Lecture 7 - Pre and Post Abscission Losses

Question 1

Total loss are from:

Answer 1

1. Failures to pollinate
2. abortion of fruits and seeds within fruits
3. resource limitation
4. granivory

Question 2

Ovule abortion: genetically related to pollen

Answer 2

Not all ovules within a fruit will develop into seeds. Some fraction will be aborted. In some cases, whole fruits will be aborted (not just ovules).

Genetically related pollen can potentially exacerbate pollen limitation: pairing deleterious alleles will lead to death of developing embryo.

Question 3

Ovule abortion: too few seeds

Answer 3

Aborted fruits in some species is a funcion of number of fertilized ovules: Fruits with fewest fertilized ovules will be aborted.

Question 4

Resource Limitation

Answer 4

Competition among seeds for maternal provision.
TWO level of competition:
* among seeds within a single fruit
* among fruits on an inflorescence or plant. Positioning can also influence size.

Phenology of fertilization: Timing of fertilization can influence losses because of limited resources at the end of the growing season.

Question 5

After fertilization, more death

Answer 5

In addition to perils of pollination failure, ovule abortion because of genetic incompabilities and limitations in resources, most seeds will be eaten by granivores before they even disperse from the mother plant.

Once dispersed, they will face death again by another suite of granivores.

Those that germinates, face herbivory.

Phenology of flowering and seed set can influence rates of losses from seed-eaters.

Question 6

Seeds are concentrated packet of nutrients

Answer 6

Some animals who normally eat few seeds become granivores when seeds are abundant: for example, during a mass year for acorns, these seeds will comprise 75% of a deer’s diet during september.

Macronutrient include complex carbohydrates, fat and protein that can be metabolized for energy. Seeds also contain little water-therfore concentrated packets of food.

Question 7

First generalization

Answer 7

Pre-abscission predators tend to be specialists (e.g. the cross-bill in the finch family), limited to eating one species or one genus. Diversity of invertebrate predators higher than in post-dispersal granivory

Post-abscission predators are generalists.

Question 8

Second generalization

Answer 8

For specialist seed-eaters, developing seeds are an easily accessible (and reliable) source of food.
In post-abscission, seed retrieval becomes increasingly difficult as seeds disperse farther from the source.

Question 9

Third Generalization

Answer 9

Small seeds are taken by small granivores while large seeds are taken by large animals.

Question 10

The main pre-abscission seed predators are:

Answer 10

1. insects-especially weevils (a type of beetle) and hemipterans (true bugs) and wasps.
2. Birds (especially the finch finally and crow family).
3. Mammals (especially rodents)

Question 11

Pre-abscission losses from flower-bud-infesting insects

Answer 11

Common pre-abscission predators are insects that lay eggs on the flower. Offspring spend their larval stage as sedentary feeders entirely within a single pod, capsule, nuts, etc.

Losses from this form of seed predation often reach over 90% losses: the rate of infestation is much lower in mass seedling years because the insect populations cannot expand fast enough to take adavantage of this suddenly increased resource. Furthermore the longer-term impact of weevils on their host-plant may not be as great: as we will see, seeds of herbaceous plants escape in time with long-lived solid seed banks.

As a general rule, the larger the inflorescence, the greater the rates of predation for laying insects.
BUT, larger flowers also means greater pollination success.

Question 12

Pre-abscission losses: Cotton-stainers

Answer 12

These arboreals seed predators insert their proboscis (probably trough mircopyle or hilum) and extract some of the endosperm (starch).
The seed is not necessarily killed.

Question 13

Pre-abscission losses: Stink Bugs

Answer 13

Stink bugs are a significant pest for cotton and soybeans in the US with annual control costs exceeding 14 million dollars in these crops

Question 14

Pre-abscission losses: Squirrels

Answer 14

Some granivores wait until the seeds are mature, then grab them from the branches.

Squirrels start harvesting spruce cones a month before seed abscission begins; the cones are still green when the squirrels begin-but the seeds are almost mature.

Question 15

Phenology of pre-abscission seed predation

Answer 15

Losses can drive plants to flower at times when risk of granivory is low.

Infestation rates can be high at the start and end of flowering season, lowest when density of flowers high.

But of course, pollination is also influencing flowering phenology as is resource limitation.

Question 16

Pre-abscission defences

Answer 16

Seed require greater investement in anti-predatory defences.
Physical deterrence is generally a pre-abscission defence.

The defensive pericarp does not dehisce until the seed is ready to abscisse. The pericarp can be hard and or contain spikes, bristles, irritant hairs, or in the case of some conifers, resin.

Question 17

Pre-abscission defences: Enlisting others

Answer 17

Extarfloral nectaries are common as reward for protective ants. The ants get fed, and they try to bite any herbivorous insect that lands on the plant . Hollow horns of wood along branches that serves as nests for ants.

Question 18

The main post-abscission seed predators

Answer 18

Granivorous mammals (eg rodents)
Birds (eg finches)
Insects (eg beetles and ants)

Question 19

Post-abscission seed losses

Answer 19

The large majority of seeds that make it to the ground die;
This stage of seed losses is not as well understood.

Some are eaten, others get buried and die of old age (though many herbaceous plants persist in the seed bank) and some small fraction is attacked by pathogens.

Question 20

Post-abscission defences: Camouflage

Answer 20

Rate of seed loss is dependent on how well one can find seeds against a background.
The seeds are doing what they can to avoid being found: they are frequently the color of the forest floor.

Question 21

Post-abscission defences: dispersal is an escape from granivores in space

Answer 21

Single seeds on the ground are encountered less often than groups of seeds (say, 10 or more).

If density falls below a certain level, granivores give up-** effort of searching outweight reward. **

Question 22

Post-abscission defenses: Mass seeding is an escape
from granivores in time

Answer 22

Following a big crop, there can be so many seeds on the branches or on the ground that the granivores are simply swamped-satiated.

The mass seedling event leads to a rare pulse of recruits-a baby boom in effects-that remains distinct decades later.

Question 23

Plants’ chemical defenses

Answer 23

Granivory directly affects reproductive success: eating a few leaves is not the same as eating a few seeds.
Plants therefore also produce toxins to deter granivores.

Most are quite mild, i.e. they do not kill the herbivore/granivore.

Rather, these substances are intended to slow down the growth rate of the insect of small mammals and impede their fecundity.

The two main substances are alkaloids and tannins.

Question 24

Alkaloids –deleteriously affect the nervous system

Answer 24

Alkaloids are a chemical group that includes thousands of bitter, nitrogenous substances.
Almost every plant family has at least a few of these in its tissues.

The alkaloids are similar in composition to certain neurotransmitters crucial in mammal brain chemistry - especially mood and the ability to concentrate.

Question 25

Tannins cause indigestion

Answer 25

Tannins are a group of compounds far less dangerous to animals; the molecules are carbon-based and devoid of any nitrogen. Relative to nitrogenous-based alkaloids, they are cheaper to produce.

When consumed by an animal, tanning bond to digestive proteins within the animal’s gut and render those proteins useless to the animal.

Question 26

Nothing is free: toxins come with a cost to plant growth

Answer 26

A major difference between plants and animals:
Plants accumulate secondary compounds (“poisons”) while animals try to get rid of them (e.g. in mammal hair) or detoxify, or sequester them.

The greater the investmnet in the seed coat (which includes toxins) the more likely the seeds are to persist in the soil seed bank. But this investement again comes at a cost: these seeds are generally smaller-fewer nutrients are offered to the embryo.
Consequently germinants are smaller.