BIOTIC RESPONSES TO SHIFTING ECOLOGICAL DRIVERS IN A DESERT COMMUNITY
S.K. Morgan Ernest
Dept of Biology & Ecology Center, Utah State University
@skmorgane
Shifting drivers, biotic responses
• Two case studies:
• Short-term driver shifts• Eruption of an exotic species
• Long-term driver shift• Increasing shrub cover
LTREB Portal Project - Background
• Initiated 1977 by Jim Brown and associates• Monitor rodent and plant dynamics• Diverse rodent & winter and summer annual communities
Portal
Portal Project - Background
• Control plots, kangaroo rat exclosures, rodent exclosures
• Responses to climate and plant responses to granivory
Case Study 1: Short-term Driver ShiftsGinger Allington, David Koons, Morgan Ernest, Michelle Schutzenhofer, Thomas Valone
State Change:
Exotic species
increase
Exotic species:Erodium cicutarium
Allington et al (2013) Ecology Letters
Population increase
Short-term Driver Shifts• Biotic Driver:
• When kangaroo rats or all rodents experimentally reduced, Erodium increases.
Allington et al (2013) Ecology Letters
Population increase
Short-term Driver Shifts• Biotic Driver:
• Considerable variation in kangaroo rats and other rodents through time
Allington et al (2013) Ecology Letters
Population increase
Short-term Driver Shifts• Abiotic Driver:
• No variation across site, but considerable temporal variation in precipitation.
Allington et al (2013) Ecology Letters
Population increase
Short-term Driver Shifts• Biotic Driver:
• Extended drop in kangaroo rat consumption before eruption
Allington et al (2013) Ecology Letters
Population increase
Short-term Driver Shifts• Abiotic Driver:
• Preferred levels of precipitation before kangaroo rats could recover
Allington et al (2013) Ecology Letters
Niche Opportunities• Driver shifts create
niche opportunities (Shea & Chesson 2002)
• How many drivers and what magnitude to create niche opportunity?
Driv
er S
hift
Mag
nitu
de
Number of shifting drivers
Case Study 2: Long-term Driver Shifts
Brown et al (1997) PNAS
19951977
Year
Num
ber
of s
hrub
s
The State Change: Arid Grassland to Shrubland
The Driver: Precipitation
• Long-term increase in winter precipitation
• Winter precipitation and shrub growth
Brown et al (1997) PNAS
PDO and Southwest Precipitation
• Pacific Decadal Oscillation: 20-30 year cycle
• Warm Phase:• La Nina – average winter
precipitation• El Nino – higher than
average precipitation
• Cool Phase: • La Nina – drought• El Nino – average winter
precipitationGutzler et al. (2002); Flagstaff NWS Office: http://www.wrh.noaa.gov
Long-term Driver Shifts & Reversals
• Multiple drivers and the problem of ‘reversing’ state changes
Driver Condition 1
Driver Condition 2
Driver Condition 1
fire
Ecosystem State:Single Driver
Ecosystem State:Multiple Drivers
Regime Shift Theory
• Focused on non-linear state-driver relationships
• Multiple driver shifts• Natural shifts & anthropogenic
shifts
Ecosystem Driver
Eco
syst
em S
tate