Showing papers on "Productivity (ecology) published in 2012"

Biodiversity impacts ecosystem productivity as much as resources, disturbance, or herbivory

Abstract: Although the impacts of the loss of biodiversity on ecosystem functioning are well established, the importance of the loss of biodiversity relative to other human-caused drivers of environmental change remains uncertain. Results of 11 experiments show that ecologically relevant decreases in grassland plant diversity influenced productivity at least as much as ecologically relevant changes in nitrogen, water, CO2, herbivores, drought, or fire. Moreover, biodiversity became an increasingly dominant driver of ecosystem productivity through time, whereas effects of other factors either declined (nitrogen addition) or remained unchanged (all others). In particular, a change in plant diversity from four to 16 species caused as large an increase in productivity as addition of 54 kg⋅ha−1⋅y−1 of fertilizer N, and was as influential as removing a dominant herbivore, a major natural drought, water addition, and fire suppression. A change in diversity from one to 16 species caused a greater biomass increase than 95 kg⋅ha−1⋅y−1 of N or any other treatment. Our conclusions are based on >7,000 productivity measurements from 11 long-term experiments (mean length, ∼ 13 y) conducted at a single site with species from a single regional species pool, thus controlling for many potentially confounding factors. Our results suggest that the loss of biodiversity may have at least as great an impact on ecosystem functioning as other anthropogenic drivers of environmental change, and that use of diverse mixtures of species may be as effective in increasing productivity of some biomass crops as fertilization and may better provide ecosystem services.

Carbon dioxide and the uneasy interactions of trees and savannah grasses

Abstract: Savannahs are a mixture of trees and grasses often occurring as alternate states to closed forests. Savannah fires are frequent where grass productivity is high in the wet season. Fires help mainta...

Production and fate of kelp detritus

Abstract: The flow of detritus between habitats is an important form of connectivity that affects regional productivity and the spatial organization of marine ecosystems Kelps form highly productive beds or forests that produce detritus through incremental blade erosion, fragmentation of blades, and dislodgement of whole fronds and thalli Rates of detrital production range from 8 to 2657 g C m !2 yr !1 for blade erosion and fragmentation, and from 22 to 839 g C m !2 yr !1 for loss of fronds and thalli The estimated global average rate of detrital production by kelps is 706 g C m !2 yr !1 , accounting for 82% of annual kelp productivity Detrital production rates are regulated by current and wave-driven hydrodynamic forces and are highest during severe storms and follow- ing blade weakening through damage by grazers and encrusting epibionts Detritus settles within kelp beds or forests and is exported to neighboring or distant habitats, including sandy beaches, rocky intertidal shores, rocky and sedimentary subtidal areas, and the deep sea Exported kelp detritus can provide a significant resource subsidy and enhance secondary production in these communities ranging from tens of meters to hundreds of kilometers from the source of production Loss of kelp biomass is occurring worldwide through the combined effects of climate change, pol- lution, fishing, and harvesting of kelp, which can depress rates of detrital production and subsidy to adjacent communities, with large-scale consequences for productivity

Timing of climate variability and grassland productivity

Abstract: Future climates are forecast to include greater precipitation variability and more frequent heat waves, but the degree to which the timing of climate variability impacts ecosystems is uncertain. In a temperate, humid grassland, we examined the seasonal impacts of climate variability on 27 y of grass productivity. Drought and high-intensity precipitation reduced grass productivity only during a 110-d period, whereas high temperatures reduced productivity only during 25 d in July. The effects of drought and heat waves declined over the season and had no detectable impact on grass productivity in August. If these patterns are general across ecosystems, predictions of ecosystem response to climate change will have to account not only for the magnitude of climate variability but also for its timing.

