BACKGROUND
conservation biologists are constantly faced with a conflict of interests: on the one hand, they need information on population abundance, species richness etc.
for implementing conservation programs; while on the other, they should be cautious on reliability of findings from published work  <cit> .
‘additional surveys’ is the most often sought after option in order to fill the gaps in knowledge.
since this is often not feasible due to limited resources, robustness of inventory is a desirable attribute of any faunal survey  <cit> .
robustness depends on unbiased and precise measurements of parameters that describe the biological community.
however, it is a well known fact that biases in sampling are common and sampling assumptions are not always met with  <cit> .
since most empirical studies in community ecology use some form of sampling to derive estimates of species richness, diversity, and abundance, the properties of these samples have great practical importance  <cit> .
particularly, when the inventory of a community is based on a non-random fraction of a population, it results in sampling error, which precludes direct comparisons of population size and species diversity measures across studies.
to address this, some fundamental questions about survey methods need to be answered: how are bias and precision of estimates of diversity and abundance related to survey methods and effort?
how do they affect our interpretation of estimates of population sizes and subsequent conservation decisions?
answering these questions and overcoming the problems in sampling are central to good-practice in data collection and study design.
we illustrate this problem using a small tropical island herpetofauna.
small island fauna compose of a finite community and they could be sampled using quadrat sampling with varying levels of intensity.
several studies in the tropics have used quadrats as sampling units to document patterns in species richness, distribution, and abundance of forest-floor herpetofauna .
most of these studies have assumed that they recorded all individuals and species in the quadrats during sampling, i.e.
no sampling error, which is a major assumption .
we explored the effect of sampling error on species richness, diversity, and density estimates from such ‘open quadrats’  by combining them with ‘bounded quadrats’  that gave total counts of individuals in the sampled area  <cit> .
our study revealed that intensive sampling using bq had varying effects on species richness and diversity, depending on the estimator, while it significantly improved density estimates.
RESULTS
effect of quadrat characteristics
boxplots of environmental variables showed no major difference between oq and bq .
further, ancova indicated quadrat type to be the most significant factor explaining the variation in number of individuals , multiple r2 =  <dig> , adjusted r2 =  <dig> , f =  <dig>  on  <dig> and  <dig> df, p =  <dig>  × 10−09; frogs—residual standard error =  <dig>  on  <dig> df, multiple r2 =  <dig> , adjusted r2 =  <dig> , f =  <dig>  on  <dig> and  <dig> df, p =  <dig>  × 10−08; table 1).
canopy cover also influenced number of both lizards and frogs recorded.
number of buttressed trees and number of trees influenced abundance of lizards and frogs respectively.
in general, habitat characteristics did not overwhelmingly influence abundance of frogs and lizards recorded in quadrats.figure  <dig> variation in environmental variables between open and bounded quadrats a elevation b canopy cover c number of tress d number of buttresses.
table  <dig> analysis of covariance table for number of individuals recorded in quadrats with number of individuals of lizards and frogs as the independent variable and quadrat type, forest type, elevation, number of trees, canopy cover and number of tree buttresses as predictor variables
species richness
observed species richness, of both lizards and frogs, were similar in bq and oq .
for both lizards and frogs, chao <dig> estimators were significantly different between oq and bq for both lizards  and frogs .
fisher’s α was also different between oq and bq for both lizards  and frogs .
second order jackknife estimate was not influenced by sampling methods i.e., the way individuals were included in the samples .
species richness of both frogs and lizards were correlated with sampling effort in bq .
abundances of frogs and lizards were not correlated with sampling effort in both the methods .
accumulation of rare species in samples  was faster for frogs and lizards in bq than in oq .table  <dig> summary of analysis of species richness and density using bounded quadrats and open quadrats
though  <dig> bounded quadrats were sampled, for the purpose of comparison of estimated species richness, we use a sample size of  <dig> for lizards because beyond this, the standard deviation of the estimated species richness became zero for jackknife <dig> estimate.
