Population survey of the spider monkeyAteles geoffroyi at Tikal,Guatemala

Intensive strip census methods were used to estimate population density and age-sex composition of a natural population of the spider monkeyAteles geoffroyi, in seasonally dry forest at Tikal, Guatemala. An objective procedure for determining effective strip width is discussed, and various census methods, including direct count and strip census, are evaluated as to merits and disadvantages     

AERIAL CENSUS OF BLACK RHINOCEROS USING STRATIFIED RANDOM SAMPLING

During 1967–1968 a census was conducted of the black rhinoceros (Diceros bicornis L.) population in part of the Tsavo ecological unit. The region concerned covers an area of approximately 23,300 km² (9,000 square miles). Large parts of the region were systematically covered in a light aircraft and all rhinoceros observed were recorded on a 1:250,000 map. Because of the visibility bias inherent in such censuses correction factors were placed on the raw aerial data and crude estimates obtained. These crude estimates were then checked by intensive work on the ground in several sectors, and adjustments made where necessary. Using these crude estimates approximately two-thirds of the area under consideration were stratified into five density strata: very high, high, medium, low, and very low. Within this area 250 randomly located 2.59 km² (1 square mile) plots were intensively searched from the aircraft and all rhinoceros observed on these plots recorded. In the very high, high, and medium density strata these intensive searches resulted, at the 95% level of significance, in a population estimate of 4,200±25% of the mean or an estimate of between 3,150 and 5,255 rhinoceros within these strata. In the low stratum the statistical analysis resulted in a population estimate of 306±83% of the mean or an estimate of between 47 and 565 rhinoceros at the 90% level of significance. In the very low stratum the population estimate, at the 50% level of significance, was 266 ± 50% of the mean or an estimate of between 133 and 400 rhinoceros. A comparison of the crude estimates and the refined statistical estimates illustrates close agreement. The remaining third of the area was covered systematically and correction factors were placed on the raw aerial data. Crude estimates suggest a population of 2,800–3,000 occupying this part of the area. Assuming that this crude estimate, together with the statistical estimates for the low and very low strata, are rough approximations of the actual populations, it is statistically probable that the true population of black rhinoceros in this region lies between 6,130 and 9,220.     

AERIAL CENSUS OF PACIFIC WALRUSES IN THE CHUKCHI SEA, 1985

The U.S. Fish and Wildlife Service conducted a survey of the walruses in the pack ice of the Chukchi Sea between 16 September and 2 October 1985, as part of a joint effort with the Soviet Union to estimate the size of the Pacific walrus population. American observers conducted censuses from two aircraft along randomly selected north–south lines over the pack ice. The observers counted walruses within a constant viewing angle that corresponded to a total strip width of 1.38 km at an altitude of 152 m.
In nine days of flying, 15,312 walruses were observed, of which 10,140 were on 5,764 km² of census strips. Few walruses were observed east of 161°W longitude or west of 170°W longitude, hence the census effort was stratified. Walrus concentrations between 161° and 170° shifted slightly westward during the 2-wk duration of the censuses. The differences among days in observed walrus density were due to changes in the numbers of walruses on the ice within 37 km of the ice edge. The number of observable walruses in pack ice of the eastern Chukchi Sea was estimated to be 62,177 (SD = 19,480), based on censuses conducted on 29 and 30 September and 1 October. At that time there were also at least 15,238 in Bristol Bay, Bering Sea. The Soviets counted 39,572 on the shores of the western Chukchi and Bering seas and estimated 115,531 in pack ice of the western Chukchi Sea. Summing U.S. and Soviet estimates, the total population of Pacific walruses in 1985 was 232,518. This number was comparable with earlier estimates from censuses conducted jointly by the U.S. and the Soviets. However, information on fraction hauled out, segregation, and movements is needed for more precise estimates.     

Estimating abundance of introduced Chinese muntjac Muntiacus reevesi and native roe deer Capreolus capreolus using portable thermal imaging equipment

• 1 Managing the conservation impacts of deer requires knowledge of their numbers. However, estimating densities in forested areas is problematic, with pellet‐group‐based methods subject to error and uncertainty in estimating defecation and decay rates in addition to sampling variance. Use of thermal imaging equipment allows direct census by distance sampling.
• 2 Densities of introduced Chinese muntjac Muntiacus reevesi and native roe deer Capreolus capreolus were estimated in 12.8 km² of conifer forest in eastern England by thermal imaging distance transects. Estimated density of introduced muntjac (±95% confidence intervals [CI]: 20 ± 8 km⁻²) exceeded that of native roe deer (16 ± 6 km⁻²); 95% CI of c. 40% were achieved in 10 days of fieldwork.
• 3 Density estimates were not sensitive to the number of width bands applied during analysis. Detectability functions differed, with narrower effective strip widths (ESW) for muntjac (74 m) than roe deer (123 m). Thus, it is important to discriminate between species when censusing mixed assemblages. Detectability and ESW also differed among plantation growth stages. Stratification by habitat may therefore improve accuracy of density estimates but would require additional survey effort.

     

Complete tag loss in capture–recapture studies affects abundance estimates: An elephant seal case study

1. In capture–recapture studies, recycled individuals occur when individuals lose all of their tags and are recaptured as though they were new individuals. Typically, the effect of these recycled individuals is assumed negligible.
2. Through a simulation‐based study of double‐tagging experiments, we examined the effect of recycled individuals on parameter estimates in the Jolly–Seber model with tag loss (Cowen & Schwarz, 2006). We validated the simulation framework using long‐term census data of elephant seals.
3. Including recycled individuals did not affect estimates of capture, survival, and tag‐retention probabilities. However, with low tag‐retention rates, high capture rates, and high survival rates, recycled individuals produced overestimates of population size. For the elephant seal case study, we found population size estimates to be between 8% and 53% larger when recycled individuals were ignored.
4. Ignoring the effects of recycled individuals can cause large biases in population size estimates. These results are particularly noticeable in longer studies.

     

The hidden side of diversity: Effects of imperfect detection on multiple dimensions of biodiversity

1. Studies on ecological communities often address patterns of species distribution and abundance, but few consider uncertainty in counts of both species and individuals when computing diversity measures.
2. We evaluated the extent to which imperfect detection may influence patterns of taxonomic, functional, and phylogenetic diversity in ecological communities.
3. We estimated the true abundance of fruit‐feeding butterflies sampled in canopy and understory strata in a subtropical forest. We compared the diversity values calculated by observed and estimated abundance data through the hidden diversity framework. This framework evaluates the deviation of observed diversity when compared with diversities derived from estimated true abundances and whether such deviation represents a bias or a noise in the observed diversity pattern.
4. The hidden diversity values differed between strata for all diversity measures, except for functional richness. The taxonomic measure was the only one where we observed an inversion of the most diverse stratum when imperfect detection was included. Regarding phylogenetic and functional measures, the strata showed distinct responses to imperfect detection, despite the tendency to overestimate observed diversity. While the understory showed noise for the phylogenetic measure, since the observed pattern was maintained, the canopy had biased diversity for the functional metric. This bias occurred since no significant differences were found between strata for observed diversity, but rather for estimated diversity, with the canopy being more clustered.
5. We demonstrate that ignore imperfect detection may lead to unrealistic estimates of diversity and hence to erroneous interpretations of patterns and processes that structure biological communities. For fruit‐feeding butterflies, according to their phylogenetic position or functional traits, the undetected individuals triggered different responses in the relationship of the diversity measures to the environmental factor. This highlights the importance to evaluate and include the uncertainty in species detectability before calculating biodiversity measures to describe communities.

