Lauren L. Sullivan

Potential Negative Ecological Effects of Corridors

Despite many studies showing that landscape corridors increase dispersal and species richness for disparate taxa, concerns persist that corridors can have unintended negative effects. In particular, some of the same mechanisms that underlie positive effects of corridors on species of conservation interest may also increase the spread and impact of antagonistic species (e.g., predators and pathogens), foster negative effects of edges, increase invasion by exotic species, increase the spread of unwanted disturbances such as fire, or increase population synchrony and thus reduce persistence. We conducted a literature review and meta-analysis to evaluate the prevalence of each of these negative effects. We found no evidence that corridors increase unwanted disturbance or non-native species invasion; however, these have not been well-studied concerns (1 and 6 studies, respectively). Other effects of corridors were more often studied and yielded inconsistent results; mean effect sizes were indistinguishable from zero. The effect of edges on abundances of target species was as likely to be positive as negative. Corridors were as likely to have no effect on antagonists or population synchrony as they were to increase those negative effects. We found 3 deficiencies in the literature. First, despite studies on how corridors affect predators, there are few studies of related consequences for prey population size and persistence. Second, properly designed studies of negative corridor effects are needed in natural corridors at scales larger than those achievable in experimental systems. Third, studies are needed to test more targeted hypotheses about when corridor-mediated effects on invasive species or disturbance may be negative for species of management concern. Overall, we found no overarching support for concerns that construction and maintenance of habitat corridors may result in unintended negative consequences. Negative edge effects may be mitigated by widening corridors or softening edges between corridors and the matrix. Other negative effects are relatively small and manageable compared with the large positive effects of facilitating dispersal and increasing diversity of native species.

Can dispersal mode predict corridor effects on plant parasites

Habitat corridors, a common management strategy for increasing connectivity in fragmented landscapes, have experimentally validated positive influences on species movement and diversity. However, long-standing concerns that corridors could negatively impact native species by spreading antagonists, such as disease, remain largely untested. Using a large-scale, replicated experiment, we evaluated whether corridors increase the incidence of plant parasites. We found that corridor impacts varied with parasite dispersal mode. Connectivity provided by corridors increased incidence of biotically dispersed parasites (galls on Solidago odora) but not of abiotically dispersed parasites (foliar fungi on S. odora and three Lespedeza spp.). Both biotically and abiotically dispersed parasites responded to edge effects, but the direction of responses varied across species. Although our results require additional tests for generality to other species and landscapes, they suggest that, when establishing conservation corridors, managers should focus on mitigating two potential negative effects: the indirect effects of narrow corridors in creating edges and direct effects of corridors in enhancing connectivity of biotically dispersed parasites.

Climate modifies response of non-native and native species richness to nutrient enrichment

Ecosystem eutrophication often increases domination by non-natives and causes displacement of native taxa. However, variation in environmental conditions may affect the outcome of interactions between native and non-native taxa in environments where nutrient supply is elevated. We examined the interactive effects of eutrophication, climate variability and climate average conditions on the success of native and non-native plant species using experimental nutrient manipulations replicated at 32 grassland sites on four continents. We hypothesized that effects of nutrient addition would be greatest where climate was stable and benign, owing to reduced niche partitioning. We found that the abundance of non-native species increased with nutrient addition independent of climate; however, nutrient addition increased non-native species richness and decreased native species richness, with these effects dampened in warmer or wetter sites. Eutrophication also altered the time scale in which grassland invasion responded to climate, decreasing the importance of long-term climate and increasing that of annual climate. Thus, climatic conditions mediate the responses of native and non-native flora to nutrient enrichment. Our results suggest that the negative effect of nutrient addition on native abundance is decoupled from its effect on richness, and reduces the time scale of the links between climate and compositional change.

Comment on "Worldwide evidence of a unimodal relationship between productivity and plant species richness"

Fraser et al. (Reports, 17 July 2015, p. 302) report a unimodal relationship between productivity and species richness at regional and global scales, which they contrast with the results of Adler et al. (Reports, 23 September 2011, p. 1750). However, both data sets, when analyzed correctly, show clearly and consistently that productivity is a poor predictor of local species richness.

