Whole-system nutrient enrichment increases secondary production in a detritus-based ecosystem

Although the effects of nutrient enrichment on consumer-resource dynamics are relatively well studied in ecosystems based on living plants, little is known about the manner in which enrichment influences the dynamics and productivity of consumers and resources in detritus-based ecosystems. Because nutrients can stimulate loss of carbon at the base of detrital food webs, effects on higher consumers may be fundamentally different than what is expected for living-plant-based food webs in which nutrients typically increase basal carbon. We experimentally enriched a detritus-based headwater stream for two years to examine the effects of nutrient-induced changes at the base of the food web on higher metazoan (predominantly invertebrate) consumers. Our paired-catchment design was aimed at quantifying organic matter and invertebrate dynamics in the enriched stream and an adjacent reference stream for two years prior to enrichment and two years during enrichment. Enrichment had a strong negative effect on standing crop of leaf litter, but no apparent effect on that of fine benthic organic matter. Despite large nutrient-induced reductions in the quantity of leaf litter, invertebrate secondary production during the enrichment was the highest ever reported for headwater streams at this Long Term Ecological Research site and was 1.2-3.3 times higher than predicted based on 15 years of data from these streams. Abundance, biomass, and secondary production of invertebrate consumers increased significantly in response to enrichment, and the response was greater among taxa with larval life spans < or = 1 yr than among those with larval life spans >1 yr. Production of invertebrate predators closely tracked the increased production of their prey. The response of invertebrates was largely habitat-specific with little effect of enrichment on food webs inhabiting bedrock outcrops. Our results demonstrate that positive nutrient-induced changes to food quality likely override negative changes to food quantity for consumers during the initial years of enrichment of detritus-based stream ecosystems. Longer-term enrichment may impact consumers through eventual reductions in the quantity of detritus.     

Cicada emergence in Southwestern riparian forest: influences of wildfire and vegetation composition.

Annually emerging cicadas are a numerically and ecologically dominant species in Southwestern riparian forests. Humans have altered disturbance regimes that structure these forests such that floods are less common and wildfires occur more frequently than was historically the case. Impacts of these changes on primary consumers such as riparian cicadas are unknown. Because cicadas are consumed by a variety of animal species, disturbances that alter timing of their emergence or abundance could have consequences for species at higher trophic levels. We trapped emerging cicadas (Tibicen dealbatus) in burned and unburned riparian forest plots along the Middle Rio Grande in central New Mexico (USA) to determine effects of wildfire and vegetation structure on their density and phenology. We measured vegetation variables and soil temperature at cicada traps and related these variables to variation in emergence density and phenology. We also experimentally heated soil under emergence traps to examine the relationship between soil temperature and emergence phenology. Emergence density was similar in wildfire and unburned plots, though emergence date averaged earlier in wildfire plots and experimentally heated traps. We identified models containing cottonwood proximity (distance from the nearest cottonwood tree) and cottonwood canopy coverage as the most parsimonious explanations of emergence density at each trap. Model selection results were consistent with the literature and field observations that showed that cottonwood trees are an essential resource for T. dealbatus. Cottonwood canopy was also correlated with low soil temperatures, which are associated with later emergence dates. Failure of cottonwoods to reestablish following wildfire could result in cicadas emerging at lower densities and at earlier dates. For cicadas to emerge at densities and times that provide the greatest benefits to birds and other riparian-obligate secondary consumers, riparian forests should be protected from fire, and native vegetation in wildfire sites should be restored.     

Riparian shading and groundwater enhance growth potential for smallmouth bass in Ozark streams.