     

The population trend of southern elephant seals (<Emphasis Type="Italic">Mirounga leonina</Emphasis> L.) at Macquarie Island (1952–2004)

Total numbers of adult female southern elephant seals (cows) breeding at Macquarie Island were determined for 19 of the 52 year period between 1952 and 2004. Totals for 1952–1987 (exc. 1959 and 1985) were estimated from the relationship between censuses of the isthmus study area and concurrent censuses for the whole island. Totals for 1987–2004 were obtained by direct census of the entire island in mid-October. Cow numbers decreased from a maximum of about 40,000 in the 1950s to a minimum of 18,300 in 2000, but then increased slightly to 19,200 in 2004. Nonlinear and post-hoc linear analysis of the count data identified 1999 as the year when the exponential rate of change (r) slowed from −1.4% per annum to near zero. The rate of change was not uniform for each census sub-area counted (1987–2004), suggesting that certain terrestrially based density-dependent mechanisms were influencing the annual distribution of cows.     

Unmanned Aerial Survey of Elephants

The use of a UAS (Unmanned Aircraft System) was tested to survey large mammals in the Nazinga Game Ranch in the south of Burkina Faso. The Gatewing ×100™ equipped with a Ricoh GR III camera was used to test animal reaction as the UAS passed, and visibility on the images. No reaction was recorded as the UAS passed at a height of 100 m. Observations, made on a set of more than 7000 images, revealed that only elephants (Loxodonta africana) were easily visible while medium and small sized mammals were not. The easy observation of elephants allows experts to enumerate them on images acquired at a height of 100 m. We, therefore, implemented an aerial strip sample count along transects used for the annual wildlife foot count. A total of 34 elephants were recorded on 4 transects, each overflown twice. The elephant density was estimated at 2.47 elephants/km² with a coefficient of variation (CV%) of 36.10%. The main drawback of our UAS was its low autonomy (45 min). Increased endurance of small UAS is required to replace manned aircraft survey of large areas (about 1000 km of transect per day vs 40 km for our UAS). The monitoring strategy should be adapted according to the sampling plan. Also, the UAS is as expensive as a second-hand light aircraft. However the logistic and flight implementation are easier, the running costs are lower and its use is safer. Technological evolution will make civil UAS more efficient, allowing them to compete with light aircraft for aerial wildlife surveys.     

ON THE COEXISTENCE AND COEVOLUTION OF ASEXUAL AND SEXUAL COMPETITORS

The coexistence and coevolution of sexual and asexual species under resource competition are explored with three models: a nongenetic ecological model, a model including single locus genetics, and a quantitative-genetic model. The basic assumption underlying all three models is that genetic differences are translated into ecological differences. Hence if sexual species are genetically more variable, they will be ecologically more variable. Under classical competition theory, this increased ecological variability can, in many cases, be an advantage to individual sexual genotypes and to the sexual species as a whole. The purpose of this paper is to determine the conditions when this advantage will outway three disadvantages of sexuality: the costs of males, of segregation, and of the additive component of recombination. All three models reach similar conclusions. Although asexuality confers an advantage, it is much less than a two-fold advantage because minor increases in the overall species niche width of the sexual species will offset the reproductive advantage of the asexual species. This occurs for two reasons. First, an increase in species niche width increases the resource base of the sexual species. Second, to the extent that the increase in niche width is due to increased differences between individuals, a reduction in intraspecific competition will result. This is not to imply that the sexual species will always win. The prime conditions that enable sexual species to stably coexist with or even supplant an asexual sister species are:

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Fine-layer depth relationships of lakewater chemistry, planktonic algae and photosynthetic bacteria in meromictic Lake Fidler, Tasmania

SUMMARY.