The productivity, metabolism and carbon cycle of tropical forest vegetation

Abstract: Summary 1. Tropical forests account for one-third of the total metabolic activity of the Earth’s land surface. Hence, understanding the controls on tropical forest photosynthesis and respiration, and the allocation of the products of photosynthesis to canopy, woody tissue and rhizosphere, is important to understand global ecosystem functioning. 2. I review how studies in tropical ecosystem ecology have progressed since their inception in the 1960s towards developing a quantitative, mechanistic and global description of the carbon cycle of tropical vegetation. 3. I present a synthesis of studies in tropical forest sites in the Americas and Asia for which gross primary productivity (GPP) has been reported, and a subset of these sites for which net primary productivity (NPP) and ecosystem carbon use efficiency (CUE) have been estimated. GPP ranges between 30 and 40 Mg C ha−1 year−1 in lowland moist tropical forests and declines with elevation. CUE in tropical forests is at the low end of the global range reported for forests. 4. A pathway and framework are presented to explain the link between photosynthesis and tropical forest biomass, and to explain differences in carbon cycling and biomass between forests. Variation in CUE and allocation of NPP can be as important as variation in GPP in explaining differences in tropical forest growth rates between sites. 5. Finally, I explore some of the key questions surrounding the functioning and future of tropical forests in the rapidly changing conditions of the early Anthropocene. 6. Synthesis.There have been significant recent advances in quantifying the carbon cycle of tropical forests, but our understanding of causes of variation amongst forests is still poor. Moreover, we should expect all tropical forests in the 21st century, whether intact or disturbed, to be undergoing rapid change in function and composition; the key challenge for tropical ecosystem ecologists is to determine and understand the major and most fundamental aspects of this change.

The 2010 spring drought reduced primary productivity in southwestern China

Abstract: Many parts of the world experience frequent and severe droughts. Summer drought can significantly reduce primary productivity and carbon sequestration capacity. The impacts of spring droughts, however, have received much less attention. A severe and sustained spring drought occurred in southwestern China in 2010. Here we examine the influence of this spring drought on the primary productivity of terrestrial ecosystems using data on climate, vegetation greenness and productivity. We first assess the spatial extent, duration and severity of the drought using precipitation data and the Palmer drought severity index. We then examine the impacts of the drought on terrestrial ecosystems using satellite data for the period 2000?2010. Our results show that the spring drought substantially reduced the enhanced vegetation index (EVI) and gross primary productivity (GPP) during spring 2010 (March?May). Both EVI and GPP also substantially declined in the summer and did not fully recover from the drought stress until August. The drought reduced regional annual GPP and net primary productivity (NPP) in 2010 by 65 and 46?Tg?C?yr?1, respectively. Both annual GPP and NPP in 2010 were the lowest over the period 2000?2010. The negative effects of the drought on annual primary productivity were partly offset by the remarkably high productivity in August and September caused by the exceptionally wet conditions in late summer and early fall and the farming practices adopted to mitigate drought effects. Our results show that, like summer droughts, spring droughts can also have significant impacts on vegetation productivity and terrestrial carbon cycling.

Key canopy traits drive forest productivity

Abstract: Quantifying the mechanistic links between carbon fluxes and forest canopy attributes will advance understanding of leaf-to-ecosystem scaling and its potential application to assessing terrestrial ecosystem metabolism. Important advances have been made, but prior studies that related carbon fluxes to multiple canopy traits are scarce. Herein, presenting data for 128 cold temperate and boreal forests across a regional gradient of 600 km and 5.4°C (from 2.4°C to 7.8°C) in mean annual temperature, I show that stand-scale productivity is a function of the capacity to harvest light (represented by leaf area index, LAI), and to biochemically fix carbon (represented by canopy nitrogen concentration, %N). In combination, LAI and canopy %N explain greater than 75 per cent of variation in above-ground net primary productivity among forests, expressed per year or per day of growing season. After accounting for growing season length and climate effects, less than 10 per cent of the variance remained unexplained. These results mirror similar relations of leaf-scale and canopy-scale (eddy covariance) maximum photosynthetic rates to LAI and %N. Collectively, these findings indicate that canopy structure and chemistry translate from instantaneous physiology to annual carbon fluxes. Given the increasing capacity to remotely sense canopy LAI, %N and phenology, these results support the idea that physiologically based scaling relations can be useful tools for global modelling.

