Grupo de Investigación en Ecología de Comunidades de Desierto

1. Grupo de Investigación en Ecología de Comunidades de Desierto Desert Community Ecology Research Team Granivory in the central Monte Desert Part I. Bottom-up effects Part II. Top-down effects Part III. The Research Program Luis Marone Ecodes http://www.ege.fcen.uba.ar/Ecodes

2. 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

3. RESEARCH PROBLEM What drives population fluctuations of seed-eating birds?

4. Theoretical framework for inspiration Biological interactions (granivory), Bottom-up control, Natural history of grasses Idea Resource (seed) availability controls bird population fluctuations

5. Which seeds are available for granivorous birds? Diet Foraging behavior

6. Prediction Positive correlation between seed availability [o its surrogates] and the abundance of granivorous bird populations

7. Surrogate: previous rainfall Seed standing crop

8. Suggestive although weak associations r = 0.51, gl = 18, p < 0.01

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

10. 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 conclusion Seed 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.

11. Part II. PATTERN - PROBLEM What drives changes in grass (seed and plant) abundance? Decadal changes in grass cover under natural conditions

12. Theoretical framework for inspiration Biological interactions (granivory), top-down control Ideas Seed consumption controls grass (seed and plant) abundance Granivory is a major mechanism determining transient seed reserves

13. The sieve seed-eating animals impose on seed reserves Granivory sieve BSS + seed traps Previous SSB 2400 seeds/m2 Seed production 3000 seeds/m2 Spring SSB 2700 seeds/m2 The sieve 2700 seeds/m2 Seed loss 50% SG (selected grasses), NSG (non-selected grasses), SF (selected forbs), NSF (non-selected forbs), NCS (non-consumed seeds)

14. 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

15. Why does the grass SSB is a transient (1 yr) one? • High and “relatively stable” seed production • Low germination (<1%, exceptional <5%) • Selective but not-so-high granivory (40-60%) • Low deep burial

16. Fungal attack

17. A model for grass soil seed banks

18. Conclusions and Reflections • 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)

19. A model for grass soil seed banks in the central Monte

20. We need a research program… 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

21. 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 News Why 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.