Grupo de Investigación en Ecología de Comunidades de Desierto

Desert Community Ecology Research Team

Granivory in the central Monte Desert

Part I. Bottom-up effectsPart II. Top-down effects

Part III. The Research Program

Luis MaroneEcodes

http://www.ege.fcen.uba.ar/Ecodes

Community consequences of granivory

Such [studies], however, will not be entirely successful unless the assessment of the impact on seed reserves of some underrated granivores like birds, bacteria and fungi, and of germination losses are increased, involving deserts throughout the world.

Marone & Horno 1997

Population interactions between seeds and avian granivores in the Monte desert may therefore be more important than hitherto appreciated… Granivory may have a significant community role in the Monte and other South American deserts, despite the probable lack of highly specialized seedeaters.

Marone, López de Casenave & Cueto. 2000

This suggests that despite the absence of specialized granivorous rodents, there still may be considerable seed consumption in the system [Fray Jorge], and this should be considered in studies of community dynamics. Meserve, Kelt, Milstead & Gutiérrez. 2003

RESEARCH PROBLEMWhat drives population fluctuations of seed-

eating birds?In

d./h

a

year year

Theoretical framework for inspirationBiological interactions (granivory), Bottom-up control, Natural

history of grasses

Idea

Resource (seed) availability controls bird population fluctuations

Which seeds are available for granivorous birds?

Grasses

Forbs

OthersSoil

Diet Foraging behavior

Prediction

Positive correlation

between seed availability [o its surrogates]

and the abundance of granivorous bird populations

WINTER r = 0.32, p = 0.37 SPRING r = 0.48, p = 0.16

0

1

2

3

4

50 150 250 350 450

Previous rainfall (mm)

Bird

den

sity

(in

d/ha

)

0

1

2

3

4

50 150 250 350 450

Previous rainfall (mm)

Bird

den

sity

(in

d/ha

)

Surrogate: previous rainfall

WINTER r = 0.36, p = 0.55 SPRING r = 0.79, p = 0.03

0

1

2

3

4

0 2 4 6 8 10 12 14

Grass seed biomass (kg/ha)

Bird

den

sity

(in

d/ha

)

0

1

2

3

4

0 2 4 6 8 10 12 14

Grass seed biomass (kg/ha)

Bird

den

sity

(in

d/ha

)

Seed standing crop

Standarized density

0

0,5

1

1,5

2

500 600 700 800 900 1000Cumulative rainfall (mm)

Den

sity

(in

d/ha

)

.

r = 0.51, gl = 18, p < 0.01

Suggestive although weak

associations

Weak association. Small effect size.Why? Resorting to natural history

Diuca diuca 22% (9)

Salatricula multicolor 11% (16)

Zonotrichia capensis 9% (6)

Poospiza ornata 8% (53)

Poospiza torquata 8% (97)

Milvago chimango 100% (5)

Nesting success

0

20

40

60

GS 2001/02 GS 2002/03 GS 2003/04

10 6 s

eeds/h

a

.

10

Rainfalls 302 mm 154 mm 222 mm

Grass seed production

1. Bird populations are open (vagrancy, migration). Scale.2. Seed resources are usually super-abundant (pulse

paradigm under debate)3. Plasticity, opportunism (Sporobolus)4. Intense nest predation (top-down on bird consumers)

- Very infrequent crunch conditions. Small bottom-up (seed) effect

Plausible conclusionSeed bottlenecks occur, but they are rare, occasional.

Most of the time, bird populations fluctuate above a “seed limitation threshold” probably owing to the populations are not able to track

super-abundant resources (e.g., limited reproduction).Besides, denso-independent local causes (e.g., winter mortality) or

factors affecting birds elsewhere.

Part II. PATTERN - PROBLEMWhat drives changes in grass (seed and plant)

abundance?

Decadal changes in grass cover under natural conditions

0

5

10

15

1987 1997 2002 2009

Coverture (%

) .

Pappophorum Stipa Trichloris

Digitaria Aristida Sporobolus

Setaria

0

5

10

15

1987 1997 2002 2009

Cov

ertu

re (%

) .

