Sampling design trade-offs in occupancy studies with imperfect detection: examples and software.

Researchers have used occupancy, or probability of occupancy, as a response or state variable in a variety of studies (e.g., habitat modeling), and occupancy is increasingly favored by numerous state, federal, and international agencies engaged in monitoring programs. Recent advances in estimation methods have emphasized that reliable inferences can be made from these types of studies if detection and occupancy probabilities are simultaneously estimated. The need for temporal replication at sampled sites to estimate detection probability creates a trade-off between spatial replication (number of sample sites distributed within the area of interest/inference) and temporal replication (number of repeated surveys at each site). Here, we discuss a suite of questions commonly encountered during the design phase of occupancy studies, and we describe software (program GENPRES) developed to allow investigators to easily explore design trade-offs focused on particularities of their study system and sampling limitations. We illustrate the utility of program GENPRES using an amphibian example from Greater Yellowstone National Park, U.S.A.     

Comparison of some homogeneity tests in analysis of over-dispersed binomial data

This paper compares the procedures based on the extended quasi-likelihood, pseudo-likelihood and quasi-likelihood approaches for testing homogeneity of several proportions for over-dispersed binomial data. The type I error of the Wald tests using the model-based and robust variance estimates, the score test, and the extended quasi-likelihood ratio test (deviance reduction test) were examined by simulation. The extended quasi-likelihood method performs less well when mean responses are close to 1 or 0. The model-based Wald test based on the quasi-likelihood performs the best in maintaining the nominal level. The score test performs less well when the intracluster correlations are large or heterogeneous. In summary: (i) both the quasilikelihood and pseudo-likelihood methods appear to be acceptable but care must be taken when overfitting a variance function with small sample sizes; (ii) the extended quasi-likelihood approach is the least favourable method because its nominal level is much too high; and (iii) the robust variance estimator performs poorly, particularly when the sample size is small.     

Reproductive value and appropriateness of response to predators by White-Crowned Sparrows

Summary Twenty-five pair of breeding Whitecrowned Sparrows were presented with a live snake and models of a hawk, jay, and junco. Pairs were tested either when the female was brooding eggs, feeding nestlings, or fledglings. It was found that the snake was responded to the most when the pair had nestlings, very little when the female was brooding eggs, and at a moderate level with fledglings. The hawk and jay models were responded to the least with eggs, more with nestlings, and the most with fledglings. The junco model elicited little response. Both the pattern and level of response is influenced by whether or not the stimulus represents an effective predator at the particular stage of the breeding cycle. They also are influenced by the reproductive value of the progeny. A model is suggested that includes two factors: stimulus value and reproductive value. We propose that this combined model is more suitable than a single factor one.     

Aggressiveness and foraging mode of young-of-the-year brook charr,Salvelinus fontinalis (Pisces,Salmonidae)

Summary Young-of-the-year brook charr in streams use either an active or a sit-and-wait foraging tactic and exhibit a range of resource defense from territoriality to tolerating conspecifics. We use simple graphical models, based on encounter rate with drift and the theory of economic defendability, to predict qualitative changes in the aggressiveness and mobility of brook charr in relation to current velocity. Aggressiveness (percent of conspecifics eliciting an overt response) initially increases with increasing current velocity, as does drift rate and foraging rate. However, aggressiveness decreases at high current velocities, probably because of increased costs of defense at these velocities. In standing water areas, brook charr use primarily an active foraging tactic, but mobility (percent time spent moving) decreases rapidly as current velocity increases. These results are generally consistent with the simple graphical models. A literature survey suggests that the models can be generalized for most species of stream salmonids.     

Demography of northern flying squirrels informs ecosystem management of western interior forests.

We studied northern flying squirrel (Glaucomys sabrinus) demography in the eastern Washington Cascade Range to test hypotheses about regional and local abundance patterns and to inform managers of the possible effects of fire and fuels management on flying squirrels. We quantified habitat characteristics and squirrel density, population trends, and demography in three typical forest cover types over a four-year period. We had 2034 captures of flying squirrels over 41 000 trap nights from 1997 through 2000 and marked 879 squirrels for mark-recapture population analysis. Ponderosa pine (Pinus ponderosa) forest appeared to be poorer habitat for flying squirrels than young or mature mixed-conifer forest. About 35% fewer individuals were captured in open pine forest than in dry mixed-conifer Douglas-fir (Pseudotsuga menziesii) and grand fir (Abies grandis) forests. Home ranges were 85% larger in pine forest (4.6 ha) than in mixed-conifer forests (2.5 ha). Similarly, population density (Huggins estimator) in ponderosa pine forest was half (1.1 squirrels/ha) that of mixed-conifer forest (2.2 squirrels/ha). Tree canopy cover was the single best correlate of squirrel density (r = 0.77), with an apparent threshold of 55% canopy cover separating stands with low- from high-density populations. Pradel estimates of annual recruitment were lower in open pine (0.28) than in young (0.35) and mature (0.37) forest. High recruitment was most strongly associated with high understory plant species richness and truffle biomass. Annual survival rates ranged from 45% to 59% and did not vary among cover types. Survival was most strongly associated with understory species richness and forage lichen biomass. Maximum snow depth had a strong negative effect on survival. Rate of per capita increase showed a density-dependent response. Thinning and prescribed burning in ponderosa pine and dry mixed conifer forests to restore stable fire regimes and forest structure might reduce flying squirrel densities at stand levels by reducing forest canopy, woody debris, and the diversity or biomass of understory plants, truffles, and lichens. Those impacts might be ameliorated by patchy harvesting and the retention of large trees, woody debris, and mistletoe brooms. Negative stand-level impacts would be traded for increased resistance and resilience of dry-forest landscapes to now-common, large-scale stand replacement fires.     

Interspecific associations of Cape ground squirrels with two mongoose species: benefit or cost?

Mixed-species associations have been described in many vertebrate species, but few behavioral studies have investigated associations between species from different mammalian orders. Cape ground squirrels (Xerus inauris) are highly social rodents that inhabit burrows with two species of mongoose, but the benefits of these interspecific associations to ground squirrels remain unresolved. We compared the behavior of squirrels while solitary, with conspecifics, and in the presence of suricates (Suricata suricatta) and yellow mongooses (Cynictis pencillatus). Squirrels spent less time alert and more time feeding when suricates were present, but increased vigilance in the presence of yellow mongooses. In a series of mobbing trials with a puff adder (Bitis arietans), a common predator of all three species, Cape ground squirrels were the most active in mobbing the snake. Our results suggest that Cape ground squirrels benefit from associating with suricates, but not necessarily with yellow mongooses. Both mongoose species benefit from the burrowing activities of the squirrels for thermoregulation and escape from predators, and a suite of other organisms may similarly benefit from the habitat modifications by Cape ground squirrels, suggesting they could be considered ecosystem engineers of the arid and semi-arid regions of southern Africa. Thus, the association between Cape ground squirrels and suricates appears mutually beneficial, whereas yellow mongooses may merely be commensals of the squirrels.     

Social network theory in the behavioural sciences: potential applications

Social network theory has made major contributions to our understanding of human social organisation but has found relatively little application in the field of animal behaviour. In this review, we identify several broad research areas where the networks approach could greatly enhance our understanding of social patterns and processes in animals. The network theory provides a quantitative framework that can be used to characterise social structure both at the level of the individual and the population. These novel quantitative variables may provide a new tool in addressing key questions in behavioural ecology particularly in relation to the evolution of social organisation and the impact of social structure on evolutionary processes. For example, network measures could be used to compare social networks of different species or populations making full use of the comparative approach. However, the networks approach can in principle go beyond identifying structural patterns and also can help with the understanding of processes within animal populations such as disease transmission and information transfer. Finally, understanding the pattern of interactions in the network (i.e. who is connected to whom) can also shed some light on the evolution of behavioural strategies.     

Sorption of the natural endocrine disruptors,oestrone and 17 beta-oestradiol in the aquatic environment

Adsorption of hydrophobic contaminants at the particle/water interface is one of the key processes controlling their fate in the aquatic environment. The sorption of the natural female hormones oestrone and 17-oestradiol has been studied under simulated riverine conditions. Both the kinetics and the effects of varying fundamental environmental parameters (e.g. sediment properties) on the thermodynamic equilibrium partition coefficient (K _p) have been studied in continuous and batch sorption experiments, respectively. Results showed that the sorption of oestrone and 17-oestradiol by sediment was relatively slow, reaching equilibrium in 50 days. In addition, relatively small adsorption of both oestrone and 17-oestradiol onto the sediment was observed, with K _p values between 200 and 250 mL g^–1. The comparable K _p values of the two compounds reflect their structural similarity. It can be concluded that the two endocrine disruptors, oestrone and 17-oestradiol remain primarily in association with the aqueous phase.     

Population dynamics and growth of juveniles of the velvet swimming crab Necora puber (Decapoda: Portunidae)

Recruitment variability plays a critical role in determining local population densities of benthic organisms, but extreme vulnerability at the onset of juvenile life is a trait that is largely responsible for population survivorship trends. The aim of the present study was to determine the role of juvenile recruitment in the population structure of Necora puber. Juveniles of N. puber were collected from the lower intertidal of rocky shores of Plymouth Sound (southwest coast of the UK) and monthly size–frequency distribution were used to determine the dynamics and the growth of the population. The parameters of the von Bertalanffy growth function were estimated (K=0.281 year⁻¹; t ₀=0.043; C=0.103; and t _s=0.268) assuming a L _∞=105 mm. Growth was markedly seasonal and present results indicated a slower juvenile growth rate than described previously for N. puber. The recruitment period was extensive and was two times higher in 2001 than in 2000 at the start of the 1+ year, but levelled off at the end of the 1+ year class on three of the four shores studied. Instantaneous mortality as high as 5.1 year⁻¹(99.4% year⁻¹) was observed during the higher recruitment year. Early juvenile mortality appears to be density dependent and a demographic bottleneck appears to limit the number of juveniles on some shores.