Use of Occupancy Models to Estimate the Influence of Previous Live Captures on DNA-Based Detection Probabilities of Grizzly Bears

Abstract: Large carnivores potentially change their behavior following physical capture, becoming less responsive to the attractants that resulted in their capture, which can bias population estimates where the change in behavior is not appropriately modeled. We applied occupancy models to efficiently estimate and compare detection probabilities of previously collared grizzly bears (Ursus arctos) with bears captured at DNA hair-snag sites that were not previously collared. We found that previously captured bears had lower detection probabilities, although their detection probabilities were still >0, implying that they were still visible to be sampled via the DNA hair-snag grid, which was able to detect finer differences in capture probabilities of previously collared bears compared with Huggins closed-captures population models. To obtain relatively unbiased population estimates for DNA surveys, heterogeneity caused by previous live capture should be accounted for in the population estimator. (JOURNAL OF WILDLIFE MANAGEMENT 72(3):589–595; 2008)     

Predicting patch occupancy in fragmented landscapes at the rangewide scale for an endangered species: an example of an American warbler

AIM: Our objective was to identify the distribution of the endangered golden-cheeked warbler (Setophaga chrysoparia) in fragmented oak-juniper woodlands by applying a geoadditive semiparametric occupancy model to better assist decision-makers in identifying suitable habitat across the species breeding range on which conservation or mitigation activities can be focused and thus prioritize management and conservation planning. LOCATION: Texas, USA. METHODS: We used repeated double-observer detection/non-detection surveys of randomly selected (n = 287) patches of potential habitat to evaluate warbler patch-scale presence across the species breeding range. We used a geoadditive semiparametric occupancy model with remotely sensed habitat metrics (patch size and landscape composition) to predict patch-scale occupancy of golden-cheeked warblers in the fragmented oak-juniper woodlands of central Texas, USA. RESULTS: Our spatially explicit model indicated that golden-cheeked warbler patch occupancy declined from south to north within the breeding range concomitant with reductions in the availability of large habitat patches. We found that 59% of woodland patches, primarily in the northern and central portions of the warbler's range, were predicted to have occupancy probabilities ≤0.10 with only 3% of patches predicted to have occupancy probabilities >0.90. Our model exhibited high prediction accuracy (area under curve = 0.91) when validated using independently collected warbler occurrence data. MAIN CONCLUSIONS: We have identified a distinct spatial occurrence gradient for golden-cheeked warblers as well as a relationship between two measurable landscape characteristics. Because habitat-occupancy relationships were key drivers of our model, our results can be used to identify potential areas where conservation actions supporting habitat mitigation can occur and identify areas where conservation of future potential habitat is possible. Additionally, our results can be used to focus resources on maintenance and creation of patches that are more likely to harbour viable local warbler populations.     

Dealing with detection error in site occupancy surveys: what can we do with a single survey?

Aim Site occupancy probabilities of target species are commonly used in various ecological studies, e.g. to monitor current status and trends in biodiversity. Detection error introduces bias in the estimators of site occupancy. Existing methods for estimating occupancy probability in the presence of detection error use replicate surveys. These methods assume population closure, i.e. the site occupancy status remains constant across surveys, and independence between surveys. We present an approach for estimating site occupancy probability in the presence of detection error that requires only a single survey and does not require assumption of population closure or independence. In place of the closure assumption, this method requires covariates that affect detection and occupancy.Methods Penalized maximum-likelihood method was used to estimate the parameters. Estimability of the parameters was checked using data cloning. Parametric boostrapping method was used for computing confidence intervals.Important findings The single-survey approach facilitates analysis of historical datasets where replicate surveys are unavailable, situations where replicate surveys are expensive to conduct and when the assumptions of closure or independence are not met. This method saves significant amounts of time, energy and money in ecological surveys without sacrificing statistical validity. Further, we show that occupancy and habitat suitability are not synonymous and suggest a method to estimate habitat suitability using single-survey data.     

Parasites of the collared peccary from Texas

Results of a survey of the parasites of the collared peccary (Dicotyles tajacu angulatus) in Texas are presented. Three ectoparasites, Amblyomma cajennense, Dermacentor variabilis, and Pulex porcinus were very common on peccaries from south Texas, but less common or absent in arid west Texas. Sucking lice, Pecaroecus javalii, were common on peccaries from west Texas, but were not found in south Texas. The known range of this louse in Texas is extended into the Big Bend area. Two ticks, Amnblyomma inornatumn and Haemaphysalis leporis-palustris, were found infrequently. Five of nine species of endoparasites found in this survey (Dirofilaria acutiuscula, Parabronema pecatriae, Parostertagia heterospiculum, Physocephalus sp., and Texicospirura turki) were prevalent. Three species, D. acutiuscula, Gongylonema baylisi, and Fascioloides magna, are reported from North American peccaries for the first time. The geographic distribution of the large American liver fluke, F. magna, coincided with an area where the parasite is enzootic in white-tailed deer. It is concluded that parasitism was of little importance in population control of peccaries during the period of the study.     

