BIOS 3010: Ecology Lecture 23: Patterns of Biodiversity & conservation: • Lecture summary: Male Passenger Pigeon – Patterns of diversity. – Richness relationships & gradients: • Resource productivity. • Spatial heterogeneity. • Climate. • Latitude, altitude & depth. • Succession. – Conservation. • Counting species. • Threats, Uncertainty & Risk. • Population Viability Analysis.
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2. Simple patterns:
• Why do some communities contain more species than others? – Are there patterns or gradients of species richness? – If so, what are the reasons for these patterns? • “Geographical”, “Climatic”, “Independent” and “Biological” reasons. – Species richness (number of species present at a site) can be related to a resource availability axis (R) as in Fig. 21.1 that is partitioned (saturated) according to niche breadths (n) that overlap by varying amounts (o) as a measure of specialization.
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3. Richness relationships:
• Resource productivity: – More productive environments have more species with narrower niches but higher densities (Fig. 24.2). – But evidence also suggests that highest diversity occurs at intermediate levels of productivity in some communities (Fig. 24.6) because competitive exclusion is less likely and immigration rates are higher.
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1 4. Richness relationships:
• Increased spatial heterogeneity also generates more species diversity (Fig. 24.9). • Climatic variation influences species richness: – Unpredictable climatic variation is a form of disturbance and species richness may be highest at intermediate levels. – But some work shows that species richness increases as climatic variation decreases (Fig. 21.9).
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5. Gradients of richness:
• Latitude: – Increase in species diversity with a decrease in latitude towards the tropics (Fig. 24.13). • Perhaps because of more intense predation (top-down) and also reduced competition, as well as increased productivity (bottom-up) but with most productivity locked up in biomass releasable by high decomposition rates after death. • Altitude: – Decrease in species richness with increased altitude (Fig. 24.17).
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6. Gradients of richness:
• Depth: – Generally species richness in lakes decreases with water depth. – But for benthic invertebrates richness is highest on the continental shelf at about 2000m (Fig. 24.19). • Succession: – Species richness increases with time through successional series because of a shift in dominance (Fig. 24.20) from smaller numbers of dominant species to more species of equivalent dominance. • Then usually a decline in richness at successional maturity (Fig. 24.21).
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2 7. Conservation ecology
• The need for conservation: – As human population density has increased worldwide, extinctions of other species have increased for the following reasons: • Overexploitation by hunting and harvesting. • Habitat destruction. • Introduction of exotic pest species. • Pollution.
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8. The need for conservation:
• Extinctions impact “biodiversity” – Species richness: • Number of species in a geographical area. – Species diversity: • Species richness + relative species abundances, biomasses or productivities. – Scale of ecological organization: • From genes and populations through communities to landscapes and biomes. • Fig. 7.16 and Table 7.4.
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9. The basic conservation problem:
• Realistically, the only viable way to conserve species and habitats is to address the issue politically through economics driving legislation: – This is a good example of Garrett Hardin’s class of “no technical solution” problems in which science can explain the problem but cannot solve it: • Solutions are implemented through legislation.
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3 10. The basic conservation problem:
• The problem is how to assess the economic value of species and habitats (ecological economics). • We need to assess: – (1) Direct economic value through conserved resource consumption (objective). – (2) Indirect economic value, including amenity value without the need for resource consumption (objective). – (3) Ethical value of species (subjective).
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11. Counting species:
• How do we count the number of species alive today? • Needed to estimate rates of extinction (Table 7.4). • About 1.8 million species have been named. • Estimates of how many exist are based on: – 2 tropical species for each temperate species: • = 3-5 million species. – Rate of discovery of new species, group by group: • = 6-7 million species. – Species size:species richness relationship >0.2 mm: • = 10 million species. – Ratio of beetle:non-beetle species richness: • = 30 million tropical arthropod species: – but see Zuckerman in the 9 June 2010 issue of New Scientist at: http://www.newscientist.com/article/dn19019-global-biodiversity-estimate-revised-down.html – Estimates go as high as 80 million species.
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12. Threats to species:
• Rarity (Fig. 7.17) - categories of threat: – Vulnerable: • 10% probability of extinction within 100 years. – Endangered: • 20 % probability of extinction within 20 years or 10 generations (whichever is longer). – Critical: • 50% probability of extinction within 5 years or 2 generations.
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4 13. Threats to species:
• Both prevalence (proportion area covered by a species) and intensity (density) are important. • Absence can be just as important as presence for different spatial and temporal distributions. • 8 types of commonness or rarity: – (Table 25.2). • Abundance and distribution appear to be positively correlated (Fig. 25.3).
