Browsing by Subject "Community ecology"

Now showing 1 - 3 of 3
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    Community assembly, invasion, and management of aquatic plant communities
    (2022-12) Verhoeven, Michael
    Determining what mechanisms drive native species declines and what governs their recovery is foundational to understanding community change, and successfully applying this knowledge to limit further losses or restore degraded ecosystems. Efforts to reduce invasive plant populations are often considered critical for halting degradation of native plant communities and fostering their subsequent recovery or restoration. To assess whether management of two invasive plants—Eurasian watermilfoil (Myriophyllum spicatum) and curlyleaf pondweed (Potamogeton crispus)—is likely to foster recovery of native aquatic plant communities, I integrate experimental and observational methods to study community assembly processes in aquatic plants. Chapter 1 builds the foundation for subsequent studies by constructing an observational monitoring database, compiled from more than 500,000 plant observations collected by disparate sources over a 19-year period. In Chapter 2, I use niche models to unpack how patterns of dominance seen in P. crispus and M. spicatum have likely arisen through different mechanisms, predicting direct competition with native species is less likely for P. crispus than M. spicatum. The aquatic plants database is used in Chapter 3 to assess invader control for boosting native plant communities in real-world management projects, with a focus on comparing the two invaders to test predictions from niche models. I show that limitations of monitoring data constrain estimation of causal effects of management. This limits the generalizability of the findings, highlighting the need for more strategic allocation of aquatic plant monitoring efforts and improved tracking of management interventions. In Chapter 4 I synthesize results of a 4-year, in-lake field experiment and the 19-year, statewide observational data, using community assembly theory to ascribe changes in plant communities to three major mechanisms (invader competition, environmental conditions, and regional species pools) and assess the scales at which these mechanisms shape aquatic plant communities. The results highlight complexity and interactivity of community assembly in this system, with mixed evidence for each mechanism and strong differences across scales. This research demonstrates that contrary to common dogma in aquatic plant management, invaders’ relationships with recipient communities are nuanced, and that invader control alone is insufficient to achieve restoration.
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    Effects of vertebrates, insects, and pathogens on patterns of early plant recruitment in tropical forests.
    (2010-07) Beckman, Noelle Gabriele
    Vertebrate seed dispersers and seed predators, insect seed predators, and pathogens are known to influence plant survival, population dynamics, and species distributions. The selective pressure of these mutualists and antagonists have resulted in a myriad of plant adaptations, including morphological and nutritional fruit traits to attract seed dispersers, and plant defenses to deter seed predators and pathogens. The importance of vertebrates, insects, and pathogens for plant communities has long been recognized, but their absolute and relative importance in early recruitment of multiple coexisting tropical plant species has not been quantified. Further, little is known about the relationship of fruit traits to seed dispersal and natural enemy induced seed and seedling mortality in tropical plants. My dissertation investigates the importance of these groups of organisms in the sequential stages of early plant recruitment (i.e. from fruit developing in the crown to seedlings on the ground) in tropical forests. I used a combination of empirical and theoretical studies: an experimental study of pre-dispersal seed mortality in plant canopies of seven species, a bioassay experiment examining patterns of fruit toxicity for eleven species, a simulation study of the interacting effects of seed dispersal and enemy attack on spatial patterns of surviving seedlings in theory, and a field study of the effects of partial defaunation of vertebrates (by hunting) on pre-dispersal seed predation and seed removal in two tree species. To determine the influences of vertebrates, insects, and pathogens on reproduction of plants varying in fruit traits, I investigated reductions in fruit development and seed germination due to vertebrates, insects, and fungal pathogens through experimental removal of these enemies using canopy exclosures, insecticide, and fungicide, respectively at the Canopy Crane Access System in Parque Natural Metropolitano in Central Panama. Results suggest that predispersal seed mortality is attributable to different natural enemies in different canopy species. Fruit morphology explained some of the interspecific variation in fruit development and seed survival in response to natural enemy removal treatments. This is the first experimental test of the relative effects of vertebrates, insects, and pathogens on seed survival in the canopy. To investigate patterns of fruit toxicity, I used bioassays involving brine shrimp (Artemia franciscana) and