Gary A. Polis

Why Are Parts of the World Green? Multiple Factors Control Productivity and the Distribution of Biomass

This paper evaluates the multiple factors that determine the production of plant biomass and its distribution among producers and various trophic groups of con_ ?^)^ Osumers. In rough order of their importance, water and nutrient availability, factors 0*S that deter herbivores (plant defenses, environmental heterogeneity and disturbance, nutrient stoichiometry), and consumption by herbivores appear to be the most universal determinants of the production and distribution of plant biomass. In some times and places, indirect effects from enemies of herbivores (predators, parasites, parasitoids and pathogens) propagate through the food web to influence plant biomass, in a manner somewhat consistent with green world and exploitation ecosystem mechanisms. I discuss why such food web dynamics appear to be much more important in water than on land. The only demonstrated cases of communitylevel trophic cascades occur in water. Although species-level cascades are moderately frequent on land, community-level cascades rarely or never occur.

Extreme climatic events shape arid and semiarid ecosystems

6 Climatic changes associated with the El Nino Southern Oscillation (ENSO) can have a dramatic impact on ter- restrial ecosystems worldwide, but especially on arid and semiarid systems, where productivity is strongly lim- ited by precipitation. Nearly two decades of research, including both short-term experiments and long-term studies conducted on three continents, reveal that the initial, extraordinary increases in primary productivity percolate up through entire food webs, attenuating the relative importance of top-down control by predators, providing key resources that are stored to fuel future production, and altering disturbance regimes for months or years after ENSO conditions have passed. Moreover, the ecological changes associated with ENSO events have important implications for agroecosystems, ecosystem restoration, wildlife conservation, and the spread of disease. Here we present the main ideas and results of a recent symposium on the effects of ENSO in dry ecosystems, which was convened as part of the First Alexander von Humboldt International Conference on the El Nino Phenomenon and its Global Impact (Guayaquil, Ecuador, 16-20 May 2005).

BOTTOM-UP DYNAMICS OF ALLOCHTHONOUS INPUT: DIRECT AND INDIRECT EFFECTS OF SEABIRDS ON ISLANDS

Allochthonous marine input is a key component of the dynamics of islands and terrestrial coastal ecosystems on islands in the Gulf of California, Mexico, where an unproductive desert juxtaposes a highly productive ocean. In this area, seabirds are a major conduit bringing marine productivity to land. Seabirds leave guano on roosting and nesting islands, and carrion on nesting islands. We analyze how seabirds affect the abundance of a dominant consumer group, tene- brionid beetles, on 25 islands in the Gulf of California over a three-year period. Tenebrionid densities vary by three orders of magnitude among islands. Beetles are most abundant in areas influenced by seabirds: they are approximately five times more dense on nesting and roosting islands than on other islands and on mainland sites, and approximately six times more dense inside vs. outside colonies. Path analyses show that seabirds significantly affect beetles by two distinct pathways. On roosting islands, effects are mainly indirect: guano, a fertilizing nutrient, significantly enhances plant productivity; beetles eat plant detritus. On nesting islands, significant effects are primarily direct: beetles eat seabird carrion; the indirect pathway (guano and plants) contributes little on these islands. By providing energy and nutrients to fuel a diverse array of consumer populations, seabirds are central to the dynamics of these island ecosystems. Scavengers and avian parasites directly increase by eating seabird products. Likewise, populations of consumers that eat detritus, plant tissues, and seeds are enhanced indirectly via the fertilizing effects of guano on plants. Increases in these primary consumers indirectly facilitate high densities of many predators. Thus, consumer populations on these islands are deeply subsidized by substantial input from seabirds. Because consumers cannot influence the renewal rate of their allochthonous resources, the dynamics of these consumers and their food webs are largely donor controlled.

Frontiers of Ecology

integration and collaboration as we meet the challenge of understanding the great complexity of biological systems. Ecological subdisciplines are rapidly combining and incorporating other biological, physical, mathematical, and sociological disciplines. The burgeoning base of theoretical and empirical work, made possible by new methods, technologies, and funding opportunities, is providing the opportunity to reach robust answers to major ecological questions. In December 1999 the National Science Foundation convened a white paper committee to evaluate what we know and do not know about important ecological processes, what hurdles currently hamper our progress, and what intellectual and conceptual interfaces need to be encouraged. The committee distilled the discussion into four frontiers in research on the ecological structure of the earth’s biological diversity and the ways in which ecological processes continuously shape that structure (i.e., ecological dynamics). This article summarizes the discussions of those frontiers and explains why they are crucial to our understanding of how ecological processes shape patterns and dynamics of global biocomplexity. The frontiers are 1. Dynamics of coalescence in complex communities 2. Evolutionary and historical determinants of ecological processes: The role of ecological memory 3. Emergent properties of complex systems: Biophysical constraints and evolutionary attractors 4. Ecological topology: Defining the spatiotemporal domains of causality for ecological structure and processes Each of the four research frontiers takes a different approach to the overall ecological dynamics of biocomplexity, and all require integration and collaboration among those approaches. These overlapping frontiers themselves are not necessarily new. Within each frontier, however, are emerging questions and approaches that will help us understand how ecological processes are interconnected over multiple spatial and temporal scales, from local community structure to global patterns.

Effects of partitioning allochthonous and autochthonous resources on food web stability

The flux of energetic and nutrient resources across habitat boundaries can exert major impacts on the dynamics of the recipient food web. Competition for these resources can be a key factor structuring many ecological communities. Competition theory suggests that competing species should exhibit some partitioning to minimize competitive interactions. Species should partition both in situ (autochthonous) resources and (allochthonous) resources that enter the food web from outside sources. Allochthonous resources are important sources of energy and nutrients in many low productivity systems and can significantly influence community structure. The focus of this paper is on: (i) the influence of resource partitioning on food web stability, but concurrently we examine the compound effects of; (ii) the trophic level(s) that has access to allochthonous resources; (iii) the amount of allochthonous resource input; and (iv) the strength of the consumer–resource interactions. We start with a three trophic level food chain model (resource–consumer–predator) and separate the higher two trophic levels into two trophospecies. In the model, allochthonous resources are either one type available to both consumers and predators or two distinct types, one for consumers and one for predators. The feeding preferences of the consumer and predator trophospecies were varied so that they could either be generalists or specialists on allochthonous and/or autochthonous resources. The degree of specialization influenced system persistence by altering the structure and, therefore, the indirect effects of the food web. With regard to the trophic level(s) that has access to allochthonous resources, we found that a single allochthonous resource available to both consumers and predators is more unstable than two allochthonous resources. The results demonstrate that species populating food webs that experience low to moderate allochthonous resources are more persistent. The results also support the notion that strong links destabilize food web dynamics, but that weak to moderate strength links stabilize food web dynamics. These results are consistent with the idea that the particular structure, resource availability, and relative strength of links of food webs (such as degree of specialization) can influence the stability of communities. Given that allochthonous resources are important resources in many ecosystems, we argue that the influence of such resources on species’ and community persistence needs to be examined more thoroughly to provide a clearer understanding of food web dynamics.

BROOD PARASITISM AMONG SPIDERS: INTERACTIONS BETWEEN SALTICIDS AND DIGUETIA MOJAVEA

Brood parasitism is not well documented in arthropods. We studied interactions among two species of salticid spiders (Habronattus tranquillus and Metaphidippus manni) and a diguetid spider (Diguetia mojavea). Observations and experiments show that these salticids invade a web and deposit their own eggs after D. mojavea mothers die. Although salticid eggs are laid after diguetid eggs are produced, their eggs are larger than those of diguetids, develop much more rapidly, and invariably emerge before the first diguetid spiderlings hatch. Salticid spiderlings also emerge at a significantly larger size than diguetid spiderlings. Thus, the salticid spiderlings have both timing and size advantages over the diguetid hatchlings. In addition, salticid spiderlings prey on resident diguetid juveniles and thereby significantly reduce the hosts fitness. We document experimentally that one defense against this parasitism is the mothers presence after her egg sacs are deposited. Other possible diguetid defenses include high fecundity and asynchronous hatching of egg sacs. Our results suggest that these salticids are obligate brood parasites of D. mojavea and provide one of the first experimental examples of host–parasite coupling among arthropods.