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Seed Dispersal and Dispersal Syndromes in Manzanitas, and Other Higher Plants
AdvisorVander Wall, Stephen B
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Dispersal is an emergent property of biological systems existing in a spatial world. Specifically, if rules governing autonomous biological agents are to (i) acquire and utilize resources to respectively (ii) metabolize and reproduce, then space is a limiting factor in areas with finite resources. Further, if parents have a greater ability to acquire resources than filial generations, only offspring of the ultimate reproduction event would have a survival probability greater than zero. Therefore, we observe movement away from parents (dispersal) as a ubiquitous phenomenon observed in all biological systems that increases the ability of biological agents to acquire resources and reproduce.From a perspective of multiple parents reproducing over time in a finite area (i.e., an ecological population) dispersal is a collective property that is generally relative to two factors: (i) population turnover (i.e., availability of previously used resources [occupied patches]) and (ii) environmental heterogeneity (i.e., availability of new resources [new patches]). If fecundity is normalized, dispersal distributions (frequency or density distribution as a function of distance expressed in one or two dimensions) are examined as the kurtosis of the sum of all parents' offspring individually or collectively. Further, dispersal metrics (kurtosis of individuals or populations) can be measured demographically where individuals are the unit or genetically where genes from an individual contribute to the gene pool.To disperse, organic life has evolved an array of mechanisms (e.g., structures like flagella, fins; behaviors like density dependent dispersal) at all life stages (e.g., diploid zygotes or diaspores, haploid gametes, reproductive adults, non-reproductive juveniles). In the seed plants (spermatophytes) that macroscopically dominate and energetically support virtually all terrestrial ecosystems through primary productivity, dispersal can occur at two life stages: haploid pollen dispersal and diploid seed dispersal. The outcome of each type of dispersal is different, with pollen being of uniparental (paternal) origin and strictly contributing to genetic (gene flow) but not demographic dispersal, and seeds being of biparental origin and contributing to demographic and likely genetic dispersal if ecological establishment it achieved.Plants have evolved different means of dispersing seeds, generally by adaptation to abiotic and biotic vectors. Abiotic vectors include fluids like air and water, and biotic vectors include a wide variety of animal taxa. Animals generally disperse seeds through digestion, through incomplete recovery of stored food, and through discarding seeds after they have been dispersed intentionally or incidentally by attachment. Adaptations to various modes dispersal by plants result in suites of traits that match the behavior, physiology, or morphology of the vectors, and referred to as dispersal syndromes.This core of this dissertation examines dispersal syndromes across different scales of biological organization. The first chapter attempted to determine how a single species of plant (greenleaf manzanita, Arctostaphylos patula) with an ambiguous dispersal syndrome is dispersed. I conducted a series of experiments to determine how the species' seeds are dispersed and what are the consequences of that particular mode of dispersal. The main conclusion redrew the boundary of how a scatter-hoarding syndrome is understood. Specifically, seeds need not be fused and can still be smaller than what has traditionally been understood as a nut syndrome dispersed by scatter-hoarding animals. Curiously, there is a relatively small endosperm reward, which should warrant further investigation by those attempting to study scatter-hoarding syndromes. A secondary main conclusion is that this syndrome appears to benefit seeds in fire-prone ecosystems because scatter-hoarding immediately deposits the seeds under the soil, and area that buffers the heat of fire. This is opposed to passive or abiotic dispersal, where seeds are thought to self-incorporate; the rates of which are highly variable and need to be studied in greater depth.The second chapter expands beyond the single-species perspective and used a morphometric analysis to compare the focal species of chapter one (greenleaf manzanita, Arctostaphylos patula) to other species in the same genus (manzanitas, Arctostaphylos). Because I had studied one species in depth, I conclude with high certainty that a number of species in the same genus are very likely to have the same mode of dispersal; namely, a scatter-hoarding syndrome. I additionally examined seed and fruit characteristics (diaspores) to find patterns across environmental gradients that are known to affect diaspore morphology, and only one was found (elevation). I compared manzanita seed mass to another, similar clade of sympatric plants (Ceanothus) and found that Ceanothus followed the predicted patterns and manzanita did not. This leads me to conclude that other factors are responsible for the diaspore morphology in manzanitas, and the hypothesis remains that it is animal-mediated seed dispersal.Lastly, in chapter 3, I scaled-out further and examined patterns of seed dispersal syndromes across a continent, with specific foci on patterns of distribution of all plant species, plant species without mutualist seed dispersers, plant species with mutualist seed dispersers, plant species distributions of the three major modes of animal dispersal (frugivory, scatter-hoarding, myrmecochory), and the differences between species and interactions across several environmental gradients at a large spatial scale. This was the largest investigation into seed dispersal syndromes of its kind. I not only found spatial patterns of different kinds of seed dispersal syndromes, but I found provocative patterns along environmental gradients, such as relatively fewer seed-dispersal mutualisms at higher elevations. One of the findings that I believe will resonate the most with the scientific community was our use of interaction abundance and diversity. I used what I believe to be a novel method that retains a maximal amount of information to describe patterns of interaction diversity. The most interesting finding of which is that there is a stronger relationship between interactions and latitude than species and latitude.