estimated species richness of frogs is compared at a sample size of  <dig> as the remaining  <dig> bounded quadrats were in small islands where no amphibian has ever been recorded.
fisher’s α is compared at a common number of individuals .
the standard deviations for density estimates presented here are unadjusted for sample size difference.
n number of quadrats based on which the estimates were arrived at, s
obs observed species richness, s
chao <dig> chao <dig> estimate, s
jack <dig> second order jackknife estimate, α fisher’s alpha, d density .
figure  <dig> relationship between sampling effort and species richness a lizards in oq b lizards in bq c frogs in oq d frogs in bq.
figure  <dig> relationship between sampling effort and abundance a lizards in oq b lizards in bq c frogs in oq d frogs in bq.
figure  <dig> detection of rare species in the community expressed as reduction in percentage of singletons against sample size for frogs and lizards in open and bounded quadrats.
solid diamonds frogs in bq, open diamonds frogs in oq, solid circles lizards in bq, open circles lizards in oq.
density and relative abundance
we detected  <dig> lizards and  <dig> frogs in  <dig> and  <dig> bq respectively.
from  <dig> oq, we detected  <dig> lizards and  <dig> frogs.
when the variance was adjusted for sample size difference, density estimates from bq were significantly greater than that from oq for both lizards and frogs .
in the case of lizards, the density estimate from bq was more than twice that from oq .
in the case of frogs, density estimate from bq was more than seven times that from oq .
however, the effort involved in a bq was significantly more  than that for an oq .
there was a noticeable difference in the relative abundance of some species between the two methods.
among lizards, the small, fossorial species lygosoma bowringii showed the greatest increase in relative abundance within bq, while the larger and more conspicuous coryphophylax subcristatus and eutropis andamanensis had greater relative abundance in oq .
among frogs, the small leaf-litter dwelling limnonectes cf.
doriae had greater relative abundance in bounded quadrats, while the arboreal and brightly coloured ingerana charlesdarwini had greater relative abundance in oq than in bq .figure 5  percentage difference in relative abundance of  lizards and  frogs in open and bounded quadrats.
species ranks are those from bounded quadrats.
positive deviations show greater relative abundance in open quadrats and negative deviations show greater relative abundance in bounded quadrats.
a open square, coryphophylax subcristatus; open circle, lygosoma bowringii; open triangle cyrtodactylus rubidus; solid square, coryphophylax brevicaudus; solid circle, cnemaspis sp.
; solid triangle, eutropis andamanensis; solid diamond, sphenomorphus maculatus.
b open square, limnonectes cf.
doriae; open circle, ingerana charlesdarwini; open triangle, bush toad; solid square, duttaphrynus melanostictus; solid circle, microhyla chakrapanii; solid triangle, fejervarya cancrivora; solid diamond, fejervrya cf.
andamanensis; multiplication sign, kaloula baleata ghoshi.
relative bias and precision
density estimates obtained from oq had high relative bias and low relative precision for both lizards  and frogs .
second order jackknife estimate of species richness had low relative bias for frogs  and lizards .
the relative precision of the estimate for lizards was also low  and in frogs it was high .
species diversity estimated as fisher’s α, of frogs and lizards, the relative bias and relative precision ranged from − <dig>  to  <dig> , implying no difference in the efficiency of the methods for this parameter.
CONCLUSIONS
stemming rapid loss of biodiversity in the tropics requires robust scientific findings that bolster conservation decisions.
it is up to the field biologists to ensure that the results provided are accurate so that decisions on resource utilization and management are not based on biased or imprecise results.
in most community studies on herpetofauna, sampling methods are chosen arbitrarily, as a result, little attention has been paid to the effect of sampling error and its implications on inferences drawn .
recently, kellner and swihart  <cit>  showed that 77% of ecological studies ignored imperfect detection, while the rest accounted for this through estimation of detection probability through statistical modelling.
a majority of studies  also reported detection probability to be highly variable, inflating the error in estimated parameters  <cit> .
our study showed that sampling error in open quadrats resulted in bias in estimated population density and in detection of less conspicuous and small frogs and lizards.
open quadrats have often been used repeatedly for sampling forest floor herpetofauna and these studies report accurate estimates of density from such plots.
however, rodda et al.
