Trophic cascades in terrestrial ecosystems. Reflections on Polis et al

Flowering and trichome development share hormonal and transcription factor regulation

Gibberellins (GAs) and cytokinins (CKs) are plant hormones that act either synergistically or antagonistically during the regulation of different developmental processes. In Arabidopsis thaliana, GAs and CKs overlap in the positive regulation of processes such as the transition from the vegetative to the reproductive phase and the development of epidermal adaxial trichomes. Despite the fact that both developmental processes originate in the rosette leaves, they occur separately in time and space. Here we review how, as genetic and molecular mechanisms are being unraveled, both processes might be closely related. Additionally, this shared genetic network is not only dependent on GA and CK hormone signaling but is also strictly controlled by specific clades of transcription factor families. Some key flowering genes also control other rosette leaf developmental processes such as adaxial trichome formation. Conversely, most of the trichome activator genes, which belong to the MYB, bHLH and C2H2 families, were found to positively control the floral transition. Furthermore, three MADS floral organ identity genes, which are able to convert leaves into floral structures, are also able to induce trichome proliferation in the flower. These data lead us to propose that the spatio-temporal regulation and integration of diverse signals control different developmental processes, such as floral induction and trichome formation, which are intimately connected through similar genetic pathways.

Toward a trophic theory of species diversity

Efforts to understand the ecological regulation of species diversity via bottom-up approaches have failed to yield a consensus theory. Theories based on the alternative of top-down regulation have fared better. Paine's discovery of keystone predation demonstrated that the regulation of diversity via top-down forcing could be simple, strong, and direct, yet ecologists have persistently failed to perceive generality in Paine's result. Removing top predators destabilizes many systems and drives transitions to radically distinct alternative states. These transitions typically involve community reorganization and loss of diversity, implying that top-down forcing is crucial to diversity maintenance. Contrary to the expectations of bottom-up theories, many terrestrial herbivores and mesopredators are capable of sustained order-of-magnitude population increases following release from predation, negating the assumption that populations of primary consumers are resource limited and at or near carrying capacity. Predation sensu lato (to include Janzen-Connell mortality agents) has been shown to promote diversity in a wide range of ecosystems, including rocky intertidal shelves, coral reefs, the nearshore ocean, streams, lakes, temperate and tropical forests, and arctic tundra. The compelling variety of these ecosystems suggests that top-down forcing plays a universal role in regulating diversity. This conclusion is further supported by studies showing that the reduction or absence of predation leads to diversity loss and, in the more dramatic cases, to catastrophic regime change. Here, I expand on the thesis that diversity is maintained by the interaction between predation and competition, such that strong top-down forcing reduces competition, allowing coexistence.

BOTTOM-UP AND TOP-DOWN EFFECTS ON INSECT HERBIVORES DO NOT VARY AMONG SITES OF DIFFERENT SALINITY

It has been suggested, but rarely tested, that the relative strength of top-down and bottom-up factors in communities varies along an environmental stress gradient. We compared the strength of bottom-up and top-down effects on the densities of insect herbivores along a range of sites of different salinities in west-central Florida. We used a 2 x 2 factorial design with plots divided into four treatments: (1) bottom-up manipulation, where fertilizer was applied to increase plant quality; (2) top-down manipulation, where sticky traps were used to reduce the effects of natural enemies (parasitoids); (3) bottom-up and top-down manipulation, where fertilizer was applied and sticky traps were used; and (4) control plots. These plots were established along a range of salinities among seven different sites containing the salt marsh plant Borrichia frutescens. In each plot, we determined the parasitism levels and abundances of the sap sucker Pissonotus quadripustulatus, the gall maker Asphondylia borrichiae, and the lepidopteran stem borer Argyresthia spp. Gall density, Pissonotus density, and stem borer density were significantly higher in lower salinity sites, suggesting a strong effect of environmental stress. There was a significant increase of galls and Pissonotus and a marginally significant increase of bored stems on fertilized plots but not on trapped plots. There was a significant interaction of site and fertilizer on gall parasitism. There were no interactions of either treatment with salinity on herbivore densities. The general lack of interaction between salinity level and other treatments on herbivore densities contrasts with our previous result where treatment effects did vary with salinity level on a large experimentally generated salinity gradient at one site. Thus, the results of the present paper suggest that, while environmental stress can modify top-down and bottom-up effects on herbivores at single sites, variation in site-to-site factors, possibly including clonal identity of plant, affects herbivore densities so much as to swamp out any observable interaction between environmental stress and top-down or bottom-up factors.

