species most responsible for sampling effects did change with fertility. These results suggest that positive effects of species diversity on ecosystem productivity in natural systems are likely to be masked by variation in environmental factors among habitats.

Epiphyte host preferences and host traits: mechanisms for species-specific interactions

Abstract: We investigated species-specific relationships among two species of vascular epiphytes and ten host tree species in a coastal plain forest in the southeastern United States. The epiphytes Tillandsia usneoides and Polypodium polypodioides were highly associated with particular host species in the field, but host traits that favored colonization were inadequate to fully explain the epiphyte-host associations for either epiphyte. Field transplant experiments that bypassed epiphyte colonization demonstrated that the growth of epiphytes was significantly higher on host tree species that naturally bore high epiphyte loads than on host species with few or no epiphytes. These species-specific relationships were highly correlated with the water-holding capacity of the host tree's bark. Positive and negative effects of throughfall, light attenuation by the canopy, and bark stability did not explain the overall patterns of host specificity, but did correlate with some epiphyte-host species relationships. The relative importance of particular host traits differed between the "atmospheric epiphyte" Tillandsia, and the fern Polypodium, which roots in the bark of its hosts. Species-specific interactions among plants, such as those described here, suggest that communities are more than individualistic assemblages of co-occurring species.

Stable isotope enrichment (δ15N and δ13C) in a generalist predator (Pardosa lugubris, Araneae: Lycosidae): effects of prey quality

Abstract: Analysis of the natural variations in stable isotope ratios in animal tissue may be a powerful tool to reveal the trophic position and feeding preferences of generalist predators such as lycosid spiders. In the present study, changes in 15N/14N and 13C/12C ratios in the lycosid spider Pardosa lugubris (Araneae: Lycosidae), fed with prey of different quality, were investigated. Experimental food chains included three trophic levels: prey media, prey organism and predator. In order to analyse the time course of stable isotope enrichment, different life stages of P. lugubris feeding on Drosophila melanogaster were studied. The 15N content of hatchlings of P. lugubris was significantly lower than that of their mothers, indicating the existence of nitrogen pools with different 15N signatures in female P. lugubris. With duration of feeding and progressing development, age and body weight, the 15N content in spiderlings increased. Starvation resulted in 15N and 13C enrichment in P. lugubris. Being fed with prey of different quality resulted in varying patterns of stable isotope enrichment. 15N but not 13C content consistently increased when fed on high quality prey (Heteromurus nitidus, D. melanogaster and a mixed diet consisting of H. nitidus and D. melanogaster). In contrast, low quality prey (Rhopalosiphum padi, Folsomia candida and a mixed diet consisting of H. nitidus, D. melanogaster and F. candida) resulted in deviations from postulated patterns of stable isotope enrichment with 15N content being similar to that of starving spiderlings. When fed on high quality prey, 15N enrichment of P. lugubris was close to ca. 3‰. It is concluded that the analysis of variations in the natural abundance of stable isotopes is particularly helpful in revealing the trophic structure of terrestrial food webs in which polyphagous feeders predominate, as is the case in litter and soil.

Effects of habitat and food resources on morphology and ontogenetic growth trajectories in perch

Abstract: Studies on resource polymorphism have mainly been considered at the end stage of ontogeny, whereas many species undergo diet changes as they grow We conducted a field survey to analyze the role of adaptive variation during ontogeny in Eurasian perch (Perca fluviatilis) We caught perch from the littoral and pelagic zones of a lake to investigate whether perch differ in morphology and diet between these habitats We also investigated whether there were any differences in morphological trajectories during the ontogeny of perch collected from the two habitats We found that perch caught in the littoral habitat, independently of size, had a deeper body, larger head and mouth and longer fins than perch caught in the pelagic zone Macroinvertebrates and fish dominated the diet of littoral perch, whereas the diet of the pelagic perch consisted mainly of zooplankton and to some extent fish Independently of size, the more streamlined individuals had a larger proportion of zooplankton and a smaller proportion of macroinvertebrates in their diet than the deeper-bodied individuals, indicating a relation between diet and morphology Some morphological characters followed different ontogenetic trajectories in the two habitats; eg the changes to a deeper body and a larger head were faster in the littoral than in the pelagic perch The relationship between the length of perch and the size of the mouth and fins also differed between perch from the two habitats, where the increase in the length of the pelvic fin and the area of the mouth increased faster with size in the littoral perch Our findings show that variation in morphology between habitats differs during ontogeny in a way that corresponds to functional expectations for fish species that occupy these habitats

