Biomedical subjects

John L Maron

Publications and source records attributed to John L Maron.

7 recordsLinked to original sources

PubMed2006

Herbivory: effects on plant abundance, distribution and population growth.

Plants are attacked by many different consumers. A critical question is how often, and under what conditions, common reductions in growth, fecundity or even survival that occur due to herbivory translate to meaningful impacts on abundance, distribution or dynamics of plant populations. Here, we review population-level studies of the effects of consumers on plant dynamics and evaluate: (i) whether particular consumers have predictably more or less influence on plant abundance, (ii) whether particular plant life-history types are predictably more vulnerable to herbivory at the population level, (iii) whether the strength of plant-consumer interactions shifts predictably across environmental gradients and (iv) the role of consumers in influencing plant distributional limits. Existing studies demonstrate numerous examples of consumers limiting local plant abundance and distribution. We found larger effects of consumers on grassland than woodland forbs, stronger effects of herbivory in areas with high versus low disturbance, but no systematic or unambiguous differences in the impact of consumers based on plant life-history or herbivore feeding mode. However, our ability to evaluate these and other patterns is limited by the small (but growing) number of studies in this area. As an impetus for further study, we review strengths and challenges of population-level studies, such as interpreting net impacts of consumers in the presence of density dependence and seed bank dynamics.

Animals↗

PubMed2006

Consumers limit the abundance and dynamics of a perennial shrub with a seed bank.

For nearly 30 years, ecologists have argued that predators of seeds and seedlings seldom have population-level effects on plants with persistent seed banks and density-dependent seedling survival. We parameterized stage-based population models that incorporated density dependence and seed dormancy with data from a 5.5-year experiment that quantified how granivorous mice and herbivorous voles influence bush lupine (Lupinus arboreus) demography. We asked how seed dormancy and density-dependent seedling survival mediate the impacts of these consumers in dune and grassland habitats. In dune habitat, mice reduced analytical lambda (the intrinsic rate of population growth) by 39%, the equilibrium number of aboveground plants by 90%, and the seed bank by 98%; voles had minimal effects. In adjacent grasslands, mice had minimal effects, but seedling herbivory by voles reduced analytical lambda by 15% and reduced both the equilibrium number of aboveground plants and dormant seeds by 63%. A bootstrap analysis demonstrated that these consumer effects were robust to parameter uncertainty. Our results demonstrate that the quantitative strengths of seed dormancy and density-dependent seedling survival--not their mere existence--critically mediate consumer effects. This study suggests that plant population dynamics and distribution may be more strongly influenced by consumers of seeds and seedlings than is currently recognized.

Animals↗

PubMed2006

What have exotic plant invasions taught us over the past 20 years?

Invasive organisms have become a focal interest in ecology, owing not only to the tremendous destruction that they can cause, but also because we do not yet understand fully how they change from being minor components of their native communities to dominant components of invaded communities. Here, we discuss our perceptions of how the study of exotic plant species has contributed to the changing face of ecology over the past 20 years. Research on invasive organisms has promoted synthetic efforts between fields that have historically operated in isolation. Most importantly, the study of invasions has resulted in significant intellectual shifts in the way that old paradigms are perceived by ecologists and have led us into new and uncharted territory.

Animals↗

PubMed2006

Biotic interactions and plant invasions.

Introduced plant populations lose interactions with enemies, mutualists and competitors from their native ranges, and gain interactions with new species, under new abiotic conditions. From a biogeographical perspective, differences in the assemblage of interacting species, as well as in abiotic conditions, may explain the demographic success of the introduced plant populations relative to conspecifics in their native range. Within invaded communities, the new interactions and conditions experienced by the invader may influence both its demographic success and its effects on native biodiversity. Here, we examine indirect effects involving enemies, mutualists and competitors of introduced plants, and effects of abiotic conditions on biotic interactions. We then synthesize ideas building on Darwin's idea that the kinds of new interactions gained by an introduced population will depend on its relatedness to native populations. This yields a heuristic framework to explain how biotic interactions and abiotic conditions influence invader success. We conclude that species introductions generally alter plants' interactions with enemies, mutualists and competitors, and that there is increasing evidence that these altered interactions jointly influence the success of introduced populations.

