Lesson 6: Allocation and life history
patterns
Principal of allocation
Allocation to resource acquisition
Allocation to survival
Allocation to reproduction and reproductive structures
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Plant
strategies and life history patterns
Seed dispersal and dormancy
Classification of life history patterns
r- and k-selected species (McArthur and Wilson)
R-, C-, and S-selected species (Grime)
Allocation and Strategy
•
Principle of Allocation: Individual plants have a limited amount of
resources to spend on
– growth,
– maintenance (survival),
– reproduction.
•
Strategy (or life history pattern) is a genetically inherited pattern
of resource allocation to these basic functions that has evolved
through the process of natural selection.
Allocation: analogy with principles of economic
theory
“Plant growth has been considered analogous to a business operating under
the principles of economic theory. Individuals that have the greatest profit
(grow fastest) gain an advantage (usurp more space and increase their
resource base). However, allocation to growth is typically associated with
trade-offs (e.g., decreased reproduction). In other words, growth,
reproduction, and survival represent competing demands for a finite
supply of carbohydrates, fats or waxes, and proteins produced by
synthetic processes in the plant. Thus, we could view the acquisition of
resources as income, with expenditures partitioned between maintenance (e.g.,
storage and defenses), growth, and reproduction. Allocation can then be
treated by a cost/benefit analysis, allowing the prediction of optimal
allocation patterns. The ultimate measure of successful allocation, however, is
reproductive success.”
Barbour et al., 1999, p. 89.
Three examples of plant strategies
(a) Even allocation to reproduction,
growth and maintenance throughout
the growth period (probably no plant
has this pattern).
(b)Pattern of a typical annual. Most
resources are allocated to growth in
the early part of the growing season.
Late in the growing season, nearly all
the resources are allocated to
reproduction.
(c ) Pattern of a typical stress-tolerant
tree or shrub. Most resources are
allocated to maintenance , less to
growth. They allocate resources to
reproduction in growing seasons that
are high in resource availability. A lot
of tundra or desert plants have this
sort of allocation pattern.
Wilson 1983 cited in Barbour et al. 1999.
Trade-offs in resource allocation
1. Allocating resources to the development of some plant modules
(roots, shoots, seeds) restricts the ability of plant to develop
others.
2. Allocation for the capture of some resources, (e.g. root production
for capture of nutrients or water) will necessarily inhibit allocation
to structures to capture other resources (e.g. leaves for carbon
acquisition).
Trade-offs associated with allocation in Poa
pratensis (Kentucky bluegrass)
Inflorescences in year 1 vs.
Inflorescences in Year 2
Inflorescences in year 1 vs.
Size of plant in Year 2
• High levels of
reproduction in first
year correspond to
fewer flowers and
smaller plants in
second year.
• Allocating more
nutrients to
reproductive organs
in the first year
results in less
available nutrients in
the second year.
Law. 1979. The cost of reproduction effort in clonal plants: a benefit-cost model. Oikos 49: 199-208.)
Problems associated with economic analogy in cost/benefit
analysis of plant allocation
1. There are three currencies associated with plant growth (versus one in
standard economic theory): carbon (CO2 acquisition using solar energy
via photosynthesis and stored in organic molecules), water, and soil
nutrients.
2. The net cost of biomass production is difficult to access. For example,
reproductive cost of flowers may not consider that some flowers have
photosynthetic parts that help offset the cost of flower making.
3. The costs of plasticity are difficult to assess. For example, nutrients are
often resorbed from leaves and flowers before abscission, making the true
costs difficult to measure.
Justis Liebig
Law of the minimum
•
The growth or distribution of a plant is
dependent on the one environmental
factor most critically in demand.
Concept of limiting resources applied to competing
species (Armstrong and McGeehee (1980)
• m is the mortality rate
(rate of population growth
below which mortality of a
species occurs).
• R* is the level of
resource R, below which
a species cannot maintain
itself.
• For species A and B, B is
able to maintain itself at a
lower level of Resource R
and hence is considered
the better competitor.
Population growth
curves for Species
A and B
Tilman 1982 cited in Barbour 1999.
An unusual aspect of plants with regard to
competition for resources
• The structures responsible for acquiring essential resources (water,
energy, and nutrients) are in different parts of the plants one above ground
and one below ground. Energy acquisition is done in the leaves through
photosynthesis. Water and nutrients are acquired mainly through the roots.
• Plants must balance allocating resources towards roots (water and
nutrients) and shoots (energy, photosynthesis, and carbon gain).
