Lecture 9 - evidence and some key studies Flashcards
what experiment showed evidence for local density being mediated by natural enemies?
- 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
describe over compensating density dependence
- 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
describe experimental evidence for over compensating density dependence
- 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
describe host specialisation
- also critical for JC to work
- natural enemies attack host - enemies have many hosts
describe experimental evidence for host specialisation
- 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
describe degree of specialisation
- 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
describe the experiment by Bagchi et al 2009 - host specialisation
- 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
describe the Bagchi et al 2014 experiment looking at varying host specialisation between ecosystems
- 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
describe the importance of community level density dependence
- 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)
describe the harms et al 2000 experiment looking at the importance of community level density dependence
- 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
describe how you can look at pairwise interactions in a tropical forest
- 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
describe the relationship between DD and abundance
strength of DD (resulting from natural enemies) correlates with abundance
describe pros and cons of the neutral theory
- 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
describe an example of trophic structure in hyperdiverse forest experiment
- 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
examples of various measures of trophic complexity
- linkage density
- compartment diversity
- evenness
- parasitism rates
- connectance