Plant And Animal Interactions- Test 3

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Limiting resource model

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Wise and Abrahamson (2007)
To predict the resource conditions under which plants will more fully compensate for herbivore damage, an ecologist needs to ask

Herbivory most strongly affects acquisition of which abiotic resource?

Which abiotic resource limits host plant performance in the low resource environment (focal resource)?

Which abiotic resource limits host plant performance in the high resource environment (alternate resource)?

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Techniques for Quantifying Mammalian Herbivores’ Effects upon Plant Communities

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Descriptive approach
Compare plant community composition at sites with naturally low herbivore densities with community composition at sites with naturally high herbivore densities

Experimental approaches
Herbivore exclusion experiments: prevent naturally-occurring herbivores from consuming plants; compare plant community composition where herbivores have been excluded with plant community composition where herbivores are present

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Problems and considerations with exclosure studies for mammalian herbivores

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What is actually being excluded? -- Does a fence prevent access by herbivores other than the ones that you intend to exclude?

Confounding effects of exclosures on abiotic resources and conditions -- altered light regime, wind speed, humidity

Sufficient replication – replication is often low because vertebrate exclosures are expensive and time-consuming to construct

Sufficient area enclosed – avoidance of edge effects

Fenceline effects – avoid placing control plots immediately adjacent to fences

Exclosure studies only tell us what the effects of totally removing an herbivore is. No information on effects of different herbivore densities

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Generalized Grazing Model for Grasslands

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evolutionary history of grazing and ecosystem productivity are the primary factors affecting grazer impacts on plant species diversity, composition and the extent to which grazing promotes invasion by exotic plants

Key observation in making predictions of grazers’ impacts – herbivory and drought produced convergent selection pressures on plants

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Granivory (seed predation)

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consumption of seeds by animals

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Granivores (seed predators)

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animals that eat seeds

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Some empirical patterns concerning pre-dispersal granivory vs. post-dispersal granivory

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Greater diversity of animal taxa feed on seeds before dispersal than after dispersal – especially for invertebrate seed predators


A higher proportion of seeds are attacked by post-dispersal seed predators than pre-dispersal seed predators (this varies greatly among plant species)


Intensities of pre- and post-dispersal seed predation suffered by a plant are generally positively correlated

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How is granivory distinctive from other forms of herbivory?

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Granivory almost always results in the death of the plant (seed).

Nutritional differences – seeds are a concentrated source of nutrients, often higher in protein

Seeds are heavily defended physically and chemically

Seed availability is highly variable in space and time

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Masting

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intermittent, synchronous production of large seed crops by individual plants in a population

A second defining characteristic of masting = switch in resource allocation patterns from vegetative growth to reproduction

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Factors hypothesized to affect granivores’ foraging preferences among plant species

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Benefits of foraging of a particular species
- seed energy content
- acquisition of scarce minerals or amino acids

Costs of foraging on a particular species
- time and energy spent handling a seed
- energy expended transporting seeds
- secondary compounds

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Density independent granivory

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Direct density dependent granivory

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Inverse density dependent granivory

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Granivory will be unimportant in determining plant population size if

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Plants regenerate primarily by vegetative reproduction

A large persistent seed bank buffers or compensates for seeds lost to seed predators

Regeneration is microsite- rather than seed-limited

Seed predators are satiated by large seed crops

Granivore densities are limited by factors other than seed availability, like predation or parasitism

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Seed predators can affect spatial structure of plant populations by

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1.Re-distributing seeds through caching behavior

2. Affecting removal of seeds or fruits by animals that would disperse seeds

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larder-hoarders:

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seed predator that cache seeds in one location (burrow)

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scatter-hoarders

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seed predators that store seeds individually or in many small cache scattered throughout the home range

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Mechanisms by which granivores may affect plant community composition

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Trade-off between susceptibility of seeds to granivory and ability to survive in a competitive environment

Granivores consume seeds of the most abundant plant species – frequency-dependent granivory

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Strict masting

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Kelly 1994
reproduction is either very large or zero and the frequency distribution of reproductive output is unambiguously bimodal

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Normal masting

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Kelly 1994
pronounced temporal variation in reproduction, but there is some reproduction in all years

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Hypotheses for why masting may be adaptive

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Synchrony in seed production satiates (overwhelms) granivores’ capacities to consume seeds – a higher proportion of seeds escape granivory than if there were moderate amounts of seed production in each year

Synchrony in reproduction enhances the efficacy of wind pollination, which is only efficient if large numbers of receptive stigmas are available when pollen is released

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Hypothesis that masting may not be an adaptation

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Large temporal variation in seed production simply reflects tracking of resource availability in the environment

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Pollination

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transfer of pollen from male to female reproductive structures of plants

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Fitness benefits of animal pollination to plants

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Increased efficiency of pollen transfer (relative to wind pollination) -- the same degree of reproductive success can be achieved with less pollen production

Increased levels of out-crossing – more pollen reaches con-specifics at long-distances

Colonization and exploitation of unused resources in habitats where wind is not strong (e.g. forest interior habitats)

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What are some fitness costs to plants of animal pollination?