<cit>  reported high densities of reptiles using their ‘total removal plots’, which is a form of bounded quadrat.
bounded quadrats have invariably reported high densities of herpetofauna .
our data revealed that bounded quadrats are efficient in sampling rare species and we expect that its performance will be better in tropical mainland areas than in islands, where a large proportion of the community is composed of rare species.
this study provides empirical evidence for second order jackknife estimate for species richness and fisher’s α for diversity as robust measures as they were not sensitive to the number of individuals included in the samples.
intensive sampling influenced measures of relative abundance of species and thereby altered community structure.
relative bias and precision point at high overall efficiency of bounded quadrats.
we anticipate that our findings would prompt more field ecologists to consider careful selection of methods, analyses of data, and comparison of results from past studies.
a promising direction of further research is the use of bounded quadrats for calibration of estimates obtained from other less intensive sampling methods  <cit> .
this would pave way for developing new and more efficient sampling methods suitable for use in the tropics.
at present, there is no alternative to an intensive sampling method to obtain precise or unbiased estimates of abundance and species richness of herpetofauna in tropical areas.
we emphasize the need for refinement in sampling techniques so that credible findings strengthen tropical conservation.

 conservation biologists are constantly faced with a conflict of interests: on the one hand, they need information on population abundance, species richness etc. for implementing conservation programs; while on the other, they should be cautious on reliability of findings from published work. ‘additional surveys’ is the most often sought after option in order to fill the gaps in knowledge. since this is often not feasible due to limited resources, robustness of inventory is a desirable attribute of any faunal survey. robustness depends on unbiased and precise measurements of parameters that describe the biological community. however, it is a well known fact that biases in sampling are common and sampling assumptions are not always met with. since most empirical studies in community ecology use some form of sampling to derive estimates of species richness, diversity, and abundance, the properties of these samples have great practical importance. particularly, when the inventory of a community is based on a non-random fraction of a population, it results in sampling error, which precludes direct comparisons of population size and species diversity measures across studies. to address this, some fundamental questions about survey methods need to be answered: how are bias and precision of estimates of diversity and abundance related to survey methods and effort? how do they affect our interpretation of estimates of population sizes and subsequent conservation decisions? answering these questions and overcoming the problems in sampling are central to good-practice in data collection and study design. we illustrate this problem using a small tropical island herpetofauna. small island fauna compose of a finite community and they could be sampled using quadrat sampling with varying levels of intensity. several studies in the tropics have used quadrats as sampling units to document patterns in species richness, distribution, and abundance of forest-floor herpetofauna. most of these studies have assumed that they recorded all individuals and species in the quadrats during sampling, i.e. no sampling error, which is a major assumption. we explored the effect of sampling error on species richness, diversity, and density estimates from such ‘open quadrats’ by combining them with ‘bounded quadrats’ that gave total counts of individuals in the sampled area. our study revealed that intensive sampling using bq had varying effects on species richness and diversity, depending on the estimator, while it significantly improved density estimates.  effect of quadrat characteristics boxplots of environmental variables showed no major difference between oq and bq. further, ancova indicated quadrat type to be the most significant factor explaining the variation in number of individuals, multiple r2 = , adjusted r2 = , f =  on and df, p =   × 10−09; frogs—residual standard error =  on df, multiple r2 = , adjusted r2 = , f =  on and df, p =   × 10−08; table 1). canopy cover also influenced number of both lizards and frogs recorded. number of buttressed trees and number of trees influenced abundance of lizards and frogs respectively. in general, habitat characteristics did not overwhelmingly influence abundance of frogs and lizards recorded in quadrats.figure  variation in environmental variables between open and bounded quadrats a elevation b canopy cover c number of tress d number of buttresses. table  analysis of covariance table for number of