Loreau, M., S. Naeem, P. Inchausti, J. Bengtsson, J.P. Grime, A. Hector, D.U. Hooper, M.A. Huston, D. Raffaelli, B. Schmid, D. Tilman, and D.A. Wardle. 2001. Biodiversity and ecosystem functioning: current knowledge and future challenges. Science 294: 804-808.

Abstract. The ecological consequences of biodiversity loss have aroused considerable interest and controversy during the past decade.  Major advances have been made in describing the relationship between species diversity and ecosystem processes, in identifying functionally important species, and in revealing underlying mechanisms. There is, however, uncertainty as to how results obtained in recent experiments scale up to landscape and regional levels and generalize across ecosystem types and processes. Larger numbers of species are probably needed to reduce temporal variability in ecosystem processes in changing environments. A major future challenge is to determine how biodiversity dynamics, ecosystem processes, and abiotic factors interact.
 

Thompson, J.N., O. J. Reichman, P.J. Morin, G.A. Polis, M.E. Power, R.W. Sterner, C.A. Couch, L. Gough, R. Holt, D.U. Hooper, F. Keesing, C.R. Lovell, B.T. Milne, M.C. Molles, D.W. Roberts, S.Y. Strauss. 2001. Frontiers in Ecology. BioScience 51: 15-24.

Abstract. In December 1999 the National Science Foundation convened a white paper committee to evaluate what we know and do not know about important ecological processes, what hurdles currently hamper our progress, and what intellectual and conceptual interfaces need to be encouraged. The committee distilled the discussion into four frontiers in research on the ecological structure of the earth's biological diversity and the ways in which ecological processes continuously shape that structure (i.e., ecological dynamics).  This article summarizes the discussions of those frontiers and explains why they are crucial to our understanding of how ecological processes shape patterns and dynamics of global biocomplexity. The frontiers are 1. Dynamics of coalescence in complex communities. 2. Evolutionary and historical determinants of ecological processes: The role of ecological memory. 3. Emergent properties of complex systems: Biophysical constraints and evolutionary attractors. 4. Ecological topology: Defining the spatiotemporal domains of causality for ecological structure and processes.
 

Hooper, D.U., D.E. Bignell, V.K. Brown, L. Brussaard, J.M. Dangerfield, D.H. Wall, D.A. Wardle, D.C. Coleman, K.E. Giller, P. Lavelle, W.H. v. d. Putten, P.C. d. Ruiter, J. Rusek, W.L. Silver, J.M. Tiedje, and V. Wolters. 2000. Interactions between aboveground and belowground biodiversity in terrestrial ecosystems: patterns, mechanisms, and feedbacks. BioScience 50:1049-1061.

Abstract. An international workshop convened in October 1998 under the auspices of the SCOPE (Scientific Committee on Problems of the Environment) Committee on Soil and Sediment Biodiversity and Ecosystem Functioning sought to address such questions about relationships between aboveground and belowground diversity. This article takes a two-step approach to evaluating these relationships. First we review the evidence for correlations between diversity of aboveground and belowground organisms, both on local and across larger biogeographical scales. If correlations exist, we ask whether they result from direct linkages among groups of organisms above and below the surface. Second, where cause-and-effect relationships are apparent, we synthesize what is known about the mechanisms involved in those relationships.
 

Chapin, F.S. III, E.S. Zavaleta, V.T. Eviner, R.L. Naylor, P.M. Vitousek, H.L. Reynolds, D.U. Hooper, S. Lavorel, O.E. Sala, S.E. Hobbie, M.C. Mack, and S. Díaz. 2000. Functional and societal consequences of changing biotic diversity. Nature 405: 234-242.

Abstract. Human alteration of the global environment has triggered the sixth major extinction event in the history of life and caused widespread changes in the global distribution of organisms. These changes in biodiversity alter ecosystem processes and change the resilience of ecosystems to environmental change. This has profound consequences for services that humans derive from ecosystems. The large ecological and societal consequences of changing biodiversity should be minimized to preserve options for future solutions to global environmental problems.
 
