“Top-down” vs. “bottom-up control”:

evolution of alternative scenarios for

community dynamics in a semiarid

system

Peter L. Meserve, Douglas A. Kelt, M. Andrea

Previtali, W. Bryan Milstead, and Julio R.

Gutiérrez

Study Site: Fray Jorge National ParkFray Jorge National Park

Quebrada de las Vacas

(71º 41' W, 30º 39' S)

north-central Chile

Semiarid thorn scrub community

• Arid-mediterranean climate

• Evergreen/drought-deciduous shrubs

• Herbaceous understory after winter rains

• Semiarid-mediterranean climate

• Evergreen/drought-deciduous shrubs

• Herbaceous understory after winter rains

Principal “players” include a diverse group of small mammals

Thylamys elegansThylamys elegans AbrocomaAbrocoma bennettibennetti

Octodon degusOctodon degus

Abrothrix longipilisAbrothrix longipilis

Phyllotis darwiniPhyllotis darwini Abrothrix olivaceusAbrothrix olivaceus

Oligoryzomys Oligoryzomys longicaudatuslongicaudatus

And their vertebrate predators…

Lycalopex culpaeusLycalopex culpaeusAtheneAthene

cuniculariacunicularia Tyto albaTyto albaBubo Bubo

magellanicusmagellanicus

Geranoaetus Geranoaetus melanoleucusmelanoleucus Philodryas Philodryas

chamissonischamissonisCallopistes Callopistes

pallumapalluma

Genesis of a large-scale manipulative field experiment…

Our original questions:

What biotic factors are mostimportant in structuring small

mammals and plants here?

Biotic interactions:• Predation• Competition• Herbivory

Our premise was that the system was primarily

“top-down”- controlled

Selective exclusion of predators and/or degus

+P+D

+P+D

+P+D

+P+D+P -

D

+P -D

+P -D

+P -D

-P -D

-P -D

-P -D

-P -D

-P+D

-P+D-P+D

-P+D

Predator exclusion grids with high fencing & netting overhead

Controls with low fencing

• Presence or absence of ca. 5 cm d holes permitting selective entry or exclusion of degus

Small mammals:

• Monthly mark-recapture; density estimates from minimum number known alive (MNKA)

Vegetation:

• Point intercept method for shrub & herbaceous plants (ephemerals)

• Soil samples for seed separation

Sampling procedures

Predators:

• Olfactory lines & scat collection (foxes)

• Sightings & pellet collections (raptors)

Responses of small mammals to exclusion of predators and/or degus

• Transitory effects of predation on degu numbers & survival

• Little or no effects of degu exclusion on other species

Responses to high rainfall events

• Dramatic increases during/after high rainfall years with time lags for degus

• Declines during ensuing dry periods

Rainfall-driven changes

A dry year A wet year

In wet years, rainfall may increase 2-3 times the normal 110-120 mm

Changes in vegetation relative to rainfall

Strong fluctuations in herbaceous (ephemeral) cover & seed density

Relatively constant perennial shrub cover

Time (years)19891990199119921993199419951996199719981999200020012002200320042005200620072008

Pe

rce

nt C

ove

r

0

20

40

60

80

100

De

nsi

ty s

ee

ds /

m2

x 1

03

0

5

10

15

20

25

30

35

40

45

50Rainfall (mm)Perennial Seeds Ephemeral

233 229 77 35 54 77 330 11 58 2363289 356209 84 168 90 147 48 96

What happens in nearby

habitats such as fog forest and more mesic aguadas?

Thorn scrubFog forestAguadas

But we also failed to consider spatial

dynamics…

Fog forest

Aguadas

Many small mammal species persist in nearby habitats during drought periods

Min

imum

Num

ber

Kn

own

Aliv

e /

Grid

O. degus

P. darwini

A. olivaceus

O. longicaudatus

T. elegans

A. longipilis

10

100

10

100

10

100

10

100

10

100

Aug Oct

Dec

Feb

Apr Ju

lO

ctFe

b

Aug Feb

May Ju

lN

ovFe

bM

ayA

ug Mar

Aug Feb

Aug

Feb

Aug

10

100

AguadaEcotoneForest

1996 1997 1998 1999 2000 2001 2002 2003

Min

imum

Num

ber

Kno

wn

Aliv

e / G

rid

O. degus

A. longipilisA

ugOct

Dec

Feb

Apr Ju

lO

ctFe

b

Aug Feb

May Ju

lN

ovFe

bM

ayA

ug Mar

Aug Feb

Aug

Feb

Aug

10

100

1996 1997 1998 1999 2000 2001 2002 2003

10

100 Thorn scrub

• Dramatic changes in the thorn scrub

• Different representation in the fog forest

• Less variation in the aguadas

Aguada

Forest

Peripheral habitats may be refuges during droughts

Two species with different spatial dynamics

Characterization of residency status

A. longipilis & some other species persist in peripheral habitats, and may colonize thorn scrub during rainy years

O. degus along with some other species maintain breeding populations in the thorn scrub

“Core species”

