The Effects of Bird and Bat Arthropod Predation on Sapling Black Cottonwoods in the Context of Restoration – Sacha Heath (Ecology) Phytophagous arthropod consumption by birds and bats can induce cascading effects that reduce plant damage or accelerate plant growth. We examined tri-trophic interactions between birds and bats, arthropods, and sapling black cottonwoods (Populus balsamifera ssp. trichocarpa) along two streams undergoing restoration at Mono Lake, California, USA. We built mesh cages (exclosures) over black cottonwood saplings to remove the direct effects of bird and bat predation on arthropods. Over 12 weeks during the growing season, we compared arthropod density and biomass, herbivore-caused foliage damage, and shoot growth between saplings inside the exclosures and paired control saplings to which birds and bats had access. Avian foraging surveys conducted in the riparian vegetation along the streams revealed that 49 percent of foraging attacks were made in black cottonwoods compared to other substrates but that only four percent of black cottonwood foraging attacks occurred in saplings comparable in height to experimental saplings. Contrary to predictions, bird and bat predation did not significantly reduce the density or biomass of arthropods, even when analyses were restricted to certain arthropod groups (i.e., herbivores, predators and parasitoids, leaf chewers). It followed that bird and bat exclusion did not significantly increase herbivore-caused leaf damage or decrease sapling shoot growth.
Resistance of N and P cycling microbes to environmental perturbation – Priyashiela Singh (Soil and Biogeochemistry) The relationship between agricultural management, and resistance and resilience of microbial communities in agricultural soils when exposed to a severe environmental perturbation is key to long-term agricultural sustainability. Agricultural management practices generally have large impacts on soil through physical disturbance, inputs of fertilizers and pesticides, and cultivation of monoculture or low-diversity plant systems. Resistance of soil microbial communities to disturbance events is a topic of growing importance with predicted rising temperatures and large unpredictability in rainfall patterns associated with global climate change. Diverse microbial communities are essential for the sustainability of agriculture. Previous research has focused on the resistance of soil systems in relation to total microbial biomass but has ignored relationships with specific functional groups of microbes. Nitrifiers and denitrifiers are key organisms in N cycling and these organisms control the pools of plant-available N in soil, while alkaline phosphatase is a key microbially produced enzyme involved in the regulation of pools of available phosphate. In this study, numbers of nitrifying and denitrifying genes as well as gene involved in alkaline phosphatase production were quantified after subjecting different managed agricultural soils to severe temperature perturbation.
2012 Symposium Presentation Abstracts
The effects of soil inoculation and topsoil depth on grasses at a mercury mine restoration site – Taraneh Emam (Ecology) Former mine sites can be particularly challenging to restore, as they are often contaminated with heavy metals and devoid of sufficient topsoil, nutrients, and soil biota. I applied two treatments commonly used in mine restoration, topsoil addition and soil inoculation, to a grassland site containing tailings from a former mercury mine. Varying depths of stockpiled topsoil were placed atop mine tailings to simulate possible reclamation practices. Subplots containing the topsoil depth gradient were then inoculated with either arbuscular mycorrhizal (AM) inoculum from a commercial source, rhizosphere soil from nearby native grassland, or were left uninoculated as a control. Results to date show that colonization of grasses by AM fungi was highest at both shallow and deep extremes of the topsoil depth gradient, but did not significantly differ by inoculum type. Commercial inoculum had little effect on grass biomass and density, and non-native grasses were more responsive to both inoculum types than native grasses. These findings indicate that caution must be exercised when using mycorrhizal inoculum for restoration purposes. In addition, topsoil depth may affect interactions between native plants, non-native plants, and the soil community in complex ways.
