Climate change and primary production: Forty years in a bunchgrass prairie.

Over the past 109 years, a Montana intermountain bunchgrass prairie annually became warmer (0.7°C) and drier (27%). The temperature and precipitation trends continued since 1978, as we studied nitrogen availability, annual aboveground primary production (ANPP), plant phenology and species composition. Given the annual increase in temperature and decrease in precipitation, ANPP might be expected to decline; however, it increased by 110%, as the period of greatest production (late-May-June) became wetter and cooler, counter to the annual pattern, and this was strongest at lower elevations. Grass production increased by 251%, while dicot production declined by 65%, which increased grass relative abundance by 54%. Summer temperatures increased 12.5% which increased plant senescence by 119% and decreased fall plant regrowth by 68%. More intense summer senescence changed plant species composition in favor of more drought tolerant species. The greater ANPP and summer senescence may increase susceptibility for fire, but fire tolerance of the plant species composition did not change. Invasive plant species increased 108% over the study with annual grasses accounting for >50% of this increase, which further increased summer plant senescence. Therefore, seasonal climate changes at a smaller geographical scale (local), rather than average annual climate changes over a larger geographical scale (regional), may better reflect plant community responses, and this makes ecological forecasting of climate change more difficult.

Biotic Versus Abiotic Control of Primary Production Identified in a Common Garden Experiment.

Understanding drivers of ecosystem primary production is a foundational question in ecology that grows in importance with anthropogenic stresses (e.g., climate change). Traditionally, ecosystem production is considered to be abiotically controlled at large spatial scales (e.g., precipitation, temperature, etc.), which underlies forecasting climate change impacts. Using a "common garden" experiment over 10 years at two sites with the same plant and grasshopper species, we show that primary production is strongly influenced by biotic factors (herbivory and plant adaptations to it) at finer spatial scales by creating positive feedbacks, which reverse relative productivity of ecosystems expected from abiotic conditions alone. Our results without herbivory indicate that one site has 26% less annual net primary production (ANPP) than the other site. With herbivory, the sites reverse in ANPP, so the site with lower ANPP without herbivory now is 15% greater than the site with higher ANPP without herbivory, as they respectively increase by 6% and decline by 33%. This reversal is due to changing nitrogen availability (N), as N becomes 16% greater at the higher ANPP site with herbivory, respectively a 3% increase and 41% decline in N. The ANPP and N changes are observed, even though the sites are a few kilometers apart and have the same grasshopper and plant species.

Herbivore and detritivore effects on rainforest plant production are altered by disturbance.

Consumer effects on rainforest primary production are often considered negligible because herbivores and macrodetritivores usually consume a small fraction of annual plant and litter production, even though consumers are known to have effects on plant production and composition in nontropical systems. Disturbances, such as treefall gaps, however, often increase resources to understory food webs, thereby increasing herbivory and feeding rates of detritivores. This increase in consumption could lead to more prominent ecosystem-level effects of consumers after disturbances, such as storms that cause light gaps. We determined how the effects of invertebrate herbivores (walking sticks) and detritivores (litter snails) on understory plant growth may be altered by disturbances in a Puerto Rican rainforest using an enclosure experiment. Consumers had significant effects on plant growth, but only in light gaps. Specifically, herbivores increased plant growth by 60%, and there was a trend for detritivores to reduce plant growth. Additionally, plant biomass tended to be 50% higher with both consumers in combination, suggesting that herbivores may mediate the effects of detritivores by altering the resources available to detritivore food webs. This study demonstrates that disturbance alters the effects of rainforest consumers, and, furthermore, that consumer activity has the potential to change rainforest successional processes.

A management case study for a new commercial fishery: brine shrimp harvesting in Great Salt Lake, Utah, USA.

A fishery for brine shrimp (Artemia franciscana) cysts to supply the aquaculture industry considerably expanded in the late 1980s in the Great Salt Lake, Utah, USA. With this expansion, concerns emerged in the 1990s about the fishery's sustainability, especially its impact on the abundant western North American waterbirds that use the lake and feed on brine shrimp. We track the development of management strategies using adaptive management by the Utah Division of Wildlife Resources (UDWR), which focused on the biology of the system and development of biology-based harvesting models. The models and their rationale are presented, their success in forecasting is evaluated, and implications for managing the harvest and conserving waterbirds are examined. We view this as an interesting case study because it transpired over a short time in a relatively simple system. This permitted us to clearly track management from the onset of a harvest market, through realization that the harvest had to be managed in the absence of needed biological knowledge, to the adaptive development of management strategies as biological knowledge was accumulated. The outcome illustrates the success that harvest management can attain with careful monitoring of the resource and terminating the harvest when a necessary escapement stock is attained.

