Browsing by Subject "Mutualism"
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Item Open Access Broad Scale Conservation: Protected Areas and Species Interactions(2009) Joppa, Lucas N.This dissertation consists of four chapters. The first three chapters examine protected areas (or parks) from multiple perspectives. Parks are the first, and often only, line of defense in efforts to conserve biodiversity. Understanding of their promise and problems is necessary to achieve conservation outcomes. Chapter One determines vegetation patterns in and around parks of differing management categories across the Amazon, Congo, South American Atlantic Coast, and West African forests. Within these forests, protected areas are the principle defense against forest loss and species extinctions. In the Amazon and Congo, parks are generally large and retain high levels of forest cover, as do their surroundings. In contrast, parks in the Atlantic Coast forest and West Africa show sharp boundaries in forest cover at their edges. This effective protection of forest cover is partially offset by their very small size: little area is deep inside park boundaries. Compared to West Africa, areas outside parks in the Atlantic Coast forest are unusually fragmented.
Chapter Two addresses a human dimension of protected areas. Given certain characteristics, parks areas may either attract or repel human settlement. Disproportionate increases in population growth near park boundaries may threaten their ability to conserve biodiversity. Using decadal population datasets, we analyze population growth across 45 countries and 304 parks. We find no evidence for population growth near parks to be greater than growth of rural areas in the same country. Furthermore, we argue that what growth does occur near parks likely results from a general expansion of nearby population centers. Parks may experience unusual population pressures near their edges; indeed, individual case studies provide examples. There is no evidence, however, of a general pattern of disproportionate population growth near their boundaries.
Chapter Three provides a review of common approaches to evaluating protection's impact on deforestation, identifies three hurdles to empirical evaluation, and notes that matching techniques from economic impact evaluation address those hurdles. The central hurdle derives from the fact that protected areas are distributed non-randomly across landscapes. Matching controls for landscape characteristics when inferring the impact of protection. Applications of matching have revealed considerably lower impact estimates of forest protection than produced by other methods. These results indicate the importance of variation across locations in how much impact protection could possibly have on rates of deforestation.
Chapter Four departs from the focus of protected areas and instead addresses a more theoretical aspect of community ecology. Ecological theories suggest that food webs might consist of groups of species forming blocks, compartments or guilds. Chapter Four considers ecological networks (subsets of complete food webs) involving species at adjacent trophic levels. Reciprocal specializations occur when (say) a pollinator (or group of pollinators) specializes on a particular flower species (or group of such species) and vice versa. We characterize the level of reciprocal specialization for various classes of networks. Our analyses include both antagonistic interactions (particularly parasitoids and their hosts), and mutualistic ones (such as insects and the flowers that they pollinate). We also examine whether trophic patterns might be palimpsests. That is, there might be reciprocal specialization within taxonomically related species within a network, but these might be obscured when these relationships are combined. Reciprocal specializations are rare in all these systems even when tested using the most conservative null model.
Item Open Access The Ecology of Sharing Mutualists: Consequences for Plant Performance and Population Dynamics(2010) Fleming-Davies, Arietta EliseAlthough we often study mutualisms (interactions in which both species benefit) at the level of the individual partners, mutualistic interactions take place in the context of populations and communities. Sharing mutualists with others in a population could result in indirect interactions in the form of mutualist-mediated competition or facilitation. In my dissertation work I asked whether intraspecific competition or facilitation for ants might occur in an extrafloral nectary-bearing (EFN) plant, and what the consequences would be for long-term population dynamics of the plant. My focal species was Colubrina spinosa (Rhamnaceae), a neotropical treelet on which I observed 69 ant species at La Selva Biological Station, Costa Rica.
Demonstrating intraspecific competition for mutualists requires that 1) neighbor densities affect mutualist visits to an individual, and 2) change in mutualist visits results in reduced benefit. To determine how mutualist density affects plant benefit, I experimentally manipulated ant abundances on plants over two years and measured growth and survival. To assess competition for mutualists, I excluded ants from conspecific neighbors and followed ant abundance on focal plants. To consider long-term facilitation, in which greater local nectar resources increase local ant abundance, I manipulated nectar resources in a two-year field experiment and estimated ant abundance on C. spinosa plants and on baits.
