Biodiversity Chapter 2: Species Richness, Stability and other Community Properties Patterns and Experiments

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1 Bio200B Laurel Fox Biodiversity Chapter 2: Species Richness, Stability and other Community Properties Patterns and Experiments Main themes for new research efforts in last years on: Properties of systems such as: stability, S, cover, productivity, nutrients. 1. Link across levels populations, communities, ecosystems (e.g.: the role of spp in community or ecosystem functioning) 2. Functional roles of species are some spp. redundant? - Do some spp contribute in similar ways to comm./ecosys Functioning? - How similar? 3. Conservation: concern for S loss needs strong ecological basis Focus of research: S and properties of communities. Tilman #1: Tilman & Downing 94 (Nature) 11 yrs study Question: what factors control S in native grasslands Assessed: S, dynamics, diversity Started in 1982: 207 plots 4 x 4 m 5-6 replicates per field 4 fields During drought : Before drought Treatment: rate of Nitrogen application (7 levels) 1987/88 Drought (worst in 50 yrs) 45% decrease in living plant biomass 35% decrease in S Conclusion #1 T1: More speciose communities are more resistant to disturbance 1

2 Resistance After drought : Before drought Conclusion #2 T1: More speciose communities are more resilient following a disturbance. Some are still well below pre-drought levels BUT: 1. Experiment manipulated N 2. So: plots with different S had spp that functioned differently with respect to N S Biomass S C3 grasses Drought sensitive C4 grasses Drought resistant Nitrogen level SO: Biomass decreased in drought because of N treatment (which selected for spp with particular properties) and not necessarily because S high or low. 2

3 3. Another interpretation pattern of resource availability - N additions are constant through time (added each year) - Rain: equally variable for all plots within a year, but varied among years ( a hidden treatment) So: low-n plots: - limited by N and can never increase much even in wet years. - interannual variability in biomass low because still N-limited even if more water is available high-n plots - biomass varies interannually - increase biomass in wet years (not limited by N or water) - decreases in dry years (limited by water) Biomass During drought Biomass: similarly low in all treatments during drought (in 96 paper, not 94 paper) Fertilizer level Conclusions from T1: 1. Expt NOT show that stability and S are directly related 2. S is NOT a treatment variable 3. Variation in biomass (=net primary productivity) related to variation in limiting resources and not to S per se. 3

4 Naeem et al Nature m 3 each Ecotron all environments the same same # of trophic levels (bacteria, nematodes, collembola, earthworms, insects, plants, snails ) Less diverse communities are subsets of the more diverse communities Evaluate mechanisms of changing functions w/ high or low S Property S Plants # reps Nested design (Low S subsets of higher S communities) Community Properties w/ in Diversity Ecosystem Properties w/ in Diversity % ground cover Respiration Volume of plants Decomposition 0 # snails, insects, parasites Nutrient retention 0 Productivity Water retention 0 BUT: Replicates sets of identical chambers w/ same spp composition. So they are not replicating S. are replicating specific set of spp SO: can t separate differences between particular spp and S itself. 4

5 Tilman #2 : 147 plots 3 x 3 m 7 tmts manipulate S 24 plant spp add S as seed to bare ground (+ some supplemental water and weeding to maintain S) replicates 23 reps 24 reps e H measure of diversity = # of equally abundant spp needed to give H Experimental plots select S for each plot by random draw from spp pool Productivity Removes soil nutrients Similar patterns in adjacent, natural grasslands: Conclusion: S matters BUT 5

6 BUT..subtle problems: 1. Probability of selecting same spp in replicates of a given S-level increases as S itself increases spp in pools - all reps w/ 24 S are the same - reps w/ few spp.more likely to be different SO: as S increases, variance declines. At low S may select spp w/ particular properties in different plots, and so greater variance among replicates. 2. Size bias: more large spp as S increases Cover More large plants at increased S.set by size of the largest spp.and not an effect of the total S NO 3 pattern NO 3 in plant zone S Many multispp groups are dominated by individuals of the largest species. SO: most biomass is from a few, dominant spp. This may also reflect biomass (more root mass) Note: Tilman & Wedin used monocultures of 5 spp and got similar results as plant biomass increased, NO 3 declined. S By the mid-90s, the discussion of what controls bidiversity had started to include: what is biodiversity? what is the best way to think about it? and does this affect how it may be conserved?? IS IT THE # OF SPECIES OR SOME OTHER ASPECT OF COMMUNITY STRUCTURE THAT LINKS W/ STABILITY AND OTHER PROPERTIES? 6

7 Types of Biodiversity 1. # of species 2. particular spp 3. # of Functional Groups 4. particular FGs Do any one or a combination most determine properties of ecological systems? Tilman #3: Many more plant species so that problem with the design is fixed. Also had several functional groups.c3 plants, C4 plants, N-fixers, other herbaceous plts. Conclusion: variation in both S and functional groups? Functional Groups C3 grasses C4 grasses Legumes (N-fixing) Other herbaceous plts Woody plts BUT Design too complex to easily tease apart effects of S and FG. Tilman concludes S more important (despite his data), others say FGs. Tilman #4 7

8 Tilman #4 = The latest iteration of this problem (Reich et al 2004) Cedar Creek (same site as T1, T2 and T3) Design: 359 plots 2 x 2 m 16 spp used (4 per each of 4 FGs) Randomly selected for each plot S and FG: statistically independent F = 1 (S = 1,4) F = 4 (S = 4, 9, 16) Other tmts: N, CO 2 Effects of S (for different F): Effects of F (for different S): Both S and F influence/determine community properties Reading for Tuesday: Zavaleta et al (2010): what is a functional group in these experiments? Are they too narrowly conceived? 8

9 Where are we now? --- Current Thinking about Biodiversity 1. S per se - most ecologists think this is not the driver of community/ecosystem responses, but that properties of spp that are correlated w/ S affect functions. 2. Special properties matter (Functional groups, keystone spp.) 3. Interaction strengths : - Strong interactions (high IS) can destabilitize commuities (e.g., strong trophic cascades) - Weak interactions (low IS) dampen effects of perturbations (e.g., buffering, coexistence of species with shared consumer) A weak interaction C R1, R2 show negative covariance (a dampening effect): R1 R2 variation in the abundance of R1 + R2 is lower than the variation of individual spp; reduces overall variation. 4. Niche complementarity (also functional groupings.) spp or fg s differ so use of available resources is maximized (saturated community). 5. Insurance Effect redundancy of spp w/ particular roles, including the ability of some spp to respond differentially to perturbations. 6. Variability of spp responses (related to #5). With increased variability of responses to perturbations, the fewer spp are needed to buffer the community. Perhaps paradoxically, increased population fluctuations act to dampen changes in communities. 7. Higher-order interactions (mutualisms, herbivory, predation ) increase complexity of the interaction web, and this enhances stability. 9