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Produce Rewards, Draw Attention (big floral displays)
Might get more damage from herbivores

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Fitness benefits to animals of animal pollination

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Efficient foraging for spatially concentrated, nutritious rewards

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What are some fitness costs to animals of animal pollination?

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Some predators locate themselves on plants "sit and wait"

Pollen may weight down flying insects

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What is the primitive and evolved type of pollination?

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Animal pollination is hypothesized to be the primitive condition among angiosperms – wind pollination evolved from an animal pollinated ancestor

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How might animal pollination have evolved?

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Animals probably first visited flowers not as pollinators, but as pollen herbivores

Plant reproductive parts, including pollen, adhere to the exterior of pollen herbivores’ bodies

Increased efficiency of gamete transfer through pollen adhering to pollen herbivores’ bodies acts as a selective force on plants to attract only those animals that are most effective in transferring pollen

Nectar evolves later as an adaptation to reduce the energy and nitrogen costs to the plant of rewarding animal pollinators with pollen

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Animal pollinated plants may show adaptations to manipulate pollinator behavior to avoid

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High levels of self-pollination – inbreeding depression



High levels of receipt of heterospecific pollen – clogs stigma preventing adherence of con-specific pollen

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Adaptations to reduce self-pollination by pollen transfer within a hermaphroditic flower

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1. Dichogamy: temporal separation of male and female function in hermaphroditic flowers

2. Herkogamy: spatial separation of anthers and stigmas in hermaphroditic flowers

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Adaptations to reduce self-pollination by pollen transfer among different flowers on non-dioecious plants

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Gender phase synchronized across all open flowers

Flowers presented in small sub-sets within larger inflorescenses – decreases the tendency for the pollinator to remain on the same plant

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Adaptations to reduce heterospecific pollen receipt

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Limit the range of pollinators that can pollinate the flower by

1. complex flower shape
Actinomorphic-radially symmetric, more generalists
Zygomorphic- Bilaterally symmetric, specialists

2. deep floral tube or nectar spur

3. flower color matches color perception
of a narrow range of pollinators

4. specific olfactory cues

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Floral constancy

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pollinator visits only flower of a single plant species during each foraging bout

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Why would floral constancy have evolved?

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Limited capacity of insects’ nervous systems means that they cannot retain the procedure for access many different floral morphologies

It is more efficient to only forage on a single floral morphology after that foraging procedure is learned rather than learning many foraging procedures during a single foraging bout

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Reward provided to animal pollinators by plants

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Nectar – aqueous solution primarily composed of water and carbohydrates

Pollen

Fatty Oils

Modified floral parts that are stocked with lipids, sugar or protein

Seeds

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Mechanisms of deceit in attracting pollinators

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Mimic a plant species that does provide rewards

Take advantage of innate behavioral responses in pollinators (provide stimuli for mating, oviposition or feeding)

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Why doesn’t natural selection favor greater ability to discriminate against non-rewarding plants in animal pollinators?

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Fitness cost of being deceived is very small

Fitness cost of passing up opportunities for rare resources (mates, oviposition sites) outweighs the fitness cost of being deceived

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Mechanism for maintenance of variation in floral phenotypes within angiosperm populations

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Temporal variation in the composition of the pollinator guild


Different floral traits are not genetically independent – pleitropic effects


Phenotypic selection by pollinators may be counteracted by natural selection later in the life cycle

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Could pollinators function as agents of disruptive selection producing sympatric speciation in plants?

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Changes in pollinator interactions would not cause sympatric speciation because:
Pollinators often have broad preferences

Most plants receive flora visits from a taxonomically diverse group of pollinators

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Why has it been difficult to establish whether plants act as agents of natural selection on their pollinators?

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Very difficult to keep track of individual pollinators until they reproduce to quantify fitness

Very difficult to keep track of feeding behavior of individual pollinators

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Effect of inflorescence-feeding insects on the demography and lifetime fitness of a native plant

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Louda and Potvin 1995

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Masting and trophic cascades: interplay between rowan trees, apple fruit moths and their parasitoid in southern Norway

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Satake and Kobro. 2004

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Patterns of annual seed production by northern hemisphere trees: a global perspective

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Koenig and Knops. 2000. Evidence that masting is not simply the result of environmental tracking

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How does pollen compare to other floral parts? Why might this have led to pollination strategies?

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High nitrogen content of pollen means that it is very nutritionally valuable to herbivores and potentially costly for plants to produce

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Insect's can access reward faster after learning the first time... causes...

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Herrera and Pellmyr (2002)

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Components of phenotypic selection: pollen export and flower corolla width in Ipomopsis aggregata.

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Campbell DR, NM Waser, MV Price, EA Lynch and RJ Mitchell. 1991

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Pollinator preference and the evolution of floral traits in Monkeyflowers

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Schemske DW, HD Bradshaw Jr. 1999

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An epigenetic mutation responsible for natural variation in floral symmetry.

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Cubas P, C Vincent and E Coen. 1999
Mutation causes a radially symmetric off-spring in a bilaterally symmetric plant species

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Evolutionary consequences of extinctions in populations of a Hawaiian honeycreeper

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Smith TB, LA Freed, JK Lepson and JH Carothers. 1995

Evidence of plants effecting evolution of animals