Exceptions

1
Exceptions The most common form of exception to
the simple predictions of the various approaches
to finding life histories that produce an
advantage to iteroparity is a variety of
long-lived semelparous species a variety of
bamboo species Agave, the century plant
2
The text appropriately points out that you need
to distinguish really semelparous species from
those that reproduce vegetatively, where each
ramet is semelparous, but the genet spreads
reproduction out across ramets, so that the
species is almost iteroparous. The two examples
are really semelparous. The bamboo does reproduce
vegetatively, but all the ramets in the genet
flower simultaneously (in the same year). Youll
see the arguments both documenting the
semelparity and explaining why. The reason the
agave is a perennial semelparous (or monocarpic)
species is completely different, but the
resulting life history is similar.
3
The Bamboos Many bamboo species are iteroparous
perennials these grasses have more or less
extended pre-reproductive periods, then flower
and set seed annually until senescence. There
are, however, a number of perennial, monocarpic
bamboos, and included among them seem to be all
of the economically important bamboo species.
Populations of these bamboos are, therefore,
managed, and natural cohorts are inevitably mixed
with agriculturally selected strains. Long-term
genetic implications of the apparent strategy may
not, as a result, be easily testable. Therefore,
much of the study of semelparous bamboos has had
to be historical, rather than experimental.
4
Phyllostachys nigra
Phyllostachys bambusoides
5
Historical records indicate that a major Chinese
bamboo species, Phyllostachys bambusoides,
flowered en masse (that is simultaneously over
hundreds of square miles) in 919 and again in
1114, but not at any point in between. Cuttings
of the rhizomes of this species were brought to
Japan and established there. Those cuttings
flowered during the period between 1716 and 1735,
then again in 1844-1847, but not during any
intervening year (what happened between 1114 and
1716 is not known). Transplants from Japan, as
well as the parental stock, flowered next in the
1950's. Those transplants were scattered in
England, Russia, and Alabama among other places.
All flowered within 3-4 years of each other.
6
Flowering appears to be genetically programmed
and fixed, essentially unaffected by the enormous
variation in environmental conditions represented
at its flowering sites (Japan, England, European
Russia, Alabama, etc.). Many other bamboo species
also flower in relative synchrony, and with long
intermast intervals. Many are exotic iteroparous
life histories. A partial list The range of
intermast intervals in bamboos which flower
synchronously over large areas
. Genera
Locations Intermast
Interval Arundinaria spp. Kenya,
Himalayas 11 - gt50 Bambusa spp.
India, Burma,Brazil 31 - 150 Chusquea
spp. Jamaica, Chile,Brazil 15 -
34 Dendrocalamus spp. India, Burma
15 - 117 Phyllostachys spp. China, Japan
13 - 120
7
The flowering in species like P. bambusoides is
'unique' in 2 ways. One is its freedom from
environmental perturbation. Unlike most other
mast reproducing species like oaks, beeches, and
many fruit tree species (all of which have far
shorter inter-mast periods) there is no apparent
environmental cue to initiate mast year
reproduction unlike the others few (almost
certainly none) of the potential seed predators
are likely to survive the inter-mast period. Yet
seed predation is hypothesized by Janzen to be
the selective force behind this, as well as other
masting phenomena. Why? Janzens basic
reasons 1) the seed crop can be extraordinarily
large, and 2) the response and variety of
seed predators can be similarly
extraordinarily large.
8
Why so many different seed predators? Bamboo seed
is slightly more nutritious than either rice or
wheat among commonly consumed grains in the
human diet. Among the 'natural consumers are
small rodents, wild pigs, and jungle fowl (the
progenitor of the domestic chicken). The
response of natural seed predators to this mast
crop is dramatic. The functional response
includes an increase of 50-100 in the number of
eggs/clutch in the jungle fowl (an indeterminate
egg layer, but has a fixed brood size of 2).
Numerical responses through migrations are
anecdotally reported in the historical
literature. Rat 'plagues' follow mast years as a
result of migration plus reproduction in Africa
movements of flocks of weaver finches numbering
in the millions follow geographic 'migration' of
the mast crops.
9
How large is the seed crop? Seed crops 5-6 inches
deep (a solid layer of seeds) below parental
stalks are observed. Larger seeded species
prevented accurate surveys by endangering the
workers seeds fell in such profusion that
equipment was damaged and workers injured. A crop
of this size can satiate seed predators and
permit some of the seeds to escape predation to
establish the next generation. But why is the
masting cycle 1) so long and 2) so tight in
timing?
