Lecture 3: Describing Populations

FIRST,What are the unique characteristics of sessile organisms?

….Sessile?....

How plants differ from (mobile) animals

•Modular GrowthImportance of meristem tissue

•MovementsHow do plants disperse? Consequences?

•Phenotypic Plasticity

•Resting Stage

•Ecological neighborhoods

Modular Growth

Semelparous vsIteroparous

Clonal growth:ramet vs genet

Implications of Modular Growth

•Loss of a part is not necessarily lethal

•Reproductive output related to size(also true of animals such as fishes)

•Age of a genet may be very greatyoung ramets perpetuate the genet

•If all meristems on all the ramets flower,the genet dies: semelparous

Phenotypic plasticity

Allows organisms to respond to local conditions

Can lessen the effects of natural selection on theunderlying genotype

Definition

Examples….

Movements- How do plants get around?-Seed dispersal

•Often characterized by dispersal agentabiotic vs. biotic transportactive vs. passive

•Animals disperse seeds of 60-90% of tropical plant species

•Animals responsible for patchy occurrenceof plants

Tapirs and Palms- an example

Fragoso, Silvius, Correa. 2003. Ecol 84:1998-2006

Resting stage- what are the consequencesof reproduction by seeds?

•How long do the seeds survive?

•Dormancy may be broken by environmentalconditions or after some amount of timehas passed

“DISPERSAL THROUGH TIME”

•The seed bank- implications for conservation and evolution

Ecological Neighborhoods

For plants….

For mobile animals…

2*STD of the mean dispersal distance during theorganism’s reproductive lifetime

REGARDLESS OF THE ORGANISM

All populations may be described as the sumof four key processes:

•Birth•Death•Immigration•Emigration

N = B + I - D - E

Demographic rates

•Reproduction

•Survival

These processes are common to both plantsand animals.

How many young are produced and recruitinto the population?

What is the probability of not dying inthe current time interval?

Movement

Dispersal- natal vs. post-breeding (adult)

Issues…

Closed Versus Open Populations

Closed populations:

•No immigration or emigration

•Special case: no births or deaths

Population is static: Membership is constant

Most populations are not closed except forvery short periods of time

Life History Strategies

Trade-offs between reproduction and survival

Effects of size or age on reproduction andsurvival

Survivorship Curves

Type III

Type I

Type II

Total lifespan

Log

(su

rviv

al)

(after Pearl 1927)

A conceptual model:

Model: abstract representation that includesonly key features of system

Conceptual models emphasize understandingover mechanistic explanation

An idealized plant life cycle

Adults Seedlings Seeds

Adults Seedlings Seeds

Time

t

t+1

Survival:

Reproduction:

An idealized plant life cycle

Adults Seedlings Seeds

Adults Seedlings Seeds

Time

t

t+1

Survival:

Reproduction:

Survival

Growth

Germination

Fecundity

The components of N(t+1)

N(t+1) = N(t) + B + I - D - E

Ignoring I and E, and looking more closely at B and D

N(t+1) = N(t) – N(t)*(1-S) + N(t)*F

BirthDeath

A BOOK KEEPING PROBLEM!

Life Tables

•Life Tables summarize the survival andreproduction of individuals accordingto their age.

Cohort life tableFollow a group of individuals of the same agefrom birth through to when the last one dies

Static life tableFollow a group of individuals of mixed agesOver one year or less (a “snapshot” in time)

Diagram of a cohort study

age

t = 0 t = 1 t = 2 t = 3

n = 5 n = 3 n = 1 n = 0

Diagram of a static studyag

e

t = 0 t = 1 t = 2 t = 3

n = 2

n = 3

n = 2

n = 1

0

1

2

3

Problems with life table calculations

•Static: assume that no’s individuals that areborn and that survive do not changefrom year to year

•Both types assume that all individualsthat are alive are counted

•There are now much better ways to calculatesurvival (mark-recapture, for example)that do not depend on these assumptions

Why are they still useful?

Under some conditions, still can be used tocalculate survivorship

Still good for keeping track of individualsand vital rates for groups

Historical perspectives

SUMMARY“Plant” perspectives:

modular growthphenotypic plasticitymovementresting stagesecological neighborhoods

The “BIDE” equationdemographic rates plus movements

Dispersal- types and issues

SUMMARY continued

Open versus closed populations

Life history strategies:survivorship, reproduction

Keeping track: conceptual models and life tables