Browsing by Author "Morris, William F"

Alien species are considered one of the primary threats to native plant populations and their control is often prominent among proposed management actions. While negative alien effects are well documented, there are also many ways that alien species can have positive effects on native plant populations that may actually contribute to their persistence. Moreover, the effect of alien species on native plants can change in magnitude and direction over varying abiotic conditions. The success of native plant populations is determined by a mix of ecological and genetic factors. Alien (and native) species and abiotic conditions could also drive selection of plant traits. In order to understand the drivers of native plant population success in the face of changing climate and increasing prevalence of alien species, it is vital to understand the relationship between genotype, phenotype, and fitness of native plants. In chapter one, I quantified the effects of neighboring alien and native plants on all demographic rates in a population of the Hawaiian endemic plant Schiedea globosa, performing biannual censuses for 4 years to encompass relatively harsh and as well as benign seasons and years. The effects of alien neighbors were mixed but most often positive across many demographic rates in both harsh and more benign abiotic conditions, suggesting that alien neighbors benefit S. globosa plants through multiple mechanisms, such as nurse plant effects and associational resistance. The effects of heterospecific native neighbors were less often positive, indicating fundamentally different effects of native and alien neighbors on the demography of the focal native. These mixed effects highlight the need to consider potential benefits of alien species in the management of threatened native plants and that those benefits may be altered by changing abiotic conditions. In chapter two, I constructed population models for multiple Schiedea species across populations and years, using demographic rate regressions driven by the effects of alien and native neighbors, integrating the mixed effects of alien and native species on demographic rates of populations to project the net effect on population growth of native populations. The effects of alien and heterospecific native plant neighbors were mixed but most often positive across many demographic rates in both harsh and more benign abiotic conditions, suggesting that alien and native neighbors benefit native plants through multiple mechanisms, such as nurse plant effects and associational resistance. The effect of alien and heterospecific native neighbors on population growth was generally positive-- the mixed, but largely positive, net effects of alien and native neighbors on population growth highlight the need to consider potential benefits of alien, as well as native, species in the management of at-risk native plant populations, and that those benefits may be altered by changing abiotic conditions, as indicated by differing effects across (and within) years and populations. In chapter 3, I used paternal half-sibship pairs to measure the heritability of morphological traits under field conditions of the critically endangered Schiedea adamantis, which were found to be heritable in prior studies in greenhouse conditions, in reintroduced populations. I also performed a selection analysis, regressing fitness components against traits of outplants that I hypothesized might influence response to climate and alien and native neighbors to study the relationship between genotype, phenotype, and fitness of plants in restoration outplantings and assessing potential for evolutionary rescue. I found no significant heritability of any of the morphological traits. I did find evidence of selection, as leaf shape, area, and whole plant morphology had significant effects on fitness components (growth, survival, and reproduction), and significant interaction effects showing traits influenced fitness components differently at different levels of shade. Together, these results suggest that while variation in traits benefit individual plants in differing field conditions, these outplantings may not have the ability to respond to selection through evolution.

Phenology, the timing of biological events across the year, is shifting in response to climate change. Not all species within a community are responding to the same environmental cues by shifting their phenology to the same degree. As a consequence, the strength and direction of species’ interactions are also undergoing rapid changes. In this dissertation, I used observations, experiments, and demographic modeling to explore the relationship between climate, phenology, and species interactions in one terrestrial and one aquatic field system. With these two systems, I attempted to answer the central question, “What are the environmental drivers and ecological consequences of phenological shifts?”

In chapter two, I examined how co-flowering subalpine spring ephemerals (Claytonia lanceolata, Mertensia brevistyla, and Mertensia fusiformis) in the Colorado Rocky Mountains, USA compete with and facilitate one another for biotic and abiotic resources. These flowering species were the first to emerge and flower shortly after snowmelt. As a result of phenological shifts, these species showed greater temporal overlap under early snowmelt conditions. Using field experiments, I found that these species did not facilitate one another or compete for pollinator resources, but they did affect each other’s vital rates in two years.

In chapter three, I simulated how the presence of neighbors, including all heterospecific neighbors, affected population growth of Claytonia and Mertensia under future predictions of spring snowmelt date. I used data from field experiments to parameterize integral projection models and forecast population growth into the future. I found that neighbors significantly influenced population growth rate under average snowmelt conditions, but not under early snowmelt conditions. Under future predictions of early snowmelt, populations declined rapidly regardless of neighbor presence.

In chapter four, I worked with the Northeast Climate Adaptation Science Center to determine the environmental drivers of alewife (Alosa pseudoharengus) migration in Massachusetts. First, I used field-collected daily fish counts to assess how the timing of migration has changed. I found that some of the streams showed significant advances in run timing, while others did not. Second, I combined migration timing metrics with publicly available climate data. I found that shorter, wetter winters and mild spring temperatures were correlated with earlier run initiation dates.