 

Hooper, D.U. and L. Johnson. 1999. Nitrogen limitation in dryland ecosystems: responses to geographical and temporal variation in precipitation. Biogeochemistry 46: 247-293.

Abstract. We investigated the relationship between plant nitrogen limitation and water availability in dryland ecosystems. We tested the hypothesis that at lower levels of annual precipitation, aboveground net primary productivity (ANPP) is limited primarily by water whereas at higher levels of precipitation, it is limited primarily by nitrogen. Using a literature survey of fertilization experiments in arid, semi-arid, and subhumid ecosystems, we investigated geographic gradients, as well as across year-to-year variation in precipitation within sites. We used four different indices to assess the degree of N limitation: (1) Absolute Increase of plant production in response to fertilization (the slope of ANPP vs. amount of added N at different levels of annual precipitation); (2) Relative Response (the percent increase in fertilized over control ANPP at different levels of N addition); (3) Fertilizer Use Efficiency (FUE, the absolute gain in productivity per amount of fertilizer N), and (4) Maximum Response (the greatest absolute increase in ANPP at saturating levels of N addition). Relative Response to fertilization did not significantly increase with increasing precipitation either across the geographic gradient or across year-to-year variation within sites. Nor did the Maximum Response to fertilization increase with increasing precipitation across the geographic gradient. On the other hand, there was a significant increase in the Absolute Increase and FUE indices with both geographical and temporal variation in precipitation. Together, these results indicate that there is not necessarily a shift of primary limitation from water to N across the geographic water availability gradient. Instead, our results support the hypothesis of co-limitation. The apparently contradictory results from the four indices of N limitation can best be explained by an integration of plant ecophysiological, community, and ecosystem mechanisms whereby plants are co-limited by multiple resources, species shifts occur in response to changing resource levels, and nitrogen and water availability are tightly linked through biogeochemical feedbacks.
 

Hooper, D.U. 1998. The role of complementarity and competition in ecosystem responses to variation in plant diversity. Ecology 79: 704-719.

Abstract. To investigate how plant diversity affects ecosystem-level processes such as primary production and nutrient cycling, I established an experimental plant diversity gradient in serpentine grassland using four functional groups of plants: early season annual forbs (E), late season annual forbs (L), perennial bunchgrasses (P), and nitrogen fixers (N). These groups differ in growth form, phenology, and other traits relevant to nutrient cycling (e.g., rooting depth, litter C:N ratio). Two or three species of each type were planted in single-group treatments, and in two-, three-, and four-way combinations, giving a range of richness from zero to nine species. I tested the hypothesis that, because of complementary resource use, increasing functional group diversity will lead to higher net primary production. At the scale of this experiment (a -diversity and yearly production), more diverse treatments were not necessarily the most productive. Live plant biomass varied more within than among levels of diversity. In most two-, three-, and four-way mixtures of functional groups, overall productivity did not differ significantly from the average of the yields of component one-group treatments. This pattern apparently resulted from competition: early season annuals and late season annuals reduced the biomass of perennial bunchgrasses (the most productive group in monoculture) below levels expected from monoculture yields.

Relative Yield Total (RYT) indicated complementary resource use in the EL and LP two-way and ELPN four-way mixtures. In the EL mixture, complementarity appeared to result from temporal rather than spatial partitioning of resources. Because of shifts in root:shoot ratio in mixtures, however, only the LP treatment had consistently significant RYT>1 when assessing total (roots plus shoots) productivity and nitrogen yield. These results show that (1) composition (the density of the species present) can be at least as important as richness (the number of species present) in effects on ecosystem processes; (2) competition during critical parts of the growing season may prevent absolute increases in net primary production with increasing diversity, despite complementary resource use at other times of the year; and (3) shifts in belowground allocation in species mixtures can have significant effects on estimates of productivity and resource use as species diversity changes.
 