Animals↗

PubMed2006

Habitat-specific impacts of multiple consumers on plant population dynamics.

Multiple consumers often attack seeds, seedlings, and adult plants, but their population-level consequences remain uncertain. We examined how insect and small mammal consumers influenced the demography and abundance of the perennial shrub, bush lupine (Lupinus arboreus). In grassland and dune habitats we established replicate experimental lupine populations in 81-m2 plots that were either protected from, or exposed to, herbivorous voles and granivorous mice (via fencing) and/or root feeding insects (via insecticide treatment). Populations were initiated with transplanted seedlings in 1999 and 2000. We followed the demography of these cohorts, subsequent generations, and the seed bank for 5.5 years. Voles and insects killed many seedlings in dune (1999 only) and grassland (1999 and 2000) habitats. After 2000, insects and voles had minimal effects on seedling or adult survival. Seed predation by granivorous mice, however, greatly depressed seedling recruitment, resulting in lower adult lupine abundance in control plots vs. those protected from rodents. In grasslands, initial effects of voles and insects on seedling survival produced large differences among treatments in adult plant density and the cumulative number of seeds produced throughout the experiment. Differences among grassland populations in seed rain, however, had little influence on the magnitude of seedling recruitment into this habitat. Instead, recruitment out of a preexisting seed bank compensated for the lack of seed production in populations exposed to consumers. Shading by dense adults in plots protected from consumers limited seedling establishment within these populations. Although differences among populations in cumulative seed rain did not influence adult establishment, populations protected from consumers accumulated substantially larger seed banks than controls. These results illustrate how density dependence, habitat-specific seed dynamics, and particular demographic impacts of consumers interact to shape plant population responses to consumers.

Analysis of Variance↗

PubMed2004

Evidence for the enemy release hypothesis in Hypericum perforatum.

The enemy release hypothesis (ERH), which has been the theoretical basis for classic biological control, predicts that the success of invaders in the introduced range is due to their release from co-evolved natural enemies (i.e. herbivores, pathogens and predators) left behind in the native range. We tested this prediction by comparing herbivore pressure on native European and introduced North American populations of Hypericum perforatum (St John's Wort). We found that introduced populations occur at larger densities, are less damaged by insect herbivory and suffer less mortality than populations in the native range. However, overall population size was not significantly different between ranges. Moreover, on average plants were significantly smaller in the introduced range than in the native range. Our survey supports the contention that plants from the introduced range experience less herbivore damage than plants from the native range. While this may lead to denser populations, it does not result in larger plant size in the introduced versus native range as postulated by the ERH.

California↗

PubMed2003

Are alien plants more competitive than their native conspecifics? A test using Hypericum perforatum L.

The evolution of increased competitive ability hypothesis predicts that introduced plants that are long liberated from their natural enemies may lose costly herbivore defense, enabling them to reallocate resources previously spent on defense to traits that increase competitive superiority. We tested this prediction by comparing the competitive ability of native St John's wort (Hypericum perforatum) from Europe with introduced St John's wort from central North America where plants have long grown free of specialist herbivores, and introduced plants from western North America where plants have been subjected to over 57 years of biological control. Plants were grown in a greenhouse with and without competition with Italian ryegrass (Lolium multiflorum). St John's wort from the introduced range were not better interspecific competitors than plants from the native range. The magnitude of the effect of ryegrass on St John's wort was similar for introduced and native genotypes. Furthermore, introduced plants were not uniformly larger than natives; rather, within each region of origin there was a high variability in size between populations. Competition with ryegrass reduced the growth of St John's wort by >90%. In contrast, St John's wort reduced ryegrass growth <10%. These results do not support the contention that plants from the introduced range evolve greater competitive ability in the absence of natural enemies.

Biological Evolution↗