• Root:shoot ratios often give insights to the allocation strategies of plants.
Example of very different aboveground and belowground environments
for resource acquisition
•
•
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Ranunculus adoneus, a snowbed plant in the Colorado alpine, can experience very
hot aboveground conditions and simultaneous very cold soils conditions (from snow
meltwater)
Mismatch between the plants ability to produce phytosynthate in the leaves, and its
ability to absorb the essential water and nutrients in the roots to support this
production.
So in this case, the plant may have to allocate more resources to belowground roots.
Resource-ratio hypothesis
(Huston and Smith 1987, Tilman 1988)
Differences in the relative supply rates of limiting resources
should lead to differences in the composition of plant
communities:
– Species allocation patterns: Species with allocation patterns
focusing on shoots are assumed to be relatively effective
competitors for light, and those allocating more heavily to
roots are assumed to be good competitor for below-ground
resources (water, nutrients).
– Landscape implications: Various habitats within landscapes
differ in their level of key resources, and hence will favor either
root or shoot specialists depending on the local resource
supply.
– Succession implications: Resource supply ratios also vary
systematically through successional series to first favor root
specialists (because soil nutrition is more limiting than light in
primary succession) and then shoot specialists because light
is more limiting in later stages of succession.
Examples of different allocation
mechanisms for survival
Life-history patterns
Length of life
Age at reproduction
Differing growth rates
Plant size
Patterns of reproduction
Monocarpic (semelparous) vs. Polycarpic (iteroparous) reproduction
Seasonal timing of reproduction (early vs. late summer)
Seed size
Dispersal mechanisms
Defense mechanisms
Toleration of herbivory (increased nutrient uptake or photosynthesis) vs.
chemical defenses
Defensive plant structures (spines, hairs, tough leaves)
Life-history patterns
How could each of these factors affect a plants ability to survive?
•
•
•
•
Length of life (annual, biennial, perennials)
Age at reproduction
Differing growth rates
Plant size
Allocation to maintenance: High root-shoot ratios
•
Courtesy of Niwot LTER web site
Plants in highly
stressful
environments
such as the
alpine or arctic
often have high
root to shoot
ratios.
Other plants that allocate lots of resources to
maintenance
Sequoiadendron giganteum (Giant Sequoia)
www.ginnyprior.com
•
Long-lived woody plants
•
Plants with a lot of protective chemical
compounds and unpalatable leaves.
Empetrum nigrum (Crowberry)
www.borealforest.org/ shrubs/crowberry.jpg
Plants with high reproductive strategies
•
Many annuals and biennials
(ruderal species) in weedy
environments; produce
abundant early germinating
seeds; palatable leaves; fast
growing; need lots of light;
low root to shoot ratios.
•
Desert annuals, plants able to
take advantage of short moist
periods with rapid growth,
flowerin, and seed set. Can
endure long periods of time
in dormant state as seeds.
Alliaria petiolata, garlic mustard
www.uwgb.edu/.../allpet_ seedlings01_tnail.jpg
Papaver escholtziii
http://dplu-mscp.sdcounty.ca.gov/speciesinfo_6/bio13desert_scrubportal.htm
Monocarpic species
Frasera speciosa
(green gentian,
monument plant)
Yucca whipplei
(Yucca)
www.laspilitas.com/ plants/pictures/a1077.jpg
www-plb.ucdavis.edu/.../ Yucca_whipplei.jp
Polycarpic vs. monocarpic reproduction
•
Polycarpic reproduction (plants that reproduce more than
once)
– Favored when adult mortality is low.
– Potential to outproduce monocarpic species over a long
lifetime.
•
Monocarpic reproduction (plants that reproduce once and
then die)
– Favored when adult mortality rates are high.
– A huge burst of flowering allows the individual to gain a
greater share of pollinators in a competitive system (e.g.
Agave and Yucca).
– Huge seed output can also be good strategy to coop space in
open habitats.
Many ways to allocate to reproduction:
Each has its tradeoffs
•
Sexual reproduction
– High investment in seed production and/or fertilization
• High pollen production vs. large ovules (male vs. female fitness)
• High pollen numbers vs. large pollen size (wind vs. animal
dissemination)
• Gamete production vs. pollinator or disperser reward (ovule
number vs. nectar or food reward)
– Investment in potential seedling success
• Seed size (provisioning) vs. seed number
• Dispersal vs. provisioning (e.g., wings, or bristles, downy
appendages vs. fruit quality or size)
•
Vegetative reproduction (sprouting vs. seed production)
Acacia seeds adapted for ant and bird
dispersal
•
•
ODowd & Gill 1986, cited in Barbour et al. 1999
Acacia seeds
apparently adapted for
dispersal by ants
(above) and birds
(below).