Moderation of stream temperatures by riparian shading and groundwater are known to promote growth and survival of salmonid fishes, but effects of riparian shade and groundwater on to be growth of warmwater stream fishes are poorly understood or assumed to be negligible. We used stream temperature models to relate shading from riparian vegetation and groundwater inflow to summer water temperatures in Missouri Ozark streams and evaluated effects of summer water temperatures on smallmouth bass, Micropterus dolomieu, growth using a bioenergetics model. Bioenergetics model simulations revealed that adult smallmouth bass in non-spring-fed streams have lower growth potential during summer than fish in spring-fed streams, are subject to mass loss when stream temperatures exceed 27 degrees C, and will likely exhibit greater interannual variation in growth during summer if all growth-influencing factors, other than temperature, are identical between the two stream types. Temperature models indicated that increased riparian shading will expand the longitudinal extent of thermal habitat capable of supporting adult smallmouth bass growth in spring-fed stream reaches when mean daily air temperatures exceed 27 degrees C. Optimum growth temperature (22 degrees C) will be present only in spring-fed streams under these conditions. Potential for increasing shade through riparian restoration is greatest for streams <5 m wide and along north-south reaches of larger streams. However, temperature models also indicated that restoring riparian shading to maximum levels throughout a watershed would increase the total stream mileage capable of supporting positive growth of adult smallmouth bass by only 1-6% when air temperatures are at or near average summer maxima; increases in suitable thermal habitat would be greatest in watersheds with higher spring densities. Riparian management for maintenance or restoration of the thermal habitat of adult smallmouth bass during summer should be focused in areas strongly influenced by groundwater. Restoring riparian shading along spring-fed warmwater streams will likely benefit adult smallmouth bass growth and may ultimately influence population sizes.     

Feedbacks between bivalve density, flow, and suspended sediment concentration on patch stable states

We explore the role of biophysical feedbacks occurring at the patch scale (spatial scale of tens of meters) that influence bivalve physiological condition and affect patch stability by developing a numerical model for the pinnid bivalve, Atrina zelandica, in cohesive sediments. Simulated feedbacks involve bivalve density, flow conditions (assumed to be primarily influenced by local water depth and peak current speed), suspended sediment concentration (evaluated through a balance between background concentration, deposition, and erosion), and changes in the physiology of Atrina derived from empirical study. The model demonstrates that high bivalve density can lead to skimming flow and to a concomitant decrease in resuspension that will affect suspended sediment concentration over the patch directly feeding back on bivalve physiology. Consequently, for a given flow and background suspended sediment load, the stability of a patch directly depends on the size and density of bivalves in the patch. Although under a range of conditions patch stability is ensured independently of bivalve density, simulations clearly indicate that sudden changes in bivalve density or suspended sediment concentration can substantially affect patch structure and lead to different stable states. The model highlights the role of interactions between organisms, flow, and broader scale environmental conditions in providing a mechanistic explanation for the patchy occurrence of benthic suspension feeders.     

Responses of the whipscorpion, Mastigoproctus liochirus (Arachnida, Uropygi) to environmental humidity

Experiments were conducted to determine the responses of third-instar nymphs of the whipscorpion Mastigoproctus liochirus to various moisture (relative humidity) levels under constant temperature conditions, using a linear humidity gradient apparatus. No previous data exist on the water relations of this uropygid. Under saturated conditions (100% RH) animals showed no preference for any section of the chamber. When a humidity gradient was established (20 to 100% RH), whipscorpions exhibited a marked preference for an area of the chamber characterized by 70 to 80% RH.     

Ecological-Economic Modeling for Biodiversity Management: Potential, Pitfalls, and Prospects

Abstract:  Ecologists and economists both use models to help develop strategies for biodiversity management. The practical use of disciplinary models, however, can be limited because ecological models tend not to address the socioeconomic dimension of biodiversity management, whereas economic models tend to neglect the ecological dimension. Given these shortcomings of disciplinary models, there is a necessity to combine ecological and economic knowledge into ecological-economic models. It is insufficient if scientists work separately in their own disciplines and combine their knowledge only when it comes to formulating management recommendations. Such an approach does not capture feedback loops between the ecological and the socioeconomic systems. Furthermore, each discipline poses the management problem in its own way and comes up with its own most appropriate solution. These disciplinary solutions, however, are likely to be so different that a combined solution considering aspects of both disciplines cannot be found. Preconditions for a successful model-based integration of ecology and economics include (1) an in-depth knowledge of the two disciplines, (2) the adequate identification and framing of the problem to be investigated, and (3) a common understanding between economists and ecologists of modeling and scale. To further advance ecological-economic modeling the development of common benchmarks, quality controls, and refereeing standards for ecological-economic models is desirable.     

