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Grace Charles Ecological Society of America 2017

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Diversity does not always beget diversity: multiple herbivore guilds maintain low woody diversity in a Kenyan savanna

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Grace Charles Ecological Society of America 2017

  1. 1. DIVERSITY DOES NOT ALWAYS BEGET DIVERSITY: MULTIPLE HERBIVORE GUILDS MAINTAIN LOW WOODY DIVERSITY IN A KENYAN SAVANNA Grace Charles, University of California, Davis TrumanYoung, Duncan Kimuyu, Corinna Riginos, KariVeblen
  2. 2. BIODIVERSITY IS DECLINING WORLDWIDE Plant species are also declining
  3. 3. UNDERSTANDING TROPHIC INTERACTIONS AND NETWORKS CAN HELP US UNDERSTAND DIVERSITY PATTERNS Scherber et al. 2010 Nature
  4. 4. CAN WE PREDICT HOW HERBIVORE DIVERSITY IMPACTS WOODY DIVERSITY? Bakker et al. 2016, PNAS
  5. 5. HOW DOES THE LOSS OF LARGE HERBIVORE DIVERSITY IMPACT WOODY DIVERSITY? • Herbivores have multiple positive and negative, direct and indirect pathways to influence woody species diversity: • Consumption • Alteration of competition • Native vs. exotic species; trees vs. grasses; dominant vs. rare species • Habitat creation and heterogeneity • Seed dispersal or other mutualisms • Seed predation • Disease spread • Fire intensity changes • We need to manipulate herbivore diversity not just herbivore presence/absence
  6. 6. MW C WMWC WCO Six combinations of large mammalian herbivores 200m C = Cattle allowed W = Wildlife allowed M = Mega-herbivores allowed (elephants and giraffes) Three replicate blocks KLEE – KENYA LONG-TERM EXCLOSURE EXPERIMENT
  7. 7. STUDY SITE Semi-arid, Acacia drepanolobium wooded grasslandKariVeblen
  8. 8. METHODS: HOW DOES HERBIVORE DIVERSITY INFLUENCE WOODY DIVERSITY? • Mapped and recorded all woody species in each KLEE plot (6 herbivore treatments x 3 blocks) • Allowed us to compare woody diversity in plots with 0, 1, 2, and 3 herbivore guilds • Calculated woody species richness and Shannon diversity • Modeled effects of herbivore treatments and herbivore diversity (i.e., number of herbivore guilds) on woody diversity
  9. 9. RESULTS: HERBIVORE DIVERSITY CAN SUPPRESS WOODY DIVERSITY • Woody plant diversity nearly 3x higher in total exclusion plot than plots with wildlife and megaherbivores One"of"Jost’s"preferred"metrics"[exp(Shannon)]"
  10. 10. INDIVIDUAL WOODY SPECIES RESPOND DIFFERENTLY TO DIFFERENT HERBIVORES 0 500 1000 1500 2000 2500 3000 3500 4000 O C W WC MW MWC Acacia drepanolobium density 0 10 20 30 40 50 60 70 80 O C W WC MW MWC Acacia mellifera density
  11. 11. INDIVIDUAL WOODY SPECIES RESPOND DIFFERENTLY TO DIFFERENT HERBIVORES 0 10 20 30 40 50 60 70 80 90 100 O C W WC MW MWC Grewia tenax density
  12. 12. INDIVIDUAL WOODY SPECIES RESPOND DIFFERENTLY TO DIFFERENT HERBIVORES 0 20 40 60 80 100 120 O C W WC MW MWC Cadaba farinosa density
  13. 13. HERBIVORES ARE POWERFUL SUPPRESSORS OF WOODY DIVERSITY Out of 20 woody species found in 2016 survey: • 12/20 (60%) of woody species excluded by megaherbivores (only found in O , C, W, or WC plots) • 5/20 (25%) of species excluded by any wildlife herbivory (only found in O or C plots) • 3/20 (15%) excluded by any herbivory (only found in O plots)
  14. 14. HOW IS WOODY DIVERSITY MAINTAINED IN THIS ECOSYSTEM? • Physical refugia • Biological refugia • Dispersal from different soil type • Important herbivore x edaphic interactions • Wildlife replacement with cattle STRI
  15. 15. Acacia brevispica Black cotton soil (poorly drained) Red Soil 0 2 4 6 8 10 12 14 Acacia brevispica density
  16. 16. Grewia sp. Black cotton soil (poorly drained) Red Soil 0 20 40 60 80 100 Grewia tenax density
  17. 17. CASCADING BIODIVERSITY LOSS: HERBIVORE DIVERSITY SUPPRESSES BIRD DIVERSITY Herbivores suppress tree canopy area, in turn reducing bird diversity Ogada et al. 2008, Oecologia O/C W/WC MW/MWC
  18. 18. A BRIEF META-ANALYSIS OF THE LITERATURE: WHEN DOES DIVERSITY BEGET DIVERSITY? Criteria • Diversity was experimentally manipulated • 3 or more levels of diversity • Surprisingly few studies!! • Measured the diversity response of another group of organisms
  19. 19. META-ANALYSIS: TROPHIC RELATIONSHIPS MATTER WHEN WE MEASURE BIODIVERSITY • 30 tests from 20 different experiments • Mesocosm and exclosure experiments • 𝒳2 P = 0.01 • Intuitive result, yet rarely tested experimentally • In cases where diversity of a higher trophic level was positively correlated with a lower trophic level, the higher trophic level had other functions • Seed dispersal, habitat creation Negative effect Positive effect NS Lower trophic level measured 4 2 2 Higher trophic level measured 0 13 2
  20. 20. CONCLUSIONS • Herbivore diversity does not beget woody diversity • Herbivory ≠ Herbivore diversity; lots of individual interactions make up broader diversity patterns and relationships • Need more studies that manipulate multiple levels of diversity • Incorporating trophic interactions gives us a better understanding of biodiversity patterns
  21. 21. KLEE: • TrumanYoung • KariVeblen • Corinna Riginos • Duncan Kimuyu Smithsonian: • David Kenfack • Paul Musili Field assistance: • Mathew Namoni • Jackson Ekadeli Thanks!

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