The food reward in the
appendage (eliaosome)
on the seed is larger
and usually more
colorful (red, orange, or
yellow vs. white) in the
bird dispersed species.
Example of poor reproductive resource allocation
• Idiot fruit, Idiospermum
australiensis.
• Ancient 100 million year-old
species.
• Puts all reproductive
resources into a large toxic
fruit that now is impossible
to disperse.
• Toxic to all animals except
an extinct species of bird.
• Highly endemic to a very
small area of the Daintree
Rainforest in NE Australia.
Life history patterns:
Longevity vs. age at first reproduction and size of plant
Longevity vs. age at first reproduction
Size vs. longevity
Loehle 1988, cited in Barbour et al. 1999
• Correlation between typical
age of sexual maturity
versus typical longevity for
North American angiosperm
trees and shrubs. Early
investment in growth and
maintenance has cost to
early reproduction.
• The longer an adult plant is
likely to survive, the more
likely it is to postpone
reproduction in favor of
production of woody and
other maintenance costs.
(greater woodiness of
longer-lived plants). Trees
with longer life spans also
produce wood that is more
likely to be better defended
against decay and disease.
Classification of Life History Attributes
Why?
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Evolution leads to adaptations that maximize “fitness”.
Life history classification schemes aid in our effort to understand
and communicate information about plant adaptations.
Plants growing in similar spatial and temporal microhabitats often
have similar life history patterns.
There have been many approaches to classify life-history
patterns. Some focus on single aspects of life history such as
reproduction, maintenance, or growth.
r- and K-selected Life History Patterns (MacArthur
and Wilson 1967)
MacArthur and Wilson (1967) in their analysis of island biogeography,
developed the concept of placing organisms within a spectrum between
extremes of allocation to reproduction.
•
r- selected plants are those that maximize their intrinsic rate of
reproductive increase. This is done through high seed production, and
minimizing costs for maintenance. They generally grow in highly
unpredictable climates or habitats, have low long term survivorship, are
poor competitors, short development times, short life-span, strong
reproductive focus with a monocarpic reproductive effort.
•
K-selected species tend to grow in more predictable climates. Mortality is
density dependent, They have Type I or II survivorship. The population is
near constant, near the carrying capacity. They are strongly affected by
competition. They have a long development time, long life span, and
generally a small seed bank. Allocation is to survivorship and delayed
reproduction, which is typically polycarpic.
Most plants are somewhere between these extremes
Survivorship types
Type I: characteristic of organisms with
mortality concentrated in the later
stages of life (e.g., annuals with seed
dormancy).
Type II: characteristic of organisms with
constant mortality rates.
Type III: characteristic of organisms with
high juvenile mortality (e.g., most
trees).
Summary of r- and k-selected adaptations for
different traits
Pianka 1970, cited in Barbour
Example of how variation in allocation
to biomass affects on longevity of two
desert ephemerals
Camissonia boothii (Booths evening
primrose) Onagraceae
Clark & Burk 1980, cited in Barbour et al. 1999
Plantago ovata (Desert Indianwheat) Polygonaceae
Photos courtesy of USDA Plants Database
Comparison of life history patterns in two desert
ephemerals
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Early in the season, Camissonia allocates
relatively more biomass to maintenance in
the form of abovground and belowground
biomass.
•
Plantago allocates more to reproductive
structures.
•
Camissonia has over twice the life span
because it has allocated more biomass to
storage and vegetative tissues.
•
Plantago has higher short-term
reproducutive output because of early,
heavy commitment of resources to
reproduction.
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Comparatively, Plantago is more of an rselected species, and presumably will have
high reproductive output even in years
where there is no late season rainfall.
Whereas, Camissonia should realize more
reproduction in years with more late-season
rainfall and an extended growing season.
•
Both plants can survive in the same habitat
because of high year-to-year variability in the
timing of rainfall.
C-, S-, and R-, selected species (J.P. Grime 1977)
Grime described a third strategy. He essentially split the k-selected
species of McArthur and Wilson into two groups C- and S-selected
species, and developed three groups, each specializing in one of
the three main growth strategies: reproduction, growth and
maintenance.
•
Competitors (C ), plants that allocate most available resources to
growth and that can capitalize on readily available resources.
They have high efficiency for capturing resources that makes it
difficult for other species to compete.
