Plant-soil microorganism interactions: heritable relationship between plant genotype and associated soil microorganisms

Although soil microbial communities are known to play crucial roles in the cycling of nutrients in forest ecosystems and can vary by plant species, how microorganisms respond to the subtle gradients of plant genetic variation is just beginning to be appreciated. Using a model Populus system in a common garden with replicated clones of known genotypes, we evaluated microbial biomass and community composition as quantitative traits. Two main patterns emerged. (1) Plant genotype influenced microbial biomass nitrogen in soils under replicated genotypes of Populus angustifolia, F1, and backcross hybrids, but not P. fremontii. Genotype explained up to 78% of the variation in microbial biomass as indicated by broad-sense heritability estimates (i.e., clonal repeatability). A second estimate of microbial biomass (total phospholipid fatty acid) was more conservative and showed significant genotype effects in P. angustifolia and backcross hybrids. (2) Plant genotype significantly influenced microbial community composition, explaining up to 70% of the variation in community composition within P. angustifolia genotypes alone. These findings suggest that variation in above- and belowground traits of individual plant genotypes can alter soil microbial dynamics, and suggests that further investigations of the evolutionary implications of genetic feedbacks are warranted.     

Application of tree-ring isotopic analyses to reconstruct historical water use of riparian trees.

Historical patterns of water source use by trees inferred from long-term records of tree-ring stable isotopic content could assist in evaluating the impact of human alterations to natural stream flow regimes (e.g., water impoundments, stream flow diversions, and groundwater extraction). Our objective was to assess the utility of the hydrogen stable isotopic composition (SD) of tree rings as an index of historical water source use by riparian trees. We investigated the influence of site conditions that varied in climate and hydrology on the relationship between deltaD of Populus xylem water (deltaD(xyl)) and tree-ring cellulose (deltaD(cell)). deltaD(xyl) and deltaD(cell) were strongly correlated across sites (r2 = 0.89). However, the slope of this relationship was less than 1, indicating that factors other than deltaD(xyl) influenced deltaD(cell). Inverse modeling with an isotopic fractionation model for tree-ring cellulose suggested that the lack of one-to-one correspondence between deltaD(xyl) and deltaD(cell) was due to the influence of the hydrogen isotopic content of the atmospheric water vapor (deltaD(atm)). Empirically measured values of deltaD(cell) were typically within the seasonal range of deltaD(cell) predicted from the fractionation model. Sensitivity analyses showed that changes in deltaD(xyl) generally had a greater influence at high-elevation montane sites, whereas deltaD(xyl) and deltaD(atm) had about equal influence on deltaD(cell) at low-elevation desert sites. The intrasite relationship between deltaD(cell) and deltaD(xyl) among individual trees was poor, perhaps because of the within-site spatial variation in hydrologic conditions and associated tree physiological responses. Our study suggests that historical variation in deltaD(cell) of Populus provides information on historical variation in both time-integrated water source use and atmospheric conditions; and that the influence of atmospheric conditions is not consistent over sites with large differences in temperature and humidity. Reconstruction of xylem water sources of Populus in riparian ecosystems from deltaD(cell) will be more direct at higher elevation mountain sites than at low-elevation desert sites.     

Effectiveness of territorial polygyny and alternative mating strategies in northern fur seals, <Emphasis Type="Italic">Callorhinus ursinus</Emphasis>

