Comparison of three vegetation monitoring methods: Their relative utility for ecological assessment and monitoring

Vegetation cover and composition are two indicators commonly used to monitor terrestrial ecosystems. These indicators are currently quantified with a number of different methods. The interchangeability and relative benefits of different methods have been widely discussed in the literature, but there are few published comparisons that address multiple criteria across a broad range of grass- and shrub-dominated communities, while keeping sampling effort (time) approximately constant. This study compared the utility of three field sampling methods for ecological assessment and monitoring: line-point intercept, grid-point intercept, and ocular estimates. The criteria used include: (1) interchangeability of data, (2) precision, (3) cost, and (4) value of each method based on its potential to generate multiple indicators. Foliar cover by species was measured for each method in five plant communities in the Chihuahuan Desert. Line- and grid-point intercept provide similar estimates of species richness which were lower than those based on ocular estimates. There were no differences in the precision of the number of species detected. Estimates of foliar cover with line- and grid-point intercept were similar and significantly higher than those based on ocular estimates. Precision of cover estimates with line-point intercept was higher than for ocular estimates. Time requirements for the three methods were similar, despite the fact that the point-based methods included cover estimates for all canopy layers and the soil surface, while the ocular estimates included only the top canopy layer. Results suggest that point-based methods provide interchangeable data with higher precision than ocular estimates. Moreover these methods can be used to generate a much greater number of indicators that are more directly applicable to a variety of monitoring objectives, including soil erosion and wildlife habitat.     

Use of sonar transects to improve efficiency and reduce potential bias in visual surveys of reef fishes

A sonar transect method has been found to be a useful alternative to traditional tape-transect methods in quantification of the species richness and abundance of reef fish. No significant difference was found between species richness measured by the methods, and high Jaccard’s similarity coefficients were calculated for each site using the two methods, indicating a similar response to fish of both the sonar and tape transects. Abundance estimates of one species only (Chaetodon tricinctus) were greater when the transect tape method was used, probably because fish were being attracted to the divers who deployed the transect tape. Substantial time saving was also noted for the sonar transects, indicating overall that estimating distance using sonar rather than transect tape may enable increased efficiency and accuracy in␣underwater visual surveys for both ecological␣and management-related research on reef fishes.     

Improving Aquatic Warbler Population Assessments by Accounting for Imperfect Detection

Monitoring programs designed to assess changes in population size over time need to account for imperfect detection and provide estimates of precision around annual abundance estimates. Especially for species dependent on conservation management, robust monitoring is essential to evaluate the effectiveness of management. Many bird species of temperate grasslands depend on specific conservation management to maintain suitable breeding habitat. One such species is the Aquatic Warbler (Acrocephalus paludicola), which breeds in open fen mires in Central Europe. Aquatic Warbler populations have so far been assessed using a complete survey that aims to enumerate all singing males over a large area. Because this approach provides no estimate of precision and does not account for observation error, detecting moderate population changes is challenging. From 2011 to 2013 we trialled a new line transect sampling monitoring design in the Biebrza valley, Poland, to estimate abundance of singing male Aquatic Warblers. We surveyed Aquatic Warblers repeatedly along 50 randomly placed 1-km transects, and used binomial mixture models to estimate abundances per transect. The repeated line transect sampling required 150 observer days, and thus less effort than the traditional ‘full count’ approach (175 observer days). Aquatic Warbler abundance was highest at intermediate water levels, and detection probability varied between years and was influenced by vegetation height. A power analysis indicated that our line transect sampling design had a power of 68% to detect a 20% population change over 10 years, whereas raw count data had a 9% power to detect the same trend. Thus, by accounting for imperfect detection we increased the power to detect population changes. We recommend to adopt the repeated line transect sampling approach for monitoring Aquatic Warblers in Poland and in other important breeding areas to monitor changes in population size and the effects of habitat management.     

A new potential method to estimate abundance of small game species

This study tested the hypothesis that nonlinear transects, such as the L‐, V‐ and W‐shaped transect, which are widely used in soil sampling but rarely in ecology, may perform better in capturing habitat heterogeneity potentially resulting in more accurate small game species estimates, than straight‐line transects. To test this, we computed and compared the Normalized Difference Vegetation Index (NDVI) captured by the straight‐line, L‐shaped, V‐shaped and W‐shaped transects for Kyle Game Reserve (Zimbabwe), inside the home ranges of two small game species, namely species A and species B. These transects differed in geometry but had the same length. We also performed simulations in a geographic information system environment to compare the detection ability of the different transect geometry. One‐way analysis of variance followed by Tukey's honest significant difference test was used to test for significant differences in the mean number of detections and NDVI variance captured by each transect geometry. Simulations results indicate that for both home ranges, the V‐shaped transect resulted in significantly higher detections than the widely used straight‐line transect. Nonlinear transects also yielded higher NDVI variances than the straight‐line transect in both home ranges.     

