A.
Trade-offs in reproduction
1.
Reproduction and mortality
2.
Timing of reproduction
3.
Reproductive value
4.
Risk of death
B. Age states
Principle of Strategic Allocation
a. Variations in fecundity can
be seen as a result of a partitioning of resources
b. Principle of strategic
allocation: organisms under natural
selection optimize partitioning of limited resources and time in a way that
maximizes fitness: tradeoffs
Tradeoffs
Individual plants can't do everything at once.
There exists onflicts among 3 things:
limited time, resources, energy
1. Principle of allocation:
allocate resources among alternative demands: conflicting demands lead to
trade-offs between different activities
2. Virtually universal trade-off
classes:
-reproduction and other activities: manifested as cost of
production
-size and number of offspring
Life history
tradeoffs: characteristics of plants that evolution has shaped by means of trade-offs
1. Reproduction:
-when they reproduce
-how often they reproduce seed crops
-how large the seeds are (linked to seed numbers)
2. Birth and Growth:
-when seeds germinate
-how plants grow
3. Mortality:
when plants die
4. Each individual weed species has
a unique life history which is the evolutionary consequence of
a trade-offs between these life history characteristics; each component
above affects reproduction and survival
The timing of
reproduction tradeoffs:
1. Between reproduction and
growth-survival
2. Between early flowering and
later growth and reproduction
Key compromises between conflicting life history goals
-Fitness tradeoffs: reproduction
Vs. competition Vs. predator avoidance
-Flowering vs. vegetative growth (precocity)
-Seed number vs. seed size (see following unit)
Flowering as an alternative to vegetative growth .
-seed production at the expense of vigorous vegetative growth: fitness
relies on both survival during vegetative growth (competition) as well as seed
production
-allocation of time and energy critical, but limits of one can drive the system
-perennials and annuals can reallocate resources within the plant when changing
from vegetative to reproductive:
plant stature, size, nutrient and quality
-colonizers allocate more resources to repro vs. veg than those in mature
habitats
-amount allocated to repro plastic
-clonal growth in perennials is an alternative to seed production in annuals
Reproduction
and Mortality
Two (2) major components of fitness
that the life history of a particular plant are correlated with:
survival and reproduction
1. Net
reproductive rate: a summation of reproduction
and survival/mortality:
a. fitness includes reproduction
and survival which are the consequences of trade-offs of conflicting goals/ends
for a plants life history
b. there is much more to fitness
than net reproductive rate discussed here
2. Using Ro as equaling net reproductive rate, an optimal life history under particular ecological conditions can be defined as one which maximizes:
Σlxmx
3. Components of Σlxmx :
lxmx = Ro = net reproductive rate
lx = proportion of individuals surviving to age x
mx = fecundity of an individual of age x
[Reference: Silvertown & Doust (1993) Ch. 9]
Timing
of Reproduction
Life history features of
reproduction and survival that push a plant to precocious reproduction or push
it to wait to reproduce in a particular set of ecological conditions
Optimum age
of reproduction
1. Optimum age when individual
should begin reproduction depends on how reproduction at a particular age would
affect later survival and reproduction
2. Optimum age of reproduction
reached when no further increase in lxmx
can be obtained by further delay
Maximizing
Σlxmx
1. lxmx
may be maximized by delaying reproduction until plant reaches a size to be
able to survive and complete (1st bout of) reproduction
2. lxmx
= Ro = net reproductive rate
-lx
= proportion of individuals surviving to age x
-mx
= fecundity of an individual of age x
3. survival expectation mitigates
otherwise precocious preproduction
Delaying
reproduction incurs a demographic penalty when
the annual rate of population size is > 1 (demographic penalty is
inability to exploit available opportunity space)
1. When the annual rate of
population size increasing (is > 1): the greater its value (greater its rate of annual population
increase), the greater the demographic penalty against plants which delay, and
the more plants with precocious reproduction are favored
2. When the annual rate of
population size decreasing (is < 1): there is an advantage to delaying reproduction
3. EX:
mortality in corn field is greater early as weed seedlings are killed by
herbicides, tillage
Precocious
reproduction: If reproduction
incurs no costs, and a population is in a phase of increase (annual rate of
population size increase is > 1), the earlier reproduction occurs the better
for fitness
-but reproduction does incur costs
-cost of reproduction may slow its growth and increase its risk of death because
small plants are more vulnerable
Reproductive
Value and its affect on the time
of reproduction for an individual species in its life history
1. Vx = reproductive
value: contribution an average individual aged x will make to the next
generation before it dies
2. Vx
has two components:
-current fecundity (mx)
[= fecundity of an individual of age x]
-residual reproductive value (Vx
- mx), potential
reproductive contribution an individual might yet make; equivalent to the
chances that remain to it to produce further offspring in following seasons
The species that uses all the resources available to it over the entire season is the one that will predominate. Precocious growth and quitting early lets those that continue (even with greater hazards of mortality) will ultimately win because there are unused resources.