Survival and detectability bias of avian fence collision surveys in sagebrush steppe

We used female ring-necked pheasant (Phasianus colchicus) carcasses as surrogates for greater sage-grouse (Centrocercus urophasianus) to study factors influencing survival and detection bias associated with avian fence collision surveys in southern Idaho, USA, during spring 2009. We randomly placed 50 pheasant carcasses on each of 2 study areas, estimated detection probability during fence-line surveys, and monitored survival and retention of carcasses and their associated sign over a 31-day period. Survival modeling suggested site and habitat features had little impact on carcass survival, and constant survival models were most supported by the data. Model averaged carcass daily survival probability was low on both study areas and ranged from 0.776 to 0.812. Survival of all carcass sign varied strongly by location, and the top sign survival model included a site effect parameter. Model averaged daily survival probability for collision sign on the 2 study sites ranged from 0.863 to 0.988 and varied between sites. Logistic regression modeling indicated detection probability of carcasses during fence-line surveys for avian collision victims was influenced by habitat type and microsite shrub height at the carcass location. Carcasses located in big sagebrush (Artemisia tridentata) habitats were detected at a lower rate (0.36) than carcasses in little (A. arbuscula) and black sagebrush (A. nova) habitats (0.71). Increasing shrub height at the carcass location from the little sagebrush mean of 16.5 cm to the big sagebrush mean of 36.0 cm reduced detection probability by approximately 30%. Avian fence collision surveys in sagebrush-steppe habitats should be conducted at ≤2-week sampling intervals to reduce the impact of survival bias on collision rate estimates. Two-week sampling intervals may be too long in areas with low carcass and sign survival, therefore survival rates should be estimated on all study areas to determine the appropriate sampling interval duration. Researchers should be aware of the effects of local vegetation on detection probabilities, and methods to correct detection probabilities based on collision site attributes should be applied to ensure more accurate collision rate estimates. Additionally, caution should be used when aggregating or comparing uncorrected collision data from areas with differing vegetation, as detection probabilities are likely different between sites. © 2011 The Wildlife Society.     

Factors affecting detectability of river otters during sign surveys

Sign surveys are commonly used to study and monitor wildlife species but may be flawed when surveys are conducted only once and cover short distances, which can lead to a lack of accountability for false absences. Multiple observers surveyed for river otter (Lontra canadensis) scat and tracks along stream and reservoir shorelines at 110 randomly selected sites in eastern Kansas from January to April 2008 and 2009 to determine if detection probability differed among substrates, sign types, observers, survey lengths, and near access points. We estimated detection probabilities (p) of river otters using occupancy models in Program PRESENCE. Mean detection probability for a 400-m survey was highest in mud substrates (p = 0.60) and lowest in snow (p = 0.18) and leaf litter substrates (p = 0.27). Scat had a higher detection probability (p = 0.53) than tracks (p = 0.18), and experienced observers had higher detection probabilities (p > 0.71) than novice observers (p < 0.55). Detection probabilities increased almost 3-fold as survey length increased from 200 m to 1,000 m, and otter sign was not concentrated near access points. After accounting for imperfect detection, our estimates of otter site occupancy based on a 400-m survey increased >3-fold, providing further evidence of the potential negative bias that can occur in estimates from sign surveys when imperfect detection is not addressed. Our study identifies areas for improvement in sign survey methodologies and results are applicable for sign surveys commonly used for many species across a range of habitats. © 2010 The Wildlife Society     

Description of collared peccary (Tayassu tajacu) milk composition

Composition of collared peccary (Tayassu tajacu) milk was described for samples obtained from a group of four captive females maintained on a high quality diet and four wild-caught females inhabiting semiarid rangelands in southern Texas. The gross composition of peccary and porcine milks are fairly similar, with some differences in mineral composition. Results suggest that bottle formulations developed for domestic swine piglets would provide adequate nutrition for hand-rearing nursling peccaries.     