Understanding global change: Lessons learnt from the European landscape

Abstract. Europe is characterized not only by large geomorphological variability but also by a long history of land use. This resulted in a highly variegated landscape. Based on the IGBP‐transect initiative, a north south transect was established across Europe ranging from north Sweden to central Italy in order to study effects of global change. Mainly process oriented studies were established on plots along the transect, and these were used to establish functional relationships as basis for landscape integration. However, it became apparent that the transect approach was not sufficient to mirror the European environment. From early on, the assessment of land cover was not constrained to this transect but complemented with measurements focused on Europe at a continental scale. Also, continental networks of flux measurements and ecosystem experiments were established, because these were able to encompass a larger inherent variability of climate, geology and land use. Resulting from the Kyoto protocol emphasis moved stronger from observations along transects towards a continental scale quantification of fluxes given the reporting needs. In this process the transect and the networks merged into research clusters, which provided a scaling and verification mechanism. Thus, the European situation may serve as an example of how the initial idea of transects has further evolved and broadened to continental scale studies in a region where anthropogenic land use dominates over climate change.     

Remote sensing of vegetation cover change in islands of the Kerguelen archipelago

The plant communities in the Iles Kerguelen (South Indian Ocean) have been extensively modified by human activities, particularly through the deliberate release of rabbits, and the intentional or accidental introduction of several plant species. During the 1990 and 2000s, a decrease in precipitation resulted in a drastic reduction of some native plant species and in the increase in alien taxa. To monitor at a wide spatial scale the rapid changes of vegetation cover induced by summer droughts, we developed a method combining field data and satellite image analysis. A long-term field monitoring of plant communities was initiated on five small islands in 1992, and annually continued for over 15?years on a total of 161 line transects. Among these islands, the rabbit??which was the only introduced herbivore??was eradicated on three, remained on one control island, and had never been present on a second control island. We computed a linear model to link remote sensored vegetation indexes to plant cover deduced from line transects in numerous habitat types. After testing 14 vegetation indexes, we used a model based on the normalized difference vegetation index to precisely map the vegetation cover at several dates. A map of differences and spatial statistics indicated that vegetation cover, as a whole, decreased over the 15-year period. This study provides a reliable tool for long-term monitoring of the dynamics of plant cover in relation to climate change on the Iles Kerguelen.     

Comparison of survey methods for wintering grassland birds

ABSTRACT Although investigators have evaluated the efficacy of survey methods for assessing densities of breeding birds, few comparisons have been made of survey methods for wintering birds, especially in grasslands. In winter, social behavior and spatial distributions often differ from those in the breeding season. We evaluated the degree of correspondence between density estimates based on different survey methods. Surveys were conducted during two winters (2001–2002 and 2002–2003) on 16 grassland sites in southwestern Oklahoma. Line‐transect (using a detection function to account for birds present but not detected) and area‐search (where density was based on the total count within a given area) methods were employed. Observations on line transects were also analyzed as strip transects, where density was based on total count within a given strip width and no detection function was used. Savannah Sparrows (Passerculus sandwichensis), LeConte's Sparrows (Ammodramus leconteii), Song Sparrows (Melospiza melodia), Smith's Longspurs (Calcarius pictus), Chestnut‐collared Longspurs (C. ornatus), and Eastern Meadowlarks (Sturnella magna) were sufficiently abundant to allow comparison. Area‐search density estimates tended to be higher than line‐transect estimates for Savannah Sparrows, Song Sparrows, and Eastern Meadowlarks, suggesting that some individuals initially located close to the transect line were not detected on line transects. The area‐search and line‐transect methods gave similar density estimates for Chestnut‐collared and Smith's longspurs. Area‐search estimates of Eastern Meadowlarks were significantly higher in the second year of the study only. For this species, area‐search estimates did not differ from those of strip transects covering an equal area, so the reason for the differing meadowlark estimates is not clear. Higher density estimates using the area‐search method likely resulted from: (1) birds that might escape detection by hiding were more likely detected (flushed) during area searches because of the repeated passes through the area, and (2) birds close to the line in line transects escape detection by hiding, biasing those estimates low. We also evaluated the correspondence of density rankings for the six species as determined by the different survey methods and for the same species across survey sites. Correlations among the six species of the area‐search results with those of line transects and strip transects generally were high, increasing in 2002–2003 when densities of birds were greater. All three methods provided similar density rankings among species. Density rankings within species across sites for the four non‐longspur species generally were concordant for the three methods, suggesting that any of them will adequately reflect among‐site differences, especially when densities vary greatly across sites. Further research is needed to determine the extent to which grassland birds are missed on line transects. We suggest that workers using line transects to study these species give careful consideration and make additional efforts to satisfy the distance‐sampling assumption that all birds on or near the line are detected. If density is measured as a total count in a fixed area, we recommend that observers pass within <10 m of all points in the area.     

