In this review we compile results on interactions above and below ground and food web functioning in an arid environment at the Baza Basin, in the Iberian southeast. Our data reveal that herbivory is ...difficult to estimate in our system. Some herbivores (rabbits) and granivores (
Messor ants) create nutrient and detritus-rich patches, with important effects on the diversity and abundance of species both above and below ground. Fluctuations of prey availability, especially those caused by changes in their circadian rhythms, provoke seasonal migrations and omnivory in predators. We also present experimental evidence of the effects of interactions between aboveground and belowground macroarthropods on litter degradation; belowground detritivores are responsible for a high percentage of ground surface litter decomposition. We demonstrate that belowground herbivores and decomposers can dramatically change aboveground multitrophic interactions. Finally, we identify the biotic and abiotic factors controlling aboveground and belowground macrofaunal distribution at a broad scale. We conclude that trophic interactions in this semi-arid area are numerous and complex with many of the interactions involving more than two or three organisms. The interactions between above- and belowground organisms added complexity to this system. These habitats in which organisms deal with extreme abiotic conditions promote odd interactions resulting in an increase of biodiversity. Finally, despite the large research effort devoted to understanding food web structure and dynamics in this area recently, our state of knowledge is still far from providing a complete picture of interactions and their implications in the regulation and functioning of the system.
► Review of above- and belowground trophic interactions in an arid system in Spain. ► Rabbits and ants create nutrient patches that modulate above and belowground biodiversity. ► Fluctuations of prey availability provoke seasonal migrations and omnivory in predators. ► Belowground herbivores and decomposers change aboveground multitrophic interactions.
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.
The goal of this paper is to provide a general description of the terrestrial arthropod fauna of arid areas of SE Iberian Peninsula. The available data show that invertebrate communities in these ...environments are dominated by arthropods and that the composition of the assemblages are similar to those described in other arid regions of the world. The number of species of different arthropod groups (Coleoptera, Hymenoptera, Orthoptera, Lepidoptera, Araneae) in these environments show that arid regions of SE Iberian Peninsula are inhabited by a diverse terrestrial arthropod fauna. An ecological feature of arthropod communities in arid zones of SE Spain is the spatial and temporal variability of assemblage composition at different scales, a characteristic of faunal assemblages in arid environments. Similarity in composition of assemblages between sites (Beta diversity) shows a high variability in faunal composition between apparently similar environments. The distribution patterns of species show that 8.4–9.6% of the species in these environments are local and SE Iberian Peninsula endemics. A number of eremic Mediterranean or Palaearctic species are also associated exclusively with these environments, highlighting the singularity of the invertebrate fauna in these arid habitats. Differences in assemblage composition of arid areas with more mesic surrounding environments show that arid zones of SE Spain are inhabited by a distinct fauna, increasing faunal diversity in SE Spain. A recent analysis of endangered and threatened invertebrate species in southern Spain recognizes the arid environments of SE Spain as priority areas for conservation, where habitat loss and fragmentation, changes in agricultural and grazing uses, and inadequate management constitute the main threats. We conclude that the fauna of the SE Iberian arid zones represents an important component of the high ecological diversity of the Mediterranean ecosystem in SE Spain, not only because of their arthropod diversity but also due to the important implications that these areas have for conservation in the entire SE region of the Iberian Peninsula.
► The goal is to describe the terrestrial arthropod fauna of arid areas of SE Spain. ► A feature of the fauna is the spatio-temporal variability in assemblage composition. ► These arid areas are inhabited by a distinct fauna including 8–10% endemic species. ► SE Spain arid habitats are priority areas for arthropod conservation but threatened. ► The fauna of arid habitats is an overlooked component of the diversity of SE Spain.
Stable isotope analysis is a powerful tool for unravelling the complex structure of food webs. This technique is particularly well suited for studies at ecosystem boundaries, where physical processes ...and mobile consumers link the dynamics of seemingly disparate systems. In coastal and insular environments, seabirds play a crucial role in transporting marine-based energy and nutrients to islands. Here we show using stable isotopes that nutrients from the ocean drive the dynamics of terrestrial food webs on small islands. The indirect effects of seabird-derived nutrients on plant productivity are particularly prominent during wet El Niño Southern Oscillation years on our Gulf of California study sites. During dry years that characterize the region, many terrestrial consumers are subsidized by carrion and prey from the ocean. Shifts in trophic structure related to El Niño Southern Oscillation could only be elucidated because of the distinct nitrogen isotope ratios associated with seabird islands. The contributions of seabirds and other marine sources are reflected in the isotope signatures of terrestrial consumers in ways that challenge conventional interpretations of stable isotope results in studies of food webs.