• 1 Use of a multi-column thin-layer pneumatic sampler and modified analytic procedures has enabled resolutions of chemical and biological strata at 2.5 or 5.0 cm depth intervals. Examination of meromictic Lake Fidler, Tasmania, indicates the presence of a thin, intense stratum of bacteriochlorohyll d in the upper monimolimnion, associated with a discrete stratum of Chlorobium cf. limicola at the microaerobic interface between the oxygenated and sulphide-rich zones.
• 2 Algae included small populations of Chlorophyceae, Chrysophyceae, Bacillariophyceae and Cryptophyceae. Bacteria included microaerophils and obligate anaerobes, pigmented and colourless, in well-defined strata in the upper monimolimnion. A population of the microcrustacean Calamoecia tasmanica tasmanica was present in the mixolimnion. Chaoborus larvae were concentrated within the Chlorobium layer.
• 3 The chemical profile of Lake Fidler was stable, with a chemocline constant in position relative to the lake bottom. The surface water levels rose and fell through a distance of 1m in conjunction with heavy rainfall in the rainforest, and with river level variation, but had no measurable effect on the absolute position of the chemocline. Marked heterogeneity of dissolved substances at depths in the vicinity of bacteria suggested endogenous influence on pH and gelbstoff (‘gilvin’ in Australia).
• 4 Downwelling light attenuation was influenced primarily by gelbstoff (‘gilvin’) in the mixolimnion, with only red light (peak at 700 nm) measurable below 2 m. Light was absorbed mainly by Chlorobium in the monimolimnion, and was unmeasurable deeper than 3 m.
• 5 The absorption spectrum of the bacteriochlorophyll d in vivo, with a maximum absorbance at 721 nm, corresponds with the available downwelling light penetrating the mixolimnion to the Chlorobium layer.

     

Presence of the invasive brown marmorated stink bug Halyomorpha halys (Stål) (Hemiptera: Pentatomidae) on home exteriors during the autumn dispersal period: Results generated by citizen scientists

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The effect of fixed‐count subsampling on macroinvertebrate biomonitoring in small streams

• 1 When rigorous standards of collecting and analysing data are maintained, biological monitoring adds valuable information to water resource assessments. Decisions, from study design and field methods to laboratory procedures and data analysis, affect assessment quality. Subsampling ‐ a laboratory procedure in which researchers count and identify a random subset of field samples ‐ is widespread yet controversial. What are the consequences of subsampling?
• 2 To explore this question, random subsamples were computer generated for subsample sizes ranging from 100 to 1000 individuals as compared with the results of counting whole samples. The study was done on benthic invertebrate samples collected from five Puget Sound lowland streams near Seattle, WA, USA. For each replicate subsample, values for 10 biological attributes (e.g. total number of taxa) and for the 10‐metric benthic index of biological integrity (B‐IBI) were computed.
• 3 Variance of each metric and B‐IBI for each subsample size was compared with variance associated with fully counted samples generated using the bootstrap algorithm. From the measures of variance, we computed the maximum number of distinguishable classes of stream condition as a function of sample size for each metric and for B‐IBI.
• 4 Subsampling significantly decreased the maximum number of distinguishable stream classes for B‐IBI, from 8.2 for fully counted samples to 2.8 classes for 100‐organism subsamples. For subsamples containing 100–300 individuals, discriminatory power was low enough to mislead water resource decision makers.

     

Validation of observer and aircraft calibration for aerial surveys of animals

Calibration procedures to determine strip widths for use by the Department of Resource Surveys and Remote Sensing (DRSRS) in its aerial surveys are validated by statistical analysis. Tests were made for differences in observer, aircraft and between using photography (camera) and direct observation (naked eye) in the determination of strip widths on the ground. The relationship between strip widths measured on the ground and in the air was determined using regression analysis. Tests were also made for differences between population estimates reported by DRSRS and those recalculated using reported strip widths for elephant, kongoni, wildebeest and cattle. There appears to be no significant differences between observer performance and between reported and recalculated population estimates for all species except cattle. A significant difference was found between strip widths obtained by photography and those through direct observation, with photography values being 14% higher at ground level. Strip widths measured while the aircraft was in the air were highly correlated to those measured while on the ground (r = 0.97). Overall, it can be concluded that the DRSRS standardization of observers, calibration of the aircraft and calculation of population estimates were carried out correctly. Finally, it is recommended that the difference in strip widths determined from photography and through direct observation needs to be considered in the final calculation of population estimates.     

