MODULE 021131Learning Objectives: Lecture 2

• The individual is the focus of selection and the controlling element of the dynamics of populations

• Distinction between unitary organisms (determinate growth, increase largely in size) and modular organisms (indeterminate growth, increase largely in numbers of modules)

• Ecological significance of body size in unitary organisms: why do organisms vary so greatly in size and how does size determine their biology?

• Flexibility of life-cycles in modular organisms: ability of modules to be committed to growth or reproduction.

• Relative and absolute growth rate: exponential and linear growth.

The individual organism

•Focus of selection

•Distinction between genotype and phenotype

•Birth and death population dynamics

Unitary organisms• Determinate growth, differentiated organs

• Grow by ? size; number of parts fixed

• Reproduce sexually: genets

• Vary greatly in size

Photo: John White

Variation in size of Unitary Organisms

insects

•Ranges over several orders of magnitude

• determined by

physiology

ecology

mechanics

phylogeny

y ? x 0.25

nematodes

Consequences of body size variation•Linked to life cycle and biological strategy

• generation time

• environmental scale

• predation risk

• prey size

• competitive ability

Modular organisms

Sand sedge Carex arenaria

rhizome

flowers

• indeterminate growth

• grow by ? size and ? in number of parts

• reproduce both sexually (genets) and asexually (ramets)

• ramets may be potentially independent individuals . . . or

Modular organisms

• . . . inter-dependent “individuoids”

• many animals are modular

• cf. colonies (ants, bees)

Bryozoan

feeding module

reproductive module

Growth of modular organisms• potential for exponential growth in number of modules (e.g. growing points)

• environmental constraints

• modular organism as a population

Growth and population dynamics

Population dynamics of Carex arenariaon a sand-dune: the population size is the balance of births and deaths of ramets

control fertilised births

net

deaths

Life-cycle of Obelia

Life-cycles of unitary and modular organisms

Growth and development linked

Growth the central phenomenon: flexible life-cycles

Relative and absolute growth ratelinear scale log scale

MODULE 021131Learning Objectives: Lecture 3

• 1. Growth is affected by the environment, by resource deficiency and by factors that cause damage.

• 2. Plasticity is especially important as a response to environmental stress for modular organisms

• 3. Resource allocation and the economics of growth: organisms must allocate resources to particular functions

• 4. Life history is determined by frequency and timing of sexual reproduction, and by longevity and growth rate.

Growth and environment

Growth requires resources• autotrophs, heterotrophsGrowth limited by• resource deprivation• environmental severity (“stress”)Genotype-environment interaction:

phenotypic plasticity

Unitary organisms and environmental stress

Respond to stress by ?in

growth ratesizedevelopmentbehavioursurvival

NOT by ? in form

Plasticity: e.g. in colour

Modular organisms and environmental stress

? in number of modules as response to stress

“Self-thinning” in dense populations of sedentary organisms

The –3/2 power law

slope = -3/2

Growth rate and stress

rate of module production: cf. relative growth rate

colony size growth rate

Polygonum lapathifolium

grown in low (left) and high (right) light

and the quantitative changes in its morphology

Sultan SE 2000 TIPS 5, 537

Phenotypic plasticity in response to environment

Polygonum persicaria

on

(a) a nutrient-rich soil

(b) a nutrient-poor pond margin

(c) a shaded site

Phenotypic plasticity: multiple environmental factors

Resource allocation

Finite resources: energy, nutrients, time etc

Allocation of resources

• time, energy - prey capture, optimal foraging

• biomass: roots, shoots, flowers Groundsel Senecio vulgaris (from Harper

& Ogden 1970)

Reproductive allocation

flowering plants

non-flowering plants

Reproductive allocation an alternative to resource capture

In modular organisms: fate of meristems

Reproduction only possible at high resource supply

Life historiesTrade-off between reproduction and survival

in a grass, Poa annua . .

. . . among mammals

Life history strategies: r and Kselection

Pioneers, opportunists, weeds: rapid growth, early reproduction, massive commitment to reproduction; poor offspring survival

Dominant and climax species: slower growth; greater investment in defence, support, survival; delayed reproduction

Does everything fit? Bamboos? Cicadas?

Life histories