Janneke Hille Ris Lambers: Current Research
The Role of Stabilizing and Equalizing Processes in Maintaining Diversity
It's safe to say that Steve Hubbell threw the ecological world on its head when he introduced community ecologists to the idea that species coexistence does not necessarily result from species interactions, but may simply occur because speciation balances extinction over long time scales. Traditionally, community ecologists had focused their research on "stabilizing" processes; those which lead to more negative intra-specific interactions than inter-specific interactions (for example: resource niches, density-dependent natural enemies, and the storage effect). Hubbell's work has forced community ecologists to consider the role of chance events and equalizing processes in maintaining diversity (i.e. random walks in population sizes when species differ little in overall fitness). Today, much ecological debate centers on the relative roles of stabilizing and equalizing processes in fostering coexistence locally.
I am collaborating with Jonathan Levine to experimentally disentangle the contribution of stabilizing vs. equalizing processes to the coexistence of California serpentine annuals. By manipulating seed production in experimentally assembled communities and monitoring temporal population dynamics, we hope to infer the relative importance of species interactions vs. drift in maintaining diversity in these communities. We will combine our experimental approach with observations of natural serpentine hummocks nearby to pinpoint which stabilizing processes (if any) are driving species interactions and community structure. Although many theoretical and empirical studies demonstrate that particular diversity-promoting mechanisms operate in natural communities, few studies attempt to disentangle the relative roles of stabilizing vs. equalizing processes in maintaining diversity, and none (that we are aware of) have attempted an experimental approach.
Why are Exotic Annuals Common and Native Annuals Rare in California Grasslands?
The domination of California grasslands by Mediterranean annual grasses is one of the most spectacular examples of the displacement of a native flora by nonnative plant species. The conversion of California's bunchgrass dominated systems to virtual monocultures of a few Mediterranean annual grass species (e.g. MAvena barbata, Bromus diandrus, Bromus hordeaceous) was thought to have been mediated by a combination of drought, grazing and disturbance associated with intensive landuse accompanying European settlement of the area. A somewhat surprising fact is that exotic, rather than native annuals, responded dramatically to this disturbance. Potential reasons include resource uptake (exotic annuals are superior competitors for limiting resources); natural enemies (exotic annuals are less negatively affected by natural enemies); and coevolution with grazers (exotic annuals can better cope with grazing, as grazers were absent from California prior to their introduction by European settlers). The question of why exotic annuals are so common when native annuals are rare forms the basis of a manipulative experiment I (in collaboration with Jonathan Levine) am conducting in the Santa Ynez valley, California (Santa Barbara county). I am measuring species traits of native and exotic annual grasses in monocultures to identify species traits that correlate with dominance in mixtures. I am considering phenology, plant-soil feedbacks, susceptibility to grazing and resource uptake as potential traits that differentiate native from exotic annuals.
Effects of Global Change on Plant Reproduction
I am investigating how anthropogenic environmental change (elevated CO2, nitrogen deposition and declining species' diversity) affects seed production of thirteen dominant prairie species in the BioCON experiment. So far, I have found that elevated CO2, elevated nitrogen and declining diversity all have strong effects on seed production that are not dependent on functional group status. Effects of global change on seed production are strong enough to alter hierarchies in colonization ability (e.g. seeds produced per m2) - suggesting there will be consequences for successional trajectories in Minnesota grasslands. I have also quantified seedling relative growth rates from these seeds to determine whether the CO2 and/or nitrogen conditions maternal plants are exposed to affects seedling quality. The CO2 environment of maternal plants does not affect seedling relative growth rates, but elevated nitrogen maternal environments positively affects seedling relative growth rates of several species - potentially due to higher nitrogen concentrations in seeds. Because differences among species during recruitment affect species diversity, rates of succession, invasibility and plant fitness, results suggest that the effects of global change on plant seed production could have dramatic consequences for plant community structure and dynamics.
The Effects of Species Diversity on Ecosystem Function
The relationship between species diversity and ecosystem function has been intensely debated. Few studies focus on quantifying species interactions, although this is critical for determining which mechanisms cause ecosystem function to increase with diversity. In a recent Ecology Letters publication, I focused on the role of species interactions in driving thepositive diversity-productivity relationship the longest running 'Biodiversity' experiment at Cedar Creek Natural History Area. I found that a third of the plant species in the Cedar Creek Biodiversity experiment benefit from inter-specific interactions and contribute to overyielding in diverse plots, and that these species are not simply the most productive monoculture species. This suggests that sampling effects, the increased likelihood that highly productive species are present in diverse plots (often hypothesized to drive the positive diversity-productivity relationship), cannot explain greater productivity at high diversity. Differences among species in resource use are important: plant species that overyield are good competitors for nitrogen OR nitrogen fixers. I also found evidence of species redundancy: in the absence of dominant nitrogen competitors, several species switched from underyielding to overyielding. All in all, this suggests that the complicated interplay of competition for limiting resources (nitrogen in these grasslands), niche differences in resource use, phenology and positive interactions between legumes and the other herbaceous perennials will affect how these grassland ecosystems respond to species extinctions.