The interaction of temperature and precipitation determines productivity and diversity in a bunchgrass prairie ecosystem.

Over the past century at the National Bison Range, temperature has increased by 0.6 °C, and annual precipitation has decreased by 26%, despite increases in May-June precipitation over the past 35 years. Limited experimental work to date has explored plant responses produced by the interaction of changes in both temperature and precipitation, and of the existing studies, none have focused on the endangered bunchgrass ecosystem. Using a 2-year climate change manipulation experiment, we show that bunchgrass productivity increased with supplemental growing season precipitation, while warming alone exerted no significant effect. More importantly, the ratio of June precipitation to minimum temperature, a representation of the interaction of climate variables, predicted bunchgrass productivity better than either climate variable individually. This ratio acted as a surrogate index reflecting increased evapotranspiration with rising temperatures and thus better predicting soil moisture available for plant growth. Experimental warming over the summer led to significantly lower plant species richness and biodiversity, while increased precipitation, when applied over the entire summer, counteracted some of these declines. Warming also led to greater and more rapid plant senescence over the summer, resulting in greater litter production [an increase of 47.82 ± 17.82 (± SE) percentage points] and potential fire risk. Given the simultaneous changes in precipitation and temperature predicted for the next century, multi-factor experiments are essential to understand how ecosystems will respond to future climate scenarios.

Tropical herbivorous phasmids, but not litter snails, alter decomposition rates by modifying litter bacteria.

Consumers can alter decomposition rates through both feces and selective feeding in many ecosystems, but these combined effects have seldom been examined in tropical ecosystems. Members of the detrital food web (litter-feeders or microbivores) should presumably have greater effects on decomposition than herbivores, members of the green food web. Using litterbag experiments within a field enclosure experiment, we determined the relative effects of common litter snails (Megalomastoma croceum) and herbivorous walking sticks (Lamponius portoricensis) on litter composition, decomposition rates, and microbes in a Puerto Rican rainforest, and whether consumer effects were altered by canopy cover presence. Although canopy presence did not alter consumers' effects, focal organisms had unexpected influences on decomposition. Decomposition was not altered by litter snails, but herbivorous walking sticks reduced leaf decomposition by about 50% through reductions in high quality litter abundance and, consequently, lower bacterial richness and abundance. This relatively unexplored but potentially important link between tropical herbivores, detritus, and litter microbes in this forest demonstrates the need to consider autotrophic influences when examining rainforest ecosystem processes.

Impacts of harvesting on brine shrimp (Artemia franciscana) in Great Salt Lake, Utah, USA.

Selective harvesting can cause evolutionary responses in populations via shifts in phenotypic characteristics, especially those affecting life history. Brine shrimp (Artemia franciscana) cysts in Great Salt Lake (GSL), Utah, USA are commercially harvested with techniques that select against floating cysts. This selective pressure could cause evolutionary changes over time. Our objectives are to (1) determine if there is a genetic basis to cyst buoyancy, (2) determine if cyst buoyancy and nauplii mortality have changed over time, and (3) to examine GSL environmental conditions over time to distinguish whether selective harvesting pressure or a trend in environmental conditions caused changes in cyst buoyancy and nauplii mortality. Mating crosses between floating and sinking parental phenotypes with two food concentrations (low and high) indicated there is a genetic basis to cyst buoyancy. Using cysts harvested from 1991-2011, we found cyst buoyancy decreased and nauplii mortality increased over time. Data on water temperature, salinity, and chlorophyll a concentration in GSL from 1994 to 2011 indicated that although water temperature has increased over time and chlorophyll a concentration has decreased over time, the selective harvesting pressure against floating cysts is a better predictor of changes in cyst buoyancy and nauplii mortality over time than trends in environmental conditions. Harvesting of GSL A. franciscana cysts is causing evolutionary changes, which has implications for the sustainable management and harvesting of these cysts. Monitoring phenotypic characteristics and life-history traits of the population should be implemented and appropriate responses taken to reduce the impacts of the selective harvesting.