Evidence for a marine tri-trophic interaction mediated by an induced defense: examining 50+ years of diet data in a Southern Ocean seabird assemblage – Matthew Savoca (Ecology) Trophic cascades are frequently documented in freshwater ecosystems, but less so in marine environments. In particular, evidence for a trophic cascade mediated by an induced plant volatile in a marine environment has never been substantiated. Dimethyl sulfide (DMS) is a plant-derived, volatile chemical abundant in the world’s oceans that has importance in global climate regulation and interspecific biological interactions. Using sensory and diet data collected on 18 species of Antarctic and sub-Antarctic Procellariiform seabirds over the course of 50 years, I conducted a meta-analysis to test for a potential tri-trophic interaction with DMS as the mediating infochemical. Some seabird species use DMS released by depredated phytoplankton as a cue to find hotspots of their crustacean prey. I predicted that the diets of DMS-tracking species would contain significantly higher proportions of crustacea (including euphausiids, copepods, decapods, and other zooplankton) than those of non DMS-tracking species. Indeed, I found that the diets of DMS-tracking species contain significantly higher proportions of crustacea. Additionally, I also found that smaller, more maneuverable species had more crustacea in their diets than did larger, less agile species. The maneuverability of smaller DMS-tracking species enables them to follow discrete odor patches to their source and to exploit foraging sites before being outcompeted by larger, more aggressive species. I will also discuss potential benefits of DMS production to phytoplankton: aside from reducing predation through a trophic cascade, it may encourage reverse nutrient cycling – the fertilization of the surface layer of the ocean by seabird excrement from dense feeding flocks.
Arthropod mesograzers control algal biomass in a temperate seagrass bed – Matthew Whalen (Ecology) A long-standing challenge in ecology, with increasing importance to management, is to understand how bottom-up and top-down processes interact to regulate the structure and dynamics of ecological systems. In coastal marine food webs, small invertebrate herbivores (mesograzers) have long been hypothesized to occupy a key position facilitating dominance of foundation macrophytes by grazing competitively superior algae. But, because of the difficulty of manipulating these small and abundant animals in the field, their impacts on community organization remain largely unknown. Understanding mesograzer impacts has taken on increasing urgency in seagrass systems due to their global decline, caused in part by widespread eutrophication favoring seagrass overgrowth by faster–growing algae. Using novel, cage-free field methods we present rigorous experimental confirmation that mesograzer reduction promotes algal blooms and thereby seagrass decline, whereas nutrient addition stimulated algal blooms only in the absence of mesograzers. Using structural equation modeling, we highlight the strong top-down role of mesograzers in the three-way interaction between seagrass, algae, and mesograzers. Our results confirm that mesograzers can be key mutualists controlling proliferation of nuisance algae under eutrophication, and thus indirectly facilitating dominance of habitat-forming macrophytes.
Dearest neighbors: adaptive nest clustering in waterfowl – Kevin Ringleman (Ecology) Ground-nesting waterfowl are non-territorial, and researchers have anecdotally noted that nests appear to be clustered in space and time. However, local nest clustering and its adaptive significance have never been subjected to a rigorous analysis. We used three years of waterfowl nesting data (2,038 nests) from Grizzly Island, CA to study nest clustering at small spatial and temporal scales. Our results show that, in all three years, simultaneously active nests were clustered at local spatial scales (50-400 m). Furthermore, nests with closer nearest neighbors were more likely to successfully hatch, a result that challenges the commonly held belief that nest predation in waterfowl is positively density-dependent (i.e. clusters of nests attract predators). In an attempt to better understand how clusters of nests are created, we examined the relative influence of vegetation and social cues (conspecific attraction) on habitat selection decisions. Our results suggest that in a year when vegetation cues were unreliable, waterfowl relied on presence of conspecifics to choose nest sites. This is the first study to suggest that social cues may, in some cases, be more important than environmental variables in waterfowl nest-site selection.
Postfire Forest Recovery in the Sierra Nevada – Kevin Welch (Ecology) Due to fire suppression policies and other management practices over the last century, many low- to mid-elevation forest types in the Sierra Nevada have accumulated high fuel loads that promote stand-replacing high-intensity fires. Current and future projected trends in climate are predicted to increase the occurrence of such fires. We established over 1,000 plots in a range of elevations, environments, forest types, climate zones and fire severity classes to provide insight into the factors that promote natural tree regeneration after wildfires, the limiting factors in species establishment, and the differences in post-fire responses of conifers and hardwoods. We employed a standardized protocol that measured site characteristics, seedling densities, and woody plant growth. Preliminary results reveal that fire severity generally has a unimodal relationship with rates of natural regeneration, although effects of site and local environment act to modulate the shape of the relationship. Above low to moderate severities, natural regeneration rates of all tree species decrease with increasing severity, possibly due to a combination of factors including seed mortality, increasing distance to the nearest living seed tree, and more severe microclimatic conditions. Understanding forest recovery and regeneration processes after high severity fires is critical to appropriately applying management strategies on National Forest lands.