Amazon forest carbon dynamics predicted by profiles of canopy leaf area and light environment

Abstract: Tropical forest structural variation across heterogeneous landscapes may control above-ground carbon dynamics. We tested the hypothesis that canopy structure (leaf area and light availability) – remotely estimated from LiDAR – control variation in above-ground coarse wood production (biomass growth). Using a statistical model, these factors predicted biomass growth across tree size classes in forest near Manaus, Brazil. The same statistical model, with no parameterisation change but driven by different observed canopy structure, predicted the higher productivity of a site 500 km east. Gap fraction and a metric of vegetation vertical extent and evenness also predicted biomass gains and losses for one-hectare plots. Despite significant site differences in canopy structure and carbon dynamics, the relation between biomass growth and light fell on a unifying curve. This supported our hypothesis, suggesting that knowledge of canopy structure can explain variation in biomass growth over tropical landscapes and improve understanding of ecosystem function.

Temperature, resources, and phytoplankton size structure in the ocean

Abstract: We conducted a meta-analysis of temperature, phytoplankton size structure, and productivity in cold, temperate, and warm waters of the world’s oceans. Our data set covers all combinations of temperature and resource availability, thus allowing us to disentangle their effects. The partitioning of biomass between different size classes is independent of temperature, but depends strongly on the rate of resource use as reflected in the rate of primary production. Temperature and primary production explained 2% and 62%, respectively, of the variability in the contribution of microphytoplankton to total biomass. This contribution increases rapidly with total biomass and productivity, reaching values . 80% when chlorophyll a concentration is . 2 m gL 21 or primary production is . 100 m gCL 21 d21, irrespective of water temperature. Conversely, picophytoplankton contribution is substantial (. 40%), at all temperatures, only when chlorophyll a concentration is , 1 m gL 21 or primary production is , 50 m gCL 21 d21. The temperature–size rule cannot explain these changes, which instead reflect fundamental reorganizations in the species composition of the assemblage, arising from taxon- and sizedependent differences in resource acquisition and use. Given that resource availability, rather than temperature per se, is the key factor explaining the relative success of different algal size classes, there will be no single, universal effect of global warming on phytoplankton size structure. Phytoplankton size structure largely determines the trophic organization of pelagic ecosystems and thus the efficiency with which organic matter produced by photosynthesis is channeled towards upper trophic levels or

Seasonal dynamics of above‐ and below‐ground biomass and nitrogen partitioning in Miscanthus × giganteus and Panicum virgatum across three growing seasons

Abstract: The first replicated productivity trials of the C4 perennial grass Miscanthus × giganteus in the United States showed this emerging ligno-cellulosic bioenergy feedstock to provide remarkably high annual yields. This covered the 5 years after planting, leaving it uncertain if this high productivity could be maintained in the absence of N fertilization. An expected, but until now unsubstantiated, benefit of both species was investment in roots and perennating rhizomes. This study examines for years 5–7 yields, biomass, C and N in shoots, roots, and rhizomes. The mean peak shoot biomass for M. × giganteus in years 5–7 was 46.5 t ha−1 in October, declining to 38.1 t ha−1 on completion of senescence and at harvest in December, and 20.7 t ha−1 declining to 11.3 t ha−1 for Panicum virgatum. There was no evidence of decline in annual yield with age. Mean rhizome biomass was significantly higher in M. × giganteus at 21.5 t ha−1 compared to 7.2 t ha−1 for P. virgatum, whereas root biomass was similar at 5.6–5.9 t ha−1. M. × giganteus shoots contained 339 kg ha−1 N in August, declining to 193 kg ha−1 in December, compared to 168 and 58 kg ha−1 for P. virgatum. The results suggest substantial remobilization of N to roots and rhizomes, yet still a substantial loss with December harvests. The shoot and rhizome biomass increase of 33.6 t ha−1 during the 2-month period between June and August for M. × giganteus corresponds to a solar energy conversion of 4.4% of solar energy into biomass, one of the highest recorded and confirming the remarkable productivity potential of this plant.

Plant Preference for Ammonium versus Nitrate: A Neglected Determinant of Ecosystem Functioning?