Colony growth of the bumblebee, Bombus terrestris, in improved and conventional agricultural and suburban habitats

Abstract: Many bumblebee species are declining at a rapid rate in the United Kingdom and elsewhere. This is commonly attributed to the decline in floral resources that has resulted from an intensification in farming practices. Here we assess growth of nests of the bumblebee, Bombus terrestris, in habitats providing different levels of floral resources. Experimental nests were placed out in conventional farmland, in farmland with flower-rich conservation measures and in suburban areas. Nests gained weight more quickly and attained a larger final size in suburban areas compared to elsewhere. The diversity of pollens gathered by bees was highest in suburban areas, and lowest in conventional farmland. Nests in suburban areas were also more prone to attack by the specialist bumblebee parasite Aphomia sociella, suggesting that this moth is more abundant in suburban areas than elsewhere. Overall, our results demonstrate that gardens provide a greater density and diversity of floral resources than farmland, and probably support larger populations of B. terrestris. Contrary to expectation, schemes deployed to enhance farmland biodiversity appear to have little measurable impact on nest growth of this bumblebee species. We argue that B. terrestris probably forage over a larger scale than that on which farms are managed, so that nest growth is determined by the management of a large number of neighbouring farms, not just that in which the nest is located.

Root respiration in North American forests: effects of nitrogen concentration and temperature across biomes

Abstract: Root respiration rates have been shown to be correlated with temperature and root N concentration in studies of individual forest types or species, but it is not known how universal these relationships are across for- est species adapted to widely different climatic and eda- phic conditions. In order to test for broad, cross-species relationships, we measured fine root respiration, as O 2 consumption, over a range of temperatures on excised root samples from ten forested study sites across North America in 1997. Significant differences existed among study sites in root respiration rates, with patterns among sites in respiration rate at a given temperature corre- sponding to differences among sites in fine root N con- centrations. Root respiration rates were highly correlated with root N concentrations at all measurement tempera- tures (r 2 >0.81, P<0.001, for 6, 18 and 24°C). Lower root respiration rates in gymnosperms than in angiosperms were largely explained by lower fine root N concentra- tions in gymnosperms, and root N concentrations and respiration rates (at a given temperature) tended to be lower at warm sites (New Mexico, Florida, and Georgia) than at cool sites with short growing seasons (Michigan and Alaska). Root respiration rates increased exponen- tially with temperature at all sites. The Q10 for root respi- ration ranged from 2.4 to 3.1, but there were no signifi- cant differences among the forest types. The average Q10s for gymnosperms (Q10=2.7) and angiosperms (Q 10 =2.6) were almost identical, as were the average Q 10 s for roots of ectomycorrhizal species ( Q 10 =2.7) and arbuscular mycorrhizal species (Q10=2.6). In 1998, fine root respiration at the study sites was measured in the field as CO 2 production at ambient soil temperature. Respiration rates under field conditions were dependent on both ambient soil temperature and root N concentra- tion. Relationships between respiration (adjusted for temperature) and root N concentration for the field mea- surements were similar to those observed in the 1997 laboratory experiments. For root respiration in tree spe- cies, it appears that basic relationships with temperature and nitrogen exist across species and biomes.

Exclusion by interference competition? The relationship between red and arctic foxes

Abstract: The distribution of many predators may be limited by interactions with larger predator species. The arctic fox in mainland Europe is endangered, while the red fox is increasing its range in the north. It has been suggested that the southern distribution limit of the arctic fox is determined by interspecific competition with the red fox. This has been criticised, on the basis that the species co-exist on a regional scale. However, if the larger red fox is superior and interspecific competition important, the arctic fox should avoid close contact, especially during the breeding season. Consequently, the distribution of breeding dens for the two species would be segregated on a much smaller spatial and temporal scale, in areas where they are sympatric. We tested this hypothesis by analysing den use of reproducing arctic and red foxes over 9 years in Sweden. High quality dens were inhabited by reproducing arctic foxes more often when no red foxes bred in the vicinity. Furthermore, in two out of three cases when arctic foxes did reproduce near red foxes, juveniles were killed by red foxes. We also found that breeding arctic foxes occupied dens at higher altitudes than red foxes did. In a large-scale field experiment, red foxes were removed, but the results were not conclusive. However, we conclude that on the scale of individual territories, arctic foxes avoid areas with red foxes. Through interspecific interference competition, the red fox might thus be excluding the arctic fox from breeding in low altitude habitat, which is most important in years when food abundance is limited and competition is most fierce. With high altitude refuges being less suitable, even small-scale behavioural effects could scale up to significant effects at the population level.

