Biology subjects

Heckman, R. W.

Publications and source records attributed to Heckman, R. W..

7 recordsLinked to original sources

bioRxiv2024

Amplification of disease by nutrient addition: Testing mechanisms from individual to community levels

Nutrient supply can amplify disease epidemics through mechanisms from individual to community levels. Within host individuals, nutrient addition can drive pathogen replication or growth. Across a host population, nutrient addition can drive disease transmission by increasing host growth and abundance relative to defense. Furthermore, such effects may be influenced by pathogen species interactions. Understanding how nutrients impact disease epidemics requires a framework that integrates these mechanisms across biological levels. To build such a framework, we conducted a field experiment in an old field on tall fescue, Lolium arundinaceum, and used structural equation models to integrate multiple hypothesized mechanisms. Nutrient addition (NPK fertilizer) increased brown patch disease but was best modeled as a direct path and not mediated by host abundance. To expand our framework, we also re-analyzed a previous experiment. That experiment reproduced the direct path from nutrients to disease, and added an indirect path mediated by host population abundance. Nutrient addition also increased foliar nitrogen, consistent with individual-level mechanism, but this did not increase disease. Brown patch decreased with burden of another disease, anthracnose, independently of nutrients. These results partially support both individual- and population-level hypotheses, emphasizing the importance of considering multiple biological levels underlying impacts of abiotic change.

ecology↗

bioRxiv2024

The shared genetic basis of leaf morphology and tensile resistance underlies the effect of growing season length in a widespread perennial grass

PremiseLeaf tensile resistance, a leafs ability to withstand pulling forces, is an important determinant of plant ecological strategies. One potential driver of leaf tensile resistance is growing season length. When growing seasons are long, strong leaves--which often require more time and resources to construct than weak leaves--may be more advantageous than when growing seasons are short. Growing season length and other ecological conditions may also impact the morphological traits that underlie leaf tensile resistance. MethodsTo understand variation in leaf tensile resistance, we measured size-dependent leaf strength and size-independent leaf toughness in diverse genotypes of the widespread perennial grass Panicum virgatum (switchgrass) in a common garden. We then used quantitative genetic approaches to estimate the heritability of leaf tensile resistance and whether there were genetic correlations between leaf tensile resistance and other morphological traits. Key ResultsLeaf tensile resistance was positively associated with aboveground biomass (a proxy for fitness). Moreover, both measures of leaf tensile resistance exhibited high heritability and were positively genetically correlated with leaf lamina thickness and leaf mass per area (LMA). Leaf tensile resistance also increased with habitat-of-origin growing season length and this effect was mediated by both LMA and leaf thickness. ConclusionsDifferences in growing season length may promote selection for different leaf lifespans and may explain existing variation in leaf tensile resistance in P. virgatum. In addition, the high heritability of leaf tensile resistance suggests that P. virgatum will be able to respond to climate change as growing seasons lengthen.

plant biology↗

bioRxiv2022

Disease decreases variation in host community structure in an old-field grassland

Disease may modulate variation in host community structure by modifying the interplay of deterministic and stochastic processes. For instance, deterministic processes like ecological selection can benefit species less impacted by disease. When disease consistently selects for certain host species, this can reduce variation in host community composition. On the other hand, when host communities are less impacted by disease and selection is weaker, stochastic processes (e.g., drift, dispersal) may play a bigger role in host community structure, which can increase variation in structure among communities. While effects of disease on host community structure have been quantified in field experiments, few have addressed the role of disease in modulating variation in structure among host communities. To address this, we conducted a field experiment spanning three years, using a tractable system: foliar fungal pathogens in an old-field grassland community dominated by the grass Lolium arundinaceum, tall fescue. We reduced foliar fungal disease burden in replicate host communities (experimental plots in intact vegetation) in three fungicide regimens that varied in the duration of fungicide exposure and included a fungicide-free control. We measured host diversity, biomass, and variation in community structure among replicate communities. Disease reduction generally decreased plant richness and increased aboveground biomass relative to communities experiencing ambient levels of disease. Despite changes in structure of the plant communities over the experiments three years, the effects of disease reduction on plant richness and biomass were consistent across years. However, disease reduction did not reduce variation in host community structure, providing little evidence for ecological selection by competition or other deterministic processes. Instead, disease reduction tended to amplify variation in host community structure among replicate communities (i.e., within fungicide treatment groups), suggesting that disease diminished the degree to which host communities were structured by stochastic processes. These results of experimental disease reduction both highlight the potential importance of stochastic processes in plant communities and reveal the potential for disease to regulate variation in host community structure.

ecology↗

bioRxiv2021

Correlational selection and genetic architecture promote the leaf economics spectrum in a perennial grass