“Opportunistic species”

We then posited alternating “top-down” vs. “bottom-up” dynamics in this system

This view seemed supported by results of food-addition experiments

Octodon degus

0

10

100

Min

imum

Num

ber

Kno

wn

Aliv

e / G

rid

Phyllotis darwini

J S N J M M J S N J M M

0

10

100

CONTROLFOOD ADDITION

1997 1998 1999

* * *

* * * * * * * *

* * * * * *

*

Abrothrix longipilis

0

10

100

Min

imu

m N

um

be

r K

no

wn

Aliv

e /

Grid

Oligoryzomys longicaudatus

J S N J M M J S N J M M

0

10

100

CONTROLFOOD ADDITION

1997 1998 1999

*

* *

Two core species Two opportunistic species

But modeling shows that populations of many of the species here are also affected

by intrinsic regulatory factors

• There are strong interactions between rainfall and density in more numerous rodent species in the community, including Octodon degus

r r 2 2 = 0.94= 0.94

pp <<<< 0.0010.001

RRtt == -7.2-7.2 – – 0.8 ln(0.8 ln(OdOdt-1t-1//ClCltt))

-5 -4 -3 -2 -1 0 1

-2-1

01

23

4

Rt

ln(Odt-1/Clt)

+ E+ Ett+ 1.3 + 1.3 ln(Clln(Clt-1t-1))

3 4 5 6

-3-2

-10

12

Rt

lnClt-1

Model incorporating density & rainfall explains most changes in degu populations

ln(Odt-1/Clt)

65%

t 6%lnClt-129%

So where are we now?

• Biotic interactions such as predation have relatively small effects on small mammals in this assemblage

• “Bottom-up” factors due to aperiodic rainfall events have major effects on all elements of the community

• Density-dependence and lagged effects of rainfall are important drivers of small mammal population dynamics

• Spatial heterogeneity explains presence/absence of some small mammal species during wet vs. dry years

• Continued monitoring of small mammals, predators and plants, as well as studies of birds & insects

• Separation of effects of lagomorphs vs. small mammals on the vegetation community

Current directions

Selective exclusions of lagomorphs and/or small mammals

Monitoring of lagomorph activity

↑ Before counts

After counts →

• Studies of competitive interactions among shrubs, and native and introduced ephemerals

• Studies of long-term effects of climate change on community components

Recently, we have detected suggestive changes in the small mammal assemblage

Time (seasons)

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008Pro

port

ion

of to

tal s

mal

l mam

mal

bio

mas

s

0.0

0.2

0.4

0.6

0.8

1.0

Prop. Opportunistic speciesProp. Octodon degusProp. Core species other than O. degus

Core species biomass (especially O. degus) has become more constant

Time (seasons)NR89R89

NR90R90

NR91R91

NR92R92

NR93R93

NR94R94

NR95R95

NR96R96

NR97R97

NR98R98

NR99R99

NR00R00

NR01R01

NR02R02

NR03R03

NR04R04

NR05R05

NR06R06

NR07R07

NR08R08

NR09

Sha

nnon

div

ersi

ty in

dex

H'

0.0

0.5

1.0

1.5

2.0

Shannon diversity values have also become more stable

• Mean rainfall increased from 111 mm in 1989-1999 to 160 mm in 2000-2008• Global Climatic Change may bring increased rainfall to Norte Chico region of Chile

Consequences of global climatic change…

1.Increased rainfall in Norte Chico region may trigger major changes in small mammal assemblage dynamics;2.Changes in numbers of herbivorous rodents may alter competitive interactions among native and exotic ephemeral plants;3.Introduced groups such as lagomorphs and ephemeral plants may have greater impacts; and:4.Destabilization of community-level interactions may include vertebrate predators, disease vectors, and other biotic elements (e.g., birds, insects) as well as larger scale processes

We recognize our many support sources…

the contributions of many, many people…

--and a cast of literally thousands

Questions?

1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Pre

dato

r ab

unda

nce

0

25

50

75

100

125 L. culpaeus

B. magellanicus

T. alba

A. cunicularia

Rod

ent d

ensi

ty (

MN

KA

/ha)

0

50

100

150

200 P. darwini A. olivaceus

O. degus

Pla

nt c

over

, se

ed d

ensi

ty

0

20

40

60

80

100

Y A

xis

2

Ephemerals Shrubs

Seeds

Ann

ual p

reci

pita

tion

(mm

)

0

100

200

300

400

Rainfall

Average