Assessing detectability for monitoring of rare species: a case study of the cobblestone tiger beetle (Cicindela marginipennis Dejean)

Interpreting data on distribution or population trends may be difficult unless detection probability is accounted for. We wished to determine the detectability of the rare and patchily distributed cobblestone tiger beetle (Cicindela marginipennis) along the upper Genesee River in western New York for development of a monitoring strategy. We used occupancy surveys and distance sampling to examine two types of detectability. The first type was site-level detectability: the probability of detecting a single cobblestone tiger beetle on an occupied cobble bar, calculated using program PRESENCE. The second type was individual-level detectability: the probability of detecting an individual cobblestone tiger beetle in a population on a single cobble bar, calculated using program DISTANCE. Our occupancy surveys consisted of collecting presence and absence data on cobble bars along the Genesee River; these showed a relatively narrow range of site-level detection probabilities (0.60?C0.68) for cobblestone tiger beetles in 2008 and 2009. Three visits were necessary to detect cobblestone tiger beetles on 90% of occupied cobble bars. Individual cobblestone tiger beetles were detectable one-half of the time (0.50) in our surveys. It is important for ecologists to distinguish between the two kinds of detectability, as monitoring implications could differ substantially depending on which is calculated. Our monitoring recommendations include (1) continuing occupancy surveys with at least three visits to each cobble bar; (2) conducting occupancy surveys between 10:00 and 17:00 on warm sunny days in mid-July and mid-August; and (3) conducting surveys at three- to five-year intervals depending on the study objective.     

Spatial dynamics of an invasive bird species assessed using robust design occupancy analysis: the case of the Eurasian collared dove (Streptopelia decaocto) in France

Aim The study of the spatial dynamics of invasive species is a key issue in invasion ecology. While mathematical models are useful for predicting the extent of population expansions, they are not suitable for measuring and characterizing spatial patterns of invasion unless the probability of detection is homogeneous across the distribution range. Here, we apply recently developed statistical approaches incorporating detection uncertainty to characterize the spatial dynamics of an invasive bird species, the Eurasian collared dove (Streptopelia decaocto). Location France. Methods Data on presence/absence of doves were recorded from 1996 to 2004 over 1045 grid cells (28 × 20 km) covering the entire country. Each grid cell included five point counts spaced along a route, which was visited twice a year, allowing for an estimation of detection probability. Each route was assigned to one of six geographical regions. We used robust design occupancy analysis to assess spatial and temporal variation in parameters related to the spatial dynamics of the species. These parameters included occupancy rate, colonization and local extinction probabilities. Our inference approach was based on the selection of the most parsimonious model among competitive models parametrized with conditional probabilities. Results The probability of detecting the presence of doves on a given route was high. However, we found evidence to incorporate detection uncertainty in inference processes about spatial dynamics, since detection probability was neither perfect (i.e. it was < 1), nor constant over space and time. Results showed a clear positive trend in occupancy rate over the study period, increasing from 55% in 1996 to 76% in 2004. In addition, occupancy rate differed among regions (range: 37–79%) and further analysis showed that colonization probability by region was positively related to occupancy rate. Finally, local extinction probability was lower than colonization probability and showed a tendency to decrease over the study period. Main conclusions Our results emphasize the importance of estimating detection probabilities in order to draw proper inferences about the spatial and temporal dynamics of the invasion pattern of the collared dove. In contrast to the perceived spatial dynamics from national atlas surveys, we provide evidence that the range of this species is currently increasing in France. Other results, such as regional specificity in colonization probabilities and time variation in local extinction are consistent with expectations from invasion and metapopulation theory.     

Monitoring site occupancy for American mink in its native range

American mink (Neovison vison) are secretive, semi-aquatic carnivores that often require noninvasive methods based on field signs such as tracks and scat for determining their spatial distribution. Most previous assessments of survey methods for American mink have been conducted in the United Kingdom where mink are an invasive species. We evaluated survey techniques for American mink in riparian habitat in its native range in the midwestern United States. We used occupancy modeling to compare detection rates between walking surveys and mink raft surveys, and we evaluated the potential for environmental covariates and observer bias to influence detectability from walking surveys. Per-survey detection probabilities were greater for walking surveys (0.72) than for mink rafts (0.39). Walking surveys also were cheaper and easier to conduct in small streams prone to flooding when compared to mink raft surveys. However, detection probabilities from walking surveys were affected by observer bias, recent rainfall, substrate, and date. We recommend walking surveys for determining the distribution of American mink in riparian habitat in the Midwest if occupancy modeling is applied to adjust for environmental and observer effects on detectability. We used such an approach to demonstrate occupancy dynamics of mink were related to variable water depths, which has implications for how this carnivore might be influenced by climate change. Mink rafts standardize the substrate for recording mink tracks and reduce the likelihood of observer effects. For studies using many volunteers, we recommend mink rafts for determining site occupancy by American mink. © 2011 The Wildlife Society.     