The leaf economics spectrum (LES) is hypothesized to result from a trade-off between resource acquisition and conservation. Yet few studies have examined the evolutionary mechanisms behind the LES, perhaps because most species exhibit relatively specialized leaf economics strategies. In a genetic mapping population of the phenotypically diverse grass Panicum virgatum, we evaluate two interacting mechanisms that may drive LES evolution: 1) genetic architecture, where multiple traits are coded by the same gene (pleiotropy) or by genes in close physical proximity (linkage), and 2) correlational selection, where selection acts non-additively on combinations of multiple traits. We found evidence suggesting that shared genetic architecture (pleiotropy) controls covariation between two pairs of leaf economics traits. Additionally, at five common gardens spanning 17 degrees of latitude, correlational selection favored particular combinations of leaf economics traits. Together, these results demonstrate how the LES can evolve within species.

ecology↗

bioRxiv2020

Contrasting effects of nutrients and consumers on tree colonization and growth during secondary succession

For succession to proceed from herbaceous to woody dominance, trees must colonize herbaceous communities and grow. Success across these two phases of succession might result from different interactions with the herbaceous community. First, colonizing trees must compete against larger, established herbs, while subsequent growth occurs among similarly sized or smaller herbs. This shift from colonization to growth may cause three drivers of secondary succession-- nutrients, consumers, and herbaceous diversity--to differentially affect tree colonization and growth. Initially, these drivers should favor larger, established herbs, reducing colonization. Later, when established trees can better compete with herbs, these drivers should benefit trees and increase their growth. In a four-year study, we added nutrients to, excluded aboveground consumers from, and manipulated initial richness of, the herbaceous community, then allowed trees to naturally colonize these communities (from intact seedbanks or as seed-rain) and grow. Nutrients and consumers had opposing effects on tree colonization and growth: adding nutrients and excluding consumers reduced tree colonization, but later increased established tree growth (height, basal diameter). Together, this shows stage-specific impacts of nutrients and consumers that may improve predictions of the rate and trajectory of succession: factors that initially limited tree colonization later helped established trees to grow.

ecology↗

bioRxiv2019

Host community assembly modifies the relationship between host and parasite richness

Host and parasite richness are generally positively correlated, but the stability of this relationship during community assembly remains untested. The composition of host communities can alter parasite transmission, and the relationship between host and parasite richness is sensitive to parasite transmission. Thus, changes in composition during host community assembly could strengthen or weaken the relationship between host and parasite richness. Host community assembly, in turn, can be driven by many processes, including resource enrichment. To test the hypothesis that host community assembly can alter the relationship between host and parasite richness, we experimentally crossed host diversity and resource supply to hosts, then allowed communities to assemble. As previously shown, initial host diversity and resource supply determined the trajectory of host community assembly, altering post-assembly host species richness, richness-independent host phylogenetic diversity, and colonization by exotic host species. Throughout community assembly, host richness predicted parasite richness. As predicted, this effect was moderated by exotic abundance: communities dominated by exotic species exhibited a stronger positive relationship between post-assembly host and parasite richness. Ultimately, these results suggest that, by modulating parasite transmission, community assembly can modify the relationship between host and parasite richness, providing a novel mechanism to explain contingencies in this relationship.

ecology↗

bioRxiv2019

Priority effects, consumer pressure, and soil resources independently alter plant diversity and resource strategies during a multi-year successional field experiment

O_LIPlant community succession is structured by priority effects, plant consumer pressure, and soil resource supply. Importantly, these drivers may interact, their effects may vary temporally, and they may influence different facets of plant community diversity by promoting different plant tradeoff strategies.\nC_LIO_LIIn an herbaceous successional system, we manipulated priority effects by altering initial plant richness, consumer pressure via pesticide spraying, and soil resource supply via fertilization. We examined how these processes jointly influenced succession, including taxonomic diversity and functional traits, over four years.\nC_LIO_LIDiversity decreased in different years in response to more diverse priority effects, lower consumer pressure, and increased soil resource supply. Functionally, higher soil resource supply increased community height, SLA, and seed mass; higher consumer pressure decreased intraspecific community height, and increased interspecific SLA; priority effects led to decreased seed mass only when plots were unplanted.\nC_LIO_LIOur results suggest species resource strategies underlie plant diversity responses. Resource addition promoted resource-acquisitive species, consumer pressure disadvantaged resource-conservative species, and diversity of priority effects altered subsequent community composition through persistence of early residents, not via traits. We show that community responses to drivers of succession depend on underlying trait tradeoffs of resident species, and these tradeoffs influence community diversity across succession.\nC_LI

ecology↗