Signal Interception and the Use of Soft Song in Aggressive Interactions

Song sparrows, like many species of songbirds, produce songs of especially low amplitude during aggressive contests. Such ‘soft songs’ have been shown to be reliable signals of intention to attack in song sparrows. Low intensity is a paradoxical feature in a highly aggressive signal, in that it seems likely to make the signal less intimidating to an opponent. A hypothesis that resolves this paradox is that use of soft songs benefits a signaler by limiting the interception of his signals by third party receivers. Here, we test this hypothesis with respect to song sparrows and two classes of third party receivers, predators and conspecific males. We tested whether selection to avoid predation is responsible for use of soft song by examining whether male song sparrows increase production of soft song in an aggressive context when they also receive signals (alarm calls) that indicate a predator is present. We found that the proportion of soft song produced by male song sparrows was actually significantly lower in the predator context than in a control context, directly contradicting the prediction. We tested whether avoiding eavesdropping by conspecific males is a benefit of soft song by removing territory owners and simulating interactions on their territories using playback from two loudspeakers. We found that intrusions by third party males were more likely in trials in which the simulated owner countered an intruder's songs using soft songs than in trials in which the simulated owner countered with loud song, again directly contradicting the hypothesis. We conclude that limiting interception by predators and conspecific males does not provide an explanation for use of soft song by song sparrows.     

Field validation supports novel niche modeling strategies in a cryptic endangered amphibian

Many studies employ ecological niche models (ENMs) to predict species’ occurrences in undersampled regions, generally without field confirmation. Here, we use field surveys to test the relative utility of four potential refinements to the standard ENM approach: 1) altering model complexity based on AIC_c, 2) selecting background points from a biologically informed region, 3) using target‐group background to account for sampling bias in existing localities, and 4) using many rangewide localities (global model) versus fewer proximal localities (local model) to construct geographically restricted range predictions. We used Maxent to predict new localities for the California tiger salamander Ambystoma californiense, an endangered species that often goes undocumented due to its cryptic lifestyle. We followed this with a field survey of 260 previously unsampled potential breeding sites in Solano County, CA and used the resulting presence/absence data to compare all factorial combinations of the four model refinements using a new application of the Kruskal–Wallis test for ENM outputs. Our field surveys led to the discovery of 81 previously undocumented breeding localities for the California tiger salamander and demonstrated that ENMs could be significantly improved by utilizing target‐group background to account for spatial sampling bias and local models to focus model output on the subregion of the range being surveyed. Our results clearly demonstrate the potential for local models to outperform global models, and we recommend supplementing traditional Maxent global models that utilize all known localities with local models, particularly when species occupy geographically structured, heterogeneous habitat types. We also recommend using target‐group background since the improvement we observed when including it in our models was significant and very similar to that documented by previous studies. Most importantly, we emphasize the importance of field verification to enable rigorous statistical comparisons among models.     

Environmental stress destabilizes microbial networks

Environmental stress is increasing worldwide, yet we lack a clear picture of how stress disrupts the stability of microbial communities and the ecosystem services they provide. Here, we present the first evidence that naturally-occurring microbiomes display network properties characteristic of unstable communities when under persistent stress. By assessing changes in diversity and structure of soil microbiomes along 40 replicate stress gradients (elevation/water availability gradients) in the Florida scrub ecosystem, we show that: (1) prokaryotic and fungal diversity decline in high stress, and (2) two network properties of stable microbial communities—modularity and negative:positive cohesion—have a clear negative relationship with environmental stress, explaining 51–78% of their variation. Interestingly, pathogenic taxa/functional guilds decreased in relative abundance along the stress gradient, while oligotrophs and mutualists increased, suggesting that the shift in negative:positive cohesion could result from decreasing negative:positive biotic interactions consistent with the predictions of the Stress Gradient Hypothesis. Given the crucial role microbiomes play in ecosystem functions, our results suggest that, by limiting the compartmentalization of microbial associations and creating communities dominated by positive associations, increasing stress in the Anthropocene could destabilize microbiomes and undermine their ecosystem services.Subject terms: Microbial ecology, Microbial ecology     

Cytoskeletal origins in sulfur-metabolizing archaebacteria

Several of the thermophilic acidopholic sulfur-metabolizing archaebacteria lack rigid cell walls. Their irregular shapes were maintained by an internal mechanism, presumably a cytoskeleton. Apparently this is an adaptation for respiration upon elemental sulfur, which requires cell contact since sulfur is insoluble in water. Also, we speculate that there could be additional functions of the cytoskeleton, such as prevention of osmotic cell lysis, thermal stabilization of enzymes, and improvements in metabolic efficiency through specific enzyme positioning. Such a well-developed cytoskeleton, evolving first in thermophilic archaebacteria, could have been a preadaptation for the evolution of eukaryotic cells.     

Song‐type sharing in a population of Song Sparrows in the eastern United States

Male Song Sparrows (Melospiza melodia) sometimes interact with neighboring territory owners by song‐type matching or repertoire matching. In some Song Sparrow populations, levels of song sharing are high and most neighbors can interact by matching, but levels of song sharing are much lower in other populations, limiting the degree to which males can match their neighbors. One explanation for variation in sharing levels is that the importance of song‐type and repertoire matching, and therefore the extent of song sharing, varies geographically in North America, being greater in western populations than eastern populations. However, to date, two studies of eastern populations have provided conflicting evidence concerning levels of song sharing by Song Sparrows. Thus, we measured sharing of whole songs and introductory phrases of songs between males with adjacent territories in another population of Song Sparrows in the eastern United States (North Carolina), near the eastern and southern limits of the species’ breeding range. Males (N = 17) in our study shared an average of only 8.7% of their song types with neighbors, and more than half of neighbor pairs shared no whole songs. Sharing of introductory phrases was more common (mean = 22.8%). The level of whole song sharing in our study is the second lowest yet reported for any Song Sparrow population, supporting the hypothesis that sharing is generally lower in eastern than in western populations.