Ariel Appleton Research Fellowship Proposal Scott A. Chamberlain

Ariel Appleton Research Fellowship Proposal Scott A. Chamberlain

2. Research Proposal

Introduction. Fundamental to the discipline of ecology is understanding the structure and dynamics of communities (1). Much theoretical and empirical attention has been given to food webs of interacting predators and prey. Although there is still much to learn about food webs, we know far more about them than we do of the structure and dynamics of mutualistic communities. Despite this shortcoming, mutualismsare increasingly recognized to be critical to patterns and processes of biological systems, including the provision of ecosystem services (2) and the maintenance of biodiversity (3). New insights into both food webs and mutualistic communities are being made through the application of network theory. Networks depict species (nodes) with interactions (links) among them. Such networks have been recently used to describe many biological systems, including gene and protein networks, social networks of sexual interactions in humans, food web networks of predator-prey interactions, and mutualistic networks of plant-animal interactions (Fig. 1). The network approach to mutualistic communities is proving to be particularly useful (3-5), as there has been little to no other general way to describe their overall community structure. For example, mutualistic networks are nested (specialists attach to a core of generalists), while predator-prey networks are compartmentalized (interaction compartments, with species more linked within than across compartments; 4). Such differences have implications for how networks will respond to perturbations such as species extinction (9).

Objectives & Research Questions.I propose to study aspects of the structure of mutualistic networks between ants and extrafloral nectar (EFN; nectar not generally serving a pollination function) bearing plants in the Sonoran Desert. A set of pristine sites in southern Arizona, USA (Organ Pipe Cactus National Monument [ORPI]) will be contrasted with degraded sites in the state of Sonora, Mexico (Bahia de Kino [BK]). My advisor and I are intimately acquainted with these two sites, thus any unforeseeable difficulties will be avoided or easily managed. In summer 2006 at ORPI, I conducted a study of one EFN bearing plant species and its associated ants. I became interested in how the ant community as a whole was interacting with the EFN bearing plant community. Only four such ant–plant networks have been investigated at the community level (5). I plan to address the following question in this proposed research: How do the structural properties (e.g., community size, linkage density, connectance, nestedness, degree distribution) of mutualistic ant-plant networks compare with other mutualistic networks (e.g., pollination, seed dispersal) and food web networks of predator-prey interactions? Although this proposal only addresses a single question, as part of my doctoral thesis I plan to survey ant-plant networks throughout the Sonoran Desert, and conduct appropriate experiments, in future years.

Methods. To address the above questionI will study interaction networks of ants and EFN bearing plants at five sites within ORPI, as well as two sites at BK. At each site randomly selected belt transects (~5 m wide) will be used to document each pairwise interaction between ant species and EFN bearing plant species; plants along transects will be marked with inconspicuous metal tags. Based on preliminary surveys, I expect to have ~25 ant species and ~15 plant species (largely cacti and legumes). Transects at ORPI will be censused every week from April through July, and the BK sites will be censused once a month. In addition to simply determining which ant species interact with which plant species, abundance of ants and plants will be estimated as it can be important to network dynamics (9). Specifically, when censusing ant-plant interactions on a weekly basis, the number of each ant species on individual plants will be recorded, and plant abundance will be recorded once during the season. All sites will be censused in subsequent years of my dissertation research.

The structure of ant-plant networks will be described using a number of structural properties commonly used in the study of ecological networks (e.g., community size [(# ant species)*(# plant species)], linkage density [L/S, total links in network / # species], connectance [L/S2], nestedness [T, matrix temperature=from 00 (perfectly nested) to 1000 (anti-nested)], degree distribution [frequency distribution of # of interactions per species]). I will compare structural properties among ant-plant networks, as well as other mutualistic (pollinator, seed-dispersal) and predator-prey networks.

Results & Broader Impacts. This proposed research explores complex, previously neglected, mutualistic communities of interacting ants and plants using network theory. Specifically, I predict that ant-plant networks will not be randomly assembled, but will show a nested structure that varies with network size as in other mutualistic networks (4,5). More generally, these results will have implications for understanding how mutualistic networks vary through time and space, and if these patterns differ from those of antagonistic networks. In addition, these results will inform conservation with respect to how complex mutualistic communities respond to habitat loss and degradation.

Ecological Significance & Relevance to Contemporary Ecology. Mutualismsare critical to patterns and processes of biological systems, including the provision of ecosystem services (2) and the maintenance of biodiversity (3). The application of network theory to mutualistic communities shows great promise for understanding ecological patterns and processes, and notably the development of theory (3-5). This approach is allowing comparisons of networks within and across different types of species interaction, and necessary predictive modeling of the dynamic properties of networks. Species extinctions are increasing at an alarming, unprecedented rate due to habitat loss, climate change, and invasive species (10). Specifically, human-induced climate change is thought to have a relatively large influence in arid areas, including the Sonoran Desert (11,12). For example, regional climatic shifts in the Sonoran Desert have caused extinction of formerly common species and increases of previously rare species (13). Application of network theory to the study of communities has great potential to provide insight into how community structure will respond to ongoing threats, and how best to mitigate impacts. Lastly, this proposed research employs a novel synthetic and dynamic approach (network theory) to community ecology that transcends disciplines.

Timeline.

References.

1. Elton, C.S. 1927. Animal Ecology. London, Sidgwick and Jacksons.

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4. Bascompte, J., et al. 2003. PNAS, 100, 9383-9387.

5. Guimaraes, P.R., et al. 2006. Proc. R. Soc. B, 273, 2041-2047.

6. Proulx, S.R., D.E.L. Promislow & P.C. Phillips. 2005. TREE, 20, 345-353.

7. Newman, M.E.J. 2003. SIAM Rev., 45, 167-256.

8. Jordano, P., et al. 2003. Ecol. Lett., 6, 69-81.

9. Montoya, J.M., S.L. Pimm & R.V. Sole. 2006. Nature, 442, 259-264.

10. Novacek, M.J. & E.E. Cleland. 2001. PNAS, 98, 5466-5470.

11. Weiss, J.L. & J.T. Overpeck. 2005. Global Change Biol., 11, 2065-2077.

12. Weltzin, J.F., et al. 2003. Bioscience, 53, 941-952.

13. Brown, J.H., et al. 1997, PNAS, 94, 9729-9733.

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