Do Collared Peccaries Negatively Impact Understory Insectivorous Rain Forest Birds Indirectly Via Lianas and Vines?

Tropical rain forest understory insectivorous birds are declining, even in large forest reserves, yet the mechanisms remain unclear. Abundant large mammals can reshape forest structure, which degrades foraging microhabitat. We used six sites in Nicaragua, Costa Rica, and Panama with varying collared peccary (Pecari tajacu) density to test three linked hypotheses: (1) locally declining understory insectivores forage preferentially in liana tangles; (2) vine and liana density, cover, and frequency of dense tangles are lower in the presence of abundant collared peccaries; and consequently (3) abundant collared peccaries are associated with reduced understory insectivorous bird abundance. Three insectivores that declined at La Selva preferentially foraged in liana tangles: Checker‐throated Antwren (Epinecrophylla fulviventris), Dot‐winged Antwren (Microrhopias quixensis), and Ruddy‐tailed Flycatcher (Terenotriccus erythrurus). Vine density, liana cover, liana tangle frequency, and forest cover were lower in the presence of collared peccaries relative to experimental mammal exclosures, with the greatest differences at La Selva Biological Station, Costa Rica. Across sites, five of seven vine and liana measures showed negative, curvilinear relationships with peccary densities. Vine and liana measures peaked at sites with intermediate peccary density, and were low at La Selva. Structural equation models suggest negative indirect effects of the collared peccaries on focal bird densities, mediated by vine and liana density, cover, or tangle frequency. Forest area and rainfall affected both lianas and birds, but collared peccaries also contributed to the reduced abundance of understory insectivores. Indirect effects such as that suggested here may occur even in large, protected forest reserves where large mammal communities are changing.     

Use of classical bird census transects as spatial replicates for hierarchical modeling of an avian community

New monitoring programs are often designed with some form of temporal replication to deal with imperfect detection by means of occupancy models. However, classical bird census data from earlier times often lack temporal replication, precluding detection‐corrected inferences about occupancy. Historical data have a key role in many ecological studies intended to document range shifts, and so need to be made comparable with present‐day data by accounting for detection probability. We analyze a classical bird census conducted in the region of Murcia (SE Spain) in 1991 and 1992 and propose a solution to estimating detection probability for such historical data when used in a community occupancy model: the spatial replication of subplots nested within larger plots allows estimation of detection probability. In our study, the basic sample units were 1‐km transects, which were considered spatial replicates in two aggregation schemes. We fit two Bayesian multispecies occupancy models, one for each aggregation scheme, and evaluated the linear and quadratic effect of forest cover and temperature, and a linear effect of precipitation on species occupancy probabilities. Using spatial rather than temporal replicates allowed us to obtain individual species occupancy probabilities and species richness accounting for imperfect detection. Species‐specific occupancy and community size decreased with increasing annual mean temperature. Both aggregation schemes yielded estimates of occupancy and detectability that were highly correlated for each species, so in the design of future surveys ecological reasons and cost‐effective sampling designs should be considered to select the most suitable aggregation scheme. In conclusion, the use of spatial replication may often allow historical survey data to be applied formally hierarchical occupancy models and be compared with modern‐day data of the species community to analyze global change process.     

Estimating site occupancy rates for aquatic plants using spatial sub-sampling designs when detection probabilities are less than one

Estimation of site occupancy rates when detection probabilities are <1 is well established in wildlife science. Data from multiple visits to a sample of sites are used to estimate detection probabilities and the proportion of sites occupied by focal species. In this article we describe how site occupancy methods can be applied to estimate occupancy rates of plants and other sessile organisms. We illustrate this approach and the pitfalls of ignoring incomplete detection using spatial data for 2 aquatic vascular plants collected under the Upper Mississippi River's Long Term Resource Monitoring Program (LTRMP). Site occupancy models considered include: a naïve model that ignores incomplete detection, a simple site occupancy model assuming a constant occupancy rate and a constant probability of detection across sites, several models that allow site occupancy rates and probabilities of detection to vary with habitat characteristics, and mixture models that allow for unexplained variation in detection probabilities. We used information theoretic methods to rank competing models and bootstrapping to evaluate the goodness-of-fit of the final models. Results of our analysis confirm that ignoring incomplete detection can result in biased estimates of occupancy rates. Estimates of site occupancy rates for 2 aquatic plant species were 19–36% higher compared to naive estimates that ignored probabilities of detection <1. Simulations indicate that final models have little bias when 50 or more sites are sampled, and little gains in precision could be expected for sample sizes >300. We recommend applying site occupancy methods for monitoring presence of aquatic species.     