Collectively, my research from subalpine meadows and coastal streams revealed three significant conclusions. First, the temporal variability in the strength and direction of species interactions may be important for predicting future coexistence. Second, unequal phenological shifts between members of a community may not only impede coexistence, but could facilitate coexistence in the future through positive effects on population growth. Finally, experiments mechanistically linking phenology and species interactions are necessary for understanding implications of phenology on coexistence.

Climate change is expected to increase climate variability over much of the world, including the timing and magnitude of weather events such as droughts and heat waves. Although ecologists have made great strides in quantifying how climate conditions at annual or longer timescales regulate populations and communities, the role of individual weather events and intra-annual variability is still relatively unknown. In this dissertation I seek to quantify how the Chihuahuan desert summer annual plant community responds to intra-annual variability, and in doing so better understand how biodiversity is maintained in this community and how this community may respond to climate change.

In chapter one, I present a general approach to quantify how plants respond to short-term variability. Using this approach, I find that species in this community show an environmentally dependent life history tradeoff between growth in wet conditions and survival in dry conditions. This tradeoff could have important implications for both species coexistence and responses to climate change, as I show in chapters two and three.

In chapter two, I seek to understand how a diverse community of annual plants is maintained in an environment with a single primary limiting resource, water. I show that the tradeoffs found in chapter one and environmental variability lead to conditions in which each life history is favored. The differences in life history are explained in part by tradeoffs in balancing carbon uptake and water use.

In chapter three I forecast how this community of annual plants may respond to climate change. I find that while increasing variability is likely to lead to reduced growth, survival, and reproduction in almost all species, it favors species with slower growth in wet conditions but and high survival in dry condition which do comparatively better in increasingly variable conditions.

The introduction of the North American beaver (Castor canadensis) to Tierra del Fuego is a widely known example of a successful biological invasion. Beavers have impacted enormously the biodiversity of the island of Tierra del Fuego, and they are now spreading northward on the continent, prompting the governments of Argentina and Chile to seek methods to control their spread. Beavers first established in forests, where they were initially introduced, but by the 1990s they began to establish in the adjacent steppe. In this dissertation, I study the biology of invasive beavers across the two major habitat types in Patagonia and attempt to develop modeling tools that might be useful to manage their spread.

In chapter one I studied the history of the beaver introduction in Patagonia and provide evidence that the beaver introduction occurred as a single release event of 20 beavers from northern Manitoba, Canada. This not only clarifies the origin of the invasion, but also suggests that the beaver population of Patagonia descends from a smaller number of individuals than previously assumed.

In chapter two I studied the demography of invasive North American beavers in the two contrasting habitat types of the island of Tierra del Fuego, forest and steppe. Habitat differences can affect vital rates which may in turn impact the speed of the invasion, but this has been rarely addressed when managing the spread of invaders. I use repeated observations, mark-resight methods, telemetry and camera traps to estimate colony size and vital rates of beavers in the two habitats. Colony size and the number of offspring (“kits”) produced per colony per year were higher in the steppe, contrary to the belief that forest is better habitat. Here I suggest this may be the result of the longer time since invasion in the forests of Tierra del Fuego and that the forest subpopulation is showing density dependent regulation. Beaver survival was high in all age classes and was higher than survival rates recorded in North America. My work shows that beaver plasticity and predator release have likely facilitated the invasion in Patagonia.

In chapter three, I investigated the more recent invasion of beavers in an area of the Patagonian steppe. I utilized repeated high resolution satellite images to identify beaver ponds, and used them to study changes in beaver abundance and habitat use over time. The number of beaver ponds increased 85 % between 2005 and 2014. During this period, beavers changed their habitat selection pattern, presumably as a response to increased density. Beavers established on small watercourses in canyons first, but as more canyons became occupied over time, beavers moved to less preferred watercourses in plains and U-shaped valleys. Potential new beaver colonies established close to existing beaver ponds, suggesting proximity to a beaver pond is an important determinant of beaver colonization. Identifying habitat preferred by beavers in the steppe could help to increase early detection of the invader at the invasion front. This work highlights the importance of the use of high resolution remote sensing technologies to better understand and monitor biological invasions.

Finally, in chapter four, I built a spatially explicit individual-based model parameterized with data I collected in the field and use it to make management recommendations. Specifically I assessed the efficacy of a potential management strategy in which a “fire-break” (a zone beyond the current population front in which beavers are removed) perpendicular to the population front is instituted to attempt to prevent further northward spread of the beaver in continental Patagonia. I found that even a 100 km wide firebreak is insufficient to contain the spread of beavers, long dispersal events being the major cause of this failure. Further, I found that increasing the fraction of beavers culled within the firebreak does not decrease either the arrival time or the number of beavers that cross the firebreak. Counterintuitively, my model indicates that moderate levels of culling within the firebreak (rather than high) may be a more effective method to manage the invasion, likely as a result of inversely density dependent dispersal.

How genetic variation is maintained in the face of persistent natural selection is a central question in evolutionary biology. Here, I leverage a focal polymorphism, leaf chemical profile in a perennial wildflower (Boechera stricta, Brassicaceae) to investigate the ecological and genetic mechanisms that may influence the maintenance of variation in this trait.

