Biodiversity and Ecosystem Function Dr. Mathew Williams What is biodiversity? • OED: ―biodiversity Ecol., diversity of plant and animal life, as represented by the number of extant species‖ • Ricklefs & Miller: Biodiversity includes a number of different levels of variation in the natural world: genetic, species, ecosystem • Begon et al. ―The term may be used to describe the number of species, the amount of genetic variation or the number of community types present in an area‖. • But…most studies do focus on species diversity Are Aspects of Ecosystem Functioning Dependent on Biodiversity? Functional Consequences of Biodiversity: Numbers and Organismal Ecosystem Kinds of traits Processes Species Traits and Ecosystem Function • Traits may mediate energy and material flow directly • Traits may alter abiotic conditions (limiting resources, disturbance, microclimate) Trait Expression Is Determined By: • Species richness • Species evenness • Species composition • Species interaction • Temporal and spatial variation Photo: Arctic LTER Leaf area index 2.0 Pteridophyte Evergreen LAI, one-sided (m m ) -2 Deciduous 1.5 Graminoid 2 1.0 0.5 0.0 5 1 2 H 94-4 3 4 6 HV 13 14 15 16 9 Barrens Wetland Heath MNT Moist acidic tundra Shrubs 2 r =0.93 3.2 Total foliar N (g m ground area ) 2.8 2.4 2.0 -2 1.6 1.2 0.8 0.4 0.0 0.0 0.4 0.8 1.2 1.6 2.0 2.4 LAI The Ecosystem/Ecology Divide • Key ecosystem types in Arctic tundra show clear differences in key species and functional types • But at the ecosystem level there are clear patterns in the landscape irrespective of species composition • Bulk measures like LAI and foliar N are good descriptors of process rates • So, are species important? An exercise in pairs • Identify specific examples of links between biodiversity and ecosystem function/process Species Richness and Ecosystem Function: Theory • If niches are complementary, adding species could increase process rates linearly • As niches overlap the response should saturate Niche differentiation and productivity. a, A simple model — the 'snowballs on the barn' model — of niche differentiation and coexistence. The range of conditions in which each species can exist is shown with a circle, the position of which is defined by its centre. By randomly choosing locations for various numbers of circles (species), it is possible to calculate the effect of diversity on the 'coverage' of the heterogeneous habitat. The amount of such coverage is proportional to community biomass. b, Results of simulations (triangles) and of an analytical solution (solid curve) to the effects of diversity on community productivity for the snowballs on the barn model From: Tilman (2000), Nature. Tilman‘s biodiversity experiment (A) Dependence of 1996 aboveground plant biomass (that is, productivity) (mean and SE) on the number of plant species seeded into the 289 plots. (B) Dependence of 1996 above- ground plant biomass on the number of functional groups seeded into each plot. Curves shown are simple asymptotic functions fitted to treatment means. More complex curves did not provide significantly better fits From: Tilman et al. (1997) Science Problems With Richness Experiments • Disentangling interactions in natural systems is difficult • Measuring productivity (below ground) • Scale (too short and too small) • ‗Sampling effect‘ problem in constructed communities – Sampling effect may be an important biological property or an experimental artefact if natural community assembly rules are broken Hypothesized mechanisms involved in biodiversity experiments using synthetic communities. Sampling effects are involved in community assembly, such that communities that have more species have a greater probability of containing a higher phenotypic trait diversity. Phenotypic diversity then maps onto ecosystem processes through two main mechanisms: dominance of species with particular traits, and complementarity among species with different traits. Intermediate scenarios involve complementarity among particular species or functional groups or, equivalently, dominance of particular subsets of complementary species. From: Loreau et al (2001) Science Richness Conclusions • With our knowledge now, we cannot reject the hypothesis that a few dominant species suffice to provide the functional diversity that is necessary to explain the level of primary production observed in grassland ecosystems at the small spatial and temporal scales considered in recent experiments. Species Evenness • Human effects on species more commonly involve alteration of relative abundance than extinction • Little research on importance of evenness of function so far • Future richness experiments should include evenness effects Species Composition: • Species mediate pathways of energy and material flow • Examples: Introduced species can alter patterns of ecosystem processes Introduced Species Can Alter Patterns of Ecosystem Processes I • Introduction of N-fixing tree Myrica faya to N-limited Hawaiian forests led to 5-fold increase in N inputs • Significant impacts on forest structure and function Vitousek et al. (1987) Science Introduced Species Can Alter Patterns of Ecosystem Processes II • Introduction of deep-rooted salt cedar (Tamarix sp.) to Mojave and Sonaran deserts resulted in: – Increased water accessed by vegetation – Increased surface litter and salts – Inhibited many native species, reduced biodiversity Berry (1970) Introduced Species Can Alter Patterns of Ecosystem Processes III • Introduction of Agropyron cristatum, tussock grass, to US Great Plains – Reduced allocation to roots compared to native grasses – Soil N levels reduced, and 25% less total soil C compared to native prairie soil •Christian & Wilson (1999) Introduced Species Can Alter Patterns of Ecosystem Processes IV • Introduction of Bromus tectorum, cheatgrass, to western US – Fire frequency increased by a factor of 10 in the >40 million ha it now dominates •Whisenant (1990) CASE STUDY: Diversity of tropical trees and carbon water relations Exploring compositional effects on ecosystem function • Are Amazonian trees water stressed? • Does water stress depend on soil texture? • Does water stress depend on species? Ecological Diversity in Amazonian Rain Forest Diversity in Rooting Depths Dawn water potential along 2.0 a soil texture gradient Dawn leaf water potential (MPa -1) 1.5 x 1.0 0.5 0.0 -0.5 3 1 4 5 6 9 10 2 11 12 7 8 13 High clay High sand Site Williams et al (2002) Species Interactions • Mutualism • Trophic interaction – Predation – Parasitism – Herbivory • Competition Mutualism • N-fixation in plant-microbe symbiosis • Plant-mycorrhizal associations – Both increase production and accelerate succession • Decomposition is driven by highly integrated consortia of microbes CASE STUDY Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity Background • arbuscular mycorrhizal fungi (AMF) form mutualistic symbiotic associations with the roots of 80% of all terrestrial plant species, thereby acting as extensions of plant root systems and increasing nutrient uptake, especially of phosphorus • Communities vary in AMF biodiversity Experimental design • Expt 1: Greenhouse experiment, 48 microcosms simulating European calcareous grassland – Quantified plant growth with no AMF or different diversities of AMF • Expt 2: Field experiment, 70 macrocosms simulating N. American old fields. – 15 plant species per plot, random mix of different numbers of species (out of 23) AMF, quantified primary productivity Experiment 1 van der Heijden 1998 Experiment 2 van der Heijden 1998 More diverse microbial communities are more efficient Soil microbial functional diversity (Shannon index H') and metabolic quotient (qCO2 = soil basal respiration/soil microbial biomass) correlate inversely. A higher diversity in the organic plots is related to a lower qCO2, indicating greater energy efficiency of the more diverse microbial community. The Shannon index is significantly different between both conventional systems (CONFYM, CONMIN) and the BIODYN system, the qCO2, between CONMIN and BIODYN (P < 0.05). Maeder 2002 Trophic Interactions • Modify fluxes of energy and materials • Influence abundance of species that control these fluxes – e.g., predator removal can lead to a cascade of ecological effects (A) Changes in sea otter abundance over time at several islands in the Aleutian archipelago and concurrent changes in (B) sea urchin biomass, (C) grazing intensity, and (D) kelp density measured from kelp forests at Adak Island. Error bars in (B) and (C) indicate 1 SE. The proposed mechanisms of change are portrayed in the marginal cartoons--the one on the left shows how the kelp forest ecosystem was organized before the sea otter's decline and the one on the right shows how this ecosystem changed with the addition of killer whales as an apex predator. Heavy arrows represent strong trophic interactions; light arrows represent weak interactions. Estes et al. (1998) Science Experimental Lakes •Two lakes dominated by Zooplanktivorous fishes(minnows) •And two other lakes dominated by Piscivorous fish (bass) •Of each pair, one was fertilized with N+P while the other was left as a control Schindler et al (1997) Science bass + P minnows, grazers, algae, lake shifts from C source to sink Comparison of crustacean grazer length, primary production, and DPCO2 [PCO2(lake) - PCO2(air)] before, during, and after a period of low minnow abundance in Peter Lake (1994 to 1995). Data are shown as the mean ± SD. Sample sizes are given in parentheses. Dotted line in bottom panel represents dissolved CO2 in equilibrium with the atmosphere Schindler et al (1997) Science Trophic Interactions: Conclusions • All types of organisms must be considered in understanding biodiversity effects • Interactions among species must be considered • Changes in interactions can alter traits expressed by species, so presence/absence of species is insufficient to predict impact Biodiversity and Ecosystem Services • Ecosystem services are defined as the processes and conditions of natural ecosystems that support human activity and sustain human life • E.g., maintenance of soil fertility, climate regulation, natural pest control • E.g., flows of ecosystem goods such as food, timber and freshwater Attaching Value to Biodiversity • Techniques used include direct valuation based on market prices, and estimates of what individuals are willing to pay to protect endangered wildlife • Valuation of marginal losses that accompany specific biodiversity changes are most relevant to policy decisions • Predictions are highly uncertain What you should have learned today • The ways in which biodiversity can affect ecosystem function • Experimental approaches and the sampling effect • Examples of species traits that control particular processes • The concept of ecosystem services and valuation References • Chapin et al (2000) Consequences of changing biodiversity. Nature 405: 234-242 • Tilman, Wedin and Knops (1996) Productivity and sustainability influenced by biodiversity in grassland ecosystems. Nature 379: 718-720 • Naeem & Li (1997) Biodiversity enhances ecosystem reliability. Nature 390:507-509 • Van der Heijden et al. (1998) Nature 396: 69-72 • Constanza et al (1997) The value of the world's ecosystem services and natural capital. Nature 387: 253-260 • Maeder et al (2002) Soil fertility and biodiversity in organic farming. Science 296: 1694-7 Reading for next week • Pfisterer. A.B. & B. Schmid. 2002. Diversity-dependent production can decrease the stability of ecosystem functioning. Nature 416 84-86 – what insights does this experiment provide? – what are the criticisms of the approach? • McCann, K., A. Hastings G. R. Huxel. 1998. Weak trophic interactions and the balance of nature. Nature 395 794-8 – what insights does the modelling provide? – what are the criticisms of the approach?
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