Nutrient rewards Seed Defense - Adam Oliver...

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BIO 4134: Plant-Animal Interactions BIO 4134: Plant-Animal Interactions 1

Granivory

BIO 4134: Plant-Animal Interactions BIO 4134: Plant-Animal Interactions

Seeds !  Pre-dispersal (inverts)

< Post-dispersal losses (verts)

!  Intensity of the two are positively correlated by attractive features – Poor chemical

defenses, nutritious rewards

2 See Table 5.1


Nutrient rewards

3 Fig. 5.1


Seed Defense

4

Fig. 5.2


Fleshy Fruit Seed Content

5 Fig. 5.3


Chemical Defense !  Non-protein amino acids,

cyanogenic glycosides, protease and amylase inhibitors

!  Effects must be studied in taxa-specific manner – E.g. Jojoba (Simmondsia

chinensis) seeds on 2 mice (Perognathus penicillatus vs. P. baileyi)

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2


Life-History Variation in Seed Prod’n

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Fig. 5.4


Granivore Guilds

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Fig. 5.5


Vertebrate Adaptations

!  Mostly behavioural !  Rodents store caches of

seeds !  Red Crossbills (Loxia

curvirostra) morphology may favour efficiency with one type of seed however forage on a variety

9 BIO 4134: Plant-Animal Interactions BIO 4134: Plant-Animal Interactions

Post-Dispersal Granivory

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Fig. 5.6


Determinants of Granivory !  Attractants at long vs. short range !  Granivores should demonstrate Optimal

Foraging – Costs: handling time, transport, detox – Benefits: nutrient content, quality

!  Mediated by – Seed traits: mass, shape, energy, toxicity – Granivore traits: body size, susceptibility to toxins,

olfactory acuity, hunger – Habitat characteristics: seed distribution, soil

texture, physical environment 11


Frequency Effects

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Density Independent

Box 5.1

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Frequency Effects

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Density Dependence

Box 5.1


Frequency Effects

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Direct Dependence

Box 5.1


Other Positive Effects !  Inter-year

– Can have a numerical response in granivore populations due to good seed crop increasing offspring survival

–  Increased predation the following year !  Residence time

– Granivores attracted more towards patches with high seed crop and stay longer

– Disproportionally great seed removal in these cases compared to small patches


Post-Dispersal Density Effects

!  Removal rate of small seeds by generalists increases with seed production

!  Large seed removal rates relatively density-independent – Limited by handling

time 16


Effects of Other Food Sources

!  Positive – Losses in Douglas Fir

(Pseudotsuga menzisii) greater when squirrels can rely on other conifer seeds in rare times

!  Negative – Losses lessened if

neighbouring species’ seeds are more attractive


Granivory Effects on Plants

!  Will be small if – Vegetative reproduction important – Large and persistent seed bank – Granivores can be satiated – Plant colonization limited by site

characteristics, rather than seed production – Granivore population limited by factors other

than seed production

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4


Regeneration Strategies


Masting Effects

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Fraxinus excelsior

Fig. 5.8


Granivore Effects on Plants !  Larder-hoarding !  Scatter-hoarding


Interactions Pre- & Post-Dispersal

!  Some animals avoid attacked seeds –  Jays, mice avoid weevils – Not squirrels

!  Many frugivores react to colour changes in attacked fleshy fruit – Some deterred – Some attracted

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Janzen-Connell Hypothesis Fig. 5.9


Rodent Recovery Potential

Fig. 5.10

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Effects on Plant Community

!  Trade-off between granivory and plant competitiveness

!  Frequency dependence !  Spatially heterogenous granivory !  Janzen-Connell hypothesis

25 BIO 4134: Plant-Animal Interactions BIO 4134: Plant-Animal Interactions 13:38

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Granivory and Natural Selection

Num

ber o

f Ins

ects

Key Largo

Wales Lake Florida

Beak Length (mm)

!  Soapberry bug (Jadera haematoma)

!  Round fruit native, flat fruit introduced


Seed Effects on Granivores

!  Darwin’s Finches (Geospiza) on Daphne Island

!  Drought in 1977

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Num

ber o

f fle

dgel

ings

Beak Width (mm)

N = 90 4

8

30 60 90

N = 751

All birds on Daphne (1976)

Survivors (1978)


Trade-Offs in Seed Production

!  Between quality and quantity !  Between frugivore attraction and

deterring seed predators !  Between satiating seed predators and

saturating mutualist seed dispersers !  Between investing in seed nutrition vs.

protection

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Economy of Scale


Insect Adaptations to Asynchrony

!  Risk-spreading diapause – Progeny with mixed diapause

lengths !  Predictive Diapause

– Emergence from diapause triggered by environmental cue coinciding with seed production

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6


Serotiny !  Retention of seeds until

environmental trigger !  Advantages

–  acts like masting –  seedling establish in

appropriate conditions !  Disadvantage

– Predictable seeds promote pre-dispersal specialists


Pre-Dispersal Specialization !  Granivores can be more selective when

seeds on plant !  Defenses in fruit have fewer constraints

than in seed !  Pre-dispersal granivores have impact on

potential for subsequent predator effects !  Increased seed defenses in specialist

relationship more likely to lead to granivore adaptation than switching

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Lodgepole Pine,

Squirrels and Crossbills

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Fig. 5.12

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