10
There will be relative synchrony in flowering in
bamboos because they are wind-pollinated and
apparently obligate outcrossers. That alone would
impose local synchrony it would cause high
levels of local pollen flow, but severely limit
genetic exchange between demes. Seed predators
sharpen that synchrony, and impose it over larger
geographical areas. How do seed predators affect
synchrony? Plants which anticipate the mast year
(say by one year) are unlikely to produce
sufficient seed to satiate seed predators.
However, predator populations are likely to be of
moderate size, since there has been no recent
mast crop, so it's possible that a few seeds
might escape.
11
Those that delay until after the mast year will
face insurmountable problems. They face predation
from a fully expanded predator population (from
both functional and numerical responses), and are
very unlikely to escape seed predation. The loss
of (selection against) genotypes which flower
slightly out of synchrony explains why the
masting cycle is so tight. Only man, by lazily
harvesting only when its easy, i.e. during the
mast year, but not years of limited seed crops,
may select against synchrony. Mast year crops
don't, in nature, wait around for slow-witted
predators. They germinate quite rapidly, and
seedlings are not heavily predated.
12
Why long inter-mast periods? How does an
interval of approximately 120 years
evolve? Janzen hypothesizes a scenario that
begins with an annually iteroparous bamboo (the
most common life history among bamboo species).
Seed predators are common, and escape of seed
rare (unpredictable reproductive success). An
individual that switched to semelparity (a chance
mutation) should produce a larger seed crop due
to increased energy allocation to reproduction.
That crop might satiate the local, numerically
adapted population of seed predators and increase
the number of seeds escaping predation. Now we
have some annual semelparous individuals.
13
Their larger seed crop means that among escapees,
offspring of the semelparous mutant will slowly
increase their proportion in the population. The
population becomes dominated by, and eventually
comprised entirely of annual semelparous
individuals (or semelparous, but with the same a
as the iteroparous kind) . When the iteroparous
parental stock has been completely replaced,
slight further shifts in ? are strongly selected
against. This follows from the explanation for
why timing is so tight. Tails of the distribution
of seed production are more completely devoured
than the peak, since seed predator adaptations
are designed for mast reproduction.
14
Janzen believed this switch would likely have
been successful only in the tropics. Predictable
rainy seasons would bring escape through
germination, and the commonness of territoriality
among seed predator species in the tropics would
limit local numerical responses. How are
extremely long inter-mast intervals achieved?
Once semelparous, mutations that produce delay
will be selected for against the 'wild-type
(iteroparous) parental stock. The longer these
new mutants wait, the larger their energy
reserves, seed output, and success compared to
whatever increases in predation they draw to the
seed crop of the parentalmutant population. It
isnt clear that this delay should occur as part
of the initial switch to semelparity.
15
However, once entirely semelparous, delay that
multiplies the previous interval could prove
selectively advantageous. Such a mutation permits
the bearer to produce larger numbers of seeds
than those who lack it, yet achieves the
buffering (protection) of producing seed
simultaneous with the parental populations. The
same kind of advantage that led to the switch to
semelparity now leads to replacement by a
doubled-delay (or tripled, quadrupled, , but
doubled is clearly the most likely) population.
Toward the end of the replacement process,
selection against the parental stock may be quite
strong.
16

• Heres a commented diagram to indicate what
  (theoretically) happens
• The initially iteroparous population, the height
  of the vertical lines indicates the size of the
  seed crop.
• 2) Now a fraction of the population becomes
  semelparous. The seed crop of those individuals
  is indicated by the second line.
• 3) Now the population is entirely semelparous.
  Long delays evolve by multiplication of the delay
  against a background seed production of the
  older, shorter delay.

17
It's important to recognize there may be reasons
other than seed predator satiation that could
explain extended delay. Most 'tree-like' plants
increase in biomass logistically. The relative
growth rate (the realized 'r) declines with
size and age height growth and structural tissue
are supported by a 'crown' of photosynthetic
leaves that reaches a 'relatively' constant
biomass. That's not true of bamboos. They are
grasses reproducing vegetatively to produce
large genets in which each culm (ramet) grows to
full adult height, producing an adult compliment
of leaves and maintaining a green stem.