Weed Profiling: A Molecular Phylogenetic Approach to Darwin’s Naturalization Hypothesis in Asteraceae – Daniel Park (Plant Sciences) Invasive species have enormous ecological and economic impacts, making the ability to understand and predict the invasiveness of species of great import. Furthermore, rare and/or endangered species seem to be particularly impacted by invaders. According to the California Natural Diversity Database, 181 of the state’s rare plant species are under threat from invasive weeds, and it has been surmised that nearly half the species on the Threatened or Endangered species lists are at risk primarily due to alien-invasive species . Once exotic plant species are established in a new region, they are frequently proven extremely difficult to control. Hence, identifying and preventing potentially invasive exotic species from reaching ground zero is the most economically and environmentally desirable management method. Here we explore the use of phylogenetic distance as a quantifiable measure of predicting invasiveness, using the weedy cosmopolitan family Asteraceae as a model system. Phylogenies representing the entire diversity of the thistle tribe, Cardueae were generated. Patterns within this monophyletic group suggest that exotic plants more closely related to natives are more likely to become invasive. Such molecular phylogenetic research can further our understanding of biological invasions, pertaining to developing a predictive framework for screening potentially invasive taxa.
Dispersal limitation of tropical vertebrate seed-dispersers affects plant community composition – Lydia Beaudrot (Ecology) We examined communities of birds, butterflies, primates and trees in protected areas across Uganda. Our objective was to evaluate the relative importance of dispersal limitation and species sorting along environmental gradients in structuring these communities. We conducted partial Mantel tests to investigate correlations between (1) community similarity and ecological distance while controlling for geographical distance and (2) community similarity and geographical distance while controlling for ecological distance. We found that dispersal limitation strongly structures communities of primates whereas species sorting strongly structures communities of birds, butterflies and trees. Upon closer analysis of tree dispersal modes, we found that wind-dispersed trees also exhibit evidence of significant dispersal limitation but that trees dispersed by animals and other methods do not. We then sought to determine if dispersal limitation of seed dispersers influences plant community composition. Specifically we examined whether trees dispersed by primates, that were strongly dispersal-limited, were more dispersal limited than trees dispersed by birds, which were not dispersal limited. We found consistent trends that primate fruit trees exhibited greater dispersal limitation than bird fruit trees across three fruit color dispersal syndromes. Taken together, our results suggest that the dispersal abilities of primary consumers affect the distribution of primary producers at a macro-ecological scale.
Fuel treatment impacts on wildfire severity and plant communities in dry mixed conifer forests of California – Jens Stevens (Ecology) Fuel reduction treatments in forests of the Sierra Nevada are increasingly implemented as a means to reduce wildfire severity. Evaluations of the effects of fuel treatments on fire severity and plant community composition have thus far focused on modeled fire behavior, or on empirical assessments of one to several of wildfires that burned through treatments. Here I present results from the largest statewide study to date of post-wildfire fuel treatment effects, both on fire severity and on plant community dynamics. We identified 12 wildfires that burned between 2005 and 2011 between through fuel treatments in yellow pine and mixed-conifer forest in California managed by the U.S. Forest Service. At each fire, we used transects that spanned the treatment boundary within the burned area, as well as in adjacent unburned forest, to estimate fire severity, measure tree mortality and recruitment, and sample understory plant diversity. Our data show that, in 10 of the 12 fires, severity metrics were dramatically reduced in fuel treatments. Correspondingly, tree mortality was also much lower in treated areas. We used multivariate ordination analyses to demonstrate that treatments have negligible effects on understory plant species composition, especially compared with the effects of wildfire.
The seasonality of nitrogen cycling in California grasslands – Jill Baty (Ecology) Understanding how key plant traits interact with biotic components of the soil may help managers understand the ways in which plant invasions alter the provision of ecosystem services, including water quality, soil quality and forage quality. In California grasslands, the composition of plant traits has shifted as new species have invaded the landscape over the past 3 centuries. Native perennial grasses take nitrogen up gradually throughout the year; forage annuals (introduced ~200-300 years ago) take nitrogen up before mid-March; invasive weeds take nitrogen up later into the growing season than do forage annuals (Heady et al. 1991, Jackson et al. 1988, Eviner unpublished). In this study, I show that the seasonality of soil food web enrichment does not vary by plant group, nor does the seasonality of when soil nitrogen is available to plants. The resulting temporal mis-match of when nitrogen enriches the soil food web, is available to plants, and is taken up by plants could indicate the potential for plant invasions and nitrogen leaching.