Theoretical framework for inspirationBiological interactions (granivory), top-down control

IdeasSeed consumption controls grass (seed and plant) abundance

Granivory is a major mechanism determining transient seed reserves

Granivory sieveBSS + seed traps

Previous SSB 2400 seeds/m2

Seed production 3000 seeds/m2

Spring SSB 2700 seeds/m2

The sieve2700 seeds/m2

Seed loss 50%

The sieve seed-eating animals impose on seed reserves

SG (selected grasses), NSG (non-selected grasses),

SF (selected forbs), NSF (non-selected forbs),

NCS (non-consumed seeds)

Despite moderate to high bird seed predation, the number of grass seeds that remains in the soil in spring would not limit seed germination and seedling recruitment. By contrast, safe-site availability and drought may be important factors limiting grass recruitment, at least in the undisturbed habitats of Ñacuñán.

Marone,, Lopez de Casenave, Milesi & Cueto. 2008

Granivory sieve does not appear to be an effective control for grass recruitment

1. High and “relatively stable” seed production2. Low germination (<1%, exceptional <5%)3. Selective but not-so-high granivory (40-60%)4. Low deep burial

Why does the grass SSB is a transient (1 yr) one?

Fungal attack

Apr-01 May-01 Jul-01 Oct-01 Feb-02 Apr-02 Apr-03

Pappophorum 0 3 ± 1 1 ± 1 12 ± 4 52 ± 6 – –

Setaria 0 2 ± 2 4 ± 3 2 ± 2 76 ± 13 91 ± 5 91.2 ± 5

Control 0 0 0 0 0 0 0

50–90%

A model for grass soil seed banks

- A low bottom-up signal exists in the population dynamics of seed-eating birds

- Changes in the abundant of immigrant (opportunistic) bird species may explain such dynamics

- High nest predation on granivorous birds (top-down on basal consumers)

- Small effect size or magnitude (e.g., correlations)

- Opportunism as a source of “unexpected” results

- The pulse paradigm may need some revision according to the natural history of producers

- Granivory (birds and ants) is a significant force (>50% of seed production) …

- … Despite this, it seems to have a low effect on vegetation (e.g., recruitment)

- Grass recruitment would be more strongly limited by abiotic factors (e.g., precipitation)

- A plurality of mechanisms determines the transient grass SSB (granivory, fungal attack)

Conclusions and Reflections

A model for grass soil seed banks in the central Monte

0

0,2

0,4

0,6

0,8

1

1,2

December March June September December March June September December

Num

ber o

f see

ds

.

Pr

BSS

0

0,2

0,4

0,6

0,8

1

1,2

March June September December March June September December

Number of seeds .

Comparative studies depend strongly on the robustness of established patterns, and the above generalization [that total granivory is negligible in South American deserts] was not tested for robustness but inferred by using one research approach alone (bait removal experiments) … in one location (Anadalgalá) … of a particular desert (Northern Monte).

The relative importance of seed consumption or granivorous assemblages in deserts around the world cannot be assessed through one research approach alone (e.g., bait-removal experiments, energetics, species composition of the granivorous guild, or quantification of individuals’ diets). Instead, research programs that include multiple approaches involving redundancy and cross-checks of hypotheses may lead to the most robust conclusions and, therefore, to synthesis and integration.

Furthermore, any such program should involve long-term studies that take into account the spatial and temporal variability of natural communities.

Although ecological “laws” are by no means universal, ecological understanding will continue to depend on theory development… whenever possible, based on patterns as well as mechanisms”.

Marone et al. 2000. Granivory in southern South American deserts… BioScience 50: 123-132

We need a research program…

Wingtip to wingtip [atmospheric –troposphere- research] “In order for data sets to be compared, we must demonstrate that the results from the different suites of instruments [C-130 Hercules and US P-3] are comparable. Our intercomparison flight was a key part of the experimental flying”.

Kaye, A. 1998. Nerc NewsWhy most published research findings are false?

“Published research findings are sometimes refuted by subsequent evidence, with ensuing confusion and disappointment… There is increasing concern than in modern research false findings may be the majority or even the vast majority of published research claims…

Unfortunately, in some areas, the prevailing mentality until now has been to focus on isolated discoveries by single teams, and interpret research experiments in isolation… If the true effect sizes are very small in a scientific field, this field is likely to be plagued by almost ubiquitous false positive claims…

We should then acknowledge that statistical significance testing in the report of a single study gives only a partial picture, without knowing how much testing has been done outside the report and in the relevant field at large”.

Ioannidis, JPA. 2005. PLoS Medicine 8: 696-701.