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Global biodiversity patterns and changes S Richness, S = number of species Diversity, H’ = - S pi log2 pi a, b,g diversity: alpha diversity: the diversity of a site beta diversity: the change in species composition from place to place, or along environmental gradients (turnover)gamma diversity: the diversity of a region or landscape due to either or both a and b. i = 1
(Begon et al. 1996) In sea, diversity increases to 6000 m, then decreases (Sanders et al. attribute to stability, then resource deprivation…)
Macan’s Filter: If a species isn’t present at a site, is it because of… Dispersal (age, distance/isolation)? yes, propagules haven’t arrived and established. no, propagules arrived, but populations don’t persist… Behavior (specialization/heterogeneity)? yes, colonists avoid this habitat. no, colonists settle, but don’t persist… Abiotic factors (stress, productivity, heterogeneity)? yes, physico-chemical conditions outside tolerable range. no, abiotic conditions sufficiently benign… Biotic factors (productivity, disturbance, heterogeneity)? yes: predators, pathogens, parasites, competitors or lack of mutualists exclude species. no… Interactions…
Stanley 1976 hypothesis: breakthrough that ended the Pre-Cambrian was the invention of ‘jaws’—the first herbivorous protists grazed holes in algal monocultures, producing environmental heterogeneity…
Tropical vs temperate niches Are niches narrower (species more specialized) in the tropics (a)? Can more niche overlap be tolerated without competitive exclusion (b)? Are resource axes longer in the tropics? Is niche space more completely filled?
Lawton- Empty niches? England Papua New Guinea Bracken fern New Mexico
Milankovitch theory: orbital oscillations of Earth cause cyclical changes in temperature and precipitation: Eccentricity of orbit: 100 ky cycle Annual timing of aphelion (point in planet’s orbit when it is furthest from the sun) relative to summer solstice: 23 ky cycle Tilt of wobbling pole: 41 ky cycle Dynesius and Jansson 2000 Ruddiman 2000
Orbitally forced species range dynamics (Dynesius and Jansson 2000) • Mean duration of species 1-30 my, so species have endured many Milankovitch oscillations • Oscillations more severe at higher latitudes than around equator, selecting in temperate, boreal and arctic species for • More vagility • Less specialization Range restricted =< 50,000 sq. km
Dynesius and Jansson 2000 Larger effect (temperature change) at higher latitudes than near equator
Brown and Maurer 1989: Hypoth: N. Am. species with small geographic ranges are limited by local environmental conditions so should run NW, while species with large ranges limited by large scale patterns of vegetational or climatic zones that correspond to latitudinal bands (run EW) North American reptiles Europe—small ranges should run EW
Diversity vs Productivity: a hump-shaped relationship, like intermediate disturbance?
Longest running ecological experiment: 1843 - present Science 293 p. 625 (2001)
Light triangles: extinct Dark triangles: extant Parmesan, C. 1996 Censused Edith’s checkerspot butterfly throughout its range for 151 previously located populations. Striking latitudinal trends in extinction: populations in Mexico 4x more likely to have gone extinct than those in Canada. Habitat loss was similar at southern and northern ends of range, suggesting range contraction due to climate change. This hypothesis also supported by less frequent extinctions at higher elevations.
Invasive species… SF Chronicle Nov 28 2003
David Schindler whole lake manipulations in Experimental Lakes Region of southern Ontario Over a wide range of pH in acidification experiments, and of nutrient concentrations in eutrophication experiments, ecosystem respiration and photosynthesis stayed fairly constant, while species composition varied markedly.
Tilman and Downing 1994 Cedar Creek LTER Plots with higher plant diversity maintained more constant biomass across severe 1987 drought Drought resistance (biomass 1988/biomass1986)
Duffy et al. 2003: more diverse grazer assemblage more effective in removing algal epiphytes from sea grass beds
Ecosystem consequences of diversity loss…(Chapin et al. 2000). • “Rivet hypothesis” (Ehrlich): which species are crucial to ‘holding the wings on the plane’, esp. in changing environments? • Grasslands: less productivity, resilience after drought • Plant production enhanced linearly by mycorrhizal species richness • Consortia of diverse microbes each contribute different enzymes to speed organic matter decomposition and nutrient recycling • Societal costs of diversity loss (some speculative examples…) • Passenger pigeons may have suppressed deer mice by competing for acorns, reducing Lyme disease reservoirs (Ostfeld) • Cheatgrass fires reduce rangeland value and air quality (D’Antonio) • Teosinte, wild relative of corn, needed periodically to develop virus-resistant strains—nearly lost to land conversion around Jalisco, MX, and now possibly subject to genetic engineering introgression (Chapela)
Higher species diversity in tropical rainforest vertebrates because of higher rates of speciation, or lower rates of extinction? Most sister taxa arose before Pleistocene, but recent species occur in heterogeneous, geologically or climatically unstable areas Evidence for both gradient and allopatric diversification Conservation strategies need to preserve both the important genetic lineages (variation) and the ecological processes that generate species (e.g. refuges and barriers: edges and forest core) Moritz et al. 2000: