Socioecological influences on the dispersal of female mountain gorillas—evidence of a second folivore paradox

Socioecological models indicate that the group structure and female dispersal patterns of primates are determined primarily by the abundance and distribution of food, predation pressures, and infanticide risks. In response to those influences, females of folivorous primates are considered relatively free to disperse into groups with the optimal size and structure. Yet some folivores live in small groups despite a potentially higher risk of predation, an apparent inconsistency known as the folivore paradox. This paper examines the female dispersal of a folivorous primate, the Virunga mountain gorillas. Mountain gorillas currently have no natural predators, but this species presents a different version of the folivore paradox: why do 50–60% of females reside in smaller one-male groups despite a higher risk of infanticide? In this study, females left one-male groups more frequently than multimale groups, but transfer destinations were not consistently biased toward multimale groups and those groups did not have higher immigration rates. We found no evidence of dispersal to avoid feeding competition within large groups, even as they have become three to five times larger than average. Thus, the lack of a consistent bias toward multimale groups was not because they are typically larger than one-male groups. Instead, the apparent inconsistencies may reflect limited female transfer opportunities, other influences on dispersal, and possibly an evolutionary disequilibrium in which current behavior does not optimize fitness.     

Initial effects of light armored vehicle use on grassland vegetation at Fort Lewis, Washington

Sustainable use of military training lands requires understanding and prediction of the effects of military vehicles on vegetation. We examined the initial impacts of an 8-wheeled, light armored vehicle (LAV) on grassland vegetation at Fort Lewis, Washington. The LAV drove replicate spiral paths at two starting velocities, 10.3 and 5.1 m s(-1). The disturbed width (width of ground impacted by the tires) increased as turning radius decreased, but was unaffected by vehicle velocity. An inverse-exponential model predicted disturbed width (r(2)=0.68) at all turning radii for both velocities combined. In low-velocity spirals, and for straight tracking (turning radius>40 m) and moderate turns (radius 20-40 m) in high-velocity spirals, all vegetation damage was imprint (plants flattened by wheels). During sharp (radius <20 m), high-velocity turns, most or all of the disturbed width was scraped free of surface vegetation and soil, which was piled to the outside of each tire track. Total plant cover (all species) was not affected by track curvature in low-velocity spirals, but decreased in the order straight tracking>moderate turns>sharp turns in high-velocity spirals. In low-velocity spirals, post-tracking cover of several plant growth forms (non-native species, perennial species, sod-forming grasses) was similar to pre-tracking cover, but in high-velocity spirals, post-tracking cover of these growth forms decreased in the order straight > or =moderate=sharp. Cover of native species and forbs decreased more in high- than in low-velocity spirals, but was unaffected by curvature. Pre- and post-tracking cover of annual species, bunchgrasses, and shrubs was < or =3%. The most severe vegetation damage caused by operation of wheeled LAVs on grasslands is associated with sharp, high-velocity turns.     

Impacts of pesticides in a Central California estuary

Recent and past studies have documented the prevalence of pyrethroid and organophosphate pesticides in urban and agricultural watersheds in California. While toxic concentrations of these pesticides have been found in freshwater systems, there has been little research into their impacts in marine receiving waters. Our study investigated pesticide impacts in the Santa Maria River estuary, which provides critical habitat to numerous aquatic, terrestrial, and avian species on the central California coast. Runoff from irrigated agriculture constitutes a significant portion of Santa Maria River flow during most of the year, and a number of studies have documented pesticide occurrence and biological impacts in this watershed. Our study extended into the Santa Maria watershed coastal zone and measured pesticide concentrations throughout the estuary, including the water column and sediments. Biological effects were measured at the organism and community levels. Results of this study suggest the Santa Maria River estuary is impacted by current-use pesticides. The majority of water samples were highly toxic to invertebrates (Ceriodaphnia dubia and Hyalella azteca), and chemistry evidence suggests toxicity was associated with the organophosphate pesticide chlorpyrifos, pyrethroid pesticides, or mixtures of both classes of pesticides. A high percentage of sediment samples were also toxic in this estuary, and sediment toxicity occurred when mixtures of chlorpyrifos and pyrethroid pesticides exceeded established toxicity thresholds. Based on a Relative Benthic Index, Santa Maria estuary stations where benthic macroinvertebrate communities were assessed were degraded. Impacts in the Santa Maria River estuary were likely due to the proximity of this system to Orcutt Creek, the tributary which accounts for most of the flow to the lower Santa Maria River. Water and sediment samples from Orcutt Creek were highly toxic to invertebrates due to mixtures of the same pesticides measured in the estuary. This study suggests that the same pyrethroid and organophosphate pesticides that have been shown to cause water and sediment toxicity in urban and agriculture water bodies throughout California, have the potential to affect estuarine habitats. The results establish baseline data in the Santa Maria River estuary to allow evaluation of ecosystem improvement as management initiatives to reduce pesticide runoff are implemented in this watershed.     

Individual variation in foraging behavior reveals a trade-off between flexibility and performance of a top predator

There is increasing evidence that behavioral flexibility is associated with the ability to adaptively respond to environmental change. Flexibility can be advantageous in some contexts such as exploiting novel resources, but it may come at a cost of accuracy or performance in ecologically relevant tasks, such as foraging. Such trade-offs may, in part, explain why individuals within a species are not equally flexible. Here, we conducted a reversal learning task and predation experiment on a top fish predator, the Northern pike (Esox lucius), to examine individual variation in flexibility and test the hypothesis that an individual’s behavioral flexibility is negatively related with its foraging performance. Pikes were trained to receive a food reward from either a red or blue cup and then the color of the rewarded cup was reversed. We found that pike improved over time in how quickly they oriented to the rewarded cup, but there was a bias toward the color red. Moreover, there was substantial variation among individuals in their ability to overcome this red bias and switch from an unrewarded red cup to the rewarded blue cup, which we interpret as consistent variation among individuals in behavioral flexibility. Furthermore, individual differences in behavioral flexibility were negatively associated with foraging performance on ecologically relevant stickleback prey. Our data indicate that individuals cannot be both behaviorally flexible and efficient predators, suggesting a trade-off between these two traits.     

Transient dynamics of invasive competition: barred owls, spotted owls, habitat, and the demons of competition present

The recent range expansion of Barred Owls (Strix varia) into the Pacific Northwest, where the species now co-occurs with the endemic Northern Spotted Owl (Strix occidentalis caurina), resulted in a unique opportunity to investigate potential competition between two congeneric, previously allopatric species. The primary criticism of early competition research was the use of current species' distribution patterns to infer past processes; however, the recent expansion of the Barred Owl and the ability to model the processes that result in site occupancy (i.e., colonization and extinction) allowed us to address the competitive process directly rather than inferring past processes through current patterns. The purpose of our study was to determine whether Barred Owls had any negative effects on occupancy dynamics of nesting territories by Northern Spotted Owls and how these effects were influenced by habitat characteristics of Spotted Owl territories. We used single-species, multi-season occupancy models and covariates quantifying Barred Owl detections and habitat characteristics to model extinction and colonization rates of Spotted Owl pairs in southern Oregon, USA. We observed a strong, negative association between Barred Owl detections and colonization rates and a strong positive effect of Barred Owl detections on extinction rates of Spotted Owls. We observed increased extinction rates in response to decreased amounts of old forest at the territory core and higher colonization rates when old-forest habitat was less fragmented. Annual site occupancy for pairs reflected the strong effects of Barred Owls on occupancy dynamics with much lower occupancy rates predicted for territories where Barred Owls were detected. The strong Barred Owl and habitat effects on occupancy dynamics of Spotted Owls provided evidence of interference competition between the species. These effects increase the importance of conserving large amounts of contiguous, old-forest habitat to maintain Northern Spotted Owls in the landscape.     

No evidence that sperm morphology predicts paternity success in wild house wrens

Postcopulatory sexual selection (PCSS) in internally fertilizing vertebrates is a topic of great interest, yet relatively little is known about the characteristics of sperm and ejaculates that confer an advantage in PCSS. In this study, we investigated several measures of sperm morphology that potentially contribute to fertilization success under PCSS. We tested whether sperm morphology related to success in PCSS (via extra-pair paternity) in house wrens (Troglodytes aedon). We found no evidence that sperm morphology differed between extra-pair sires and the within-pair males they cuckolded, nor that sperm morphology correlated with the proportion of within-pair offspring sired, the number of extra-pair offspring sired, or the total annual reproductive success. Male behavioral strategies may affect the probability that their sperm compete with other males’ sperm and that their sperm succeed under competition. Effects of these behavioral strategies, as well as differences between males in sperm number, could mask the effects of sperm morphology on the outcome of PCSS. Despite moderate levels of extra-pair paternity, selection on sperm may be relatively weak in house wrens. Further work is needed to understand general patterns in how sperm morphology relates to fertilization success within species.