two foliar fungal pathogens (Fusarium sp, Phoma sp.) to understand how chemical defenses of coexisting canopy plants differ from the immature to the mature stage of fruit development and between the seed and pericarp. Every plant species tested in this study exhibited toxicity to at least one bioassay organism but patterns of toxicity depended on plant species and bioassay organism. To explore how patterns of natural enemy attack and seed dispersal affect seedling recruitment patterns, I developed a spatially-explicit, individual-based model to study plant life stages following seed dispersal. With this model, I explored how different seed deposition patterns and natural enemies affect the spatial patterns of surviving seedlings in a simulated model community. The seedling recruitment patterns observed in the model reproduced the range of patterns observed empirically. Recruitment patterns were sensitive to the type of natural enemy attack and the movement distances and fecundity of natural enemies, as well as to seed dispersal distances and the degree of clumping. To investigate how hunting alters seed dispersal and seed predation, I compared these processes for two canopy tree species that differ 16-fold in seed size in both hunted and protected forests in Central Panama. The results of this study suggest that in hunted areas there are greater reductions in seed removal and seed predation of the larger-seeded compared to the smaller-seeded tree. Overall, the results of my dissertation contribute to an emerging, but still very incomplete, trait-based approach to understanding interspecific variation in biotic interactions. Determining the relative roles of vertebrates, insects, and pathogens throughout the stages of plant recruitment will aid in our understanding of the mechanisms that limit plant populations, contribute to our knowledge of diversity maintenance, and is critical to predicting the consequences of anthropogenic pressures on plant communities.
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    Understanding the role of plant hydraulic traits in ecological processes
    (2021-11) Vargas Gutierrez, German
    Among all ecosystems tropical forests play a disproportionate role in Earth’s carbon cycle as they account for one third of total primary productivity. Global environmental change is causing novel precipitation patterns, shifts in the timing of rainfall events and an increase of the atmospheric water demand in tropical regions. Such changes pose a threat to the ecosystem services provided by tropical forest ecosystems as there is great uncertainty of how tropical forests will respond to warmer and drier climatic conditions. Plant responses to drought occur at different biological and temporal scales, ranging from responses at the tissue level to responses that define species and populations. Plant hydraulic traits have proven to be key in the characterization of how plants respond to drought, as they allow to group species in those that avoid drought and those that resist drought. Therefore, quantifying the sources of variation in plant hydraulic traits presents a promising way to move forward our understanding of how tropical forests will respond to climate-change. The main objective of this research was to study the aspects that drive variation in plant hydraulic traits in tropical plant species, and how nutrient availability affects tropical forest responses to drought. In Chapter 1, I performed a study in which I investigated if it is possible to generalize tree species drought tolerance and growing strategies using their leaf habit (i.e., deciduous, evergreen and semi-deciduous). In this study I found that in the tropical dry forest deciduous species tend to share similarities in their physiological properties, contrary to evergreen species that tend to be physiologically different across sites. Generally deciduous species will show a hydraulically risky growing strategy, and evergreen species a hydraulically safe growing strategy. In Chapter 2, I carried out a large-scale ecosystem manipulation experiment to study how both water and nutrient availability control primary productivity in a tropical dry forest. I found that nutrient availability had a stronger but weak effect on forest productivity that a decrease in soil moisture. Forest productivity was more sensitive to inter-annual variation in climatic conditions, as we observed large decreases in productivity in warmer and drier years when compared to the effect of the experimental treatments. In Chapter 3, I performed a data synthesis to study how environment and evolutionary history shape plant hydraulic trait variation among evergreen and deciduous species across the world tropics. Through this pantropical data synthesis, I was able to point out biases in the assessment of tropical plants drought tolerance and that environment play a stronger role in shaping plant hydraulic traits than species’ evolutionary history. I also demonstrated that deciduous species tend to have similar vulnerability to drought across environmental conditions, while evergreen species hydraulic traits vary as a function of water availability. Collectively, these three studies help us to understand the drivers of drought tolerance in tropical plants and how environmental variation shapes responses to drought in the tropical forests.