1. 10/19/071 Biodiversity at local scales Reading assignment: GSF Ch. 13

2. 10/19/072 Biodiversity at Local Scales Differing abundances of different species may result from variable competitive ability (fitness) among species, or from stochastic factors Patterns of commonness and rarity are scale-dependent and may shift over time Common species are called “dominants” Most species are not common

3. 10/19/073 Geographic range (wide/narrow) Habitat specificity (broad/restricted) Local abundance (somewhere large vs. everywhere small) Three aspects of species distributions (Rabinowitz, 1981)

4. 10/19/074 Why are species common? Ecological generalists: wide habitat tolerance; broad fundamental niches Common species are “superior organisms,” more competitive for resources This theory has been partially supported, but maybe common species happen to be well adapted to commonly found environmental conditions

5. 10/19/075 Why are species rare? Classic explanation is that species tend to be ecologically specialized – Low abundance – Small geographic ranges (endemism) – Rare species can efficiently exploit their specialized niche Lack of dispersal (spatial and temporal barriers) Historical accidents

6. 10/19/076

7. 7 Why are species invasive? Invasive species are those that expand their ranges rapidly outside their native habitat –May be “weedy” or “ruderal” but not always –May be native to the region but more often exotic Community and ecosystem properties may be altered Most invasive species have been introduced by people (2000-3000 species in US, last 100 years)

8. 10/19/078 Traits of Invasive Species (also see Mack et al., 2000) Invaders have a variety of strategies for spreading Lots of seeds, wind dispersal Rhizomes For pines, reproduction at a young age, small seeds and large, frequent seed crops Rapid growth; lack of natural predators/pathogens Empty niches Invaders may alter ecosystem properties in a way that increases their success (feedback)

9. 10/19/079 Diffuse knapweed

10. 10/19/0710 “Novel Weapons” hypothesis (Callaway and Ridenour) Certain root exudates are ineffective against natural neighbors Diffuse knapweed (Centaurea diffusa) produces 8-hydroxyquinoline, an allelopathic compound that has adverse effects on many plants outside its native range Selective advantage may result in rapid evolution of the weapon Competitive ability is further strengthened

11. 10/19/0711 Testing Allelopathy Root exudates collected from diffuse knapweed inhibited shoot and root differentiation and germination of native plant seedlings, crop plants and even other invasive species Vivanco et al. 2004

12. 10/19/0712 Allelopathic compound is less concentrated in soils of knapweed’s native range than in invaded soils Vivanco et al. 2004 Identifying allelopathic compounds

13. 10/19/0713 And to make matters even worse… Plants exposed to root boring insects for biological control produce more allelopathic chemicals, reduce native grass growth more Biocontrol actually makes these plants more competitive Thelen et al. 2005

14. 10/19/0714 Ecosystem Conditions leading to Invasion Empty niches (Elton 1958) –Species-poor communities have more ecological “space” for invaders –Evidence is mixed! Unused resources –Dalmation toadflax, tap-rooted perennial forb, invades mixed grass prairie; few other species make use of deep soil moisture Disturbance –Increases resource availability and niche space

15. 10/19/0715 What is the role of empty niches in invasion? Dominant paradigm says that species-poor areas are susceptible to invasion Species-rich communities may have higher rate of invasive species (Fig. 13.5) S. African fynbos, New Zealand beech forests, and Great Plains grasslands show similar patterns Are species-rich areas more fertile?

16. 10/19/0716 What is the role of disturbance in invasion? More work needs to be done on specific mechanisms driving disturbance-invasion cycles Factors promoting likelihood of invasion after disturbance soil surface conditions, microclimate water infiltration rates competition natural enemies resource (nutrient) availability feedbacks: kochia and cheatgrass increase N cycling rates

17. 10/19/0717 Phases of invasion Lag phase; many introduced species disappear Population increases slowly Often multiple introductions of the same species occur over time and space Invasion phase; rapid population increase in number and area Eventually the population and area stabilizes (may take centuries) The “tens” idea

18. 10/19/0718 Mack et al. 2000 Phases of invasion

19. 10/19/0719 Scales of Diversity (see Table 15.4) Inventory diversity (species density, Shannon-Weiner index, etc) –Alpha diversity: within one community –Gamma diversity: across several communities Differentiation diversity (mean similarity, turnover) –Beta diversity: difference in community composition along an environmental gradient or among communities in landscape –See coenocline slide

20. 10/19/0720 Why do some areas have high species diversity? As productivity (or resources, or area) increases, the number of species should, too Assumes that more productivity is driven by more energy in the system, which would support more individuals (or species) “Paradox of Enrichment:” at high productivity, some individuals (species) outcompete others if they get bigger or take up more space, leading to lower diversity

21. 10/19/0721 The relationship between productivity and diversity is very scale dependent

22. 10/19/0722 Productivity-Diversity Patterns Highest biodiversity was found at intermediate levels of productivity in 40% of studies Could be that areas with intermediate productivity and fertility are most common, and more species are adapted to these conditions

23. 10/19/0723 Does disturbance promote diversity? Gaps in temperate and tropical forests are important in maintaining diversity –Stochastic events lead to species coexistence Intermediate disturbance hypothesis –Competitive exclusion reduces diversity at low levels of disturbance –Recolonization may be too slow at high levels of disturbance –Recent review showed marginal support for this idea (Fig. 13.10)

24. 10/19/0724 Does increased biodiversity increase productivity? (why might this happen?) Much of the increased productivity was associated with functional groups May be dependent on particular species

25. 10/19/0725 Does increased biodiversity increase ecosystem stability? What is stability? If stability is reduced variability, there is some evidence for a relationship (Fig. 13.12)

26. 10/19/0726