We conducted a 6-year longitudinal behavioral and genetic investigation of a highly polygynous pinniped, the northern fur seal (Callorhinus ursinus), to determine the contribution of terrestrial polygyny to male fertilization success and to assess the occurrence of alternative mating strategies. Genetic samples from 37 adult males, 50 adult females, and 85 pups were collected and genotyped using five polymorphic microsatellite loci. Pup paternity was assigned using Cervus 2.0 at 99% confidence level. Paternity of 83 pups (98%) was assigned to terrestrial males who held territories or stayed temporarily in the study area during the breeding season when fertilization occurred. For 56 pups of which attendance records of their mothers were available, paternity of 45 pups (80%) was assigned to the associate males in whose territory their mothers stayed during the perioestrus period. In addition to defending breeding territories, territorial males have often been observed attempting to forcibly abduct adult females from adjacent territories (female stealing): We observed a total of 95 such cases, in which the stealers had significantly fewer females than the territorial males from whose territories they stole females. Our results indicate that terrestrial resource-defense polygyny is the major mating system in this species and that nonassociated paternity occurs mostly as a result of alternative mating strategies of less successful males. Male northern fur seals thus appear to adopt conditional alternative strategies that depend on their current social status to maximize their life-time reproductive success.     

Increasing litter species richness reduces variability in a terrestrial decomposer system

Debate on the relationship between diversity and stability has been driven by the recognition that species loss may influence ecosystem properties and processes. We conducted a litterbag experiment in the Scottish Highlands, United Kingdom, to examine the effects of altering plant litter diversity on decomposition, microbial biomass, and microfaunal abundance. The design of treatments was fully factorial and included five species from an upland plant community (silver birch, Betula pendula; Scots' pine, Pinus sylvestris; heather, Calluna vulgaris; bilberry, Vaccinium myrtillus; wavy-hair grass, Deschampsia flexuosa); species richness ranged from one to five species. We tested the effects of litter species richness and composition on variable means, whether increasing litter species richness reduced variability in the decomposer system, and whether any richness-variability relationships were maintained over time (196 vs. 564 days). While litter species composition effects controlled variable means, we revealed reductions in variability with increasing litter species richness, even after accounting for differences between litter types. These findings suggest that higher plant species richness per se may result in more stable ecosystem processes (e.g., decomposition) and decomposer communities. Negative richness-variation relationships generally relaxed over time, presumably because properties of litter mixtures became more homogeneous. However, given that plant litter inputs continue to enter the belowground system over time, we conclude that variation in ecosystem properties may be buffered by greater litter species richness.     

Global potential net primary production predicted from vegetation class, precipitation, and temperature

Net primary production (NPP), the difference between CO2 fixed by photosynthesis and CO2 lost to autotrophic respiration, is one of the most important components of the carbon cycle. Our goal was to develop a simple regression model to estimate global NPP using climate and land cover data. Approximately 5600 global data points with observed mean annual NPP, land cover class, precipitation, and temperature were compiled. Precipitation was better correlated with NPP than temperature, and it explained much more of the variability in mean annual NPP for grass- or shrub-dominated systems (r2 = 0.68) than for tree-dominated systems (r2 = 0.39). For a given precipitation level, tree-dominated systems had significantly higher NPP (approximately 100-150 g C m(-2) yr(-1)) than non-tree-dominated systems. Consequently, previous empirical models developed to predict NPP based on precipitation and temperature (e.g., the Miami model) tended to overestimate NPP for non-tree-dominated systems. Our new model developed at the National Center for Ecological Analysis and Synthesis (the NCEAS model) predicts NPP for tree-dominated systems based on precipitation and temperature; but for non-tree-dominated systems NPP is solely a function of precipitation because including a temperature function increased model error for these systems. Lower NPP in non-tree-dominated systems is likely related to decreased water and nutrient use efficiency and higher nutrient loss rates from more frequent fire disturbances. Late 20th century aboveground and total NPP for global potential native vegetation using the NCEAS model are estimated to be approximately 28 Pg and approximately 46 Pg C/yr, respectively. The NCEAS model estimated an approximately 13% increase in global total NPP for potential vegetation from 1901 to 2000 based on changing precipitation and temperature patterns.     