Mechanical Vectors Enhance Fungal Entomopathogen Reduction of the Grasshopper Pest Camnula pellucida (Orthoptera: Acrididae).

Mounting scientific evidence indicates that pathogens can regulate insect populations. However, limited dispersal and sensitivity to abiotic conditions often restricts pathogen regulation of host populations. While it is well established that arthropod biological vectors increase pathogen incidence in host populations, few studies have examined whether arthropod mechanical vectors (an organism that transmits pathogens but is not essential to the life cycle of the pathogen) influence host-pathogen dynamics. The importance of mechanical dispersal by ant scavengers, Formica fusca (L.), in a grasshopper-fungal entomopathogen system was investigated. We examined the ability of ants to mechanically disperse and transmit the pathogen, Entomophaga grylli (Fresenius) pathotype 1, to its host, the pest grasshopper Camnula pellucida (Scudder), in a series of laboratory experiments. Fungal spores were dispersed either externally on the ant's body surface or internally through fecal deposition. In addition, a third of all grasshoppers housed with fungal-inoculated ants became infected, indicating that ants can act as mechanical vectors of E. grylli. The effect of ant mechanical vectors on E. grylli incidence was also examined in a field experiment. Ant access to pathogen-exposed experimental grasshopper populations was restricted using organic ant repellent, thereby allowing us to directly compare mechanical and natural transmission. Ants increased grasshopper pathogen mortality by 58%, which led to greater pathogen reductions of grasshopper survival than natural transmission. Taken together, our results indicate that ants enhance E. grylli reduction of grasshopper pest numbers. Therefore, mechanical transmission of pathogens may be an important overlooking component of this grasshopper-fungal pathogen system.

Prey change behaviour with predation threat, but demographic effects vary with prey density: experiments with grasshoppers and birds.

Increasingly, ecologists emphasize that prey frequently change behaviour in the presence of predators and these behavioural changes can reduce prey survival and reproduction as much or more than predation itself. However, the effects of behavioural changes on survival and reproduction may vary with prey density due to intraspecific competition. In field experiments, we varied grasshopper density and threat of avian predation and measured grasshopper behaviour, survival and reproduction. Grasshopper behaviour changed with the threat of predation and these behavioural changes were invariant with grasshopper density. Behavioural changes with the threat of predation decreased per capita reproduction over all grasshopper densities; whereas the behavioural changes increased survival at low grasshopper densities and then decreased survival at high densities. At low grasshopper densities, the total reproductive output of the grasshopper population remained unchanged with predation threat, but declined at higher densities. The effects of behavioural changes with predation threat varied with grasshopper density because of a trade-off between survival and reproduction as intraspecific competition increased with density. Therefore, resource availability may need to be considered when assessing how prey behavioural changes with predation threat affect population and food web dynamics.

How will species respond to climate change? Examining the effects of temperature and population density on an herbivorous insect.

An important challenge facing ecologists is to understand how climate change may affect species performance and species interactions. However, predicting how changes in abiotic variables associated with climate change may affect species performance also depends on the biotic context, which can mediate species responses to climatic change. We conducted a 3-yr field experiment to determine how the herbivorous grasshopper Camnula pellucida (Scudder) responds to manipulations of temperature and population density. Grasshopper survival and fecundity decreased with density, indicating the importance of intraspecific competition. Female fecundity tended to increase with temperature, whereas grasshopper survival exhibited a unimodal response to temperature, with highest survival at intermediate temperatures. Grasshopper performance responses to temperature also depended on density. Peak survival in the low-density treatment occurred in warmer conditions than for the high-density treatment, indicating that the intensity of intraspecific competition varies with temperature. Our data show that changes to the temperature regimen can alter grasshopper performance and determine the intensity of intraspecific competition. However, the effects of temperature on grasshopper performance varied with density. Our data indicate the importance of the biotic context in mediating species responses to climatic factors associated with global change.

Dynamics of two Montana grasshopper populations: relationships among weather, food abundance and intraspecific competition.