Evidence for the Role of Infectious Disease in Species Extinction and Endangerment

Abstract:  Infectious disease is listed among the top five causes of global species extinctions. However, the majority of available data supporting this contention is largely anecdotal. We used the IUCN Red List of Threatened and Endangered Species and literature indexed in the ISI Web of Science to assess the role of infectious disease in global species loss. Infectious disease was listed as a contributing factor in <4% of species extinctions known to have occurred since 1500 (833 plants and animals) and as contributing to a species' status as critically endangered in <8% of cases (2852 critically endangered plants and animals). Although infectious diseases appear to play a minor role in global species loss, our findings underscore two important limitations in the available evidence: uncertainty surrounding the threats to species survival and a temporal bias in the data. Several initiatives could help overcome these obstacles, including rigorous scientific tests to determine which infectious diseases present a significant threat at the species level, recognition of the limitations associated with the lack of baseline data for the role of infectious disease in species extinctions, combining data with theory to discern the circumstances under which infectious disease is most likely to serve as an agent of extinction, and improving surveillance programs for the detection of infectious disease. An evidence-based understanding of the role of infectious disease in species extinction and endangerment will help prioritize conservation initiatives and protect global biodiversity.     

Sand stress as a non-determinant of habitat segregation of indigenous (Perna perna) and invasive (Mytilus galloprovincialis) mussels in South Africa

Periodical sand inundation influences diversity and distribution of intertidal species throughout the world. This study investigates the effect of sand stress on survival and on habitat segregation of the two dominant mussel species living in South Africa, the invasive Mytilus galloprovincialis and the indigenous Perna perna. P. perna occupies a lower intertidal zone which, monthly surveys over 1.5 years showed, is covered by sand for longer periods than the higher M. galloprovincialis zone. Despite this, when buried under sand, P. perna mortality rates were significantly higher than those of M. galloprovincialis in both laboratory and in field experiments. Under anoxic condition, P. perna mortality rates were still significantly higher than those for M. galloprovincialis, but both species died later than when exposed to sand burial, underlining the importance of the physical action of sand on mussel internal organs. When buried, both species accumulate sediments within the shell valves while still alive, but the quantities are much greater for P. perna. This suggests that P. perna gills are more severely damaged by sand abrasion and could explain its higher mortality rates. M. galloprovincialis has longer labial palps than P. perna, indicating a higher particle sorting ability and consequently explaining its lower mortality rates when exposed to sand in suspension. Habitat segregation is often explained by physiological tolerances, but in this case, such explanations fail. Although sand stress strongly affects the survival of the two species, it does not explain their vertical zonation. Contrary to our expectations, the species that is less well adapted to cope with sand stress maintains dominance in a habitat where such stress is high. GI Zardi, KR Nicastro contributed equally to the work     

Integrating quality and quantity of mutualistic service to contrast ant species protecting Ferocactus wislizeni

Generalized, facultative mutualisms are often characterized by great variation in the benefits provided by different partner species. This variation may be due to differences among species in the quality and quantity of their interactions, as well as their phenology. Many plant species produce extrafloral nectar, a carbohydrate-rich resource, to attract ant species that can act as "bodyguards" against a plant's natural enemies. Here, we explore differences in the quality and quantity of protective service that ants can provide a plant by contrasting the four most common ant visitors to Ferocactus wislizeni, an extrafloral nectary-bearing cactus in southern Arizona. The four species differ in abundance when tending plants, and in the frequency at which they visit plants. By adding surrogate herbivores (Manduca sexta caterpillars) to plants, we demonstrate that all four species recruit to and attack potential herbivores. However, their per capita effectiveness in deterring herbivores (measured as the inverse of the number of workers needed to remove half of the experimentally added caterpillars) differs. Using these among-species differences in quality (per capita effectiveness) and quantity (number of workers that visit a plant and frequency of visitation), we accurately predicted the variation in fruit production among plants with different histories of ant tending. We found that plant benefits (herbivore removal and maturation of buds and fruits) typically saturated at high levels of ant protection, although plants could be "well defended" via different combinations of interaction frequency, numbers of ant workers per interaction, and per capita effects. Our study documents variation among prospective mutualists, distinguishes the components of this variation, and integrates these components into a predictive measure of protection benefit to the plant. The method we used to average saturating benefits over time could prove useful for quantifying overall service in other mutualisms.