individuals recorded in quadrats with number of individuals of lizards and frogs as the independent variable and quadrat type, forest type, elevation, number of trees, canopy cover and number of tree buttresses as predictor variables species richness observed species richness, of both lizards and frogs, were similar in bq and oq. for both lizards and frogs, chao estimators were significantly different between oq and bq for both lizards and frogs. fisher’s α was also different between oq and bq for both lizards and frogs. second order jackknife estimate was not influenced by sampling methods i.e., the way individuals were included in the samples. species richness of both frogs and lizards were correlated with sampling effort in bq. abundances of frogs and lizards were not correlated with sampling effort in both the methods. accumulation of rare species in samples was faster for frogs and lizards in bq than in oq.table  summary of analysis of species richness and density using bounded quadrats and open quadrats though bounded quadrats were sampled, for the purpose of comparison of estimated species richness, we use a sample size of for lizards because beyond this, the standard deviation of the estimated species richness became zero for jackknife estimate. estimated species richness of frogs is compared at a sample size of as the remaining bounded quadrats were in small islands where no amphibian has ever been recorded. fisher’s α is compared at a common number of individuals. the standard deviations for density estimates presented here are unadjusted for sample size difference. n number of quadrats based on which the estimates were arrived at, s obs observed species richness, s chao chao estimate, s jack second order jackknife estimate, α fisher’s alpha, d density. figure  relationship between sampling effort and species richness a lizards in oq b lizards in bq c frogs in oq d frogs in bq. figure  relationship between sampling effort and abundance a lizards in oq b lizards in bq c frogs in oq d frogs in bq. figure  detection of rare species in the community expressed as reduction in percentage of singletons against sample size for frogs and lizards in open and bounded quadrats. solid diamonds frogs in bq, open diamonds frogs in oq, solid circles lizards in bq, open circles lizards in oq. density and relative abundance we detected lizards and frogs in and bq respectively. from oq, we detected lizards and frogs. when the variance was adjusted for sample size difference, density estimates from bq were significantly greater than that from oq for both lizards and frogs. in the case of lizards, the density estimate from bq was more than twice that from oq. in the case of frogs, density estimate from bq was more than seven times that from oq. however, the effort involved in a bq was significantly more than that for an oq. there was a noticeable difference in the relative abundance of some species between the two methods. among lizards, the small, fossorial species lygosoma bowringii showed the greatest increase in relative abundance within bq, while the larger and more conspicuous coryphophylax subcristatus and eutropis andamanensis had greater relative abundance in oq. among frogs, the small leaf-litter dwelling limnonectes cf. doriae had greater relative abundance in bounded quadrats, while the arboreal and brightly coloured ingerana charlesdarwini had greater relative abundance in oq than in bq.figure 5  percentage difference in relative abundance of lizards and frogs in open and bounded quadrats. species ranks are those from bounded quadrats. positive deviations show greater relative abundance in open quadrats and negative deviations show greater relative abundance in bounded quadrats. a open square, coryphophylax subcristatus; open circle, lygosoma bowringii; open triangle cyrtodactylus rubidus; solid square, coryphophylax brevicaudus; solid circle, cnemaspis sp.; solid triangle, eutropis andamanensis; solid diamond, sphenomorphus maculatus. b open square, limnonectes cf. doriae; open circle, ingerana charlesdarwini; open triangle, bush toad; solid square, duttaphrynus melanostictus; solid circle, microhyla chakrapanii; solid triangle, fejervarya cancrivora; solid diamond, fejervrya cf. andamanensis; multiplication sign, kaloula baleata ghoshi. relative bias and precision density estimates obtained from oq had high relative bias and low relative precision for both lizards and frogs. second order jackknife estimate of species richness had low relative bias for frogs and lizards. the relative precision of the estimate for lizards was also low and in frogs it was high. species diversity estimated as fisher’s α, of frogs and lizards, the relative bias and relative precision ranged from − to, implying no difference in the efficiency of the methods for this parameter.  