Heterogeneity in African savanna elephant distributions and their impacts on trees in Kruger National Park,South Africa

1. Though elephants are a major cause of savanna tree mortality and threaten vulnerable tree species, managing their impact remains difficult, in part because relatively little is known about how elephant impacts are distributed throughout space.
2. This is exacerbated by uncertainty about what determines the distribution of elephants themselves, as well as whether the distribution of elephants is even informative for understanding the distribution of their impacts.
3. To better understand the factors that underlie elephant impacts, we modeled elephant distributions and their damage to trees with respect to soil properties, water availability, and vegetation in Kruger National Park, South Africa, using structural equation modeling.
4. We found that bull elephants and mixed herds differed markedly in their distributions, with bull elephants concentrating in sparsely treed basaltic sites close to artificial waterholes and mixed herds aggregating around permanent rivers, particularly in areas with little grass.
5. Surprisingly, we also found that the distribution of elephant impacts, while highly heterogeneous, was largely unrelated to the distribution of elephants themselves, with damage concentrated instead in densely treed areas and particularly on basaltic soils.
6. Results underscore the importance of surface water for elephants but suggest that elephant water dependence operates together with other landscape factors, particularly vegetation community composition and historical management interventions, to influence elephant distributions.

     

Evaluation of four survey methods for estimating elephant densities

Aerial sample survey, foot survey, vehicle survey, dry-season droppings count and wet-season droppings count were used for estimating elephant (Loxodonta africana africana) densities on the Nazinga Game Ranch, Burkina Faso and evaluated against the results of an aerial total count. Results of all survey techniques except the dry-season dropping count method had low accuracies relative to the aerial total count. It is concluded that the droppings count method was the best and most cost-effective survey technique available for estimation of elephant densities.     

THE DIET OF DUCK AND COOT ON LAKE NAIVASHA

• 1 The diet of coot and duck on Lake Naivasha has been investigated to provide information on the duck/coot interaction.
• 2 Some 79 duck and coot were shot in a limited area over a period of 24 h, and their stomach contents preserved in 4% formalin.
• 3 The analysis of stomach contents was performed in two stages: large particles being completely identified and counted, small particles being sampled.
• 4 The results of the analyses are considered to be most usefully expressed as proportions of food component by number of particles.
• 5 Identification of components has been made by matching epidermal characters with collected plants, by matching shape and structures of seeds with collected seeds, and by recognition of such materials as arthropod exoskeletal fragments and molluscan shell. Some components have not been specifically identified.
• 6 A discussion has been presented on the problem of relating the results of stomach-content analysis to ecologically significant differences in feeding. The discussion hinges on four questions:

• (a) Is the result of a stomach-content analysis an accurate and appropriate indication of the stomach-contents of a bird when shot, and, if so, do the results presented indicate differences between stomach contents?
• (b) What is the relationship between stomach-contents at the time of shooting and food ingested over the immediately preceding period, and do differences between stomach contents indicate differences in food intake?
• (c) How far do differences in diet so deduced apply to the whole population of birds concerned in the study?
• (d) Are the differences in diet relatable to availability of food, and can any valid inferences be made concerning the birds' interaction?

• 7 It is concluded that at the time of the shooting, over the limited area examined, there was little overlap in diet for important components, and therefore probably little competition for food among the duck species and coot.
• 8 Some general observations have been made on the feeding biology of duck and coot, and it is pointed out that the feeding apparatus of coot differs from that of all duck species in having shearing edges. This difference is related to the dietary differences in a predictable manner.

