Erin J. Questad

Coexistence through spatio-temporal heterogeneity and species sorting in grassland plant communities

The effect of spatial heterogeneity on species coexistence relies on the degree of niche heterogeneity in the habitat and the ability of species to exploit the available niche opportunities. We studied species coexistence in a perennial grassland, and tested whether small-scale disturbances create environmental heterogeneity that affects coexistence and whether the functional diversity of species in the species pool affects the ability of community composition to reflect heterogeneity through species sorting. We manipulated the spatio-temporal heterogeneity of disturbance and the functional diversity of species added as seed and measured their impact on the spatial turnover of species composition. Disturbance increased environmental heterogeneity and spatial turnover, and the effect of heterogeneity on turnover was greatest in the presence of a functionally diverse species pool, showing the importance of trait variation among species for exploiting environmental heterogeneity, and suggesting that coexistence occurred due to species sorting among heterogeneous niches.

Floristic Quality as an Indicator of Native Species Diversity in Managed Grasslands

Abstract We undertook floristic studies of 104 grasslands in the tallgrass prairie region of Kansas to examine differences in the floristic quality of five common grassland management systems. The different grasslands were warm-season prairie hay meadows, warm-season native pastures, cool-season planted hay fields, cool-season planted pastures, and Conservation Reserve Program (CRP) fields. We recorded 383 vascular plant taxa of which 79% were native and 21% were non-native. Species richness at our sites ranged from seven for a cool season pasture to 109 for a warm-season hay meadow. Our results show that warm-season hay meadows exhibit highest species richness (256 taxa) and are habitats for highly conservative native taxa, while degraded grasslands have a higher number of alien taxa (29% in cool season planted hay fields) and lower species richness (136 for cool season hay fields). We computed Floristic Quality Assessment Index (FQI) values, which ranged from 0.3 for a cool-season pasture to 41 for warm-season prairie hay meadows while modified FQI ranged from 0.09 for a cool-season pasture to 4.48 for a warm-season prairie hay meadow. FQI values across management types differed significantly from each other (p=0.000). We conclude that native prairie hay meadows are significant reservoirs of conservative grassland species. In addition, the FQI can be a useful tool for discerning effects of land management on grassland vegetation.

Remote analysis of biological invasion and the impact of enemy release

Escape from natural enemies is a widely held generalization for the success of exotic plants. We conducted a large-scale experiment in Hawaii (USA) to quantify impacts of ungulate removal on plant growth and performance, and to test whether elimination of an exotic generalist herbivore facilitated exotic success. Assessment of impacted and control sites before and after ungulate exclusion using airborne imaging spectroscopy and LiDAR, time series satellite observations, and ground-based field studies over nine years indicated that removal of generalist herbivores facilitated exotic success, but the abundance of native species was unchanged. Vegetation cover <1 m in height increased in ungulate-free areas from 48.7% +/- 1.5% to 74.3% +/- 1.8% over 8.4 years, corresponding to an annualized growth rate of lambda = 1.05 +/- 0.01 yr(-1) (median +/- SD). Most of the change was attributable to exotic plant species, which increased from 24.4% +/- 1.4% to 49.1% +/- 2.0%, (lambda = 1.08 +/- 0.01 yr(-1)). Native plants experienced no significant change in cover (23.0% +/- 1.3% to 24.2% +/- 1.8%, lambda = 1.01 +/- 0.01 yr(-1)). Time series of satellite phenology were indistinguishable between the treatment and a 3.0-km2 control site for four years prior to ungulate removal, but they diverged immediately following exclusion of ungulates. Comparison of monthly EVI means before and after ungulate exclusion and between the managed and control areas indicates that EVI strongly increased in the managed area after ungulate exclusion. Field studies and airborne analyses show that the dominant invader was Senecio madagascariensis, an invasive annual forb that increased from < 0.01% to 14.7% fractional cover in ungulate-free areas (lambda = 1.89 +/- 0.34 yr(-1)), but which was nearly absent from the control site. A combination of canopy LAI, water, and fractional cover were expressed in satellite EVI time series and indicate that the invaded region maintained greenness during drought conditions. These findings demonstrate that enemy release from generalist herbivores can facilitate exotic success and suggest a plausible mechanism by which invasion occurred. They also show how novel remote-sensing technology can be integrated with conservation and management to help address exotic plant invasions.

Mapping habitat suitability for at‐risk plant species and its implications for restoration and reintroduction

The conservation of species at risk of extinction requires data to support decisions at landscape to regional scales. There is a need for information that can assist with locating suitable habitats in fragmented and degraded landscapes to aid the reintroduction of at-risk plant species. In addition, desiccation and water stress can be significant barriers to the success of at-risk plant reintroduction programs. We examine how airborne light detection and ranging (LiDAR) data can be used to model microtopographic features that reduce water stress and increase resource availability, providing information for landscape planning that can increase the success of reintroduction efforts for a dryland landscape in Hawaii. We developed a topographic habitat-suitability model (HSM) from LiDAR data that identifies topographic depressions that are protected from prevailing winds (high-suitability sites) and contrasts them with ridges and other exposed areas (low-suitability sites). We tested in the field whether high-suitability sites had microclimatic conditions that indicated better-quality habitat compared to low-suitability sites, whether plant-response traits indicated better growing conditions in high-suitability sites, whether the locations of individuals of existing at-risk plant species corresponded with our habitat-suitability classes, and whether the survival of planted individuals of a common native species was greater in high-suitability, compared to low-suitability, planting sites. Mean wind speed in a high-suitability field site was over five times lower than in a low-suitability site, and soil moisture and leaf wetness were greater, indicating less stress and greater resource availability in high-suitability areas. Plant height and leaf nutrient content were greater in high-suitability areas. Six at-risk species showed associations with high-suitability areas. The survival of planted individuals was less variable among high-suitability plots. These results suggest that plant establishment and survival is associated with the habitat conditions identified by our model. The HSM can improve the survival of planted individuals, reduce the cost of restoration and reintroduction programs through targeted management activities in high-suitability areas, and expand the ability of managers to make landscape-scale decisions regarding land-use, land acquisition, and species recovery.

