Study Site: Fray Jorge National Park

1. “Top-down” vs. “bottom-up control”: evolution of alternative scenarios for community dynamics in a semiarid systemPeter L. Meserve, Douglas A. Kelt, M. Andrea Previtali, W. Bryan Milstead, and Julio R. Gutiérrez

2. Study Site:Fray Jorge National Park (71º 41' W, 30º 39' S) Quebrada de las Vacas north-central Chile

3. Arid-mediterranean climate Semiarid thorn scrub community • Semiarid-mediterranean climate • Evergreen/drought-deciduous shrubs • Evergreen/drought-deciduous shrubs • Herbaceous understory after winter rains • Herbaceous understory after winter rains

4. Oligoryzomys longicaudatus Abrocomabennetti Abrothrix longipilis Thylamys elegans Phyllotis darwini Octodon degus Abrothrix olivaceus Principal “players” include a diverse group of small mammals

5. Geranoaetus melanoleucus Lycalopex culpaeus Athenecunicularia Bubo magellanicus Tyto alba Callopistes palluma Philodryaschamissonis And their vertebrate predators…

6. Genesis of a large-scale manipulative field experiment… Our premise was that the system was primarily “top-down”-controlled Original questions: Which biotic interactions are important in structuring small mammals and plants here? • Predation • Herbivory • Competition

7. +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 Selective exclusion of predators and/or degus

8. Predator exclusion grids with high fencing & netting overhead

9. Controls with low fencing • Presence or absence of ca. 5 cm d holes permitting selective entry or exclusion of degus

10. Sampling procedures • 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 • Predators: • Olfactory lines & scat collection (foxes) • Sightings & pellet collections (raptors)

11. 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

12. Responses to high rainfall events • Dramatic increases during/after high rainfall years with time lags for degus • Declines during ensuing dry periods

13. Rainfall-driven changes A dry year A wet year In wet years, rainfall may increase 2-3 times the normal 110-120 mm

14. Changes in vegetation relative to rainfall • Strong fluctuations in herbaceous (ephemeral) cover & seed density • Relatively constant perennial shrub cover

15. But we failed to consider the role of spatial dynamics… What happens in nearby habitats such as fog forest and more mesic aguadas? Thorn scrub Fog forest Aguadas

16. Fog forest

17. Aguadas

18. Many small mammal species persist in nearby habitats during drought periods

19. Two species with different spatial dynamics • Dramatic changes in the thorn scrub • Different representation in the fog forest • Less variation in the aguadas  Peripheral habitats may be refuges during droughts

20. Characterization of residency status O. degus along with some other species maintain breeding populations in the thorn scrub “Core species” A. longipilis & some other species persist in peripheral habitats, and may colonize thorn scrub during rainy years “Opportunistic species”

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

22. This view seemed supported by results of food-addition experiments Two core species Two opportunistic species

23. 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

24. Model incorporating density & rainfall explains most changes in degu populations Rt= – 0.8 ln(Odt-1/Clt) + Et -7.2 + 1.3 ln(Clt-1) Rt Rt ln(Odt-1/Clt) lnClt-1 r 2 = 0.94 p<<0.001

25. 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 plays a role in explaining the presence/absence of small mammal species during wet vs. dry years

26. Thus, principal small mammals here are responding to changes in food availability, and indirectly, precipitation

27. Current directions • 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 Selective exclusions of lagomorphs and/or small mammals

28. Monitoring of lagomorph activity After ↓ ↑ Before

29. Studies of competitive interactions among shrubs, and native and introduced ephemerals • Studies of long-term effects of climate change on community components

30. Recently, we have detected suggestive changes in the small mammal assemblage Core species biomass (especially O. degus) has become more constant

31. 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 the Norte Chico region of Chile

32. Consequences of global climatic change… Increased rainfall in Norte Chico region may be triggering major changes in small mammal assemblage dynamics Changes in numbers of herbivorous rodent may alter competitive interactions among native and exotic ephemeral plants Exotic groups such as lagomorphs and ephemeral plants may have greater impacts Destabilization of community-level interactions may involve vertebrate predators, disease vectors, and other biotic elements (e.g., birds, insects) as well as larger scale processes

33. We acknowledge our support sources…

34. As well as many, many people… --and a cast of literally thousands 

35. Questions?