Population dynamics across multiple sites

Multiple populations

• How many populations are needed to ensure a high probability of survival for a species?

• To what extent should multiple populations be clumped together in space versus spread apart?

• Can small populations or those occupying sites with low habitat quality substantially add to the regional viability of a species?

Terminology

• Site: a discrete piece of habitat that has some potential of maintaining a population of the species of interest.

Separated Juxtaposed

Terminology

• Population: the group of individuals living on a site

• While the individuals across all sites will be called the Total or multi-site population

Terminology

• Metapopulation: sets of discrete, largely (but not entirely) independent populations whose dynamics are driven by local extinction and recolonization via movement from other populations (Levin 1969)

Data needs

• Modeling the operation of a set of populations requires all the information to do a good job of a single-population PVA for each site, plus data on movement rates between populations, plus estimates of how temporal fluctuations in population processes are correlated between population

Characteristics of plant PVAs (n=90)

Characteristic Alternative % studies

Classification method

Stage or size 80

Other 68

Length of the study

1-5 years 70

>5 years 30

Number of populations

1 84

>1 16

Advanced modeling features

Environmental Stoch. 24

Density dependence 12

Spatially explicit 5

Demographic Stoch. 3

Genetics 3

Modified from Menges 2000, TREE 15: 51-56

Requirement 1:

• Site specific Population dynamics.

• Information about the quality of the population

• But how likely is that for every population of an endangered species, many years of census data, let alone estimates of all vital rates, will be available?

Common approaches:

• Assume that population growth rates or vital rates are identical at all sites, but carrying capacities differ among sites.

• Assume that most demographic rates are identical across sites, but to allow a handful of rates, about which more information is available to differ.

Cowbird parasitism

• The importance of correlations.

• Through “safety in numbers” multiple populations can strikingly decrease the risk of total extinction of a species.

• However, this benefit critically relies on a lack of correlation in the dynamics, and hence risks of extinction, of the different populations

Requirement 2:

Hypericum cumulicola

YEAR

1999199819971996199519941993

LAM

BD

A

2.2

2.0

1.8

1.6

1.4

1.2

1.0

.8

.6

.4

SITE

93

62

59

32

1

The California clapper rail

The California clapper rail

Population Numbers μ σ2 P(ex)

Mowry 70 0.043 0.051 0.06

Faber 29 -0.002 0.041 0.79

Laumeister 33 0 0.051 0.72

Harding et al. 2001

0.06*0.79*0.72=0.034

Pearson correlation coefficients

Population Mowry Faber

Mowry 1

Faber 0.995 1

Laumiester 0.896 0.938

Among sites Pearson correlation coefficients of H. cumulicola vegetative-small adult

transition (TSF>15 years)

Population 62 93 59

1 -0.681 0.476 0.599

62 -0.150 -0.527

93 -0.393

Correlations in population growth rates

Correlations in population growth rates

Joint-rank correlations for Delphinium uliginosum patch level data

World distribution of Hypericum cumulicola

Archbold

The Lake Wales Ridge

Patch level: Archbold Biological Station110 Rosemary scrub

patches

0 500 1,000

Meters

occupied

un-occupied

Patch 45

H. cumulicola occupancy = 58 %

Hypericum cumulicola occupancy related to patch size and patch isolation (p<0.001)

occupied

unoccupied

Quintana-Ascencio & Menges. (1996)

Large-Aggregated = 84 %

0 500 1,000 1,500

Meters

occupied

un-occupied

Large-Isolated = 57 %

0 500 1,000

Meters

occupied

un-occupied

Small-Aggregated = 52 %

0 500 1,000

Meters

occupied

un-occupied

Small-Isolated = 41 %

0 500 1,000

Meters

occupied

un-occupied

Fire intensity and location in burn unit 58 E, 1967, 1968, 1976 and 1980

• The importance of movement.• If movement rates are quite high, then

multiple sites do not truly harbor multiple populations, but instead a single one that utilizes a dispersed set of habitat patches.

• If movement occurs at low rates, it may nevertheless play an important role in supporting multi-site viability by allowing rescue of populations

Requirement 3:

How to quantify movement?

• Capture-recapture analysis

Capture-recapture methods

• use resighting data to estimate the actual numbers of individuals in each class including those not directly seen

• To accomplish this, you must not only count and relocate the marked animals, but also estimate the number of unmarked animals in each site

Fitting functions

Dispersal

Observed

Fitted

A classification of multi-site scenarios

Correlations

in population

Movement rates

Vital rates Essentially none

Low to medium

High

Significantly

negative

Separate pops, multiple strongly beneficial

Highly effective

Metapop

Multiple sites, very different habitat

Non different from zero

Separate pops multiple strongly beneficial

Somewhat effective Matapop

Multiple sites

Somewhat different habitat

Significantly

Positive

Separate pops

Multiple not very effective

Ineffective

Metapop

One population