Euphydryas editha bayensis: Evidence for a Metapopulation ModelHarrison et al. 1998

http://www.calacademy.org/science_now/headline_science/

AM. NAT. 1988. VOL. 132, PP. 360-382.

Preliminary indications of Metapop. dynamicsFrequent Extinction of Local Populations

Behavior and patchiness limits recolonization

* Goal: To determine the extent, dynamics, and equilibrium of the potential metapopulation*

Life CycleAdults appear in March/April, eggs hatch in 2-3 weeks

Plantago erecta ,Orthocarpus sp.

Larvae enter summer diapause in May, ends in late fall

Larvae feed from Dec-Feb and pupate

http://plants.usda.gov/java/largeImage?imageID=pler3_003_ahp.tif

http://www.laspilitas.com/nature-of-california/plants/orthocarpus-densiflorus

http://www.botany.hawaii.edu/Faculty/Carr/ofp/images2/lom_mac_9347b.jpg

http://plants.usda.gov/java/profile?symbol=LACAC2&photoID=lach3_001_ahp.tif

http://www.americanmeadows.com/WildflowerSeeds/Species/TidyTipsseeds.aspx

http://calphotos.berkeley.edu/cgi/img_query?

query_src=&enlarge=6105+1651+0932+0099

“TIDY TIPS”

“FALSE BABYSTARS”

“BIGSEED BISCUITROOT”

“GOLDFIELDS”

Habitat Distribution- 15 x 30 kmNorthwest= San Jose

Northeast= Inner Coast Range

Southwest= Santa Cruz Mountains

GOOGLE MAPS

Habitat Quality

Steep southern/southwestern slopes are very different from northern/northeastern slopes

in most years, S/SW slopes contribute little to future pop size

in unusually wet years, S/SW slopes contribute disproportionately to future pop size

Density of plant sources vary from patch to patch

History of Extinction

“50 year drought”

Severe drought: 1975-1977

3 pops in the MH area went extinct

MH patch pop size likely extremely reduced (not likely to have gone extinct)

Pops observed in 1986 likely due to 9yr recolonization

Methods

Determined number of pops, potential habitat

patches with host plants= potential (59 total)

patches with adults= “occupied”

8 sites found in 1986 were checked for larvae

Methods, ctd.

Measure habitat quality

distance from MH, % warm, moderate, and cool slopes

topography

density indices (%s) for both host and 1 nectar plant

Logistic Regression (P<0.10)

Methods, ctd.

Estimation of population sizes found extant in 1986 and 1987

7 total

Mark-recapture

Regression of pop size and patch area

Regression to separate effect of distance from MH on patch quality

Results- Distance and Quality

All patches found in ’87 were 1.4-4.4 km from MH

Patch area did not affect plant density

All 6 variables significant in explaining presence/absence of butterflies (p< 0.10)

REGRESSION EXPRESSION FOR PRESENCE/ABSENCE VS DISTANCE AND QUALITY

Results- Pop. Sizes

Log(patch area) is a significant predictor of Log(pop size)

ln(warm area) was a significant predictor of pop size

Results- Quality

Of all the patches that were examined (59), using the quality formula (below), 27 were considered “suitable”

Discussion

Likelihood of patch occupancy depends on distance from MH and quality

GOOD ENOUGH, NEAR ENOUGH

NOT GOOD ENOUGH, TOO

FAR AWAY

NOT GOOD ENOUGH,

NEAR ENOUGH

GOOD ENOUGH, TOO FAR AWAY

Annual rates of colonization as a function of distance

I= probability (per unit time) of colonization

D= distance from source to target

D’= species-specific dispersal constant (estimated from data)

The radius of the MH metapopulation is estimated to be 7-8 km

“Stepping-Stone” colonizationIn some populations, emigration is higher at low population sizes

Colonization from several small patches could be important

Use a modified immigration function to model (immigration from MH + immigration from all occupied small patches)

Stepping stone islands unlikely to increase immigration rates to distant patches

Continuous ExtinctionAre extinctions due to a major, devastating event or factors affecting local populations

Data from Jasper Ridge indicate that mean time to extinction depends on log(initial pop size)

reciprocal is expected yearly probability of extinction and can be compared to expected yearly colonization rate

determined using 1987 observed (if available) and expected population sizes

Conclusions

MH metapopulation appears to be mainland-island

Habitat variables and distance from a source pop determine patch occupancy

Stepping stone colonization appears to be unimportant

Continuous and catastrophic extinctions occur