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Announcements Homework due Wed. Extra credit due next Friday – if you want it back by final exam. Extra credit opportunity next Friday at noon, SL 110: 1. 2. 3. Processes affecting diversity - I READING: Processes affecting diversity Chap. 16 - Species Abundance A. Equilibrium and Diversity 1. Habitat heterogeneity/niche differentiation Chap. 17 – Community structure 2. Intermediate stress hypothesis 3. Food webs and trophic interactions: keystone species B. Non-equilibrium 1. Intermediate disturbance hypothesis 2. Temporal variability C. Ecosystem implications of food webs Chap. 18 – 2o production (4th: 424-9; 5th: 411-17) Tasha Johnson - Sex with Aliens: The effects of a showy invasive plant on pollination of a rare endemic wildflower 4. - D. Island biogeography (Chap. 22) 1. Effects of island size and distance 2. The balance between immigration and extinction E. The latitudinal species gradient 1. The patterns 2. The hypotheses Mutualisms lecture – posted, with notes. Focus on questions outlined at the beginning of powerpoint and in the study guide. Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. Causes of diversity variation? 1. Environmental Complexity • D. Hooper ©1992 In general, species diversity increases with environmental complexity or heterogeneity. Serpentine grassland Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. Diversity of Algae 16.9 Molles: Ecology 2nd Ed. 16.11 Molles: Ecology 2nd Ed. 1 Environmental Heterogeneity and Diversity of Plants (tropical forests) Above floodplain 2. Intermediate stress hypothesis Grime predicts a “humped-back” distribution of species richness across a gradient of productivity (degree of resource availability stress) 1-2 m above stream >2 m above stream Drought-stressed Grime 2001, pp. 263-266 16.14 Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. Highest species richness generally found in areas with low nutrient availability Addition of nutrients leads to decline in species richness and evenness in Rothamsted, England 16.15 16.16 Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. Serpentine grasslands Serpentine soils: low nutrients + high heavy metals + low Ca:Mg = tough for plants! But, high diversity because of low dominance by exotic grasses. 3. Food webs and trophic interactions Keystone species D. Hooper ©1992 Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. 2 Effects on community composition • Keystone predators 13.21 17.7 Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. Keystones species 13.21 Molles: Ecology 2nd Ed. Fig.17.16 Keystones species Fig.17.8 Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. Keystones: dependence on environment Fig.17.9 Molles: Ecology 2nd Ed. 3 Not all top predators increase diversity Keystones: factors that increase diversity 1. Selective predation 2. Preferred prey is the competitive dominant Fig.17.17 Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. B. Non-equilibrium processes 1. Intermediate disturbance hypothesis • Disturbance and Diversity Connell proposed disturbance is a prevalent feature that significantly influences community diversity. Proposed both high and low levels of disturbance would reduce diversity. • Sousa defined disturbance: Discrete, punctuated, killing, displacement, or damaging of one or more individuals that directly or indirectly creates an opportunity for new individuals to be established. Two major characteristics: Frequency Intensity Molles: Ecology 2nd Ed. Molles: Ecology 2nd Ed. 2. Temporal heterogeneity For example, coexistence of competing species in grasslands because environmental conditions (rainfall amount and timing) varies greatly from year to year. Variable conditions mean that the best competitor varies from year-to-year as well. • • Goldfields San Jose, California Boulders supporting greatest diversity of species were those subject to intermediate levels of disturbance. ¾ Molles: Ecology 2nd Ed. 25 20 15 10 5 0 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Large boulders require more force to move. 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 16.18 A nnual precip. (inches) 30 Plantago Year Molles: Ecology 2nd Ed. D. Hooper photos 2005 4 Summary • Species diversity is affected by both equilibrium and non-equilibrium processes. Equilibrium: complex environments, degree of environmental stress, trophic interactions. Non-equilibrium: levels of disturbance, temporal variability. Molles: Ecology 2nd Ed. 5