Abstract: Although nitrogen (N) availability is a major determinant of ecosystem properties, little is known about the ecological importance of plants’ preference for ammonium versus nitrate (β) for ecosystem functioning and the structure of communities. We modeled this preference for two contrasting ecosystems and showed that β significantly affects ecosystem properties such as biomass, productivity, and N losses. A particular intermediate value of β maximizes the primary productivity and minimizes mineral N losses. In addition, contrasting β values between two plant types allow their coexistence, and the ability of one type to control nitrification modifies the patterns of coexistence with the other. We also show that species replacement dynamics do not lead to the minimization of the total mineral N pool nor the maximization of plant productivity, and consequently do not respect Tilman’s R* rule. Our results strongly suggest in the two contrasted ecosystems that β has important consequences for ecosyste...

End-Cretaceous marine mass extinction not caused by productivity collapse

Abstract: An asteroid impact at the end of the Cretaceous caused mass extinction, but extinction mechanisms are not well-understood. The collapse of sea surface to sea floor carbon isotope gradients has been interpreted as reflecting a global collapse of primary productivity (Strangelove Ocean) or export productivity (Living Ocean), which caused mass extinction higher in the marine food chain. Phytoplankton-dependent benthic foraminifera on the deep-sea floor, however, did not suffer significant extinction, suggesting that export productivity persisted at a level sufficient to support their populations. We compare benthic foraminiferal records with benthic and bulk stable carbon isotope records from the Pacific, Southeast Atlantic, and Southern Oceans. We conclude that end-Cretaceous decrease in export productivity was moderate, regional, and insufficient to explain marine mass extinction. A transient episode of surface ocean acidification may have been the main cause of extinction of calcifying plankton and ammonites, and recovery of productivity may have been as fast in the oceans as on land.

Impact of precipitation dynamics on net ecosystem productivity

Abstract: Net ecosystem productivity (NEP) was measured on shortgrass steppe (SGS) vegetation at the USDA Central Plains Experimental Range in northeastern Colorado from 2001 to 2003. Large year-to-year differences were observed in annual NEP, with >95% of the net carbon uptake occurring during May and June. Low precipitation during the 2002 April to June time period greatly reduced annual net carbon uptake. Large precipitation events (>10 mm day 1 ) promoted carbon uptake, while small precipitation events ( 5 mm having a similar increase in respiration (>3.00 g m Cm 2 day 1 ). In addition, the size of the heterotrophic respiration pulse is independent of both the amount of time since the last rainfall event and the time of occurrence during the growing season.

Continuous microalgae cultivation in a photobioreactor.

Abstract: New biomass sources for alternative fuels has become a subject of increasing importance as the nation strives to resolve the economic and strategic impacts of limited fossil fuel resources on our national security, environment, and global climate. Algae are among the most promising non-food-crop-based biomass feedstocks. However, there are currently no commercially viable microalgae-based production systems for biofuel production that have been developed, as limitations include less-than optimal oil content, growth rates, and cultivation techniques. While batch studies are critical for determining basic growth phases and characteristics of the algal species, steady-state studies are necessary to better understand and measure the specific growth parameters. This study evaluated the effects of dilution rate on microalgal biomass productivity, lipid content, and fatty acid profile under steady-state conditions with continuous illumination and carbon dioxide supplemention for two types of algae. Continuous cultures were conducted for more that 3 months. Our results show that the productivity of Chlorella minutissima varied from 39 to 137 mg/L/day (dry mass) when the dilution rate varied from 0.08 to 0.64 day(-1). The biomass productivity of C. minutissima reached a maximum value (137 mg/L/day) at a dilution rate of 0.33 day(-1), while the productivity of Dunaliella tertiolecta varied from 46 to 91 mg/L/day at a dilution rate of 0.17 to 0.74 day(-1). The biomass productivity of D. tertiolecta reached a maximum value of 91 mg/L/day at a dilution rate of 0.42 day(-1). Moreover, the lipid content had no significant change with various dilution rates.

