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Seed dispersal mutualisms between plants and animals often determine the structure and diversity of ecosystems. In eastern hardwood forests of North America, the mutualism between scatter-hoarding animals and nut-bearing trees is crucial in shaping the distribution of dominant tree species and the complex wildlife communities they support. Avian seed-hoarders, in particular Blue Jays (Cyanocitta cristata), are especially important long-distance dispersers of oak (Quercus) acorns and other small nuts, through their caching of seeds in the autumn for winter consumption. Since oak-dominated forests are declining across much of the eastern U.S. – through mesophication, disease, and introduced pests – understanding their relationship with jays is critical to determining how best to manage for increased oak dispersal and recruitment. In addition, the reintroduction of American chestnut (Castanea dentata) to its former range is anticipated in the near future, and jays are likely to play a key role in their natural dispersal from founder plantings. In this dissertation, I examine dynamics of the mutualism between avian seed-hoarders and oaks/chestnuts to determine how it will shape these two processes of forest change – oak decline and chestnut reintroduction. I approach this topic in two ways. First, I examine each stage of the dispersal process from selection of seeds by birds through growth and survival of seedlings, for two common oaks (white oak [Q. alba] and black oak [Q. velutina]) and American chestnut. Second, I examine the link between mast abundance and Blue Jay abundance and survival, to determine how jay populations will be influenced by oak forest declines. When assessing the seed preferences of jays and other avian seed-hoarders in southeastern Ohio (Chapter 2), I found that jays consistently preferred black oak acorns over chestnuts, and chestnuts over white oak acorns. The strength of their preference for black oak acorns increased in late fall/early winter, when all seeds were less available, suggesting perhaps a greater need for lipid-rich resources in winter. Their preference for chestnuts over white oak acorns, however, suggests that chestnuts may become an important resource for jays and other seed-hoarders in years of black oak mast failure. Likewise, jays will likely serve as effective dispersers (Chapter 3) of chestnuts in those years, as 6.2% of cached chestnuts produced seedlings surviving through the first growing season, compared to 3.8% for black oak and 2.6% for white oak acorns. In addition, the propensity of jays to cache seeds in shrubby early-successional areas may also be a boon for early recruitment of the more competitive chestnut seedlings. On the other hand, jays provided directed dispersal for black and white oak acorns, as stem emergence of planted seeds was greater at cache sites than random sites for these species, but not chestnut. One of the most limiting factors for dispersal effectiveness of jays was pilferage by rodents, with >80% of all cached seeds being pilfered. Dispersal distances by jays (up to 460m from parent trees) at my mostly contiguous forest site were shorter than those reported by other studies in more fragmented habitats (up to 4km), but still suggest great potential for recruitment and colonization for oak and chestnut seedlings. Overwinter survival of jays (Chapter 4) at my Ohio field site was much greater for adults (88%) compared to young birds (47%), but otherwise was not influenced by annual mast abundance or other weather-related factors. In an analysis of long-term bird and mast abundance datasets (Breeding Bird Survey, Christmas Bird Count, and Regional Mast Survey) across 15 years and 8 states, I found that winter abundance of jays was positively associated with red oak subgenus mast abundance and negatively associated with the previous year’s white oak mast abundance, indicating both direct and indirect influences of acorn abundance on jays. Breeding abundance of jays, however, was not clearly associated with mast abundance, suggesting that jay populations are not necessarily limited by mast. Taken together, my results highlight the dynamic nature of the relationship between nut-bearing trees and their avian seed-dispersers, which underscores the importance of considering plant-animal mutualisms when planning forest management and restoration efforts.
Any scientific discipline needs a theoretical framework to guide its development and to sharpen the questions its researchers pursue. In biology, evolution is the grand theoretical framework, and an his torical perspective is necessary to understand present-day biological conditions. In its formative years, the modern study of the fruit-frugivore mutualism was guided by the 'specialist-generalist' paradigm developed by D. Snow, D. McKey, and H. Howe. Howe reviews the current status of this evolution ary paradigm and points out that it has been dismissed by many workers before being adequately tested. This is because ecologists working with the tropical plants and frugivorous birds for which the paradigm was originally developed rarely measure the seed dispersal effectiveness of different disperser species. He indicates that this paradigm still has heuristic value and suggests that several additional ecological paradigms, including the concept ofkeystone species ofplants and frugivores and the role that frugivores play in density-dependent mortality in tropical trees, are worth studying. The concept of seed dispersal quality has been central to discussions of fruit-frugivore coevolution. Schupp thoroughly reviews data bearing on this concept, constructs a hierarchical framework for viewing disperser effectiveness, and points out that disperser effectiveness depends on both the quantity and quality of seed dispersal. Effectiveness, in turn, affects both evolutionary and ecological relationships between dispersers and their food plants.
The relationship between scatter-hoarders and trees varies on a continuum from mutualism to antagonism and can change across time, location, and among species. We examined five tree species across five sites to quantify this variability: red oak (Quercus rubra), white oak, (Quercus alba), American chestnut (Castanea dentata), Chinese chestnut (Castanea mollissima), and a hybrid American/Chinese chestnut. In the fall of 2012, 2013, and 2015, we set up field sites across the east coast, including in Virginia, Pennsylvania, Indiana, Massachusetts, and Maine. We tested the potential for mutualism between scatter-hoarders and trees by using a simple model proposed in the literature that compares rates of seedling emergence with and without scatter-hoarders. The model uses the ratio of seed emergence from the surface divided by seed emergence from a cache (Es/Ec) as one variable to place seeds on a continuum from mutualism to antagonism. We predicted across different study sites and found that that red oak seeds will have a more mutualistic relationship with scatter-hoarders because of their long dormancy period before germination, white oak seeds will have an antagonistic relationship because they germinate readily, and chestnuts will fall in between. We hypothesized that abiotic conditions surrounding the seed would have a stronger influence on the Es/Ec ratio than the provenance of the seed would. We found that different seed provenances caused variation in the Es/Ec ratio, but abiotic conditions described more of the variation in the continuum from mutualism to antagonism. We also quantified the effect of temperature at our different study sites to measure the correlation between temperature and emergence. We found that temperature did not predict the high degree of variation in the effect of abiotic conditions on the degree of mutualism.
Theimer, an accomplished ecologist.
This textbook provides the first overview of plant-animal interactions for twenty years focused on the needs of students and professors. It discusses a range of topics from the basic structures of plant-animal interactions to their evolutionary implications in producing and maintaining biodiversity. It also highlights innovative aspects of plant-animal interactions that can represent highly productive research avenues, making it a valuable resource for anyone interested in a future career in ecology. Written by leading experts, and employing a variety of didactic tools, the book is useful for students and teachers involved in advanced undergraduate and graduate courses addressing areas such as herbivory, trophic relationships, plant defense, pollination and biodiversity.
This book presents current knowledge of seed fate in both natural and human-disturbed landscapes, from various regions of the world. Habitats considered range from mountain and arid deserts in the temperate zone, to savanna and lowland rainforests in tropical regions of the world. Particular attention is paid to plant diversity conservation when seed removal is affected by factors such as hunting, habitat fragmentation or intensive logging. Contributors include leading scientists involved in research on seed ecology and on animal-plant relationships from the perspective of both primary and secondary seed dispersal, and predation.