Lecture 9 - evidence and some key studies Flashcards

1
Q

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

A
  • 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
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2
Q

describe over compensating density dependence

A
  • 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
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3
Q

describe experimental evidence for over compensating density dependence

A
  • 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
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4
Q

describe host specialisation

A
  • also critical for JC to work

- natural enemies attack host - enemies have many hosts

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5
Q

describe experimental evidence for host specialisation

A
  • 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
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6
Q

describe degree of specialisation

A
  • 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
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7
Q

describe the experiment by Bagchi et al 2009 - host specialisation

A
  • 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
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8
Q

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

A
  • 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
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9
Q

describe the importance of community level density dependence

A
  • 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)
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10
Q

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

A
  • 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
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11
Q

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

A
  • 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
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12
Q

describe the relationship between DD and abundance

A

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

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13
Q

describe pros and cons of the neutral theory

A
  • 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
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14
Q

describe an example of trophic structure in hyperdiverse forest experiment

A
  • 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
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15
Q

examples of various measures of trophic complexity

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