24Indicator species (IS) are used to monitor environmental changes, assess the efficacy of 25 management, and provide warning signals for impending ecological shifts. Though widely 26 adopted in recent years by ecologists, conservation biologists, and environmental practitioners, 27 the use of IS has been criticized for several reasons, notably the lack of justification behind the 28 choice of any given indicator. In this review, we assess how ecologists have selected, used, and 36"bioindicator", and "biomonitor," but these and other terms often were not clearly defined.
Metabolic rates of individual animals and social insect colonies generally scale hypometrically, with mass-specific metabolic rates decreasing with increasing size. Although this allometry has wide ranging effects on social behaviour, ecology and evolution, its causes remain controversial. Because it is difficult to experimentally manipulate body size of organisms, most studies of metabolic scaling depend on correlative data, limiting their ability to determine causation. To overcome this limitation, we experimentally reduced the size of harvester ant colonies (Pogonomyrmex californicus) and quantified the consequent increase in mass-specific metabolic rates. Our results clearly demonstrate a causal relationship between colony size and hypometric changes in metabolic rate that could not be explained by changes in physical density. These findings provide evidence against prominent models arguing that the hypometric scaling of metabolic rate is primarily driven by constraints on resource delivery or surface area/volume ratios, because colonies were provided with excess food and colony size does not affect individual oxygen or nutrient transport. We found that larger colonies had lower median walking speeds and relatively more stationary ants and including walking speed as a variable in the mass-scaling allometry greatly reduced the amount of residual variation in the model, reinforcing the role of behaviour in metabolic allometry. Following the experimental size reduction, however, the proportion of stationary ants increased, demonstrating that variation in locomotory activity cannot solely explain hypometric scaling of metabolic rates in these colonies. Based on prior studies of this species, the increase in metabolic rate in sizereduced colonies could be due to increased anabolic processes associated with brood care and colony growth.
Hemlock forests of the northeastern United States are declining due to the invasive hemlock woolly adelgid (HWA) (Adelges tsugae). Hardwood species replace these forests, which affects soil properties that may influence other communities, such as red-backed salamanders (red-backs) (Plethodon cinereus). This study examined the effects of HWA invasion on soil properties and how this affects red-backs at the Hemlock Removal Experiment at Harvard Forest, which consists of eight 0.8 ha plots treated with girdling to simulate HWA invasion, logging to simulate common management practices, or hemlock-or hardwood-dominated controls. Coverboard surveys were used to determine the relative abundance of red-backs between plots during June and July 2014 and soil cores were collected from which the bulk density, moisture, pH, temperature, leaf litter, and carbon-nitrogen ratio were measured. Ordination provided a soil quality index based on temperature, pH, and carbon-to-nitrogen ratio, which was significantly different between plot treatments (p < 0.05) and showed a significant negative correlation with the red-back relative abundance (p < 0.05). The findings support the hypothesis that red-backs are affected by soil quality, which is affected by plot treatment and thus HWA invasion. Further studies should explore how salamanders react in the long term towards changing environments and consider the use of red-backs as indicator species.
Long-term ecological research (LTER) and monitoring programs accrue invaluable ecological data that inform policy and improve decisions that enable adaptation to and mitigation of environmental changes. There is great interest in identifying ecological indicators that can be monitored easily and effectively to yield reliable data about environmental changes in forested ecosystems. However, the selection, use, and validity of ecological indicators to monitor in LTER programs remain challenging tasks for ecologists and conservation biologists. Across the eastern United States of America, the foundation tree species eastern hemlock (Tsuga canadensis (L.) Carrière) is declining and dying from irruptions of a non-native insect, the hemlock woolly adelgid (Adelges tsugae Annand). We use data from the Harvard Forest LTER site’s Hemlock Removal Experiment together with information from other eastern US LTER sites to show that plethodontid salamanders can be reliable indicators of ongoing ecological changes in forested ecosystems in the eastern USA. These salamanders are abundant, they have a history of demographic stability, are both predators and prey, and can be sampled and monitored simply and cost-effectively. At the Harvard Forest LTER, red-backed salamanders (Plethodon cinereus Green) were strong indicators of intact forests dominated by eastern hemlock (Tsuga canadensis); their high site fidelity and habitat specificity yielded an indicator value (robust Dufrêne and Legendre’s “IndVal”) for this species of 0.99. Eastern red-spotted newts (Notopthalmus viridescens viridescens Rafinesque) were better indicators of nearby stands made up of young, mixed hardwood species, such as those which replace hemlock stands following adelgid infestation. At the Hubbard Brook and Coweeta LTER sites, plethodontid salamanders were associated with intact riparian habitats, which may also be dominated by eastern hemlock. We conclude that plethodontid salamanders satisfy most criteria for reliable ecological indicators of environmental changes in eastern US forests.
The metabolic costs of being an organism generally scale hypometrically, such that per‐gram metabolic rates decrease with increasing size. The drivers of this almost universal feature have yet to be clearly identified. Cohesive social groups, particularly the social insects, also show hypometric scaling with colony size. It is not known whether size is a causal factor or whether this relationship is only a correlation. We experimentally reduced the size of harvester ant colonies (Pogonomyrmex californicus), to test whether colony size is causally responsible for large colonies utilizing less energy per gram. We identified three potential parameters, all related to the organization of the work, that are candidate mechanisms for driving hypometric scaling of metabolism. Larger colonies had lower median (but not average) walking speeds, and more ants classified as stationary, potentially reducing locomotory costs. Larger colonies had reduced mass‐specific perimeters of the brood pile, consistent with reduced per‐capita brood rearing, a potentially expensive task. Finally, larger colonies had reduced network density and centrality, consistent with greater division of labor, specialization, and segregation of individuals, which may reduce metabolic costs by improving efficiency. A simulation model suggests that simple behavioral rules may cause reduced network densities that could reduce colonial metabolic rates by promoting specialization in task performance and activity levels.Support or Funding InformationThis research has been supported by Providence College, the James S. McDonnell Foundation, and the National Science Foundation under Grant No. 1110796.
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