Putting free-riding to work: A Partnership Solution to the common-property problem

The common-property problem results in excessive mining, hunting, and extraction of oil and water. The same phenomenon is also responsible for excessive investment in R&D and excessive outlays in rent-seeking contests. We propose a “Partnership Solution” to eliminate or at least mitigate these excesses. Each of N players joins a partnership in the first stage and chooses his effort in the second stage. Under the rules of a partnership, each member must pay his own cost of effort but receives an equal share of the partnership's revenue. The incentive to free-ride created by such partnerships turns out to be beneficial since it naturally offsets the excessive effort inherent in such problems. In our two-stage game, this institutional arrangement can, under specified circumstances, induce the social optimum in a subgame-perfect equilibrium: no one has a unilateral incentive (1) to switch to another partnership (or create a new partnership) in the first stage or (2) to deviate from socially optimal actions in the second stage. The game may have other subgame-perfect equilibria, but the one associated with the “Partnership Solution” is strictly preferred by every player. We also propose a modification of the first stage which generates a unique subgame-perfect equilibrium. Antitrust authorities should recognize that partnerships can have a less benign use. By organizing as competing partnerships, an industry can reduce the “excessive” output of Cournot oligopoly to the monopoly level. Since no partner has any incentive to overproduce in the current period, there is no need to deter cheating with threats of future punishments.     

Using social network analysis tools in ecology: Markov process transition models applied to the seasonal trophic network dynamics of the Chesapeake Bay

Ecosystem components interact in complex ways and change over time due to a variety of both internal and external influences (climate change, season cycles, human impacts). Such processes need to be modeled dynamically using appropriate statistical methods for assessing change in network structure. Here we use visualizations and statistical models of network dynamics to understand seasonal changes in the trophic network model described by Baird and Ulanowicz [Baird, D., Ulanowicz, R.E., 1989. Seasonal dynamics of the Chesapeake Bay ecosystem. Ecol. Monogr. 501 (59), 329–364] for the Chesapeake Bay (USA). Visualizations of carbon flow networks were created for each season by using a network graphic analysis tool (NETDRAW). The structural relations of the pelagic and benthic compartments (nodes) in each seasonal network were displayed in a two-dimensional space using spring-embedder analyses with nodes color-coded for habitat associations (benthic or pelagic). The most complex network was summer, when pelagic species such as sea nettles, larval fishes, and carnivorous fishes immigrate into Chesapeake Bay and consume prey largely from the plankton and to some extent the benthos. Winter was the simplest of the seasonal networks, and exhibited the highest ascendency, with fewest nodes present and with most of the flows shifting to the benthic bacteria and sediment POC compartments. This shift in system complexity corresponds with a shift from a pelagic- to benthic-dominated system over the seasonal cycle, suggesting that winter is a mostly closed system, relying on internal cycling rather than external input. Network visualization tools are useful in assessing temporal and spatial changes in food web networks, which can be explored for patterns that can be tested using statistical approaches. A simulation-based continuous-time Markov Chain model called SIENA was used to determine the dynamic structural changes in the trophic network across phases of the annual cycle in a statistical as opposed to a visual assessment. There was a significant decrease in outdegree (prey nodes with reduced link density) and an increase in the number of transitive triples (a triad in which i chooses j and h, and j also chooses h, mostly connected via the non-living detritus nodes in position i), suggesting the Chesapeake Bay is a simpler, but structurally more efficient, ecosystem in the winter than in the summer. As in the visual analysis, this shift in system complexity corresponds with a shift from a pelagic to a more benthic-dominated system from summer to winter. Both the SIENA model and the visualization in NETDRAW support the conclusions of Baird and Ulanowicz [Baird, D., Ulanowicz, R.E., 1989. Seasonal dynamics of the Chesapeake Bay ecosystem. Ecol. Monogr. 501 (59), 329–364] that there was an increase in the Chesapeake Bay ecosystem's ascendancy in the winter. We explain such reduced complexity in winter as a system response to lowered temperature and decreased solar energy input, which causes a decline in the production of new carbon, forcing nodes to go extinct; this causes a change in the structure of the system, making it simpler and more efficient than in summer. It appears that the seasonal dynamics of the trophic structure of Chesapeake Bay can be modeled effectively using the SIENA statistical model for network change.     