     

CROCODILE DISTRIBUTION AND STATUS IN THE MAJOR WATERS OF WESTERN AND CENTRAL UGANDA IN 1969

• 1 This report presents the results of a survey into the current status of the Nile crocodile in the major waters of central and western Uganda, made on behalf of the Uganda Fisheries Department and the Trustees of the Uganda National Parks.
• 2 The waters covered by the survey are described briefy
• 3 Crocodile numbers and distribution were assessed by diurnal aerial counting and nocturnal ground counts as described by Graham (1968). The major sources of error inherent in these techniques are recognised and discussed. Efforts to reduce bias in results included the use of a light helicopter to obtain data for comparison with those obtained from a conventional aircraft. Comparative costs are also presented.
• 4 Inaccessible areas of dense papyrus were searched at night from a helicopter equipped with a spotlight.
• 5 Information was obtained from fishermen in Lake Albert on the status of crocodile to corroborate other results.
• 6 The analysed results of aerial and ground counts give estimates of less than 100 crocodile outside the Murchison Falls National Park, indicating the species' reduction to the status of an ‘occasional’ or ‘rare’ animal. Within the National Park crocodile occur in two populations separated by the Murchison Falls. Below the Falls it is estimated that there are 1064 crocodile, and above 433.
• 7 By use of aerial photography 369 crocodile from the larger of the National Park's populations were measured for length. From Graham's (1968) age/length data, age classes are ascribed to those measured. This, together with data from the counts, permitted the computation of a survivorship curve for the population. This is presented with two possible alternatives. From these curves it does not appear that the population is declining through lack of recruitment.
• 8 From the aerial photographs river banks are classified into four crude categories. By relating crocodile in the photographs to the nearest bank category, it is shown that they have a marked preference for sandy banks over swampy shores, hard flat earth or cliffs. It is also shown that smaller crocodile make more use of swampy areas than larger specimens.
• 9 Biomass densities for both crocodile populations in the National Park are computed. The larger shows a biomass of 347 kg/km of shoreline which compares with 350.2 kg/km for Lake Rudolf (Graham's (1968) data modified), the smaller 75.3 kg/km.
• 10 The decline of crocodile in Uganda is attributed to expanding human populations and over-exploitation for skins.
• 11 Recommendations for the future conservation and management of crocodile are presented. These are based on reducing immature mortality rates by hatching eggs and artificially rearing small crocodiles for a period of 6 months after hatching. Some of these should be used to augment recruitment in the National Park populations, and the surplus placed in other waters until of an exploitable size.

     

New technologies in the mix: Assessing N‐mixture models for abundance estimation using automated detection data from drone surveys

1. Reliable estimates of abundance are critical in effectively managing threatened species, but the feasibility of integrating data from wildlife surveys completed using advanced technologies such as remotely piloted aircraft systems (RPAS) and machine learning into abundance estimation methods such as N‐mixture modeling is largely unknown due to the unique sources of detection errors associated with these technologies.
2. We evaluated two modeling approaches for estimating the abundance of koalas detected automatically in RPAS imagery: (a) a generalized N‐mixture model and (b) a modified Horvitz–Thompson (H‐T) estimator method combining generalized linear models and generalized additive models for overall probability of detection, false detection, and duplicate detection. The final estimates from each model were compared to the true number of koalas present as determined by telemetry‐assisted ground surveys.
3. The modified H‐T estimator approach performed best, with the true count of koalas captured within the 95% confidence intervals around the abundance estimates in all 4 surveys in the testing dataset (n = 138 detected objects), a particularly strong result given the difficulty in attaining accuracy found with previous methods.
4. The results suggested that N‐mixture models in their current form may not be the most appropriate approach to estimating the abundance of wildlife detected in RPAS surveys with automated detection, and accurate estimates could be made with approaches that account for spurious detections.

     

Social associations and vocal communication in wild and captive male savannah elephants Loxodonta africana

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