Zachary T. Long

CAN DISTURBANCE CREATE REFUGIA FROM HERBIVORES : AN EXAMPLE WITH HEMLOCK REGENERATION ON TREEFALL MOUNDS

and soil created by treefalls provide a better site for the survival and growth of hemlock (Tsuga canadensis L.) than the areas immediately surrounding the mounds. The xeric and unstable conditions of tip-up mounds may impede the establishment and growth of hemlock relative to surrounding areas. The size and steep walls of tipup mounds, however, may deter deer from accessing the tops of mounds, thereby allowing hemlocks to escape browsing. Nine years after a catastrophic blowdown in the Allegheny National Forest in northwestern Pennsylvania, we found that hemlocks on the mounds were larger, more abundant, and browsed less often than hemlocks found off of the mounds. The increased growth and survival of hemlocks on tip-up mounds was likely caused by decreased browsing pressure.

Population and community resilience in multitrophic communities.

Diversity-stability relationships have long been a topic of controversy in ecology, but one whose importance has been re-highlighted by increasing large-scale threats to global biodiversity. The ability of a community to recover from a perturbation (or resilience) is a common measure of stability that has received a large amount of theoretical attention. Yet, general expectations regarding diversity-resilience relations remain elusive. Moreover, the effects of productivity and its interaction with diversity on resilience are equally unclear. We examined the effects of species diversity, species composition, and productivity on population-and community-level resilience in experimental aquatic food webs composed of bacteria, algae, heterotrophic protozoa, and rotifers. Productivity manipulations were crossed with manipulations of the number of species and species compositions within trophic groups. Resilience was measured by perturbing communities with a nonselective, density-independent, mortality event and comparing responses over time between perturbed communities and controls. We found evidence that species diversity can enhance resilience at the community level (i.e., total community biomass), though this effect was more strongly expressed in low-productivity treatments. Diversity effects on resilience were driven by a sampling/selection effect, with resilient communities showing rapid response and dominance by a minority of species (primarily unicellular algae). In contrast, diversity had no effect on mean population-level resilience. Instead, the ability of a communitys populations to recover from perturbations was dependent on species composition. We found no evidence of an effect of productivity, either positive or negative, on community- or population-level resilience. Our results indicate that the role of diversity as an insurer of stability may depend on the level of biological organization at which stability is measured, with effects emerging only when focusing on aggregate community properties.

Biodiversity mediates productivity through different mechanisms at adjacent trophic levels.

Biodiversity may enhance productivity either because diverse communities more often contain productive species (selection effects) or because they show greater complementarity in resource use. Our understanding of how these effects influence community production comes almost entirely from studies of plants. To test whether previous results apply to higher trophic levels, we first used simulations to derive expected contributions of selection and complementarity to production in competitive assemblages defined by either neutral interactions, dominance, or a trade-off between growth and competitive ability. The three types of simulated assemblages exhibited distinct interaction signatures when diversity effects were partitioned into selection and complementarity components. We then compared these signatures to those of experimental marine communities. Diversity influenced production in fundamentally different ways in assemblages of macroalgae, characterized by growth-competition trade-offs, vs. in herbivores, characterized by dominance. Forecasting the effects of changing biodiversity in multitrophic ecosystems will require recognizing that the mechanism by which diversity influences functioning can vary among trophic levels in the same food web.

Fertilization and plant diversity accelerate primary succession and restoration of dune communities

Plant species richness can increase primary production because plants occupy different niches or facilitate each other (“complementarity effects”) or because diverse mixtures have a greater chance of having more productive species (“selection effects”). To determine how complementarity and selection influence dune restoration, we established four types of plant communities [monocultures of sea oats (Uniola paniculata), bitter panicgrass (Panicum amarum) and saltmeadow cordgrass (Spartina patens) and the three-species mixture] under different soil treatments typical of dune restorations (addition of soil organic material, nutrients, both, or neither). This fully factorial design allowed us to determine if plant identity, diversity and soil treatments influenced the yield of both the planted species and species that recruited naturally (volunteers). Planted species responses in monocultures and mixtures varied among soil treatments. The composition of the plantings and soils also influenced the abundance of volunteers. The mixture of the three species had the lowest cover of volunteers. We also found that the effect of diversity on production increased with fertilizer. We partitioned the biodiversity effect into complementarity and selection effects and found that the increase in the diversity effect occurred because increased nutrients decreased dominance by the largest species and increased complementarity among species. Our findings suggest that different planting schemes can be used to meet specific goals of restoration (e.g., accelerate plant recovery while suppressing colonization of non-planted species).

Body Size: The consequences of body size in model microbial ecosystems

Patterns in the sizes of coexisting organisms have always intrigued ecologists(Hutchinson, 1961). Some kinds of regularities are well known for some systems(Sheldon, Prakash & Sutcliffe, 1972), and are less appreciated or rediscovered forothers (Enquist & Niklas, 2001; Cohen, Jonsson & Carpenter, 2003). One exampleof this kind of pattern is the apparent cons tancy of total biomass within differentsizefractionsoforganismslivinginaquaticcommunities(Sheldon, etal.1972;Cyr,2000; Kerr & Dickie, 2001; Cohen et al., 2003; Sheldon, Sutcliffe & Paranjape, 1977;Tilman et al., 2001; Mulder et al., 2005). Essentially,over many orders ofmagnitudeof organism size, any particular size class holds about the same total biomass oforganisms per unit volume. The result is an inverse relationship between the logoforganism size and the logoforganism abundance per unit volume, with a slopeof 1. The apparent constancy of this relationship has even led some workers tosuggest, tongue in cheek, that it could be used to estimate the population size ofsome organisms that have proven to be notoriously difficult to observe, onceassumptions about their average size were made (Sheldon & Kerr, 1972, 1973).Whether or not the elusive Loch Ness Monster (to which these calculations wererather whimsically applied) actually exi sts, it appears that the total biomass oforganisms in some habitats is fixed by certain features of the habitat, most likelythe abundance of incoming energy and nutrients that drive productivity (Sheldonetal., 1977; Cyr, 2000; Kerr & Dickie, 2001; Cohen et al.,2003; Mulder et al., 2005).Arecent flurry of research on the allometry of metabolism seeks to explain suchpatternsmechanisticallyasaconsequenceofthewaysthatorganismscaptureandtransport energy and material (Brown, 2004). It is worth noting that none of thesepatterns suggest that diversity, the number of different organisms that totalbiomass is divided among, should necessarily influence the total standing stockof biomass supported by a particular environment and its energetic regime.