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) CharacteristicAlternative% studies Classification method Stage or size80 Other68 Length of the study 1-5 years70 >5 years30 Number of populations 184 >116 Advanced modeling features Environmental Stoch.24 Density dependence12 Spatially explicit5 Demographic Stoch.3 Genetics3 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

The California clapper rail

PopulationNumbers μσ2σ2 P(ex) Mowry Faber Laumeister Harding et al *0.79*0.72=0.034

Pearson correlation coefficients PopulationMowryFaber Mowry1 Faber Laumiester

Among sites Pearson correlation coefficients of H. cumulicola vegetative-small adult transition (TSF>15 years) Population

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 Station 110 Rosemary scrub patches 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 % Large-Isolated = 57 % Small-Aggregated = 52 % Small-Isolated = 41 %

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 ratesEssentially 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