Comparison of Methods for Estimating Bird Abundance and Trends From Historical Count Data

Abstract: The use of bird counts as indices has come under increasing scrutiny because assumptions concerning detection probabilities may not be met, but there also seems to be some resistance to use of model-based approaches to estimating abundance. We used data from the United States Forest Service, Southern Region bird monitoring program to compare several common approaches for estimating annual abundance or indices and population trends from point-count data. We compared indices of abundance estimated as annual means of counts and from a mixed-Poisson model to abundance estimates from a count-removal model with 3 time intervals and a distance model with 3 distance bands. We compared trend estimates calculated from an autoregressive, exponential model fit to annual abundance estimates from the above methods and also by estimating trend directly by treating year as a continuous covariate in the mixed-Poisson model. We produced estimates for 6 forest songbirds based on an average of 621 and 459 points in 2 physiographic areas from 1997 to 2004. There was strong evidence that detection probabilities varied among species and years. Nevertheless, there was good overall agreement across trend estimates from the 5 methods for 9 of 12 comparisons. In 3 of 12 comparisons, however, patterns in detection probabilities potentially confounded interpretation of uncorrected counts. Estimates of detection probabilities differed greatly between removal and distance models, likely because the methods estimated different components of detection probability and the data collection was not optimally designed for either method. Given that detection probabilities often vary among species, years, and observers investigators should address detection probability in their surveys, whether it be by estimation of probability of detection and abundance, estimation of effects of key covariates when modeling count as an index of abundance, or through design-based methods to standardize these effects.     

Agricultural encroachment and lack of enforcement threaten connectivity of range-wide jaguar (Panthera onca) corridor

The purpose of protected areas is to safeguard natural habitats and their biodiversity. However, few protected areas are large enough for the long-term conservation of resident species, making the landscapes surrounding protected areas equally important for species conservation. This is especially true for wide-ranging carnivores such as the jaguar (Panthera onca), for which conservation strategies stress the maintenance of range-wide habitat connectivity via protected areas and dispersal corridors. We used 587 interviews with local people in 441 36-km² sampling units to assess the presence of jaguar and six prey species in a critical corridor in Central America, linking Bosawas Biosphere Reserve and Wawashan Nature Reserve in Nicaragua. We analyzed these data using single-season false-positive detection models in a site occupancy framework. We found that agricultural encroachment, particularly pervasive within a protected area, was the main factor limiting the presence of jaguar and three large-bodied prey species: white-lipped peccary (Tayassu pecari), collared peccary (Pecari tajacu), and red brocket deer (Mazama americana). We suggest that improperly-managed protected areas can suffer the loss of apex predators and their prey, thereby jeopardizing range-wide conservation strategies for threatened species.     

Integrating ultrasonography within the reproductive management of the collared peccary (Tayassu tajacu)

Ultrasound imaging has been used to elucidate certain aspects of the reproductive biology of wild or endangered species. However, to our knowledge, this tool has not been used for reproductive monitoring of the collared peccary (Tayassu tajacu). In this study, real-time ultrasonography was used in 16 collared peccary females to diagnose early pregnancy status and predict gestational age. Based on the detection of an embryo, the earliest pregnancy diagnosis was made on Day 18 after mating, with the mean time needed for diagnosis being 22 days. Overall accuracies on Days 22, 26 and 28 were 56, 93, and 100%, respectively. On Days 26 and 28, all pregnancy and non-pregnancy diagnoses, respectively, were correct. The fetal measurements that best correlated with gestational age were crown-rump-length (CRL) and the length and diameter of the thorax. CRL was considered the most practical measurement because, contrary to thoracic fetometry, it could be determined when the embryo was first detected. Our findings revealed real-time ultrasound scanning to be a very accurate method for early pregnancy diagnosis and prediction of gestational age in the collared peccary.     