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14. Threats to species:
• Overexploitation: – Unsustainable harvests of resources such as great whales and large terrestrial herbivores (bison). – Collection of rare species. • Habitat disruption: – Through destruction, degradation or disturbance. • Introduced species: – e.g. brown tree-snake on Guam or the nile perch in Lake Victoria (Fig. 25.4).
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15. Uncertainty and the risk of extinction:
• Dynamics of small populations: (Fig. 7.18). – High levels of uncertainty and high risks of extinction through: • (1) Demographic uncertainty • (2) Environmental uncertainty • (3) Spatial uncertainty (metapopulations). • Local and global extinctions: – Local extinctions are common events (Fig. 7.21) and extinction is more likely in smaller populations.
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5 16. Uncertainty and the risk of extinction:
• Habitat fragmentation: – Populations fragment into metapopulations. – Then each constituent subpopulation gets smaller with higher risks of extinction (Fig. 25.9). – Fragmentation thus influences the probability of extinction of metapopulations and the way in which we design nature reserves (Figs. 25.11 & 25.12).
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17. Population Viability Analysis (PVA):
• What is the minimum viable population size (MVP) for a species population? • Answers based on: – (1) Clues from long-term studies: • e.g. Fig. 25.8b using criteria such as 95% probability of persistence for 100 years (Table 7.6) • Based on data from hunting and bird watching enthusiasts. – (2) Subjective assessment: • Discussion among experts on target species to develop PVA- based decision analysis for species such as the Sumatran rhinoceros (Fig. 7.23). – (3) Modelling persistence time: • Increases with population size and mean intrinsic rate of natural increase - if mean r is greater than its variance then persistence time increases dramatically (Fig. 7.24, 7.26).
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Figure 21.1:
Simple model of how species richness may vary with resource range (R), niche size (n), and niche overlap (o)
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6 Figure 24.2 (3rd ed.): Niche overlap and environmental productivity.
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Figure 24.5 (3rd ed.): Species richness of (a) seed-eating rodents and ants (b) lizards, and (c) cladocera, against measures of productivity.
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Figure 24.6 (3rd ed.) : Highest richness at intermediate levels of productivity for (a) Malaysian trees, (b) fynbos plants, (c) desert rodents.
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7 Figure 24.9 (3rd ed.) : Increase in species richness with increased spatial heterogeneity for (a) freshwater fish in Wisconsin lakes, and (b) birds in Mediterranean climates.
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Figure 21.9: Decreasing species richness with increased temperature ranges in western North America.
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Figure 24.13 (3rd ed.) : Variation in species richness with latitude (see fig. 21.21, 4th ed.).
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8 Figure 24.17 (3rd ed.): Variation in species richness with altitude in Nepal (see fig. 21.22, 4th ed.).
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Figure 24.19 (3rd ed.): Species richness with depth of marine, bottom-dwelling animals.
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Figure 24.20 (3rd ed.): Rank-abundance curves of successional shifts in species richness.
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9 Figure 24.21 (3rd ed.): Increase in species richness during old-field successions (see Fig. 21.25, 4th ed.)
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Figure 7.16: Trends in animal species extinctions since 1600.
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Table 7.4
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10 Figure 7.17: Levels of threat.
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Table 25.2 (3rd ed.):
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Figure 25.3 (3rd ed.): Abundance positively correlated with distribution of breeding birds in Britain.
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11 Figure 25.4 (3rd ed.): Effect of brown tree snake on forest birds of Guam.
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Figure 7.18: Path to extinction in small populations.
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Figure 7.21: Extinction rates against habitat area for (a) zooplankton in lakes, (b, c, e, f) birds on islands, (d) plants in Sweden.
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12 Figure 25.9 (3rd ed.):
Breeding success of birds in Michigan at different distances from a forest edge.
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Figure 25.11 (3rd ed.): Extinction probabilities (P) for same sized metapopulations subjected to different degrees of fragmentation (m = migration).
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Figure 25.12 (3rd ed.): Abundance of checkerspot butterflies in (a) a single population or (b) 3 separate populations.
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13 Figure 25.8 (3rd ed.): Extinction or persistence against population size for (a) island birds, and (b) bighorn sheep.
(b = Fig 7.22 in 4th ed.)
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Table 7.6:
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Figure 7.23: 30-year management decisions for Sumatran rhinoceros (pE = probability of extinction, EpE = expected value of pE).
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14 Figure 7.24: Population persistence time against population size under demographic and environmental stochasticity.
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Figure 7.26: Cumulative probability of elephant extinction in 6 different habitat sizes over 1,000 years.
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