Lecture 9 - evidence and some key studies

Question 1

what experiment showed evidence for local density being mediated by natural enemies?

Answer 1

• they removed pathogens by spraying fungicide on a new patch of seedlings - removing pathogens removed density dependence - in patchs with fungicide surivival was not effected by density whereas in patchs without fungicide the survival rate was much lower in high density populations

Question 2

describe over compensating density dependence

Answer 2

• its needed for the JC to work
• in an overcompensating population the number of survivors falls as the starting population is increased - as we increase the density of seeds the probability of dying increases so fast that noone can survive at high density

Question 3

describe experimental evidence for over compensating density dependence

Answer 3

• when they add fungicide there is no DD - as density increases do does survivors
• no fungicide = overcompensating DD - few survivors - maximum survival occurs at medium densities

Question 4

describe host specialisation

Answer 4

• also critical for JC to work

- natural enemies attack host - enemies have many hosts

Question 5

describe experimental evidence for host specialisation

Answer 5

• measuring the survival of species at different evolutionary distances from the parent tree
• survival of trees close to parent was higher with evolutionary distance from parent species
• evolutionary distance should measure the likelihood that other species share enemies with the parent tree

Question 6

describe degree of specialisation

Answer 6

• there are most insect species in tropical forests that temperate ones - herbivorous insects are no more specialised in the tropics than in temperate forests implying that diversity of insects is driven by tree diversity

Question 7

describe the experiment by Bagchi et al 2009 - host specialisation

Answer 7

• close relatives are more likely to share natural enemies than less closely related trees
• evidence of this represents host specialisation
• positive correlation - survival increases as evolutionary distance increases
• shared natural enemies

Question 8

describe the Bagchi et al 2014 experiment looking at varying host specialisation between ecosystems

Answer 8

• removed different types of natural enemies
• control = baseline level of diversity when all natural enemies are present - going below the line represents loss of diversity
• experiment showed that removing insects has biggest impact on increasing tree abundance
• however removing funghis has biggest effect on overall diversity

Question 9

describe the importance of community level density dependence

Answer 9

• tendency for rare species to increase in abundance and common species to become rarer
• specifically JC hypothesis generates this effect as a consequence of natural enemies
• neutral theory has no density dependence
• if DD is strong and inter-specific competition is weak, conditions for coexistence are fulfilled (Lotka Volterra)

Question 10

describe the harms et al 2000 experiment looking at the importance of community level density dependence

Answer 10

• comparing density of seedling recruits with density of seeds
• should be simple linear relationship between seed density and recruitment density
• slope less than 1 is evidence of density dependence
• this was represented in this experiment

Question 11

describe how you can look at pairwise interactions in a tropical forest

Answer 11

• take data from tropical forest systems and fit lotka volterra model
• measure effect of each species on each other
• look at interactions of coefficients accross the community
• the effect of species on itself is strong - strong DD
• weak interspecific interactions

Question 12

describe the relationship between DD and abundance

Answer 12

strength of DD (resulting from natural enemies) correlates with abundance

Question 13

describe pros and cons of the neutral theory

Answer 13

• no direct evidence to support it
• however predictions of neutral theory look like real systems
• can be difficult to test using main predictions (e.g. distributions of abundance)
• theoretically very useful - generates realistic plots
• if diversity if regulated by natural enemies then coexistence of competitors is not the problem theory suggests
• lots of singleton species

Question 14

describe an example of trophic structure in hyperdiverse forest experiment

Answer 14

• tropical forest insect food web - quantitive food web
• hosts = natural enemies, parasitoids feed on hosts
• in modified habitat there was a higher ratio of parasitoid to host species and increased parasitism rates
• the most abundant parasitoid species were more specialised in human modified habitats
• communities in tropical forests appear to be more complex with a greater diversity of interactions
• trophic networks in modified habitats are more simplified

Question 15

examples of various measures of trophic complexity

Answer 15

• linkage density
• compartment diversity
• evenness
• parasitism rates
• connectance