Patterns and consequences of re-invasion into a Hawaiian dry forest restoration

The restoration of native plant diversity may be an effective tool for weed control, but its use has not been tested in the heavily invaded Hawaiian dry forest ecosystem. In addition, the ecological mechanisms by which invasive plants may cause declines in native plant diversity are generally not well understood. We examined invasion resistance and the relationships between invasion and declines in native plant diversity at local scales in a Hawaiian dry forest restoration by experimentally removing non-native species, planting native species, manipulating resources and environmental conditions, and allowing non-native species to re-invade. We found higher invasion rates in habitats that supported the most native species, suggesting that similar mechanisms may regulate the distribution of both native and invasive species. Pennisetum setaceum, a dominant invasive perennial grass, was associated with native plant mortality and declines in native diversity. Although invasion rates were greatest in more favorable habitats, the association of invasion with native species loss was significant only in lower quality habitats, suggesting that environmental conditions may regulate competitive interactions between native and invasive species. We found that native woody plant restoration is not an effective tool for weed control in this community, and that invasion may result in declines in native diversity. Our study provides a caveat to previous suggestions that invasion impacts on native diversity will be greatest in the most diverse plant communities. At local scales in some communities it may also be effective to control invasive species in sensitive areas with low resource availability instead of, or in addition to, areas with high biodiversity.

Primary Succession on a Hawaiian Dryland Chronosequence

We used measurements from airborne imaging spectroscopy and LiDAR to quantify the biophysical structure and composition of vegetation on a dryland substrate age gradient in Hawaii. Both vertical stature and species composition changed during primary succession, and reveal a progressive increase in vertical stature on younger substrates followed by a collapse on Pleistocene-aged flows. Tall-stature Metrosideros polymorpha woodlands dominated on the youngest substrates (hundreds of years), and were replaced by the tall-stature endemic tree species Myoporum sandwicense and Sophora chrysophylla on intermediate-aged flows (thousands of years). The oldest substrates (tens of thousands of years) were dominated by the short-stature native shrub Dodonaea viscosa and endemic grass Eragrostis atropioides. We excavated 18 macroscopic charcoal fragments from Pleistocene-aged substrates. Mean radiocarbon age was 2,002 years and ranged from < 200 to 7,730. Genus identities from four fragments indicate that Osteomeles spp. or M. polymorpha once occupied the Pleistocene-aged substrates, but neither of these species is found there today. These findings indicate the existence of fires before humans are known to have occupied the Hawaiian archipelago, and demonstrate that a collapse in vertical stature is prevalent on the oldest substrates. This work contributes to our understanding of prehistoric fires in shaping the trajectory of primary succession in Hawaiian drylands.

Resource Availability, Propagule Supply, and Effect of Nonnative Ungulate Herbivores on Senecio madagascariensis Invasion

Abstract: Nonnative invasive herbivores can create complex biotic interactions by differentially feeding on native and nonnative invasive plant species. The herbivores may act as enemies of nonnative plants and prevent them from becoming invasive, or they may facilitate invasion by having a greater negative impact on native plants compared with nonnative plants. It is also possible that within the same ecosystem nonnative herbivores could either facilitate or inhibit invasion under different abiotic or biotic conditions. In this study we experimentally investigated how abiotic (soil nutrients) and biotic (propagule density) conditions influence the effect of invasive generalist herbivores on Senecio madagascariensis, an invasive plant species in Hawaiian dry forest plant communities. We used fenced exclosures to manipulate presence or absence of invasive ungulates (feral goats and sheep), and we used seed addition to manipulate propagule supply of S. madagascariensis. The experiment was replicated in a recently burned and an unburned site to examine how a resource pulse following fire may alter plant-herbivore interactions. There were very few seeds of S. madagascariensis in the seed rain of both sites, and recruitment was four times higher when seeds were experimentally added, suggesting that S. madagascariensis is dispersal limited in this area. Recruitment of S. madagascariensis was five times higher in the burned site compared to the unburned site, suggesting that increased resources promote recruitment. Recruitment was three times higher when herbivores were present compared to when they were excluded, but plants were much smaller when herbivores were present. We conclude that herbivores can alter S. madagascariensis recruitment, even during dry conditions, and that propagule availability influences where S. madagascariensis can become established.

A match made in academia: you and your graduate advisor

Joe shivered as he looked out of his window at the falling snow. Even the cockroaches that usually haunted his office had moved on to cozier places. He was working late on his dissertation proposal, as he had been doing for months. He had hoped to defend it in a weeks time, but yesterday his advisor had postponed the committee meeting and requested substantial revisions. Joe was furious, and had spent the day searching for ways to prove his advisor wrong. Now he was exhausted and alone in his cold office at 3:00 am on a Saturday, he lacked adequate proof, and he finally had to accept that his advisor was right. Like Joe, we graduate students look to our advisors for guidance, but it can sometimes be hard to accept their criticism. With good communication, you can develop a strong working relationship with your advisor, that will have positive and long-lasting impacts on your career. If you hope to shine brilliantly as a job candidate when you finish your graduate degree, the key person who can help you shine (or dull you completely) is your advisor. Most of us have good relationships with our advisors. We meet regularly, have fun exchanging ideas, collabo rate on research projects, co-author papers, and generally like each other. If a student-advisor relationship does not work out, it is usually for one of two main reasons: