The tremendous increase in precipitation associated with the 1992-1993 El Niñ o profoundly affected terrestrial communities on arid islands in the Midriff region of the Gulf of California. In 1992, winter precipitation was 5.4 times the historical mean, and winter precipitation over the entire El Niñ o was the highest two-year amount ever recorded. Increased precipitation led to an explosion of annual plant growth on the previously barren (0-4% cover) islands: plant cover increased 10-160 times over what it had been. With the resumption of arid conditions in 1994, live plant cover returned to the low levels seen before the onset of El Niño.Insect abundance tracked this pulse in plant productivity and approximately doubled in 1992 and 1993 compared to 1991 levels. In 1994, the crash of annual plants caused insect densities to drop to the lowest levels recorded during the 5-yr study. El Niñ o also affected the composition of the insect assemblage. In the dry years 1990-1991, the assemblage was dominated by insects feeding on products originating in the ocean: detritivores/scavengers on shore drift of marine algae and carcasses, avian parasites, and detritivores of bird products. Herbivores were extremely rare. The heavy plant growth in 1992 stimulated large (40-190 times pre-El Niñ o levels) increases in herbivores.The great increases in land plant biomass and insect abundance are indicative of an important change in the dynamics of this system. Previously, most material flowing through the food webs of these islands originated directly or indirectly in the ocean. In contrast, during this El Niñ o, most material originated via productivity by terrestrial plants. Thus, wet El Niñ os represent an agent that switches the system from one dependent primarily on allochthonous input to a system driven to a greater extent by in situ productivity. The influence of this pulse of terrestrial productivity extends beyond the El Niñ o years: the persistence and slow release of plant and detrital biomass reserves may also greatly affect dynamics for years after the El Niñ o event has passed. We suggest that large-scale climatic events such as El Niñ o may be long-lasting determinants of community dynamics rather than occasional disturbance events.

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, tenebrionid 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.

No abstract

Ivermectin is a veterinary pharmaceutical generally used to control the ecto- and endoparasites of livestock, but its use has resulted in adverse effects on coprophilous insects, causing population decline and biodiversity loss. There is currently no information regarding the direct effects of ivermectin on dung beetle physiology and behaviour. Here, based on electroantennography and spontaneous muscle force tests, we show sub-lethal disorders caused by ivermectin in sensory and locomotor systems of Scarabaeus cicatricosus, a key dung beetle species in Mediterranean ecosystems. Our findings show that ivermectin decreases the olfactory and locomotor capacity of dung beetles, preventing them from performing basic biological activities. These effects are observed at concentrations lower than those usually measured in the dung of treated livestock. Taking into account that ivermectin acts on both glutamate-gated and GABA-gated chloride ion channels of nerve and muscle cells, we predict that ivermectin’s effects at the physiological level could influence many members of the dung pat community. The results indicate that the decline of dung beetle populations could be related to the harmful effects of chemical contamination in the dung.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. 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, tenebrionid 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.

Spider density on desert islands in the Gulf of California is a joint function of variable productivity (bottom‐up effects) and consumer intensity (top‐down effects). Data from 20 islands and five years (1990–1994) show that the cumulative productivity of islands, a sum of the input of marine detritus and terrestrial plant productivity, varies spatially and temporally. Marine inputs are a function of the perimeter/area ratio and are thus relatively greater on smaller islands. Land plant productivity, a function of precipitation, is negligible in most years but can increase greatly (10–160× in plant cover) with heavy rains from El Niño events (e.g., 1992–1993). Consumer intensity is a function of the continual influence of predaceous scorpions and the sporadic occurrence of parasitoid spider wasps (Pompilidae); the importance of these factors varies greatly, both because scorpions are absent on some islands and because parasitoid densities show extreme year‐to‐year oscillations. El Niño rains produced large changes in the quantity and quality of plant resources and the control of spider populations. Spider densities doubled in 1992 in response to high levels of prey that followed heavy rains but crashed in 1993, despite continued high plant productivity and prey availability. The appearance of copious nectar and pollen, food of adult parasitoid wasps, caused wasp populations to erupt in 1993. Wasps emerged as a major, but previously hidden, trophic influence and greatly suppressed spider populations. Pompilids act as nonequilibrium agents—they are regulated by factors other than prey (spider) availability. In general, smaller spider populations were more variable year to year in the magnitude of their increases and decreases. In spite of the great temporal and spatial variability, consistent changes in plant, prey, spider, and wasp dynamics occurred repeatedly and apparently independently on islands throughout the archipelago. Such patterns of concerted change strongly suggest that general processes related to productivity and consumption are key to understanding the dynamics of this system.

No abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Spider density on desert islands in the Gulf of California is a joint function of variable productivity (bottom-up effects) and consumer intensity (top-down effects). Data from 20 islands and five years (1990)(1991)(1992)(1993)(1994) show that the cumulative productivity of islands, a sum of the input of marine detritus and terrestrial plant productivity, varies spatially and temporally. Marine inputs are a function of the perimeter/area ratio and are thus relatively greater on smaller islands. Land plant productivity, a function of precipitation, is negligible in most years but can increase greatly (10-160X in plant cover) with heavy rains from El Nifio events (e.g., 1992-1993). Consumer intensity is a function of the continual influence of predaceous scorpions and the sporadic occurrence of parasitoid spider wasps (Pompilidae); the importance of these factors varies greatly, both because scorpions are absent on some islands and because parasitoid densities show extreme yearto-year oscillations.El Nifio rains produced large changes in the quantity and quality of plant resources and the control of spider populations. Spider densities doubled in 1992 in response to high levels of prey that followed heavy rains but crashed in 1993, despite continued high plant productivity and prey availability. The appearance of copious nectar and pollen, food of adult parasitoid wasps, caused wasp populations to erupt in 1993. Wasps emerged as a major, but previously hidden, trophic influence and greatly suppressed spider populations. Pompilids act as nonequilibrium agents-they are regulated by factors other than prey (spider) availability. In general, smaller spider populations were more variable year to year in the magnitude of their increases and decreases.In spite of the great temporal and spatial variability, consistent changes in plant, prey, spider, and wasp dynamics occurred repeatedly and apparently independently on islands throughout the archipelago. Such patterns of concerted change strongly suggest that general processes related to productivity and consumption are key to understanding the dynamics of this system. can vary greatly in space and time. Freshwater ecologists have most clearly analyzed how variable predation and productivity interact to form a rich array of dynamics (Carpenter and Kitchell 1993, Rosemond et al. 1993, Osenberg and Mittelbach 1996). The concurrent analysis of top-down and bottom-up forces on the structure and control of terrestrial populations and communities is less common (e.g., Schmitz 1993, 1994, Polis and Hurd 1995; see Strong 1992). Pol...