In the first chapter, I present data from a suite of common garden and greenhouse experiments showing that the alleles underlying variation in chemical profile have contrasting fitness effects across environments. I identify two putative selective drivers on chemical profiles, and utilize phenotype-environment associations and molecular genetic analyses to test for additional evidence of past selection by these drivers. Together, these data are consistent with balancing selection on chemical profile, likely caused by pleiotropic effects of genes that influence secondary chemical biosynthesis on herbivore defense and drought response.

In the second chapter, I utilize a multi-year, manipulative field experiment to test for the effects of variation in selective drivers on genotypes conferring contrasting chemical profiles. I integrate variable effects of the environment on fitness components across life history and across environments to assess patterns of lifetime fitness. These data suggest that environmental conditions in which contrasting genotypes can both persist may be widespread.

Together, these chapters provide complementary perspectives on the question of persistent natural variation, suggesting that variation in secondary metabolic profiles in B. stricta may persist at present due to balancing selection, and may continue to persist in the future under variable environmental scenarios.

Plants commonly produce multiple, seemingly redundant defenses, but the reasons for this are poorly understood. The specificity of defenses to particular herbivores could drive investment in multiple defenses. Alternatively, genetic correlations between defenses could lead to their joint expression, even if possessing both defenses is non-adaptive. Plants may produce multiple defenses if putative resistance traits do not reduce damage, forcing plants to rely on tolerance of damage instead. Furthermore, resource shortages caused by herbivore damage could lead to compensatory changes in expression and selection on non-defense traits, such as floral traits. Natural selection could favor producing multiple defenses if synergism between defenses increases the benefits or decrease the costs of producing multiple defenses. Non-linear relationships between the costs and benefits of defense trait investment could also favor multiple defenses.

Passiflora incarnata (`maypop') is a perennial vine native to the southeast United States that produces both direct, physical traits (leaf toughness and trichomes) and rewards thought to function in indirect defense (extrafloral nectar in a defense mutualism with ants), along with tolerance of herbivore damage. I performed two year-long common garden experiments with clonal replicates of plants originating from two populations. I measured plant fitness, herbivore damage, and defense traits. I ran a genotypic selection analysis to determine if manipulating herbivore damage through a pesticide exclusion treatment presence mediated selection on floral traits, and if herbivore damage led to plastic changes in floral trait expression. To evaluate the role of selection in maintaining multiple defenses, I estimated fitness surfaces for pairwise combinations of defense traits and evaluated where the fitness optima were on each surface.

I found that resistance traits did not reduce herbivore damage, but plants demonstrated specific tolerance to different classes of herbivore damage. Tolerance was negatively correlated with resistance, raising the possibility that tolerance of herbivore damage instead of resistance may be the key defense in this plant, and that production of the two type of defense is constrained by underlying genetic architecture. Plants with higher levels of generalist beetle damage flowered earlier and produced proportionally more male flowers. I found linear selection for both earlier flowering and a lower proportion of male flowers in the herbivore exclusion treatment. I found that selection favored investment in multiple resistance traits. However, for two tolerance traits or one resistance and one tolerance trait, investment in only one trait was favored.

These results highlight the possibility of several mechanisms selecting for the expression of multiple traits, including non-defense traits. Resistance traits may have a non-defensive primary function in this plant, and tolerance may instead be a key defense strategy. These results also emphasize the need to consider the type of trait--resistance or tolerance--when making broad predictions about their joint expression.

Population models are powerful tools that have long been used to explore the interplay between population dynamics and environmental drivers, including human-induced changes in environmental conditions. Yet these models often require simplifying assumptions that have the potential to break down when the relationships between demographic vital rates (survival, growth, reproduction, and recruitment) and drivers become complex or when multiple co-occurring drivers interact. As humans continue to alter global ecosystems, there is a growing need to ensure that models are able to accurately capture population responses to increasingly complicated driver scenarios, including the potential for nonlinear relationships and nonadditive interactions among co-occurring drivers. In this dissertation, I explore this question of model accuracy in three ways. First, I (along with collaborators) extend a common post-hoc analysis tool – the life table response experiment – to incorporate potential second-order population responses to drivers and compare the resulting accuracy of this extended approach to the standard method that assumes only linear relationships. Second, I combine global climate model projections, a climate vs. wildfire model, and empirical demographic data for a threatened North Carolina endemic plant (mountain golden heather; Hudsonia montana) in order to assess the extent to which climate change influences population dynamics directly, as well as indirectly by altering wildfire probability. Finally, I examine the outputs of a handful of population models built using either different underlying data or different assumptions in order to characterize how management recommendations can be shaped by various aspects of data limitation. The results of the first two chapters demonstrate that extending models to account for more complex drivers and population responses can both increase the accuracy of population projections and improve the quality of our inferences about the underlying sources of observed population changes in response to shifting environmental conditions. On the other hand, the results of the third chapter reveal the extent to which modeling output is contingent on the types of assumptions necessitated when data are sparse and highlights the need to balance preconceptions of optimal management with the realities of scientific uncertainty.

Although 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.