Populations and demography

Patterns of distribution and abundance

Population fluctuationsB

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Pechmann et al. 1991

Plethodon cinereus – as stable as populations get!

Population Growth

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

where N = population size

t = time

B = births,D = Deaths,I = Immigrants,E = Emigrants

Within Population Processes:Survivorship versus fecundity The best way to think of what goes on

within a local population is in terms of survivorship and fecundity (i.e. deaths and births)

Survivorship must be balanced against fecundity for a population to remain stable

Among Population Processes:Migration

Connects populations May be sufficient to make local

populations dependent on one another for long-term persistence = as “metapopulations”

Three case studies:

Wood frogRed-spotted newtBlanding’s turtle

Case Study I: Wood frog (Rana sylvatica)

Life cycle of the wood frog

EggEgg LarvaeLarvae MetamorphMetamorph

JuvenileJuvenileAdultAdult

How do we get these data? Surrounded two ponds

in Maryland with drift fences for 7 years!

Drift fences = aluminum window screening 0.75 m high with cans buried every 3 m on each side to catch adults and recently metamorphosed juveniles.

K. A. Berven (1990), Ecology 71(4):1599-1608

K. A. Berven (1990) ~ Methods Frogs do not regenerate clipped toes --

each frog got a pond clip on left front foot and a year clip on right front foot.

What determined survival? 92-99% of mortality occurred before metamorphosis Density – when greater numbers of eggs are laid,

fewer tadpoles survive to reach metamorphosis. Larvae reached densities of 1,000/m2

Hydroperiod – in years of low rainfall, aquatic densities are especially high, in wet years less crowding, more habitat!

Size at metamorphosis - Larger metamorphs were more likely to survive and to reproduce early

Rainfall - Adult survival was largely independent of age, but was correlated with rainfall

What determined fecundity? Larger females laid more eggs than

smaller females When the adult population size was

large, female clutch sizes were smaller

Sex ratios

Male and female survival did not differ

But females matured a year later than males

So about 2.3 more males as females survived to breed

These skewed sex ratios in breeding aggregations can escalate male-male competition for mates

Exam III

Last day of class…Thurs, Dec. 9 Read Ch 8, 9, 10, and 11 from Gibbs et al.

(short chapters) Read Ch. 14 from Vitt and Caldwell Be familiar with invited lecture (Thomas) and

“Demography” and “Conservation” overview lecture by Gibbs

Know your NYS “herp regs”

How would you boost wood frog populations in an area?

Three case studies:

Wood frogRed-spotted newtBlanding’s turtle

Case Study II: Red-spotted newt

Co-evolution and population dynamics at the landscape scale

D. E. Gill,“The metapopulation ecology of the red-spotted newt, Notophthalmus viridescens

(Rafinesque),” Ecological Monographs (1978)

Methods Studied newts in a series of

mountaintop ponds in the Shenandoah Mountains, Virginia.

 Adults and juveniles captured with drift fences and dipnetting.

Individuals identified by the number and pattern of dorsal red spots

Also toe clips for pond and year of capture, identified >8,500 individuals!

Study results Older ponds had many more

individuals than newer ponds Female survival

closely tied to population density and declined in ponds with larger populations.

Female mass depressed in midsummer owing

to severe competition for food -- little fat deposited, which predisposes them to mortality in the winter.

Foraged almost exclusively on fingernail clams.

Philopatry Adults always homed to the same pond year after

year -- with > 8,500 animals marked not one ever changed ponds between years!

Reproduction was poor, and most ponds fail to produce enough recruits to replace themselves

A very few ponds produced huge numbers of recruits. One of the 12 ponds produced > 91% of all the young in the

metapopulation one year! Strong selection on adults to return to sites where

reproduction was successful (“metapopulation centers”)

All individuals have just 1, perhaps 2, seasons to breed. Therefore, no way to ever regain the penalty of 1 lost breeding season.

Selection for dispersal

Young strongly selected to disperse as efts to the new ponds created

Ponds are temporary and females have much higher breeding success in new ponds.

Essentially a colonizing species that tracks temporary pond habitats that constantly shift in time and space.

 Efts need somewhere to go: what makes the ponds that drives dispersal by newts?

Beavers ~ ecosystem engineers Beavers create ponds with finite (10-25

year) life-spans. Most likely newts have coevolved with

beavers to track their movements and pond-creation processes.

Newts have clearly “adopted” beavers recently given that the Notophthalmus lineage is ~12 million years

This is a “landscape-scale” species

Three case studies:

Wood frogRed-spotted newtBlanding’s turtle

Case Study III: Blanding’s Turtles (Emydoidea blandingii)

Congdon et al. 1993 Demographic data on long-lived

species are rare Many species can outlive the

researcher How do life history traits

constrain an organism’s ability to maintain stable populations when survival rates are reduced?

“Blanding's turtles are excellent models becausethey have life history characteristics similar to those of other freshwater turtles, sea turtles, and tortoises, and to those proposed for dinosaurs”

Congdon et al. 1993 ~ Methods Conducted over 40 years (!) at the

U. Michigan George’s Reserve Turtles trapped in hoop nets in

ponds and captured in drift fences. Caught 711 individuals 2,968

times. Marked by shell notching along

margins of carapace.

Survivorship All estimates of adult survivorship

exceeded 93% and were stable from one year to the next

Generation time was 37 years Some individuals have been

marked for over 60 years.

Fecundity Nest survivorship ranged from 0-

63% Was highly variable and averaged

44% in first half of study but only 3% in second half

Fur prices had dropped and mammal abundance had increased resulting in many more predators

Sensitivity analysis of population growth rate (r) versus survivorship and fecundity. Note that r = 0 represents a stable population

Conclusions Population stability was most sensitive to

changes in adult survival Less sensitive to juvenile survival Almost unresponsive to fecundity or

nest survival Turtles forgo reproduction to allocate

resources to shell production The tradeoff is late breeding but long life

with continuous, though delayed reproduction

Implications Historically there have been few

threats to adults, currently there are many

Demography in this species seems to be a co-evolved set of traits that makes it difficult for populations to respond to new sources of adult mortality.

 The concept of sustainable harvest for these animals is an oxymoron

Turtle head starting

Turtle Excluder Device (TED)

End:Populations and demography