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

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. BIODIVERSITY IS DECLINING
WORLDWIDE
Plant species are also declining

3. UNDERSTANDING TROPHIC
INTERACTIONS AND NETWORKS CAN
HELP US UNDERSTAND DIVERSITY
PATTERNS
Scherber et al. 2010 Nature

4. CAN WE PREDICT HOW HERBIVORE
DIVERSITY IMPACTS WOODY DIVERSITY?
Bakker et al. 2016, PNAS

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. 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

8. STUDY SITE
Semi-arid, Acacia drepanolobium
wooded grasslandKariVeblen

9. 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

10. 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)]"

11. 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

12. 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

13. INDIVIDUAL WOODY SPECIES
RESPOND DIFFERENTLY TO
DIFFERENT HERBIVORES
0
20
40
60
80
100
120
O C W WC MW MWC
Cadaba farinosa density

14. 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)

15. 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

16. Acacia brevispica
Black cotton soil (poorly drained) Red Soil
0
2
4
6
8
10
12
14
Acacia brevispica density

17. Grewia sp.
Black cotton soil (poorly drained) Red Soil
0
20
40
60
80
100
Grewia tenax density

18. 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

19. 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

20. 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

21. 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

22. KLEE:
• TrumanYoung
• KariVeblen
• Corinna Riginos
• Duncan Kimuyu
Smithsonian:
• David Kenfack
• Paul Musili
Field assistance:
• Mathew Namoni
• Jackson Ekadeli
Thanks!