The population dynamics of two grasshoppers (Melanoplus femurrubrum and M. sanguinipes) were studied using experimental microcosms over 8 years at a Palouse prairie site in Montana. Grasshopper density, survival and reproduction in the experimental populations responded in a density-dependent fashion to natural and experimental changes in food availability for all grasshopper developmental stages, both within and between all years. We observed that field populations of the grasshoppers at the site exhibited density, survival and reproductive responses similar to the experimental populations over the period of the study. Because we could not identify any differences between the field and microcosm environments or the grasshopper individuals in them, we contend that field populations were ultimately limited by food within and between years. Density-dependent food limitation occurred for all age categories over the entire summer, because food abundance declined relative to grasshopper food requirements over the summer. Food limitation occurred between years, because in years with the lowest food abundance, the populations produced more hatchlings for the next year than could be supported by the highest observed food abundance. Finally, the observed annual changes in food abundance were correlated with the annual variation in weather (rainfall and temperature), which indicated that the long established relationship between grasshopper densities and weather conditions does not imply population limitation by density-independent processes.

How good must models and data be in ecology?

Linear programming models of diet selection (LP) have been criticized as being too sensitive to variations in parameter values that have not been or may not be able to be measured with a high degree of precision (small standard error). Therefore, LP's predictions have been questioned, even though the predicted diet choices agree very well with observations in 400 published tests. The philosophical and statistical aspects of this criticism of LP are reviewed in light of the ability to test any nontrivial ecological theory. It is argued that measures of error in field data may not meet simple statistical definitions, and thereby, may make sensitivity analyses that use the error measures overly conservative. Furthermore, the important issue in testing ecological theory may not be the statistical confidence in a single test, but whether or not the theory withstands repeated tests.

Effects of spines and thorns on Australian arid zone herbivores of different body masses.

We investigated the effects of thorns and spines on the feeding of 5 herbivore species in arid Australia. The herbivores were the rabbit (Oryctolagus cuniculus), euro kangaroo (Macropus robustus), red kangaroo (Macropus rufus), sheep (Ovis aries), and cattle (Bos taurus). Five woody plants without spines or thorns and 6 woody plants with thorns were included in the study. The spines and thorns were not found to affect the herbivores' rates of feeding (items ingested/min), but they did reduce the herbivores' rates of biomass ingestion (g-dry/item). The reduction in biomass ingested occurred in two ways: at a given diameter, twigs with spines and thorns had less mass than undefended plants, and the herbivores consumed twigs with smaller diameters on plants with spines and thorns. The relative importance of the two ways that twigs with spines and thorns provided less biomass varied with herbivore body mass. Reduced twig mass was more important for small herbivores, while large herbivores selected smaller diameters. The effectiveness of spines and thorns as anti-herbivore defenses did not vary with the evolutionary history of the herbivores (i.e. native vs. introduced). Spines and thorns mainly affected the herbivores' selection of maximum twig sizes (reducing diameter and mass), but the minimum twig sizes selected were also reduced.

Sociality of Columbian ground squirrels in relation to their seasonal energy intake.

Seasonal energy intake was estimated for ten populations of Columbian ground squirrels (Spermophilus columbianus) in northwestern Montana. We calculated daily energy intake for an average ground squirrel in each population using measurements of feeding time, consumption rates of different vegetation types (monocots vs. dicots), and the proportion of monocots and dicots in the diet. These daily energy intakes were multiplied by the length of the plant growing season for each population to estimate seasonal energy intake, i.e. over the ground squirrel active season. Amicable interaction rates measured for each population varied with seasonal energy intake, but not with environmental heterogeneity, sex ratio, or the ratio of adults to juveniles. In particular, amicable interactions among adult-juvenile and juvenile-juvenile pairs increased as seasonal energy intake decreased. The proportion of females breeding as yearlings increased as seasonal energy intake increased. This suggests that "harsh" environments reduce the energy available for juvenile growth and development, leading to delayed dispersal and age at first reproduction. These responses may promote the formation of kin groups and increased amicable interactions within those groups. The length of the plant growing season may determine environmental "harshness" across elevational gradients, but at a particular elevation, "harshness" may depend on factors determining daily food intake.

Optimal activity times and habitat choice of moose.

A set of concepts was presented which could be used to model an animal's activity cycle and habitat choice as an optimization process. The model was applied to moose (Alces alces) summer activity and its predictions were consistent with observed feeding times and habitat selections. The optimization model had a goal of maximizing daily feeding time at the least possible energetic cost. This goal was consistent with a foraging strategy that maximized the intake of some nutritional component, i.e. energy, protein, etc. The optimization problem, however, was bounded. Three constraints appeared to limit the goal: body temperature must be maintained within set upper and lower limits, thermal equilibrium must be maintained over an extended period so the activity cycle strategy can be repeated and sufficient time must be spent ruminating.