The effect of network clustering on pathogen transmission in Belding’s ground squirrels – Kimberly VanderWaal (Animal Behavior) Here, we use social network analysis to examine patterns of transmission of an environmentally-transmitted parasite, Cryptosporidium spp., in Belding’s ground squirrels (Spermophilus beldingii). Across four time periods, we found that variation in the prevalence of Cryptosporidium in two different colonies was negatively correlated with transitivity, a measure of network clustering, and positively correlated with the percentage of juvenile males. Additionally, network transitivity decreased when there were higher percentages of juvenile males. We suggest that juvenile males are critical in mediating the ability of Cryptosporidium to spread through colonies because their behavior decreases the amount of clustering in the social network.
Sustainable habitat enhancement for burrowing owls: Modifying habitat structure through reestablishment of an ecosystem engineer – Sarah McCullough (Ecology) In San Diego, population declines and local extinctions of Western Burrowing Owl (Athene cunicularia hypugaea) have been recorded in southern and coastal locations, where habitat has been reduced by urban development, exotic species invasions, and increases in fire frequency. This study focuses on the California ground squirrel (Spermophilus beecheyi) as an ecosystem engineer that supports owls through its burrowing and foraging activities. We conducted a field experiment manipulating habitat structure, soil compaction, and squirrel presence. We utilized squirrel translocations and mowing treatments to re-establish and maintain low, open grassland habitat for owls. After treatments, evidence of squirrel activity was almost exclusively found on translocated plots. The number of new burrows was significantly higher in the mowing treatments relative to the control (81% of all burrows, range 68 – 97%). This study represents the first step to adaptively manage grassland habitat as a self-sustaining system of burrows, squirrels, and owls.
Does size matter? Testing the effects of body size and sex on spatial ecology parameters of Northern Pacific Rattlesnakes (Crotalus oreganus oreganus) in Central California – Bree Putnam (Ecology) The spatial ecology of animals is influenced by a combination of factors. Spatial ecology parameters are commonly calculated for rattlesnakes of the genus Crotalus, but they are highly variable within and among populations and species. Many studies show that large male snakes have the largest home range sizes and move the farthest. We quantified aspects of the spatial ecology of a population of Northern Pacific Rattlesnake (Crotalus oreganus oreganus) in the southern part of their range in order to determine the factors that influence movements and home range sizes. We found that males had larger home range sizes, made longer mean daily movements, and moved longer total distances than females for the entire 14-month study. Sex differences in movements were present during the mating season but not the post-mating season, suggesting that mate searching causes males to travel greater distances than females. We also found that within each sex, body size was not related to home range size or movements, but this effect was sensitive to sample size. Our seasonal analysis revealed that in the summer, male body size was negatively related to movements, while female body size was positively related to movements. In the spring, larger males made their first long distance movement earlier than smaller males, suggesting that body size could influence the timing of initiation of mate searching activity. Our study shows that sex may be the most important factor influencing movement patterns in C. o. oreganus, but body size may have an effect at finer scales.
Spill-over, spill-back, wild host migrations and the evolution of chemical resistance in parasites – Jaime Ashander (Population Biology) Outbreaks of sea lice in sea-cage salmon farms result in economic costs and can affect wild salmon populations. Currently, these outbreaks are controlled by chemical treatment. Resistance to this treatment is a growing economic and conservation concern. Another potential interaction between chemical treatment and wild salmon is that migratory salmon bring lice to farms from a large, basin-wide population. In the Pacific, many of these immigrating lice will come from subpopulations that are not selected for resistance. This flow of susceptible genes from lice on wild fish into lice populations on farms could delay chemical resistance in farms, raising the question of whether farm management can affect this dynamic. In order to address this question we have extended an approximation of a basic population that yields time to resistance. First, we assume a constant, yearly immigration of susceptible parasites due to wild host migration and ask how different chemical treatment strategies affect time-to-resistance. Second, we consider whether a decline in migratory wild populations due to sea lice spill-over from the farm, which reduces the number of immigrating sea lice in subsequent years, affects which treatment strategy maximizes time-to-resistance. We find that with yearly migrations, treatments that minimize the parasite population on farms when migration occurs maximize expected time-to-resistance. We also compare treatment strategies based on migration timing to strategies based on average abundance thresholds. We find that timed strategies result in longer time-to-resistance in the presence of migration.