On balanced exploitation of marine ecosystems: results from dynamic size spectra

Abstract: Fisheries are often managed to protect small young fish and to harvest big old fish. This can be wasteful, leading to large parts of catches being discarded. A recent suggestion is that it could be better to distribute fishing more widely across species and body sizes, balancing it more closely to the natural productivity of different organisms. Here, we test effects of such fishing against more traditional methods using a model of a single fish species with a dynamic size spectrum together with a fixed spectrum of plankton. This has the feature that productivity is determined by the bookkeeping of biomass in the model, and decreases as fish grow larger. The results show that harvesting smaller fish (which have higher productivity) allows a greater sustainable biomass yield than harvesting larger fish (which have lower productivity); the greater spawning-stock biomass that comes from protecting large fish contributes to this. Balanced exploitation brings fishing mortality more in line with this natural variation in productivity. In addition, the resilience of the ecosystem to perturbations can be improved, and disruption to the size distribution of organisms in the ecosystem reduced. We argue that there are potentially real benefits to be gained by moving towards more balanced exploitation of marine ecosystems, unconventional though this is.

Centuries of Anadromous Forage Fish Loss: Consequences for Ecosystem Connectivity and Productivity

Abstract: Lost biomass of anadromous forage species resulting from the seventeenth to nineteenth century damming of waterways and from overharvest in the northeastern United States contributed to significant changes in coastal marine–terrestrial ecosystems. Historic alewife populations in Maine for the years 1600–1900 were assessed using analyses of nineteenth and twentieth century harvest records and waterway obstruction records dating to the 1600s. Obstructed spawning access in nine watersheds reduced the annual alewife productivity per watershed to 0%–16% of virgin estimates, equaling a cumulative lost fisheries production of 11 billion fish from 1750 to 1900. Including preharvest production, our estimates suggest a lost flux of anadromous forage fish increasing from 10 million fish per year in 1700 to 1.4 billion annually by 1850. Our results suggest a realignment of current restoration goals is needed to recognize oceanic and freshwater ecosystem interdependence and the gap between current targets and potential productivity.

A widespread decrease in productivity of sockeye salmon (Oncorhynchus nerka) populations in western North America

Abstract: We used data on 64 stocks of sockeye salmon (Oncorhynchus nerka) from British Columbia (B.C.), Washington, and Alaska to determine whether recent decreases in abundance and productivity observed fo...

A general integrative framework for modelling woody biomass production and carbon sequestration rates in forests

Abstract: Summary 1. Forests are an important, yet poorly understood, component of the global carbon cycle. We develop a general integrative framework for modelling the influences of stand age, environmental conditions, climate change and disturbance on woody biomass production and carbon sequestration. We use this framework to explore drivers of carbon cycling in New Zealand mountain beech forests, using a 30-year sequence of data from 246 permanent inventory plots. 2. A series of disturbance events (wind, snow storms, earthquakes and beetle outbreaks) had major effects on carbon fluxes: by killing large trees, they removed significant quantities of carbon from the woody biomass pool, and by creating canopy gaps, they reduced the crown area index (CAI) of stands (i.e. canopy area per unit ground area) and woody biomass production. A patch-dynamics model, which we parameterized using permanent plot data, predicts that episodic disturbance events can create long-term (c. 100-year) oscillations in carbon stocks at the regional scale. 3. Productivity declined with stand age, as shown in many other studies, but the effect was hard to detect because of canopy disturbance. Individual trees can increase productivity by adjusting the positioning, nutrient content and angle of leaves within canopies. We show that such optimization is most effective when trees are large and suggest it reduces the impact of water and nutrient limitation in old stands. 4. We found no evidence that forests were responding to changing climatic conditions, although strong altitudinal trends in biomass production indicate that global warming could alter carbon fluxes in future. 5. Synthesis.Our study emphasizes the critical role of disturbance in driving forest carbon fluxes. Losses of biomass arising from tree death (particularly in older stands) exceeded gains arising from growth for most of the 30-year study, moving 0.3 Mg C ha−1 year−1 from biomass to detritus and atmospheric pools. Large-scale disturbance events are prevalent in many forests world-wide, and these events are likely to be a driving factor in determining forest carbon sequestration patterns over the next century.

