Henry F. Howe

Survival and growth of juvenile Virola surinamensis in Panama: effects of herbivory and canopy closure

Effects of mammalian herbivory and seasonal drought were studied for Virola surinamensis (Myristicaceae) juveniles on Barro Colorado Island, Panama. Seedlings were planted at three months of age and the juveniles were monitored for two years; Treatments included: intact plants protected from mammals by cages, defoliated plants similarly protected, and unprotected plants, each planted in treefall gaps, on gap edges, and in the shaded understorey. Juveniles planted in treefall gaps survived seasonal drought far better than those planted on gap edges or in shaded understorey. Two years after establishment, juveniles protected from mammalian herbivory showed a 78% survival in gaps (mean 6.8% skylight), 50% survival on gap edges (mean 3.0% skylight), and 33% survival in shaded understorey (1.4% skylight). This advantage was due to accelerated growth in gaps. Juveniles in gaps increased 616% in height, 1075% in leaf number, and 1800% in total leaf area. Comparable numbers in edges were 247%, 378% and 690%; in understorey 33%, 222% and 289%. Accelerated growth in gaps permitted yearlings to survive drought that killed suppressed yearlings in understorey. Mean light differentials as small as 0.6% and 0.3% skylight significantly influenced survival on edges and in shaded understorey, respectively. Mammalian herbivory killed juveniles directly, and defoliation by mammals strongly accentuated drought mortality by suppressing root development. Natural defoliation was not attributable to gap conditions. Demographic projections from experimental data suggest that mammalian herbivory kills at least 48% of the juveniles of this species over two years, and contributes to the death of 32% more that actually die of drought stress. These projections suggest that 14% of the juveniles of this species die of drought mortality, independent of herbivory, during the first two years. Herbivory most strongly affects plants The context of establishment determines the ‘shade tolerance’ of this conspicuous canopy tree. Without serious mammalian herbivory or extreme dry seasons, V. surinamensis can easily recruit as a shade tolerant plant in the understorey. Under present conditions on Barro Colorado Island, it cannot. Persistence involves both the chances of arrival in different microhabitats, and survival therein. Projections that include both the forest area represented by gaps, gap edges, and understorey and the experimental results from this study indicate that juvenile V. surinamensis can survive for two years in gaps, edges, and understorey, but that the higher proportions of vigorous individuals survive in edges, gaps and understorey, respectively.

EFFECTS OF BIRDS AND RODENTS ON SYNTHETIC TALLGRASS COMMUNITIES

We used a bird and mammal exclosure design on plowed ground to test for the effects of granivory and herbivory by small vertebrates on early stages of tallgrass prairie succession. Seed predation by birds and browsing by rodents had major and additive impacts. In high- and low-density plantings, respectively, seed-eating birds reduced plant densities by 20% and 23% and grass biomass by 24% and 34%. Meadow voles did not affect plant numbers but reduced forb biomass by 35% in high- and 57% in low-density plantings. In high- and low-density plots, respectively, birds reduced species richness by 3% and 17% without influencing diversity; selective browsing by voles on two legumes and one coneflower left species number unaffected but reduced diversity by 4% and 25% by accentuating dominance of already-prominent species. Bird effects were more pronounced in high-density plantings, while vole effects were most pronounced in low-density plantings. Results suggest that opportunistic finches and doves that seek out ...

EPISODIC DEATH ACROSS SPECIES OF DESERT SHRUBS

Extreme events shape population and community trajectories. We report episodic mortality across common species of thousands of long-lived perennials individually tagged and monitored for 20 years in the Colorado Desert of California following severe regional drought. Demographic records from 1984 to 2004 show 15 years of virtual stasis in populations of adult shrubs and cacti, punctuated by a 55-100% die-off of six of the seven most common perennial species. In this episode, adults that experienced reduced growth in a lesser drought during 1984-1989 failed to survive the drought of 2002. The significance of this event is potentially profound because population dynamics of long-lived plants can be far more strongly affected by deaths of adults, which in deserts potentially live for centuries, than by seedling births or deaths. Differential mortality and rates of recovery during and after extreme climatic events quite likely determine the species composition of plant and associated animal communities for at least decades. The die-off recorded in this closely monitored community provides a unique window into the mechanics of this process of species decline and replacement.

MASSIVE AND DISTINCTIVE EFFECTS OF MEADOW VOLES ON GRASSLAND VEGETATION

We ask whether vole herbivory in experimental grassland plots is sufficient to create an unpalatable community. In a six-year experiment, meadow voles (Microtus pennsylvanicus) reduced plant standing crop between 30% and 72%, well within the range of ungulate effects. Moreover, meadow voles reduced their available forage species by changing the plant community composition: four grass species and a legume upon which they foraged declined sharply in cover and/or number of individuals, five forbs avoided by voles increased, and two forbs neither declined nor increased with either measure. Reductions of diversity occurred when voles first defoliated the plots in 2000 but disappeared as plant species avoided by voles replaced vulnerable plants. Within six years, meadow voles created plant communities dominated by species that they did not eat.

Density and seed set in a self-compatible forb, Penstemon digitalis (Plantaginaceae), with multiple pollinators

Seed production may be limited because flowers do not get enough suitable pollen or because plants lack the resources to make seeds. We used replicated plantings to test factors that influence effects of bumblebee behavior on pollen limitation, as measured by the difference in seed set between hand- and naturally pollinated flowers, of Penstemon digitalis in patches of four to 41 flowering individuals. Seed set per flower was 376% higher in the largest as compared with the smallest Penstemon patches. This positive density dependence reflects activity of long-tongued bees, which (1) have higher effective density as patch size increases, (2) visit greater proportions of plants as patch size increases, and (3) visit smaller proportions of flowers per visited plant as patch size increases. Our results suggest that economics of flight and maneuverability of large, long-tongued bumblebees lead them to transfer more pollen between than within Penstemon plants in large patches. Density of smaller, short-tongued bumblebees was not positively associated with Penstemon seed set, but these bees may be important pollinators at low plant densities. Our experimental system indicates a clear positive relationship between activity of effective pollinators and seed set in a species capable of pollinating itself.

Obvious and Cryptic Vole Suppression of a Prairie Legume in Experimental Restorations

Ecological conditions may amplify or diminish effects of herbivory. We use replicated prairie plantings established in 1997 to test for the effects of two components of ecological conditions, planting density (high or low density at a 10‐fold difference) and planting season (spring or fall), on meadow vole (Microtus pennsylvannicus) herbivory of the prairie perennial Desmanthus illinoensis Michx. (Fabaceae). Over 3 yr, Desmanthus counts declined in plots planted at high density and in December compared with those planted at low density and in June. In 2000, obvious suppression of Desmanthus by voles was reduction of numbers: overall, Desmanthus was 58% less common in plots with vole access than in those without and was virtually eliminated where voles had access in December plantings. In June plantings, direct but cryptic suppression by voles reduced plant biomass (−51%), fecundity (−44%), and seed mass (−50%). In June plantings, Desmanthus adults produced nearly 3000 fewer seeds, each with half the mass, where voles had access compared with where they did not. Desmanthus abundance was much reduced in December plantings, where remaining individuals were smaller and had lower reproductive output than in June plantings, with vole herbivory accentuating seasonal effects. Our results offer insights into the reasons why populations of this legume are sparse in much of central North America and why restoration plantings of this plant sometimes fail.

Planting densities and bird and rodent absence affect size distributions of four dicots in synthetic tallgrass communities

Variability in the size distributions of populations is usually studied in monocultures or in mixed plantings of two species. Variability of size distributions of populations in more complex communities has been neglected. The effects of seeding density (35 or 350xa0seeds/species/m2) and presence of small vertebrates on the variability of size distributions were studied for a total of 1,920 individuals of 4 species in replicated synthetic communities of 18 species in northern Illinois. End-of season height and above-ground biomass were measured for prairie perennials Dalea purpurea (purple prairie clover), Echinacea purpurea (purple coneflower), Desmanthus illinoensis (Illinois bundleflower) and Heliopsis helianthoides (early sunflower). Variability in biomass distribution of the four target species was twice as great at low than at high densities when small vertebrates were excluded. Our results suggest that inter- and intraspecific competition may affect all individuals more under high-density conditions, thereby reducing the variability in their biomass distributions within this community. This result, a consequence of plant-plant interaction, is obscured when small birds or mammals are present, presumably because either or both add variance that overwhelms the pattern.