Photosynthetic and support tissues increase in
parallel genet growth remains exponential over
an extended period, and biomass potentially
available to allocation to reproduction also
continues to increase (exponentially) until mast
seeding.
18
To indicate how common such species are Gadgil
and Prasad (1984) found that 70 of 72 Indian
bamboo species were perennial monocarps
(long-lived and semelparous), but that only 8
were synchronized over wide geographic areas.
The basic life history is, therefore, common,
but Janzen's arguments of the importance of
mobile seed predators in producing and
synchronizing it possibly less common. Let's
consider seed production on the basis of
flowering per adult stem. Flowers on grasses are
organized on spikes, with a spike of flowers at
each node (the slightly thicker rings on a piece
of dried bamboo).
19
Gadgil found in one of the synchronized, mast
flowering species, Bambusa arundinacea, the
following flowering rates 65
flower bearing nodes/ramet
x 133 spikes per node
x 156 flowers per spike 1.3 x
106 flowers/culm at mast flowering
x 50-200 culms/genet Even with the
limitations of wind pollination, 24 of seeds had
developed endosperm, resulting in 150-800 Kg of
seeds/genet, and an allocation of biomass to
reproduction of between 20-30. Not only is the
total impressive, but the allocation to
reproduction in bamboos is far higher than in
trees (usually at most a few percent). This is
without reference to predators.
20
However, since the build-up to reproduction from
clonal growth may show variation due to
environmental conditions, the intermast interval
and/or the intensity of reproduction may vary
over time. Since there may be variation among
clones and populations, we might also expect to
see a broad peak in a curve depicting intermast
intervals. That is what Gadgil and Prasad found
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23
What determines the optimum age for
reproduction? Aging! As new culms appear, older
culms reach an age when mortality rates increase.
The optimum is set by the rate of appearance
versus age-related mortality. Genet reproduction
occurs before culm mortality increases, wasting
the biomass and energy committed to those culms.
24
The Agave story Are seed predators the only
external biotic force that drives the evolution
of long-lived semelparity? No! That brings us to
the other plant story that of the century
plant, an Agave. First, the basis of the
story In theory, any species should maximize the
sum of current fecundity (or mx) and expected
future reproductive value (which can be
determined from proportional survivorship and
reproductive value of the next age class, i.e.
px and Vx1). Graphing these two components on
separate axes, a maximum is achieved by greatest
distance from the origin. Maximization of the sum
is, of course, the way to maximize fitness.
Remember the diagram
25
If the curve is concave, maximum distance from
the origin is at one of the end points, i.e.
either retain all energy for future reproduction,
or use all available energy. Concave curves
produce semelparity, with delay if, early on,
expectation of future reproduction exceeds
possible present offspring production. For the
bamboos the age of semelparous reproduction is
set by a mortality-driven decrease in residual
reproductive value. How does this apply to
Agaves?
26
Heavy line indicates observed strategy, here all
energy reserved As residual reproductive value
until Current benefit exceeds value of retention
for later reproduction.
Residual reproductive value
Semelparous species
27
In Agaves optimal foraging by pollinators will
maximize the seed set of individuals making the
largest reproductive effort. This, too, selects
for semelparity in isolated, individual Agave
plants. If it costs a pollinator considerable
energetic output to get from one isolated plant
to another, it should logically choose those
which offer the most food for the least flight
cost, i.e. those with more flowers (or greater
reproductive effort from the plant's point of
view).
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• Evidence?
• For the group of semelparous Agaves, but not for
  congeneric iteroparous species, there is a
  significant positive correlation between the
  percent of flowers which successfully produce
  fruit and the size of the inflorescence. Note
  that the positive correlation is with percentage
  of flowers developing fruit. This corresponds to
  the
• curvilinear profit function associated with
  semelparity. Graphically

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2. The number of pollinators observed on a plant
per centimeter of inflorescence was
positively correlated with inflorescence
length (which is proportional to the number of
flowers). More pollinators were attracted to
each flower in larger floral displays. This
correlation was larger in semelparous species
of agave than in iteroparous ones, even
though the same pollinator, Bombus sonoris, works
both semelparous and iterparous agave
species. An interesting, but unanswered
question, is why pollinator selectivity should be
different in agaves with differing life histories
when the flowers look virtually identical.