Impacts of an abundant introduced ecosystem engineer within mudflats of the southeastern US coast

Abstract: Invasive ecosystem engineers can have far-reaching effects on systems, especially if they provide structure where none was before. The non-native seaweed Gracilaria vermiculophylla has proliferated on estuarine mudflats throughout the southeastern US, including areas (South Carolina and Georgia) that historically were extremely low in seaweed biomass. Quantitative field surveys across 150 km of high salinity estuaries revealed that the density of the native onuphid polychaete Diopatra cuprea and the aboveground height of its biogenic tubes, which Diopatra decorates with drifting debris and seaweed, positively influenced Gracilaria biomass. The abundance of Gracilaria epifauna, composed primarily of amphipods and small snails, increased with Gracilaria biomass at many locations in our field surveys. To examine whether epifauna were facilitated by Gracilaria we experimentally manipulated Gracilaria biomass in two locations. Consistent with the field surveys, we found that increasing Gracilaria biomass facilitated epifauna, particularly amphipods and snails. Epifaunal densities on Gracilaria were higher than on a biologically-inert structural mimic of Gracilaria (plastic aquarium alga), indicating that epifauna colonize Gracilaria because Gracilaria provisions both physical structure and a biological resource. We also quantified the seaweed’s net rate of productivity and decomposition. Primary production of Gracilaria was variable, but massive in some areas (up to 200 % net biomass increase in 8 weeks). The seaweed rapidly degraded upon burial in silty sediments (79 % loss in mass within 10 days) and thus may represent an important new addition to detrital foodwebs. As a copious, novel source of primary production, detritus, and desirable habitat for epifauna, Gracilaria has the potential to transform southeastern US estuaries.

Effects of wave exposure on population structure, demography, biomass and productivity of the kelp Laminaria hyperborea

Abstract: Patterns of potential recruitment, survival, age-structure, density, biomass and pri- mary production were studied in the kelp Laminaria hyperborea along a wave-exposure gradient in western Norway. The distribution of L. hyperborea is positively related to wave exposure, and the present work aimed to study how demography and population dynamics are affected by wave exposure. Populations at 9 sites representing 3 different levels of wave exposure were investi- gated. The biomass and production of L. hyperborea doubled along the gradient from low- to high-exposure sites. This increase was caused by an increase in plant density and individual plant size. Recruits and sub-canopy plants made up the majority of all individuals at all sites, but poten- tial recruitment increased more than mortality rate among recruits and young sub-canopy plants as wave exposure increased, leading to a higher density of these small individuals at high- exposure sites. Despite their large numbers, recruits and sub-canopy plants were not important for total biomass and production, and variations in those parameters with wave exposure were largely driven by variations in the density and size of adult canopy plants. Canopy plants suffered higher rates of mortality at low wave exposure, leading to shorter longevity and lower density than at high-exposure sites. The density and individual size of canopy plants both increased by ca. 50% with increasing wave exposure, explaining the higher biomass and productivity at high- exposure sites. We suggest that the high mortality rate, low density and small size of canopy plants at low-exposure sites is caused by a combination of high epiphytic load and self-shading, which may impair light conditions, affect nutrient uptake and increase drag on the blades during extreme wind events. The reason why plants at high-exposure sites reach a larger individual size remains unknown.

Migrate or stay: terrestrial primary productivity and climate drive anadromy in Arctic char

Abstract: A shift in the magnitude and timing of animal migrations is one of the most documented ecological effects of climate change. Although migrations are largely driven by spatial variation in resource gradients, few studies connect expected changes in primary production with geographic patterns in migratory behavior. Here, we link lake primary production to the occurrence of sea migrations in the partially anadromous salmonid Arctic char (Salvelinus alpinus L.). We compiled presence/absence records of anadromous char populations spanning productivity and temperature gradients along the Norwegian coast. The probability of anadromy decreased with increasing migration distance, maximum slope of the migration route and lake productivity. There was a significant interaction between lake productivity and migration distance. The negative effect of longer migration distances was more severe in lakes with higher productivity, indicating reduced relative profitability of migration with increased feeding opportunities in freshwater. Lake productivity was mainly driven by terrestrial primary production in the catchment. We predicted future distributions of anadromous char given downscaled temperature and precipitation changes projected by two different emission scenarios and global climate models (GCMs). Projected increases in temperature and precipitation in 2071–2100 increased terrestrial primary production and, compared to the control scenario (1961–1990), decreased the range of anadromous populations. The prevalence of anadromy decreased by 53% in the HadAm3H GCM with the A2 emission scenario, 61% in HadAm3H with the B2 scenario and 22% in ECHAM4 with the B2 scenario. Cross-ecosystem studies (e.g., terrestrial to freshwater) are critical for understanding ecological impacts of climate change. In this case, climate-driven increases in terrestrial primary production are expected to increase primary production in lakes and ultimately reduce the prevalence of anadromy in Arctic char populations.