Assessing drought-related ecological risk in the Florida Everglades

In the winter-spring of 2001, South Florida experienced one of the worst droughts in its recorded history. Out of a myriad of ecological concerns identified during this time, the potential for catastrophic peat fire and negative impacts to wading bird reproduction emerged as critical issues. Water managers attempted to strike a balance between the environment and protection of water supplies for agriculture and urban interests. It became evident, however, that a broad-scale, integrated way to portray and prioritise ecological stress was lacking in the Florida Everglades, despite this being considered a necessary tool for addressing issues of environmental protection. In order to provide a framework for evaluating various water management operations using real-time information, we developed GIS-based indices of peat-fire risk and wading bird habitat suitability. These indices, based on real physical, chemical, and biological data, describe two ecological conditions that help define the physical and biological integrity of the Everglades. In addition to providing continuous, updated assessments throughout the drought period, we incorporated predictive models of water levels to evaluate how various water management alternatives might exacerbate or alleviate ecological stress during this time.     

Synergistic interactions between edge and area effects in a heavily fragmented landscape

Both area and edge effects have a strong influence on ecological processes in fragmented landscapes, but there is little understanding of how these two factors might interact to exacerbate local species declines. To test for synergistic interactions between area and edge effects, we sampled a diverse beetle community in a heavily fragmented landscape in New Zealand. More than 35,000 beetles of approximately 900 species were sampled over large gradients in habitat area (10(-2) 10(6) ha) and distance from patch edge (2(0)-2(10) m from the forest edge into both the forest and adjacent matrix). Using a new approach to partition variance following an ordination analysis, we found that a synergistic interaction between habitat area and distance to edge was a more important determinant of patterns in beetle community composition than direct edge or area effects alone. The strength of edge effects in beetle-species composition increased nonlinearly with increasing fragment area. One important consequence of the synergy is that the slopes of species area (SA) curves constructed from habitat islands depend sensitively on the distance from edge at which sampling is conducted. Surprisingly, we found negative SA curves for communities sampled at intermediate distances from habitat edges, caused by differential edge responses of matrix- vs. forest-specialist species in fragments of increasing area. Our data indicate that distance to habitat edge has a consistently greater impact on beetle community composition than habitat area and that variation in the strength of edge effects may underlie many patterns that are superficially related to habitat area.     

Hagfish in the New Zealand fjords are supported by chemoautotrophy of forest carbon

Forest litter is often considered to be a minor energy source to marine communities due to its refractory nature. Large volumes of forest litter are deposited in the New Zealand fjords, and likely recycled into available energy by microbial activity. In this study we used evidence from stable isotope analyses to test whether recycled carbon from chemoautotrophs was an important contributor to the diet of hagfish (Eptatretus cirrhatus). We then analyzed fatty acid biomarkers from the chemoautotrophic clam Solemya parkinsoni and E. cirrhatus to further discriminate the contribution of marine, terrestrial, and chemoautotrophic sources. Bulk isotopic signatures of E. cirrhatus varied considerably (delta13C, from -29.2 per thousand to -16.7 per thousand; delta15N, from -2.8 per thousand to +15.5 per thousand; delta34S, from -21.7 per thousand to +16.7 per thousand) and indicated that a significant percentage of organic matter (38-51%) originated from chemoautotrophs (delta13C, -31.3 per thousand +/- 0.1 per thousand [mean +/- SE]; delta15N, -5.7 per thousand +/- 0.2 per thousand; delta34S, -32.per thousand +/- 3.8 per thousand). Fatty acid biomarkers were depleted in 13C, particularly cis-vaccenic acid (18:1omega7: delta13C, -39.0 per thousand) indicating specific microbial origins of carbon. A high proportion of forest litter in sediments, coupled with isotopic and fatty acid biomarker results, indicates that terrestrial organic matter is a dominant contributor to this marine benthic system. This study demonstrates a clear linkage between terrestrial and marine ecological processes.