Diet Overlap and Foraging Activity between Feral Pigs and Native Peccaries in the Pantanal

Inter-specific competition is considered one of the main selective pressures affecting species distribution and coexistence. Different species vary in the way they forage in order to minimize encounters with their competitors and with their predators. However, it is still poorly known whether and how native species change their foraging behavior in the presence of exotic species, particularly in South America. Here we compare diet overlap of fruits and foraging activity period of two sympatric native ungulates (the white-lipped peccary, Tayassu pecari, and the collared peccary, Pecari tajacu) with the invasive feral pig (Sus scrofa) in the Brazilian Pantanal. We found high diet overlap between white-lipped peccaries and feral pigs, but low overlap between collared peccaries and feral pigs. Furthermore, we found that feral pigs may influence the foraging period of both native peccaries, but in different ways. In the absence of feral pigs, collared peccary activity peaks in the early evening, possibly allowing them to avoid white-lipped peccary activity peaks, which occur in the morning. In the presence of feral pigs, collared peccaries forage mostly in early morning, while white-lipped peccaries forage throughout the day. Our results indicate that collared peccaries may avoid foraging at the same time as white-lipped peccaries. However, they forage during the same periods as feral pigs, with whom they have lower diet overlap. Our study highlights how an exotic species may alter interactions between native species by interfering in their foraging periods.     

Suckling Behavior of the Collared Peccary (Tayassu tajacu)

Based on Trivers' (1974) theory of parent/offspring conflict, we predicted that weaning would be accelerated under poor dietary conditions in an aseasonally breeding, monomorphic ungulate, the collared peccary (Tayassu tajacu). Suckling behavior was observed in 18 captive litters under a dietary treatment of high or low protein formulated to mimic forage under ecological conditions of normal rainfall vs. drought. Litters on the low protein diet weaned later (mean of 27 weeks) than litters on the high protein diet (mean of 21 weeks). Other indices of cumulative suckling effort (weaning weight, cumulative minutes suckling, cumulative bouts) did not differ between dietary treatments. No differences in suckling effort were found relative to offspring sex or litter size. Milk did not completely compensate for the low protein diet affecting growth of young in the second and third phases of lactation. Some nursers did not conserve bodily resources as would be expected if the benefits of short interbirth intervals outweighed the costs of lactation during unpredictable drought conditions.     

Population trends and calling phenology of anuran populations surveyed in Ontario estimated using acoustic surveys

Many acoustic surveys have been initiated to monitor anuran populations in North America. We used the Ontario Backyard Frog Survey to examine temporal and spatial trends, from 1994 to 2001. Our data suggest that there have been no consistent trends in site occupancy during this time period, but there were some differences among years. Both American toads and northern leopard frogs were more prevalent in 1995 than in 1994. Similarly, species richness was higher in 1995 and 1996 compared to most other years. Individual populations of species, however, often were not stable. Extinction and colonization rates varied among species, and ranged from 1.5 to 19.5% per year, and site occupancy was negatively correlated with extinction rates. Daily detection probabilities were often quite low, and were primarily driven by the perceived calling intensity. We recommend: (i) that monitoring programs attempt to preserve common survey routes, despite heavy turnover of volunteers, (ii) calling surveys be timed to maximize detection probabilities, and (iii) analyses based upon landscape features and GIS approaches should be used to determine localized changes in site occupancy or species richness.     

Population size influences amphibian detection probability: implications for biodiversity monitoring programs

Monitoring is an integral part of species conservation. Monitoring programs must take imperfect detection of species into account in order to be reliable. Theory suggests that detection probability may be determined by population size but this relationship has not yet been assessed empirically. Population size is particularly important because it may induce heterogeneity in detection probability and thereby cause bias in estimates of biodiversity. We used a site occupancy model to analyse data from a volunteer-based amphibian monitoring program to assess how well different variables explain variation in detection probability. An index to population size best explained detection probabilities for four out of six species (to avoid circular reasoning, we used the count of individuals at a previous site visit as an index to current population size). The relationship between the population index and detection probability was positive. Commonly used weather variables best explained detection probabilities for two out of six species. Estimates of site occupancy probabilities differed depending on whether the population index was or was not used to model detection probability. The relationship between the population index and detectability has implications for the design of monitoring and species conservation. Most importantly, because many small populations are likely to be overlooked, monitoring programs should be designed in such a way that small populations are not overlooked. The results also imply that methods cannot be standardized in such a way that detection probabilities are constant. As we have shown here, one can easily account for variation in population size in the analysis of data from long-term monitoring programs by using counts of individuals from surveys at the same site in previous years. Accounting for variation in population size is important because it can affect the results of long-term monitoring programs and ultimately the conservation of imperiled species.