I am continuing to develop these ideas in collaboration with Peter Reich, lead-PI of the BioCON experiment. We plan to determine how the addition of limiting resources through global environmental change (i.e. elevated atmospheric carbon dioxide and nitrogen deposition) alters species interactions and affects the dynamics of overyielding in these grasslands. Because species differences in resource uptake (i.e. resource niches) affect the relationship between diversity and ecosystem function, we predict that the addition of CO2 (which reduces limitation by water) and nitrogen (the most limiting soil nutrient at Cedar Creek) should reduce both overyielding and the number of species that overyield. We are testing these ideas using data from the BioCON experiment, which manipulates species diversity, CO2 (2 levels; using free air carbon enrichment technology) and nitrogen (2 levels) in a fully factorial manner.
Density-dependent Mortality and the Latitudinal Gradient in Species Diversity
One important hypothesis explaining the latitudinal gradient in tree diversity focuses on the actions of host-specific herbivores, pathogens and predators. If natural enemies cause negative density-dependent mortality, tree diversity is promoted because abundant species are prevented from dominating forest communities while rare species are buffered from extinction (Janzen 1970, Connell 1971). Furthermore, if pest pressure and levels of host-specificity are decreased by temperate and boreal winters, density-dependent mortality, and hence diversity, will be greatest in tropical latitudes. Many studies have found that tropical trees experience density-dependent mortality, and results have frequently been interpreted as evidence that Janzen-Connell effects are more important in tropical forests than in temperate forests. Recently, however, several studies have shown that density-dependent mortality and natural enemies may also play an important role in temperate systems. For example, I have shown that the prevalence of density-dependent mortality at early life-history stages in temperate forests is similar to that in tropical forests (research conducted at the Coweeta Hydrologic Laboratory in North Carolina. Important questions that remain are whether i) density-dependent mortality results from natural enemies or resource niches; ii) density-dependent mortality is strong enough to promote diversity; and iii) the strength of density-dependent mortality varies predictably with latitude or climatic factors. To answer these questions, I have submitted an NCEAS working group proposal (in collaboration with Helene Muller-Landau, at the University of Minnesota) to bring together theoreticians, ecologists with expertise in natural enemies and resource competition, forest ecologists with the demographic data necessary to quantify and compare the strength of density-dependent mortality in forests world-wide (North America, Japan, Australia, Central America, Latin America, and Southeast Asia). The analyses we are proposing will rigorously test a long-standing hypothesis in tropical forest ecology; that the density-dependent effects of predators and pathogens on tree populations are stronger in tropical forests, and thus, contribute to the latitudinal gradient in tree diversity.
Other Research: The Role of Early-life History Stages in the Coexistence of Temperate Forest Trees
I focused on early life-history stages of forest trees for my dissertation, because they play a pivotal role in explanations for diverse assemblages of tree species. I quantified seed dormancy, dispersal and density-dependent mortality for multiple tree species in a Southern Appalachian forest, and found evidence that each of these processes strongly affects community composition and potentially, species diversity. Contrary to the widespread belief that seed banking is unimportant for population dynamics of temperate tree species, I found that the relatively short seed longevity (<5 yrs) of several Southern Appalachian tree species allows them buffer their highly variable annual seed production. Because all tree species are strongly recruitment limited, seed banking thus provides an important advantage for these tree species. I also used demographic data and statistical models to determine the relative importance of dispersal in governing seed and seedling distributions. Differences among species in seed dormancy and dispersal appear to trade-off with seedling survival and the risk of seed predation, suggesting that a competition-colonization trade- off may mediate species coexistence in these Southern Appalachian forests. Finally, I found that the most abundant tree species in these forests are limited by density-dependent mortality, a mechanism that may promote species diversity by preventing any one species from dominating the community. Density-dependent mortality is often cited as a mechanism promoting diversity in tropical forests, and my work provides evidence that this process also operates in temperate forests.