Considering local neighbor density both within a 1m radius and in 5x5 m plots, ant densities on C. spinosa plants showed evidence for a small-scale competition effect and a contrasting plot-level facilitation effect. The small-scale competition was sized-based; smaller plants lost ants to larger plants. Ant benefit to plants also depended on plant size. For larger plants, those with greater size-adjusted ant density had higher growth and survival than those with fewer ants than expected for their size.
To determine whether these contrasting competition and facilitation effects could impact population growth or densities, I modeled population dynamics with an integral projection model (IPM). Growth and survival were functions of ant density, which in turn depended on conspecific neighbors, plant size, and mean background ants. Results suggest that larger-scale facilitation of mutualists impacts long-term population growth more than small-scale competition. Population growth rate increased with increasing background ant density, which depended on facilitation at the 5x5m plot scale. In contrast, small-scale competition caused a redistribution of mutualist ants among plants of different sizes, but had very little effect on long-term population growth.
I thus conclude that on the scale of individuals there is evidence of intraspecific competition for ants as well as facilitation in the EFN plant C. spinosa, but only facilitation effects lead to appreciable changes in population dynamics. If mutualist-mediated facilitation effects tend to occur over long time scales in other systems as well, facilitation might prove to be more important than competition in other mutualisms.
Item Open Access The Visual Ecology of the Cleaner Shrimp-Client Fish Mutualism(2018) Caves, Eleanor MaryCleaner shrimp are small, brightly-colored tropical crustaceans that attract reef fish clients to set locations on a reef called cleaning stations and then "clean" them by removing ectoparasites from their scales, gills, and mouths. Because clients benefit from the removal of potentially harmful parasites, and shrimp benefit from a meal, this interaction is considered mutualistic. The evolution of cleaner-client relationships is especially paradoxical, however, given that crustaceans make an easy meal for many reef fish. How did such an interaction arise and evolve? Additionally, how do cleaner shrimp and clients find and recognize each other, and why doesn't the client eat the cleaner? One hypothesis is that mutualistic partners evolve signals that identify them as beneficial partners. For the cleaner shrimp-client fish mutualism, it has been suggested that these signals are visual, and function to identify cleaners as helpers, rather than food, and perhaps to identify clients as seeking cleaning rather than a meal.
The broad goal of this dissertation was to examine the cleaner shrimp-client fish mutualism by combining visual physiology, visual ecology, and animal behavior. Throughout, I focus on visual acuity-the ability to perceive detail-an underexplored aspect of visual capability which should represent an important selective force on signals, as it determines what visual information can and cannot be resolved. To begin, in Chapter 2, I characterized the visual capabilities of cleaner shrimp for the first time, in particular examining spectral sensitivity, visual acuity, and temporal resolution of three species from the three primary genera where cleaning has arisen. This revealed that these cleaner shrimp have monochromatic, coarse vision, and thus that their color patterns likely do not serve an intraspecific signaling purpose, but rather may be part of signals directed at client fish. Thus, Chapter 3 examined visual acuity in fish by performing a literature synthesis of known visual acuity values across fishes and examining how acuity relates to certain aspects of morphology and ecology. Here, I found that acuity is higher in fish with larger eyes and in fish that live in spatially complex habitats.
Having examined the visual capabilities of both parties in the cleaner-client mutualism, I then explored signaling between cleaners and clients. In chapter (4), I used sequential analysis to demonstrate that certain stereotyped motions by cleaner shrimp are signals directed at clients, and provide the first evidence of potential signals on the part of client fish as well. Additionally, I developed an R package, AcuityView (Appendix A) which displays images with only the spatial information available to a receiver of given acuity from a given distance, and used it to show that cleaner and client signals are visible to their intended receivers. Lastly, in Chapter (5), I outlined what we know about acuity across species, provide primers on the anatomical basis for acuity and what factors can make acuity higher, and put forward specific predictions and hypotheses about how receiver acuity may influence signal form.