•
Stress-tolerators (S) are plants that live in habitats where
resources are limited, or where survival depends on allocating
most resources to maintenance and defense, (e.g., deserts, arctic
tundra, alpine meadows, peat bogs, serpentine soils).
•
Ruderals (R) (weedy species) are typically found in temporary,
frequently disturbed, habitats and are characterized by allocating
most resources to reproduction, and an adaptive suite identical to
that described for r-selected plants by MacArthur and Wilson.
Examples of R-selected species
Epilobium latifolium
E. angustifolium
• Soft, deciduous leaves,that are poorly defended against insects.
• Produce many flowers. Both produce many light seeds that are easily dispersed by wind.
• Generally short-lived plants that are replaced by other species during succession.
Examples of S-selected species
Ledum decumbens
Picea mariana
Rhizocarpon
geographicum
• Long-lived, slow-growing plants. Often woody. Many lichens.
• Live in sites with low-levels of nutrients.
• Relatively coarse, evergreen leaves or non deciduous structures.
• High levels of tannins and other secondary metabolites. Highly defended
against insect attack.
• Tend to have years of high flowering and years of low flowering.
Examples of C-selected species
Vicia cracca (blue vetch)
www.bruehlmeier.info/ Bilder/1201%20Vicia%20cr
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Poa pratensis
turfgrassmanagement.psu.edu/ Graphics/kblue.jpg
Allocate most available resources to growth.
Highly competive root systems (offten rhizomatous species).
Strong competitors for nitrogen. Or are nitrogen fixers.
High efficiency for capturing resources (water, light, nutrients)
that makes it difficult for other species to compete.
Summary of C-, S-, and R-, selected
characteristics
Grimes triangular model of plant
characteristics
Allows for intermediate combinations of
the R-, C-, and S-characteristics. The
corners of the triangle represent the
extremes of R, S, and C species.
Interior positions in the triangle
represent different combinations of
the characteristics.
(b) shows the position of several growth
forms. Annuals (A), biennials (B),
lichens (L), trees (T) and shrubs (S).
(c )The position of bryophytes (BR) and
perennials (P).
Grime 1990
Summary
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•
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Principle of Allocation: Individual plants have a limited amount resources
to spend on growth, maintenance (survival), and reproduction and adopt
strategies to allocate resources.
Strategy (or life history pattern) is a genetically inherited pattern of
resource allocation that has evolved through the process of natural
selection. There are trade-offs associated with different allocation
patterns.
Tilmans Resource-ratio Hypothesis is a cost/benefit analysis of different
patterns of plant allocation. It focuses on plant competition for limited
resources (more on this later).
Plants have adapted to survival in different habitats by differing allocation
patterns in terms of:
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Patterns of reproduction
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Life-history patterns
–
•
Length of life, age at reproduction, differing growth rates, plant size, seed size
Defense mechanisms
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Moncarpic (semelparous) vs. Polycarpic (iteroparous) reproduction, seasonal timing of reproduction
(early vs. late summer)
Toleration of herbivory (increased nutrient uptake or photosynthesis) vs. chemical defenses, also
defensive plant structures (spines, hairs, tough leaves)
Grimes plant strategy classification includes R-, C-, and S-selected
species. It focuses on the role of environment in relation to plant life
histories.
Literature for Lesson 7
**Grime, J.P. Evidence for the existence of three primary strategies in
plants and relevance to ecological and evolutionary theory. American
Naturalist 111: 1169-1194.
• Rejmanek, M. and D.M. Richardson. 1996. What attributes make some plant
species more invasive? Ecology 77: 1655-1661.
• Reichard, S.H. and C.W. Hamilton. 1997. Predicting invasions of woody
plants introduced into North America. Conservation Biology 11: 193-203.
**Bloom, A.J. F.S. Chapin, III., H.A. Mooney 1985. Resource limitation in
plants – An Economic analogy. Ann. Rev. Ecol. Syst., 16: 393.
**Schulze, E.D. and F.S. Chapin, III. 1987. Plant specialization to
environments of different resource availabilities. Ecol. Studies, 61: 120..
• Chapin, F.S., III, E. Schulze, and H.A. Money. 1990. Storage ecology and
economics of storage in plants. Ann. Rev. Ecol. Syst. 21:423-447.
• Chapin, F.S., III. 1991. Integrated responses of plants to stress: a
centralized system of physiological responses. BioScience 41: 29-36.
• McGraw, J.B. 1985. Experimental ecology of Dryas octopetala ecotypes:
relative response to competitors. New Phytologist, 100: 223-241.