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

Seminars

Outline 1.Amensalism 2.What is allelopathy and how is it inferred? 3.“plus-minus” interactions 4.Herbivory and importance in range and forest 5.Trophic relationships: top down VS bottom up 6.Effects of herbivory Aspects of herbivory in range and forest Herbivore functional responses Sublethal effects and compensation (example)

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

Allelopathy Inferred by: 1.Spatial pattern/arrangement of plant community 2.Presence of chemicals in leaves, roots and/or soil 3.Demonstration that chemicals cause decline in growth or survival of surrounding vegetation

Allelopathy Difficult to prove because: 1.Low [chemicals] in the field 2.Mediated by 3 rd party (e.g. microbes and litter) 3.Trophic interactions similar to apparent competition: shrub harbours seed predator – causes “allelopathic” spatial arrangement.

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

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

“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]=f 1 (V,P) dP/dt=V[kaV-d]=f 2 (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

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…

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)

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

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

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

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

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

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

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?

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 Oecologia 138:

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)

Grazing experiment Plants placed in experimental plots in April 1998 Monitored until April 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.

“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.

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.