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Community Structure BIOL400 12 October 2015 Abundance of Species in Communities: Preston’s Log-Normal Bell Curve Fig. 19.1 p. 379 6814 moths of 197 species from Rothamsted, England (not shown: a species with 1799 captures) Fig. 19.2 p. 380 Place abundance of species on log2 scale of “octaves” Defines a partial bell curve Left of the y-axis: species too rare to detect in the sample Trees of BCI, Panama Fig. 19.3 p. 380 Snakes of Panama British birds HANDOUT—Preston 1962 Factors Promoting Community Diversity A) General Latitudinal Trend Species diversity is greatest in tropical areas and declines toward either pole Fig. 19.6 p. 382 Fig. 19.7 p. 383 Fig. 19.9 p. 384 B) History Older communities have had more time for specialization and diversification of species Might explain part of the tropics-vs.temperate-zone difference Never-glaciated tropics vs. temperate zone that was glaciated repeatedly C) Spatial Heterogeneity More spatial heterogeneity of habitat = more niches to be filled Predicts less overlap in resource use in tropics HANDOUT Longenecker, 2007 D) Intermediate-Disturbance Hypothesis Highest diversity of species occurs at intermediate frequency of disturbance High frequency: several species die out due to detrimental effects of the disturbance Low frequency: competitive exclusion occurs as species approach carrying capacity of the habitat Fig. 19.23 p. 396 HANDOUT—Sousa 1979 Fig. 19.24 p. 397 The "tail" of lowered abundance on one side or the other may not exist Tide-pool data support hypothesis Emergentsubstrate data do not Fig. 19.25A p. 397 Fig. 19.25B p. 397 Regulation of Species’ Abundances in Ecosystems Regulation of Species’ Abundances in Ecosystems Ecologists generalize about effects of species on one another in nutrientvegetation-herbivore-carnivore systems 27 (333) different models of effects can be drawn using level-to-level symbols: , , and (symbolizing regulation of abundance) Ex: N V H C Fig. 21.12 p. 436 HSS (1960): Predators limit herbivores, who do not limit plants Competition (C) is greater among plants and among carnivores than among herbivores Fig. 21.12 p. 436 MS (1987): Model incorporates environmental stress as possibly trumping competition and predation in regulating populations Predation becomes progressively more important than competition as the environment becomes more benign Fig. 21.11 p. 436 Fig. 21.12 p. 436 HSS (1960): Strong competition among plants; herbivores compete only weakly and do not regulate plants MS (1987): Weak competition among plants; herbivores compete in benign environments and regulate plants Table 21.3 p. 437 In most studies, herbivore removal had strong positive effect on plants Supports MS model Two Additional Models Top-down NVHC aka “Trophic Cascade” Bottom-up Regulation Regulation NVHC Fig. 21.14 p. 439 Fig. 21.15 p. 439 Top-down regulation in Zion National Park, Utah Fig. 21.16 p. 440 Cougars common Cougars rare HANDOUT Hebblewhite et al. (2005) Table 21.4 p. 441 Food Webs and Their Linkages Food Webs and Their Linkages Complexity of food webs is limited, primarily by inefficiency of energy transfer from one trophic level to next Generally about 5-20% • Rest lost to heat of metabolism and decomposers Fig. 20.7 p. 408 Food chains are short Data on 95 species in an estuary in Scotland (5518 links) Fig. 20.8 p. 409 Food chains shorten at lower productivity Experimental tree holes that varied in added leaf litter (100%, 10%, or 1%) Fig. 20.5 p. 407 Connectance = proportion of all total possible links that occur = 10/72 = 0.20 Generally ~0.14 regardless of web’s diversity Two Alternate Hypotheses Constant Connectance L is some constant proportion of S2, which is the maximum possible L • L=S2: each species is linked to every other species, including itself via cannibalism Link-Species Scaling Posits linear relationship of L with S HANDOUT—Martinez (1992) Constant Connectance: Supported by Exponent of 1.54? S1 S1.54 Link-Species Scaling Regression 20 50 100 101 413 1,202 S2 Constant Connectance 400 2,500 10,000 Diversity-Stability Hypothesis Diversity-Stability Hypothesis Stability = resistance to change and rate of recovery from change Idea that more diverse communities resist and recover from major change better than less diverse systems Some functional redundancy with increased diversity of species Fig. 20.20 p. 419 168 experimental plots of MN prairie Community stability = Mean/SD for late-summer biomass over 10 years Fig. 20.21 p. 419 Resistance measures changes in abundance of plant species