The ecology of lizard reproductive output

Abstract: Aim We provide a new quantitative analysis of lizard reproductive ecology. Comparative studies of lizard reproduction to date have usually considered life-history components separately. Instead, we examine the rate of production (productivity hereafter) calculated as the total mass of offspring produced in a year. We test whether productivity is influenced by proxies of adult mortality rates such as insularity and fossorial habits, by measures of temperature such as environmental and body temperatures, mode of reproduction and activity times, and by environmental productivity and diet. We further examine whether low productivity is linked to high extinction risk. Location World-wide. Methods We assembled a database containing 551 lizard species, their phylogenetic relationships and multiple life history and ecological variables from the literature. We use phylogenetically informed statistical models to estimate the factors related to lizard productivity. Results Some, but not all, predictions of metabolic and life-history theories are supported. When analysed separately, clutch size, relative clutch mass and brood frequency are poorly correlated with body mass, but their product – productivity – is well correlated with mass. The allometry of productivity scales similarly to metabolic rate, suggesting that a constant fraction of assimilated energy is allocated to production irrespective of body size. Island species were less productive than continental species. Mass-specific productivity was positively correlated with environmental temperature, but not with body temperature. Viviparous lizards were less productive than egg-laying species. Diet and primary productivity were not associated with productivity in any model. Other effects, including lower productivity of fossorial, nocturnal and active foraging species were confounded with phylogeny. Productivity was not lower in species at risk of extinction. Main conclusions Our analyses show the value of focusing on the rate of annual biomass production (productivity), and generally supported associations between productivity and environmental temperature, factors that affect mortality and the number of broods a lizard can produce in a year, but not with measures of body temperature, environmental productivity or diet.

Above- and belowground responses of Arctic tundra ecosystems to altered soil nutrients and mammalian herbivory.

Abstract: Theory and observation indicate that changes in the rate of primary production can alter the balance between the bottom-up influences of plants and resources and the top- down regulation of herbivores and predators on ecosystem structure and function. The Exploitation Ecosystem Hypothesis (EEH) posited that as aboveground net primary productivity (ANPP) increases, the additional biomass should support higher trophic levels. We developed an extension of EEH to include the impacts of increases in ANPP on belowground consumers in a similar manner as aboveground, but indirectly through changes in the allocation of photosynthate to roots. We tested our predictions for plants aboveground and for phytophagous nematodes and their predators belowground in two common arctic tundra plant communities subjected to 11 years of increased soil nutrient availability and/or exclusion of mammalian herbivores. The less productive dry heath (DH) community met the predictions of EEH aboveground, with the greatest ANPP and plant biomass in the fertilized plots protected from herbivory. A palatable grass increased in fertilized plots while dwarf evergreen shrubs and lichens declined. Belowground, phytophagous nematodes also responded as predicted, achieving greater biomass in the higher ANPP plots, whereas predator biomass tended to be lower in those same plots (although not significantly). In the higher productivity moist acidic tussock (MAT) community, aboveground responses were quite different. Herbivores stimulated ANPP and biomass in both ambient and enriched soil nutrient plots; maximum ANPP occurred in fertilized plots exposed to herbivory. Fertilized plots became dominated by dwarf birch (a deciduous shrub) and cloudberry (a perennial forb); under ambient conditions these two species coexist with sedges, evergreen dwarf shrubs, and Sphagnum mosses. Phytophagous nematodes did not respond significantly to changes in ANPP, although predator biomass was greatest in control plots. The contrasting results of these two arctic tundra plant communities suggest that the predictions of EEH may hold for very low ANPP communities, but that other factors, including competition and shifts in vegetation composition toward less palatable species, may confound predicted responses to changes in productivity in higher ANPP communities such as the MAT studied here.