stemming rapid loss of biodiversity in the tropics requires robust scientific findings that bolster conservation decisions. it is up to the field biologists to ensure that the results provided are accurate so that decisions on resource utilization and management are not based on biased or imprecise results. in most community studies on herpetofauna, sampling methods are chosen arbitrarily, as a result, little attention has been paid to the effect of sampling error and its implications on inferences drawn. recently, kellner and swihart showed that 77% of ecological studies ignored imperfect detection, while the rest accounted for this through estimation of detection probability through statistical modelling. a majority of studies also reported detection probability to be highly variable, inflating the error in estimated parameters. our study showed that sampling error in open quadrats resulted in bias in estimated population density and in detection of less conspicuous and small frogs and lizards. open quadrats have often been used repeatedly for sampling forest floor herpetofauna and these studies report accurate estimates of density from such plots. however, rodda et al. reported high densities of reptiles using their ‘total removal plots’, which is a form of bounded quadrat. bounded quadrats have invariably reported high densities of herpetofauna. our data revealed that bounded quadrats are efficient in sampling rare species and we expect that its performance will be better in tropical mainland areas than in islands, where a large proportion of the community is composed of rare species. this study provides empirical evidence for second order jackknife estimate for species richness and fisher’s α for diversity as robust measures as they were not sensitive to the number of individuals included in the samples. intensive sampling influenced measures of relative abundance of species and thereby altered community structure. relative bias and precision point at high overall efficiency of bounded quadrats. we anticipate that our findings would prompt more field ecologists to consider careful selection of methods, analyses of data, and comparison of results from past studies. a promising direction of further research is the use of bounded quadrats for calibration of estimates obtained from other less intensive sampling methods. this would pave way for developing new and more efficient sampling methods suitable for use in the tropics. at present, there is no alternative to an intensive sampling method to obtain precise or unbiased estimates of abundance and species richness of herpetofauna in tropical areas. we emphasize the need for refinement in sampling techniques so that credible findings strengthen tropical conservation. 
fisher’s α measure of species diversity and second order jackknife estimate of species richness were not sensitive to number of individuals sampled. fisher’s α for species diversity and second order jackknife for species richness were robust measures. imperfect detection from less intensive sampling method results incorrect estimates of abundance of herpetofauna. by combining traditional open quadrats used for sampling forest floor herpetofauna with intensive bounded quadrats, we explore the effect of sampling error on estimates of species richness, diversity, and density in the andaman islands. it influenced relative abundance of individual species resulting in underestimation of abundance of small or camouflaged species; and also resulted in low precision in lizard species richness estimates.
 one of the basic premises of drawing samples from populations is that the samples are representative of the populations. however, error in sampling is poorly recognized, and it goes unnoticed especially in community ecology. by combining traditional open quadrats used for sampling forest floor herpetofauna with intensive bounded quadrats, we explore the effect of sampling error on estimates of species richness, diversity, and density in the andaman islands.  fisher’s α measure of species diversity and second order jackknife estimate of species richness were not sensitive to number of individuals sampled. sampling error resulted in underestimation of density in both frogs and lizards. it influenced relative abundance of individual species resulting in underestimation of abundance of small or camouflaged species; and also resulted in low precision in lizard species richness estimates.  sampling error resulted in underestimation of abundance of small, fossorial or camouflaged species. imperfect detection from less intensive sampling method results incorrect estimates of abundance of herpetofauna. fisher’s α for species diversity and second order jackknife for species richness were robust measures. these have strong implications on inferences made from previous studies as well as sampling strategies for future studies. it is essential that these shortfalls are accounted for while communities are sampled or when datasets are compared. electronic supplementary material the online version of this article contains supplementary material, which is available to authorized users. keywords rainforestlizardsfrogsprecisionrelative biaspopulation densitytropical islandsissue-copyright-statement© the author 2015 