Out of the shadows: multiple nutrient limitations drive relationships among biomass, light and plant diversity

The paradigmatic hypothesis for the effect of fertilisation on plant diversity represents a one-dimensional trade-off for plants competing for below-ground nutrients (generically) and above-ground light: fertilisation reduces competition for nutrients while increasing biomass and thereby shifts competition for depleted available light. The essential problem of this simple paradigm is that it misses both the multivariate and mechanistic nature of the factors that determine biodiversity as well as their causal relationships. We agree that light limitation, as DeMalach and Kadmon argue, can indeed be an important factor associated with diversity loss, and we presented it as an integral part of our tests of the niche dimension hypothesis. We disagree with DeMalach and Kadmon that light is the ‘main’ factor explaining diversity, because this misrepresents the causal structure represented in the design of our experiment in which multiple nutrient addition was the ultimate causal driver of a suite of correlated responses that included diversity and light, and especially live and dead biomass, which are the factors that control light depletion. Our findings highlight that multiple nutrient limitations can structure plant diversity and composition independently of changes in light and biomass. For example, approximately one-third of our sites showed no significant increase in biomass with greater number of added nutrients yet still lost diversity when nutrients were added. The important message is that while light limitation can be an important contributor to diversity loss, it is not a necessary mechanism.

Density dependence in demography and dispersal generates fluctuating invasion speeds

Significance Mitigating the spread of invasive species remains difficult—substantial variability in invasion speed is increasingly well-documented, but the sources of this variability are poorly understood. We report a mechanism for invasion speed variability. The combined action of density dependence in demography and dispersal can cause invasions to fluctuate, even in constant environments. Speed fluctuations occur through creation of a pushed invasion wave that moves forward not from small populations at the leading edge but instead, from larger, more established populations that “jump” forward past the previous invasion front. Variability in strength of the push generates fluctuating invasion speeds. Conditions giving rise to fluctuations are widely documented in nature, suggesting that an important source of invasion variability may be overlooked. Density dependence plays an important role in population regulation and is known to generate temporal fluctuations in population density. However, the ways in which density dependence affects spatial population processes, such as species invasions, are less understood. Although classical ecological theory suggests that invasions should advance at a constant speed, empirical work is illuminating the highly variable nature of biological invasions, which often exhibit nonconstant spreading speeds, even in simple, controlled settings. Here, we explore endogenous density dependence as a mechanism for inducing variability in biological invasions with a set of population models that incorporate density dependence in demographic and dispersal parameters. We show that density dependence in demography at low population densities—i.e., an Allee effect—combined with spatiotemporal variability in population density behind the invasion front can produce fluctuations in spreading speed. The density fluctuations behind the front can arise from either overcompensatory population growth or density-dependent dispersal, both of which are common in nature. Our results show that simple rules can generate complex spread dynamics and highlight a source of variability in biological invasions that may aid in ecological forecasting.

Effects of habitat and time-since-fire on recruitment, survival, and reproduction of Paronychia chartacea ssp. chartacea, a short-lived Florida scrub endemic herb1

Abstract Conservation of a threatened species requires knowledge of the factors that affect its recruitment, survival, and reproduction. We conducted a long-term study on the demography of Paronychia chartacea ssp. chartacea, a short-lived, gynodioecious Florida rosemary scrub endemic. Specifically, we assessed the effects of habitat (rosemary scrub vs. roadsides), time-since-fire, microhabitat, and rainfall on recruitment, survival, flowering, and seed production. In addition, in a shorter-term study, we assessed the effects of habitat, time-since-fire, and gender on flower and seed production. Overall, approximately one-third of individuals survived more than one year and individuals most often flowered twice indicating that P. chartacea ssp. chartacea is an iteroparous, short-lived perennial, which is in contrast to its disjunct conspecific, the annual P. chartacea ssp. minima. Recruitment was higher in roadsides, but seed production was higher in rosemary scrub. Within rosemary scrub, recruitment was highest at intermediate times-since-fire and in the center of large gaps, the microhabitat with the greatest amount of bare sand. Median lifespan was longest in long unburned sites, suggesting that P. chartacea ssp. chartacea is able to persist despite litter accumulation and competition from shrubs. Rainfall had a positive effect on annual survival, but did not affect seedling recruitment. Females produced more seeds than hermaphrodites, indicating that females contribute more to population persistence. Our study suggests P. chartacea ssp. chartacea populations will persist under the 15–40 year fire return interval characteristic of regularly burned Florida rosemary scrub, but may not persist in roadsides if greater recruitment does not balance shorter lifespans.