Hooper, D.U. and P.M. Vitousek. 1998. Effects of plant composition and diversity on nutrient cycling. Ecological Monographs 68: 121-149.

Abstract. We evaluated the effects of plant functional group richness on seasonal patters of soil nitrogen and phosphorus cycling, using serpentine grassland in south San Jose, California. We established experimental plots with four functional types of plants: early-season annual forbs (E), late-season annual forbs (L), nitrogen-fixers (N), and perennial bunchgrasses (P). These groups differ in several traits relevant to nutrient cycling, including phenology, rooting depth, root:shoot ratio, size, and leaf C:N content. Two or three species of each group were planted in single functional group (SFG) treatments, and in two-, three-, and four-way combinations of functional groups. We analyzed available nutrient pool sizes, microbial biomass nitrogen and phosphorus, microbial nitrogen immobilization, nitrification rates, and leaching losses.

We used an index of "relative resource use" that incorporates the effects of plants on pool sizes of several depletable soil resources: inorganic nitrogen in all seasons, available phosphorus in all seasons, and water in the summer dry season. We found a significant positive relationship between increasing relative resource use (including both plant and microbial uptake) and increasing plant diversity. The increase in relative resource use results because different functional groups have their maximum effect on different resources in different seasons: E's dominate reduction of inorganic nitrogen pools in winter; L's have a stronger depletion of nitrogen in spring and a dominant reduction of water in summer; P's have a stronger nitrogen depletion in summer; N-fixers provide additional nitrogen in all seasons and have a significant phosphorus depletion in all seasons except fall. Single functional group treatments varied greatly in relative resource use; for example, the resource use index for the L treatment is as high as in the more diverse treatments.

We expected a reduction of leaching losses as functional group richness increased because of differences in rooting depth and seasonal activity among these groups. However, measurements of nitrate in soil water leached below the rooting zone indicated that, apart from a strong reduction in losses in all vegetated treatments compared to the bare treatment, there were no effects of increasing plant diversity. While some single functional group treatments differed (L, N ³ P), more diverse treatments did not. Early- and late-season annuals, but not perennial bunchgrasses, had significant positive effects on microbial immobilization of nitrogen in short-term (24 h) ¹⁵N experiments.

We conclude that: (1) total resource use, across many resource axes and including both plant and microbial effects, does increase with increasing plant diversity on a yearly time-scale due to seasonal complementarity; (2) while the presence of vegetation has a large effect on ecosystem nitrogen retention, nitrogen leaching losses do not necessarily decrease with increasing functional group richness; (3) indirect effects of plants on microbial processes such as immobilization can equal or exceed direct effects of plants on microbial processes such as immobilization can equal or exceed direct effects of plant uptake on nutrient retention; and (4) plant composition (i.e., the identity of the groups present in treatments) in general explains much more about the measured nutrient cycling processes than does functional group richness alone (i.e., the number of groups present).

Verville, J.H., S.E. Hobbie, F.S. Chapin III, and D.U. Hooper. 1998. Response of tundra CH₄ and CO₂ flux to manipulation of temperature and vegetation. Biogeochemistry 41: 215-235.

Abstract. We conducted plant species removals, air temperature manipulations, and vegetation and soil transplants in Alaskan wet-meadow and tussock tundra communities to determine the relative importance of vegetation type and environmental variables in controlling ecosystem methane (CH4) and carbon dioxide (CO2) flux. Plastic greenhouses placed over wet-meadow tundra increased air temperature, soil temperature, and soil moisture, but did not affect CH4 or CO2 flux (measured in the dark). By contrast, removal of sedges in the wet meadow significantly decreased flux of CH4, while moss removal tended to increase CH4 emissions. At 15 cm depth, pore-water CH4 concentrations were higher in sedge-removal than in control plots, suggesting that sedges contribute to CH4 emissions by transporting CH4 from anaerobic soil to the atmosphere, rather than by promoting methanogenesis. In reciprocal-ecosystem transplants between the wet-meadow and tussock tundra communities, CH4 and CO2 emissions were higher overall in the wet-meadow site, but were unrelated to transplant origin. Methane flux was correlated with local variation in soil temperature, thaw depth, and water-table depth, but the relative importance of these factors varied through the season. Our results suggest that future changes in CH4 and CO2 flux in response to climatic change will be more strongly mediated by large-scale changes in vegetation and soil parameters than by direct temperature effects.
 

