evidence and some key studies: diversity models (lecture 9)

Question 1

What is the evidence for Janzen-Connel?

• assumption local density-dependence is mediated by natural enemies
• assumption density-dependence is overcompensating

Answer 1

Bell, Freckleton and Lewis (2006)

• density-dependence in presence of pathogens (natural enemies)
• no density-dependence when fungicide applied (pathogens absent)

Bagchi et al. (2010)
- density-dependence is overcompensating when fungal pathogens present

Question 2

What is the evidence for Janzen-Connel?

• host specialisation

Answer 2

Bagchi et al. 2009
- parent tree = Parashorea melaanonan

• measured survival of other species in plots
• plotted survival of other species against evolutionary distance from P. melaanonan
• evolutionary distance should measure likelihood that other species share enemies with P. melaanonan
• survival of trees close to parent was higher with evolutionary distance from parent species

Question 3

Janzen-Connel?

• degree of specialisation

Answer 3

• more insect species in tropical than temperate forests
• herbivorous insects no more specialised in tropics than temperate species
• implies insect diversity driven by tree diversity

Question 4

What is the importance of community level density-dependence?

Answer 4

• community level density-dependency = tendency for rare species to become more abundant, common species to become rarer
• according to J-C = consequence of natural enemies
• neutral theory has no-density dependence
• general theory
• strong density-dependence, weak interspecific competitions = fulfilled coexistence conditions (Lotka-Volterra)
• this is the case for pairwise interactions (Volkov et al., 2009)

Question 5

What is the relationship beween density-dependence and abundance?

Answer 5

• positive correlation between strength of density dependence resulting from natural enemies and abundance
• Mangan et al., 2010
• Cornita et al., 2010

Question 6

What support is there for neutral theory?

Answer 6

• no direct evidence
• but predictions of neutral theory look like real systems
• difficult to test using main predictions
  e. g. distributions of abundance
• theoretically very useful
• if diversity regulated by natural enemies coexistence of competitors is not the problem neutral theory suggests

Question 7

How are communities organised in hyperdiverse tropical forests?

Answer 7

• communities in tropical forests appear more complex with greater diversity of interactions
• trophic networks in modified habitats more simplified

Tylianakis et al., 2007

• in modified habitats had higher ratio of parasitoid to host species, increased parasitism rates
• most abundant parasitoid species was more specialized in human modified habitats

Question 8

What are outstanding problems?

Answer 8

• what maintains biodiversity?
• why X species?
• why did so many species evolve?
• what happens as we fragment habitats?
• what are the consequences of climate change?