Allelopathy and herbivory

1. Allelopathy and herbivory Additional readings: • Hawkes CV, Sullivan JJ . 2001. The impact of herbivory on plants in different resource conditions: A meta-analysis. Ecology 82:2045-2058.

2. Seminars

3. Outline • Amensalism • What is allelopathy and how is it inferred? • “plus-minus” interactions • Herbivory and importance in range and forest • Trophic relationships: top down VS bottom up • Effects of herbivory • Aspects of herbivory in range and forest • Herbivore functional responses • Sublethal effects and compensation (example)

4. Allelopathy • Could be considered “interference competition” for plants... • Definition: • Often mentioned, but hard to prove. Why?

5. Allelopathy • Inferred by: • Spatial pattern/arrangement of plant community • Presence of chemicals in leaves, roots and/or soil • Demonstration that chemicals cause decline in growth or survival of surrounding vegetation

6. Allelopathy • Difficult to prove because: • Low [chemicals] in the field • Mediated by 3rdparty (e.g. microbes and litter) • Trophic interactions similar to apparent competition: shrub harbours seed predator – causes “allelopathic” spatial arrangement.

7. Allelopathy • Recent work on Centauria maculata (spotted knapweed) provides good evidence for allelochemicals. • Knapweed is an important rangeland weed; what are implications of allelopathy?

8. Amensalism • Negative to one species, inconsequential to other. • Often is actually very asymmetric competition/pathogenesis • Example: allelopathy

9. “Plus-Minus” relationships • Include disease, parasitism, predation and herbivory. • Effects on population (or biomass) can be modelled using Lotka-Volterra equations to predict population of prey and predators. dV/dt=V[b-aP]=f1(V,P) dP/dt=V[kaV-d]=f2(V,P) Where: V=#prey, P=#predators, b=prey growth rate, a=prey consumption rate by predator, k=rate of increase of predator per unit prey, and d=predator death rate

10. Functional response • Relationship of predator (herbivore) consumption of prey (plants) to density of prey (plants) • 3 types – unsaturating (I), saturating(II), and sigmoidal (III) • Type of functional response has implications for community structure and stability • Discuss further in lab…

11. Herbivory • What is the importance of herbivory? • Small amount of biomass removed: 10% • Tundra/alpine 3% • Forest 4% • Grassland 10-15% • Rangeland/grazing systems 30-60% • “world is green” hypothesis (who?): there is more plant biomass than herbivores can eat. • Why? Trophic interactions. • Top down control (predators) • Bottom up control (plant forage quality)

12. Herbivory • But – trophic cascade models too simplistic; herbivory has more dramatic effects than they predict... • Defensive compounds (coevolution) • Community composition • Productivity • Seedling survival and demography • Seed predation

13. Herbivory • What are some of the effects of herbivores on plants and plant communities?

14. Effects and issues • Mortality of seedlings • Insects VS vertebrates • Herbivore outbreaks (insects especially) • Sublethal effects and herbivory escape • Compensatory growth and overcompensation • Productivity • Herbivore functional responses • Resistance/defence

15. Forests • Mortality of seedlings • Insects • Outbreaks (insects especially) • Sublethal effects and herbivory escape • Defensive compounds?

16. Rangelands • Productivity • Compensation/overcompensation • Herbivore functional responses • Toxicity and herbivore resistance • Selectivity/preference • Diversity and coexistence

17. Herbivore selectivity • Plant stress VS plant vigour hypotheses: • Herbivores attack already stressed plants • Herbivores favour plants in high resource areas and with larger “plant modules” • Evidence for both • Also influenced by herbivore defenses • Can alter species composition and lead to coexistence IF favoured species is best competitor • Differs among herbivore species, therefore management of different species can change community composition

18. Compensation • Response of plants to defoliation can vary widely: positive, negative or neutral • “Compensation” means plant growth increases after herbivory to compensate for lost tissue • “Overcompensation” much discussed: this means plants are stimulated to grow MORE after grazing/browsing. • Is overcompensation possible? • Is compensation over extended periods possible?

19. Sublethal effects of herbivory • Herbivores don’t often cause mortality of adult plants. • Can affect plants in other ways: • Reduced seed set/fruit abortion • Reduced size/growth rate • Change in architecture • Delay or prevention of maturity • These may all affect plant fitness (contribution to next generation) • Example: population consequences of herbivory on three Australian native plants. Allcock and Hik 2004. Oecologia 138:231-241.

20. Grazing experiment • Three groups of grazing animals: domestic stock (sheep and cattle), native macropods (kangaroos and wallabies), and introduced rabbits. • Four treatments: control, stock fence, kangaroo fence, rabbit fence. • Two habitats: woodland (intact Eucalyptus canopy) and grassland (cleared “native” pasture) • Three target species: kangaroo grass (Themeda australis), cypress pine (Callitris glaucophylla) and white box (Eucalyptus albens)

21. Grazing experiment • Plants placed in experimental plots in April 1998 • Monitored until April 2001. • Generalized linear modelling used to analyze factors affecting survival (habitat, grazing animals, competition) • Survival and growth data used to parameterize stage-based population models for trees.

22. “Life history” transition diagram • Transitions between 5 size (height) classes for trees; final stage is “escape from herbivory”. • Models created for each habitat and treatment combination.

23. P 35 P 25 P P 15 P 14 24 P 13 P P P S P S S S S 23 34 45 12 3 5 1 2 4 P P P P P 22 33 44 55 54 P P P P 21 18 11 43 P P P 51 52 53 P 42 P 41 P 31 S S S S S S S S S S Matrix Two 1 2 3 4 5 Matrix One 1 2 3 4 5 P P P P P S [<25 cm] 0 P 0 0 0 S [<25 cm] 11 21 31 41 51 1 21 1 P P P P P S [25 - 49 cm] 0 P 0 0 0 S [25 - 49 cm] 12 22 32 42 52 2 22 2 P P P P P S [50 - 74 cm] 0 P 0 0 0 S [50 - 74 cm] 13 23 33 43 53 3 23 3 P P P P P S [75 - 100 cm] 0 P 0 0 0 S [75 - 100 cm] 14 24 34 44 54 4 24 4 P P P P P S [ >100 cm] 0 P 0 0 0 S [ >100 cm] 15 25 35 45 55 5 25 5

25. Results • Different herbivore species affected different plant species (rabbits/kangaroos – cypress; stock – Eucalyptus) • Plants in productive environment better able to compensate (more rapid growth) • High herbivory rate in unfenced low productivity habitats prevented “escape”. • This could have population consequences even though mortality was fairly low. • Interaction between competition and herbivory…tradeoff.