Chapin, F.S. III, O.E. Sala, I.C Burke, J.P. Grime, D.U. Hooper, W.K. Lauenroth, A. Lombard, H.A. Mooney, A.R. Mosier, S. Naeem, S.W. Pacala, J. Roy, W. Steffen, and D. Tilman. 1997. Ecosystem consequences of changing biodiversity. BioScience 48: 45-52.

Abstract. We investigated the relationship bewteen plant nitrogen limitation and water availability in dryland ecosystems. We tested the hypothesis that at lower levels of annual precipitation, aboveground net primary productivity (ANPP) is limited primarily by water whereas at higher levels of precipitation, it is limited primarily by nitrogen. Using a literature survey of fertilization experiments in arid, semi-arid, and subhumid ecosystems, we investigated the response of ANPP to nitrogen addition as a function of variation in precipitation across geographic gradients, as well as across year-to-year variation in precipitation within sites. We used four different indices to assess the degree of N limitation: (1) Absolute Increase of plant production in response to fertilization (the slope of ANPP vs. amount of added N at different levels of annual precipitation); (2) Relative Response (the percent increase in fertilized over control ANPP at different levels of N addition); (3) Fertilizer Use Efficiency (FUE, the absolute gain in productivity per amount of fertilizer N), and (4) Maximum Response (the greatest absolute increase in ANPP at saturating levels of N addition); Relative Response to fertilization did not significantly increase with increaseing precipitation either across the geographic gradient or across year-to-year variation within sites. Nor did the Maximum Response to fertilization increase with increasing precipitation across the geographic gradient.  On the other hand, there was a significant increase in the Absolute Increase and FUE indices with both geographical and temporatl variation in precipitation.  Together, these results indicate that there is not necessarily a shift of primary limitation from water to N across the goeographic water availability gradient,  Instead, our results support the hypothesis of co-limitation.  The apparently contradictory results from the four indices of N limitation can best be explained by an integration of plant ecophysiological, community, and ecosystem mechanisms whereby plants are co-limited by multiple resources, species shifts occur in response to changing resource levels, and nitrogen and water availability are tightly linked through biogeochemical feedbacks.

Hooper, D.U. and P.M. Vitousek. 1997. The effects of plant composition and diversity on ecosystem processes. Science 277: 1302-1305.

Abstract. The relative effects of plant richness (the number of plant functional groups) and composition (the identity of the plant functional groups) on primary productivity and soil nitrogen pools were tested experimentally. Differences in plant composition explained more of the variation in production and nitrogen dynamics than did the number of functional groups present. Thus, it is possible to identify and differentiate among potential mechanisms underlying patterns of ecosystem response to variation in plant diversity, with implications for resource management.
 

Chapin, F.S. III, B.H. Walker, R.J. Hobbs, D.U. Hooper, J.H. Lawton, O.E. Sala, and D. Tilman. 1997. Biotic control over the functioning of ecosystems. Science 277: 500-504.

Abstract. Changes in the abundance of species--especially those that influence water and nutrient dynamics, trophic interactions, or disturbance regime--affect the structure and functioning of ecosystems. Diversity is also functionally important, both because it increases the probability of including species that have strong ecosystem effects and because it can increase the efficiency of resource use. Differences in environmental sensitivity among functionally similar species give stability to ecosystem processes, whereas differences in sensitivity among functionally different species make ecosystems more vulnerable to change. Current global environmental changes that affect species composition and diversity are therefore profoundly altering the functioning of the biosphere.

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