Think of it from the point of view of two identical plants "deciding" whether to reproduce early or wait until the end of the season. The one that reproduces early shifts its energy to flowers, its vegetative phase neighbor shades it out right away, so the loss to early reproducer happens right away. Larger, waiting, individuals that survive have much increased seed yield from larger vegetative body than earlier reproducers:
Risk
of Death
Key to precocity:
Is the risk of death greater early or late?
a. risk greatest early in crop
field
b. cost of reproduction may slow
early reproducing plants growth and increase its risk of death because small
plants are more vulnerable:
c. if you stop in the spring or summer to reproduce while your
neighbors are still growing, remaining vegetative,
you suffer a penalty while the larger plants shade you out and
out-compete you for resources
d. precocious get seed production
over quick, only advantage in this is if next generation in the same season has
a good chance of setting seed, on and on. This
is how they exploit new land.
e. but in Iowa corn fields
the competitor species will shade you out (competititive exclusion) and later
emerging seedling of the precocious species (its 2nd generation of
that season) will not do well, or die. So trade-offs shift advantage to the longer vegetative
period species and and its big seed rain at end of the season
f. high adult mortality risk favors
earlier reproduction because it lowers the residual reproductive value
g. a weed in a corn field that can
past layby (Iowa: June, early July; the time after which farmers can't get
equipment in the field because the crop is too large; they "layby"
their equipment) has a low risk of farmer mortality until the time of seed set
h. whether to reproduce early in
season or wait depends on the
particular ecological conditions of a locality: unused late season resources
will get grabbed by someone, and the fuller their use in the face of death the
more fit the species
The best time to
reproduce for a summer annual if it waits
1. Environmental causes of
mortality (not directly related to cost of repro) may determine where the upper
limit of the optimum age of 1st reproduction lies.
2. This
environmental cause of mortality is winter for summer annuals
-season length (and winter) is predictable over the time spans of the
evolutionary experience in crop fields of America and Eurasia:
-what varies now is onset of frost (Clay Co., Iowa: fair time, 2nd
week of Sept.; Story Co. Iowa: mid-Oct.)
-seed set timing hedgebet favors August and September, which clearly before
frost
-uncertain time is October and November with chance of frost; don't wait until
the autumn and chance of frost and chance of harvest destruction occurs
3. Winter:
-time of declining population size (ignoring dormancy)
-When the annual rate of population size decreasing is < 1, there is an
advantage to delaying reproduction
4. Exception:
The case of the 2 leaf foxtail plant emerging in October and producing
one seed: is this precocious is bad? Foxtail
has a multiple seedling emergence timing hedgebets,
including precocity late in the season
An organisms
reproductive value changes with age
1. Seed from a plant produced
at different times in its life cycle differ in their contributions to the future
growth of the population
2. This true for successional species (perennial woody plants, trees) and for annual weeds
3. Plants
reproductive value changes with age and it depends on the plant's life cycle:
a. Annuals:
summer annuals, winter annuals
b. Biennials
c. Perennials:
herbaceous, woody
Plant Age
How long does an annual plant live? How
old is it when it dies? Annual
plants actually can live to very old age, older than mature trees.
For example, velvetleaf can live for 20, 50, 100 years in the seed bank,
then it germinates and finishes life cycle.
Seedbanks are enormously elongated juvenille periods, providing
potentially very old plants as seedlings.
What is the oldest
plant in nature?
-bristlecone pine, 2000+ years
-olive trees
-oak: 200+
-Sequoia: ?
-lotus seed, 10,000 year old live seed found in dig
-aspen as clonal tree colony, forest: distant shoots subject to mutation, is
shoot still same genotype as other shoots attached too?
-oldest weed from evolutionary perspective:
the primitive Equisetum, spores, rhizomes, primitive photosynthetic
leaves, scales
Perennial weeds like quackgrass, johnsongrass, hemp
dogbane, milkweed, and Canada
thistle could be older than mature trees in the later phases of succession:
-vegetative clones never die, may be decades, hundreds, thousands of years old
since seed production
-if very old: somatic mutations affect different parts of plant: plant may be
composed of different genotypes in different tissues: evolution of genotype in
same individual plant
-twist to natural selection and precocity: "eternal life" of
herbaceous perennials favored because plants spend most of their lives in
declining populations