Spatial and seasonal variability of primary production on the Eastern Bering Sea shelf

Abstract: Spatial and seasonal variability in net primary production (NPP) is reported for the Eastern Bering Sea shelf in spring and summer 2008 and 2009. Euphotic zone integrated NPP in the spring, estimated from simulated in situ 14C-incubations, varied ∼30-fold across the northern and southern regions of the Inner, Middle, and Outer domains of the shelf. During spring, rates were lowest and similar in the North and South region of the Inner domain, while the highest rates were in the Southern region of the Outer domain as a result of extensive ice-edge phytoplankton blooms. Generally, phytoplankton integrated chlorophyll (Chl-a) standing stocks were dominated (>50%) by large (>5 μm) cells. With the exception of the southern Middle domain, integrated rates of NPP were consistently lower in summer than spring, while there was no consistent pattern in changes in integrated Chl-a. Conversely, phytoplankton growth rates (μ) were lower in spring (0.22±0.09 d−1) than summer (0.42±0.17 d−1), consistent with a seasonal shift from a biomass-controlled production system to a growth-rate controlled production system. Associated with this shift in control was a decrease in the size distribution of chlorophyll from ∼60% of the Chl-a>5 μm in the spring to ∼30% of the Chl-a>5 μm in the summer. Despite the widespread distribution of stations, these observations highlight the variable nature of NPP in the Bering Sea, which makes the estimation of seasonal or annual rates in any domain or across the entire shelf difficult, if not impossible, on direct observations alone. A vertically generalized productivity model (VGPM) was used to assimilate the more extensive underway dataset from two cruises to improve the spatial distribution of calculated NPP included in the regional estimates of NPP. The VGPM output captured ∼83% of the variance in measured 14C production, accurately estimated observed NPP (Model II regression slope±stdev.; 0.92±0.06), and allowed for a better constrained estimation of shelf-wide productivity due to higher data density in each region/domain. These results, when combined with published data on shelf-wide productivity suggest that the ecosystem response to climate change (whether an increase or decrease in productivity) would have to exceed a factor of two from mean conditions before being detectable from a comparable survey effort.

Biodiversity increases the productivity and stability of phytoplankton communities.

Abstract: Global biodiversity losses provide an immediate impetus to elucidate the relationships between biodiversity, productivity and stability. In this study, we quantified the effects of species richness and species combination on the productivity and stability of phytoplankton communities subject to predation by a single rotifer species. We also tested one mechanism of the insurance hypothesis: whether large, slow-growing, potentially-defended cells would compensate for the loss of small, fast-growing, poorly-defended cells after predation. There were significant effects of species richness and species combination on the productivity, relative yield, and stability of phytoplankton cultures, but the relative importance of species richness and combination varied with the response variables. Species combination drove patterns of productivity, whereas species richness was more important for stability. Polycultures containing the most productive single species, Dunaliella, were consistently the most productive. Yet, the most species rich cultures were the most stable, having low temporal variability in measures of biomass. Polycultures recovered from short-term negative grazing effects, but this recovery was not due to the compensation of large, slow-growing cells for the loss of small, fast-growing cells. Instead, polyculture recovery was the result of reduced rotifer grazing rates and persisting small species within the polycultures. Therefore, although an insurance effect in polycultures was found, this effect was indirect and unrelated to grazing tolerance. We hypothesize that diverse phytoplankton assemblages interfered with efficient rotifer grazing and that this "interference effect" facilitated the recovery of the most productive species, Dunaliella. In summary, we demonstrate that both species composition and species richness are important in driving patterns of productivity and stability, respectively, and that stability in biodiverse communities can result from an alteration in consumer functioning. Our findings underscore the importance of predator-prey dynamics in determining the relationships between biodiversity, productivity and stability in producer communities.