Predicting and correcting bias caused by measurement error in line transect sampling using multiplicative error models

Line transect sampling is one of the most widely used methods for animal abundance assessment. Standard estimation methods assume certain detection on the transect, no animal movement, and no measurement errors. Failure of the assumptions can cause substantial bias. In this work, the effect of error measurement on line transect estimators is investigated. Based on considerations of the process generating the errors, a multiplicative error model is presented and a simple way of correcting estimates based on knowledge of the error distribution is proposed. Using beta models for the error distribution, the effect of errors and of the proposed correction is assessed by simulation. Adequate confidence intervals for the corrected estimates are obtained using a bootstrap variance estimate for the correction and the delta method. As noted by Chen (1998, Biometrics 54, 899-908), even unbiased estimators of the distances might lead to biased density estimators, depending on the actual error distribution. In contrast with the findings of Chen, who used an additive model, unbiased estimation of distances, given a multiplicative model, lead to overestimation of density. Some error distributions result in observed distance distributions that make efficient estimation impossible, by removing the shoulder present in the original detection function. This indicates the need to improve field methods to reduce measurement error. An application of the new methods to a real data set is presented.     

An Evaluation of Survey Methods for Estimating Northern Bobwhite Abundance in Southern Texas

Abstract: Distance sampling has been identified as a reliable and well-suited method for estimating northern bobwhite (Colinus virginianus) density. However, distance sampling using walked transects requires intense sampling to obtain precise estimates, thus making the technique impractical for large acreages. Researchers have addressed this limitation by either resorting to the use of indices (e.g., morning covey-call surveys) or incorporating the use of aerial surveys with distance sampling. Both approaches remain relatively untested. Our objectives were to 1) compare density estimates among morning covey-call surveys, helicopter transects, and walked transects; 2) test a critical assumption of distance sampling pertinent to helicopter surveys (i.e., all objects on line are detected); and 3) evaluate the underlying premise of morning covey-call surveys (i.e., that the no. of calling coveys correlates with bobwhite density). Our study was conducted on 3 study sites in Brooks County, Texas, USA, during October to December, 2001 to 2005. Comparisons between walked transects and morning covey-call surveys involved the entire 5-year data set, whereas helicopter transects involved only the latter 2 years. Density estimates obtained from helicopter transects were similar to walked transect estimates for both years. We documented a detection probability on the helicopter transect line of 70 ± 10.2% (% ± SE; n = 20 coveys). Morning covey-call surveys yielded similar density estimates to walked transect estimates during only 2 of 5 years, when walked transect estimates were the least accurate and precise. We detected a positive relationship (R² = 0.51; 95% CI for slope: 29.5–53.1; n = 63 observations) between covey density and number of coveys heard calling. We conclude that helicopter transects appear to be a viable alternative to walked transects for estimating density of bobwhites. Morning covey-call surveys appear to be a poor method to estimate absolute abundance and to depict general population trajectories.     

Comparing survey methods for monitoring vegetation change through time in a restored peatland

Monitoring is an essential step to assess vegetation trajectories post-restoration and ultimately evaluate success. In this paper, we compare two monitoring methods, the line-point intercept (LPI) and the permanent plot (PP) methods, for evaluating plant recovery of a restored cut-over peatland (8.5 ha), following a “moss layer transfer technique”. We used the LPI method to estimate covers (from frequency measures) for each plant species using a systematic grid of approximately 5,700 points (every 3 m × 5 m). In parallel, 43 PP (3 m × 8 m) were established and used to evaluate plant covers. The post-restoration recovery of vegetation was assessed against a reference ecosystem encompassing the variation in species cover from natural undisturbed peatlands in the same region. For all plant groups considered, the LPI consistently showed higher cover estimates than the PP method. Discrepancy between the two methods was particularly evident for the Ericaceae group. A complementary sampling method, the line-intercept (LI), showed strong correlations with the visual estimations of Ericaceae covers (akin to PP), suggesting an overestimation from the LPI method. Most life form groups of the restored peatlands are developing a structure similar to the regional reference ecosystem 8 years post-restoration with the herb group being still most dissimilar. Indeed, when analyzing the temporal evolution of the different key peatland plant components, several are within the range of regional abundance values or moving positively towards range of cover abundance of the reference system such as Sphagnum cover, a key peat-accumulating plant group.     