Trophic diversity in two grassland ecosystems

The roles of consumers (top-down forces) versus resources (bottom-up forces) as determinants of alpha diversity in a community are not well studied. Numerous community ecology models and empirical studies have provided a framework for understanding how density at various trophic levels responds to variation in the relative strength of top-down and bottom-up forces. The resulting trophic theory can be applied to understanding variation in insect diversity at different trophic levels. The objective of this research was to elucidate the strengths of direct and indirect interactions between plants and entire arthropod communities to determine the effects of trophic interactions on arthropod diversity. Grassland plant and insect diversity was measured in July 2001 to document patterns of diversity at multiple trophic levels. The study site includes riparian grasslands in North-Central Colorado on the Carpenter Ranch, owned and managed by The Nature Conservancy. This pastureland consists of sites with different management regimes: unmanaged pasture intermixed along riparian forest, and cattle grazed pasture with flood irrigation. Plant abundance and richness were higher on the grazed-irrigated pasture versus the unmanaged field. Path analysis revealed strong effects of herbivore diversity on diversity of other trophic levels. For the managed fields, top-down forces were important, with increases in enemy diversity depressing herbivore diversity, which in turn depressed plant abundance. For the unmanaged fields, bottom-up forces dominated, with increases in plant diversity causing increased herbivore diversity, which in turn increased enemy diversity. These results support hypotheses from other empirical studies, demonstrating that changes in diversity of a single trophic level can cascade to effect diversity at other, nonadjacent trophic levels.

Do multitrophic interactions override N fertilization effects on Operophtera larvae?

We examined how performance of Operophtera brumata (Lepidoptera) larvae was affected by nitrogen (N) fertilization of boreal forest understorey vegetation. We monitored larval densities on Vaccinium myrtillus plants for a period of 7 years in a field experiment. Preliminary results indicated that the N effect on larval densities was weak. To examine if this was due to indirect interactions with a plant pathogen, Valdensia heterodoxa, that share the same host plant, or due to top-down effects of predation, we performed both a laboratory feeding experiment (individual level) and a bird exclusion experiment (population level) in the field. At the individual level, altered food plant quality (changes in plant concentration of carbon, N, phenolics, or condensed tannins) due to repeated infection by the pathogen had no effect on larval performance, but both survival to the adult stage and adult weight were positively affected by N fertilization. Exclusion of insectivorous birds increased the frequency of larval damage on V. myrtillus shoots, indicating higher larval densities. This effect was stronger in fertilized than in unfertilized plots, indicating higher bird predation in fertilized plots. Predation may thus explain the lack of fertilization effect on larval densities in the field experiment. Our results suggest that top-down effects are more important for larval densities than bottom-up effects, and that bird predation may play an important role in population regulation of O. brumata in boreal forests.

Seasonal shift from bottom-up to top-down impact in phytophagous insect populations

Although many studies now examine how multiple factors influence the dynamics of herbivore populations, few studies explicitly attempt to document where and when each is important and how they vary and interact. In fact, how temporal variation in top-down (natural enemies) and bottom-up (host plant resources) factors affect herbivore dynamics has been suggested as a particularly important yet poorly understood feature of terrestrial food webs. In this study we examined how temporal changes in predator density (wolf spiders, sheet-web builders, and mirid egg predators) and host-plant resources (plant quality and structural complexity) influence the population dynamics of the dominant phytophagous insects on Atlantic-coast salt marshes, namely Prokelisia planthoppers (Homoptera: Delphacidae). We designed a factorial experiment in meadows of Spartina alterniflora to mimic natural variation in vegetation quality and structure by establishing two levels of plant nutrition (leaf nitrogen content) by fertilization, and two levels of habitat complexity by adding leaf litter (thatch). We then assessed seasonal changes in the strength of bottom-up (plant quality) and top-down (predator) impacts on planthopper populations. Planthopper populations responded positively to increased plant quality treatments in late summer. Despite the greater number of planthopper adults colonizing fertilized Spartina plots compared to unfertilized controls, the offspring of these colonists were much less abundant at the end of the season in fertilized plots, particularly those with thatch. The initial colonization effect was later erased because arthropod predators selectively accumulated in fertilized plots where they inflicted significant mortality on all stages of planthoppers. Predators rapidly colonized fertilized plots and reached high densities well in advance of planthopper colonization, a response we attribute to their rapid aggregation in complex-structured habitats with readily available alternative prey. Our results suggest that plant resources not only mediate the strength of predator impacts on herbivore populations, but they also promote the coupling of predator and prey populations and thus influence when enemy impacts are realized.