Small group gender ratios impact biology class performance and peer evaluations

Women are underrepresented in science, technology, engineering, and mathematics (STEM) disciplines. Evidence suggests the microclimate of the classroom is an important factor influencing female course grades and interest, which encourages retention of women in STEM fields. Here, we test whether the gender composition of small (8–9 person) learning groups impacts course performance, sense of social belonging, and intragroup peer evaluations of intellectual contributions. Across two undergraduate active learning courses in introductory biology, we manipulated the classroom microclimate by varying the gender ratios of learning groups, ranging from 0% female to 100% female. We found that as the percent of women in groups increased, so did overall course performance for all students, regardless of gender. Additionally, women assigned higher peer- evaluations in groups with more women than groups with less women. Our work demonstrates an added benefit of the retention of women in STEM: increased performance for all, and positive peer perceptions for women.

Mammalian Herbivores Alter the Population Growth and Spatial Establishment of an Early-Establishing Grassland Species.

Plant-herbivore interactions influence the establishment context of plant species, as herbivores alter the community context in which individual species establish, and the spatial relationship between individuals and their source population as plants invade. This relationship can be described using an establishment kernel, which takes into account movement through seed dispersal, and subsequent establishment of adults. Mammalian herbivores are hypothesized to influence plant population growth and establishment through a combination of consumption of seeds and seedlings, and movement of seeds. While the movement abilities of plants are well known, we have very few empirical mechanistic tests of how biotic factors like mammalian herbivores influence this spread potential. As herbivores of all sizes are abundant on the landscape, we asked the question, how do mammalian herbivores influence the population growth, spatial establishment, and the community establishment context of an early-recruiting native prairie legume, Chamaecrista fasciculata? We planted C. fasciculata in source populations within a four-acre tallgrass prairie restoration in plots with and without herbivores, and monitored its establishment with respect to distance from the source populations. We found that herbivores decreased population growth, and decreased the mean and range establishment distance. Additionally, C. fasciculata established more often without herbivores, and when surrounded by weedy, annual species. Our results provide insight into how the interactions between plants and herbivores can alter the spatial dynamics of developing plant communities, which is vital for colonization and range spread with fragmentation and climate change. Mammalian herbivores have the potential to both slow rates of establishment, but also determine the types of plant communities that surround invading species. Therefore, it is essential to consider the herbivore community when attempting to restore functioning plant communities.

Nutrients and environment influence arbuscular mycorrhizal colonization both independently and interactively in Schizachyrium scoparium

Background and aimsArbuscular mycorrhizal fungi (AMF) are important for plant nutrient and water acquisition. Much is known about how nutrient addition and environment affect AMF, but little is known about nutrient by environment interactions. We measured AMF colonization with nutrient additions and along an environmental gradient to assess these interactions.MethodsWe measured AMF colonization in roots of little bluestem (Schizachyrium scoparium (Michx) Nash) with nutrient addition and across an environmental gradient. We assessed how AMF colonization changed across different fertilization treatments, and used ridge regression to determine nutrient, environment, and nutrient by environment interaction variables that predicted AMF colonization.ResultsThe addition of nitrogen decreased AMF colonization, while mean annual temperature (MAT) and soil pH both positively predicted the percentage of AMF colonization in Schizachyrium scoparium. Additionally, we found an interaction term between MAT and phosphorus treatments that significantly affected percent AMF colonization.ConclusionsOur results show the importance of understanding environmental conditions on AMF as well as nutrient by environment interactions when assessing how AMF respond to nutrient addition. Here we present a full-factorial nutrient addition study along an environmental gradient to assess how AMF root colonization is influenced by abiotic factors in addition to nutrients.