Interactions between fleshy fruits and frugivores in a tropical seasonal forest in Thailand

Abstract: Large frugivores are considered to be important seed dispersers for many tropical plant species. Their roles as seed dispersers are not well known in Southeast Asia, where degraded landscapes typically lack these animals. Interactions between 259 (65 families) vertebrate-dispersed fruits and frugivorous animals (including 7 species of bulbul, 1 species of pigeon, 4 species of hornbill, 2 species of squirrel, 3 species of civet, 2 species of gibbon, 1 species of macaque, 2 species of bear, 2 species of deer, and 1 species of elephant) were studied for 3 years in a tropical seasonal forest in Khao Yai National Park, Thailand. The purpose was to examine the dietary overlaps among the large frugivores and the characteristics of fruits they consumed. Most fruit species are eaten by various kinds of frugivores; no close relationship between a particular fruit and a frugivore was found. The number of frugivore groups that served a given plant species was negatively correlated with seed size. Additionally, the fruit/seed diameters consumed by bulbuls were significantly smaller than consumed by the other nine groups. These trends of fruit characteristics were consistent with those observed elsewhere in Southeast Asia: small fruits and large, soft fruits with many small seeds are consumed by a wide spectrum of frugivores while larger fruits with a single large seed are consumed by relatively few potential dispersers. Importantly, these large, single-seed fruits are not consumed by the small frugivores that thrive in small forest fragments and degraded areas in Southeast Asia. To insure the natural seed dispersal process in the forest, an evaluation of all frugivore groups in the forest is urgently needed in Southeast Asia.

Larval growth predicts the recruitment success of a coral reef fish

Abstract: While growth rates of pelagic larvae have been argued to be one of the principal determinants of the recruitment success of temperate marine fishes, it is not known if this is the case in the tropics. Here, we use lar- val growth histories derived from otoliths of a Caribbean reef fish to show that monthly variation in the intensity of settlement and recruitment of pelagic juveniles onto reefs is positively correlated with variation in growth rates 1-2 weeks after larvae begin feeding. Our results suggest that the processes thought to underlie recruitment of marine fishes in temperate regions may also operate in the tropics and contrasts with current research on the causes of recruitment variability in coral reef fishes, which emphasises the role of larval transport.

Latitudinal and climate-driven variation in the strength and nature of biological interactions in New England salt marshes.

Abstract: We examined the linkage between climate and interspecific plant interactions in New England salt marshes. Because harsh edaphic conditions in marshes can be ameliorated by neighboring plants, plant neighbors can have net competitive or facilitative interactions, depending on ambient physical stresses. In particular, high soil salinities, which are largely controlled by solar radiation and the evaporation of marsh porewater, can be ameliorated by plant neighbors under stressful conditions leading to facilitative interactions. Under less stressful edaphic conditions, these same neighbors may be competitors. In this paper, we use this mechanistic understanding of marsh plant interactions to examine the hypothesis that latitudinal and inter-annual variation in climate can influence the nature and strength of marsh plant species interactions. We quantified the relationship between climate and species interactions by transplanting marsh plants into ambient vegetation and unvegetated bare patches at sites north and south of Cape Cod, a major biogeographic barrier on the east coast of North America. We hypothesized that the cooler climate north of Cape Cod would lead to fewer positive interactions among marsh plants. We found both latitudinal and inter-annual variation in the neighbor relations of marsh plants that paralleled latitudinal differences in temperature and salinity. South of Cape Cod, plant neighbor interactions tended to be more facilitative, whereas north of Cape Cod, plant neighbor interactions were more competitive. At all sites, soil salinity increased and plant neighbor interactions were more facilitative in warmer versus cooler years. Our results show that interspecific interactions can be strikingly linked to climate, but also reveal that because the sensitivity of specific species interactions to climatic variation is highly variable, predicting how entire communities will respond to climate change will be difficult, even in relatively simple, well-studied systems.