The tremendous increase in precipitation associated with the 1992-1993 El Niño 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 Niño 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 Niño 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 Niño 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 Niño, most material originated via productivity by terrestrial plants. Thus, wet El Niños 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 Niño years: the persistence and slow release of plant and detrital biomass reserves may also greatly affect dynamics for years after the El Niño event has passed. We suggest that large-scale climatic events such as El Niño may be long-lasting determinants of community dynamics rather than occasional disturbance events.
Acorn burial and consumption by geotrupid dung beetles is an interesting trophic interaction recently reported for Iberian Thorectes and North American Mycotrupes species. In Iberian Thorectes ...species, this interaction provides not only ecophysiological and reproductive advantages to the beetles but also more effective dispersal for Quercus acorns. The genus Thorectes is particularly diverse in the Iberian Peninsula and North Africa, where most species of the genus occur. Due to the high diversity of Thorectes in North Africa, especially in Morocco, where Quercus forests are widely distributed, we investigated whether acorn burial and consumption by Thorectes beetles also occurred in this region. For this, field sampling was conducted in northern Morocco by searching for beetles buried and consuming acorns within 1 m2 quadrats in areas where Thorectes species and Quercus forests occur jointly. Three beetle species (T. distinctus, T. laevigatus and T. trituberculatus) were found consuming buried acorns of Q. suber in the field. Most of the beetles were found feeding on acorns, while only 20% were found in dung-provisioned burrows. These results suggest that acorns constitute the staple food of some North African Thorectes species. In addition, we found T. trituberculatus and T. distinctus burrows and nests provisioned with oak leaf litter. Burrows and nests provisioned with Quercus litter also suggest a tighter interaction of Thorectes beetles with oaks. Feeding and nesting with relatively predictable and abundant resources in oak forests, such as acorns and leaf litter, may be especially important to explain the diversification and success of flightless Thorectes species in Mediterranean ecosystems such as those of the Iberian Peninsula and North Africa.
This paper investigates the effect of brood parasitism in a dung beetle assemblage in an arid region of Spain. The study was conducted during the spring season (March-May 1994-1998) using mesh ...cylinders buried into the ground, filled with sand and with sheep dung on top. We quantified the proportion of nests containing larvae of parasitic beetles and their effect on host larvae survival. Experiments on the effect of parasitic larvae on host-larvae survival were conducted by placing scarab brood masses (raised from captive scarabs in the laboratory) in containers with and without aphodiid larvae. During the spring, dung desiccation is rapid, preventing aphodiids nesting in the dung, and forcing these species to adopt brood parasitism as a nesting strategy. Parasitic aphodiids were found in 12-47% of scarab nests of three species. The incidence of brood parasitization was positively related with the number of brood masses contained in the nests, being also higher in the most abundant species. Field data and experiments showed that brood parasites significantly reduced host larvae survival from 74.8% in non-parasitized nests to 8.8% in parasitized nests. Because different rates of nest parasitization and mortality were caused by parasites, brood parasitism had a differential effect on different host species. Thus, brood parasitism constitutes an important mortality factor reducing the reproductive success of the host species and potentially affecting the beetle abundance in the area.
Over a period of 2 yr, physiological and morphological traits related with nesting in two dominant dung beetle species in the Guadix-Baza Basin (Onitis ion Ol. and Onthophagus merdarius Chevr., ...Coleoptera: Scarabaeidae) were analyzed. The role of physiological conditions and resource availability for nesting as factors determining reproductive output in these species is discussed. Our results show that food does not seem to constrain sexual maturation. Both species reached sexual maturation and stored large amounts of reserves after a short period of nourishment. In addition, females were inseminated shortly after emergence, so they were ready to lay eggs only a few days after emergence. Although both species reached sexual maturation quickly, the number of nests dug by the females and the number of eggs laid were low, as indicated by the very low tibial wear and the low proportion of yellow bodies found. Although the rate of dung colonization was high, the number of beetles per dung pat was generally low. Therefore, aggregation of beetles does not seem to explain the low proportion of eggs laid by females of these species. Most dung pats in the study site were small, and only a small proportion (10.4%) was large enough to be suitable for nesting. In conclusion, both species were ready to reproduce soon after emergence, allowing beetles to increase their chances of reproductive success in a harsh environment in which high dung desiccation rates cause a low availability of suitable dung for nesting.
