Population dynamics across multiple sites Multiple populations How many populations are needed to...
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Transcript of Population dynamics across multiple sites Multiple populations How many populations are needed to...
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.
• 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
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
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
• 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:
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
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