Digitized photographs in vegetation analysis ‐ a comparison of cover estimates

Abstract. The use of digital images in measuring plant species cover and species number in boreal forest vegetation was studied. Plant cover was estimated by manual delineation on photographs and subsequent digitized measurement of the areas. This value was regarded as the reference and compared with cover obtained by visual estimate, point frequency and automatic image analysis methods. The automatic image analysis was based on scanned photographs. Supervised image classification with ERDAS software was then used to distinguish between the covers of different plant species. When comparing the ability of the methods to detect species, the visual estimate method gave values similar to the reference. The study material was collected from three sites along a heavy‐metal pollution transect in western Finland. All four methods detected, in a similar manner, differences between the plant species abundances along the transect. Compared with the reference, the digital images underestimated the cover of the lichens Cladina spp. and Cetraria islandica, but gave similar estimates for the dwarf shrubs Empetrum nigrum and Vaccinium vitis‐idaea. The point frequency method overestimated the cover of all the species studied. The visual estimates of lichens were close to the reference, while the dwarf shrub covers were overestimated. The number of species detected using supervised image analysis and the point frequency method was lower than that with visual estimation. Visual estimation was faster, and the estimate closer to the reference cover values than the others. Digital images may be useful in detecting changes in some selected species in vegetation with a simple vertical structure but with taller, multilayered vegetation and a higher species number, the reliability of the cover estimates is lower.     

The Influence of Transect Use by Local People and Reuse of Transects for Repeated Surveys on Nesting in Western Lowland Gorillas (Gorilla gorilla gorilla) and Central Chimpanzees (Pan troglodytes troglodytes) in Southeast Cameroon

Monitoring populations of endangered species over time is necessary to guide and evaluate conservation efforts. This is particularly important for nonprotected areas that ensure connectivity between protected populations but are prone to uncontrolled hunting pressure. We investigated whether use of transects by local people and transect reuse for repeated surveys influence great ape nesting and bias results. We conducted simultaneous marked nest count surveys over 12 mo on established and newly opened transects in a nonprotected area subject to traditional heavy use by local people and recorded forest composition and signs of human activity. Chimpanzee and gorilla density estimates and encounter rates per kilometer were lower on established transects than on new ones. A generalized linear model indicated that hunting activity, distance to a regularly used forest trail, and transect type (old or new) predicted chimpanzee nest abundance, and distance to the trail and transect type predicted gorilla nest abundance, with no effect of habitat type (percentage suitable habitat) for either species. We, therefore, suggest that the difference in great ape nesting is a result of high levels of hunting by local people on established transects and forest trails. Our results support the use of repeated line transect surveys for monitoring great ape populations in many circumstances, although we advocate taking precautions in nonprotected areas, to avoid the bias imposed by use of established transects for hunting.     

Familiarity may buffer the adverse effects of relocation on marmosets (Callithrix kuhlii): Preliminary evidence

The relocation of exotic species is increasingly crucial for the proper management of captive populations. We investigated the effects of relocation on urinary cortisol levels and proximity behavior in four Wied's marmosets (Callithrix kuhlii). Two of the relocated marmosets constituted a well‐established pair, whereas two (the newly established pair) were introduced when they arrived at the new facility. Four adult females served as control animals in which we assessed cortisol response to pair formation in the absence of relocation. Urinary cortisol in the relocated subjects was monitored for 4 weeks before and 12 weeks after the relocation event. Urinary cortisol in the control subjects was monitored for 4 weeks before and 8 weeks after the pairs were formed. Proximity was monitored in the relocated subjects for 12 weeks following the move. After the animals were relocated, the newly established pair had significantly elevated levels of urinary cortisol compared to the well‐established pair and control females, which lasted at least 8 weeks. For both pairs, cortisol levels decreased over time across the 12 weeks following the relocation event. The members of the well‐established pair were in contact with each other for 65% of the intervals sampled in the 2 weeks immediately following relocation, compared to only 12% in the newly established pair. These preliminary findings suggest that relocation is associated with significant adverse physiological and behavioral changes that may be ameliorated by the presence of a highly familiar social partner. The study highlights the importance of future research in this field. Zoo Biol 24:93–100, 2005. © 2005 Wiley‐Liss, Inc.     

Measurement errors in line transect surveys where detectability varies with distance and size

When using bivariate line transect methods to estimate the biomass density of a tightly clustered biological population, it is generally assumed that both the perpendicular distance from the trackline to the cluster and the cluster size, or biomass, are measured without error. This is unlikely to be the case in practice. In this article, assuming additive mean zero errors in distance and multiplicative errors in size, we develop an estimator of density that corrects for these errors. We use the method of moments for the case of gamma cluster size, randomly placed transect lines, and the generalized exponential detection function. We derive results that show that it may not be necessary to correct for errors in distance or size when the distance and size estimates are not biased. When the size estimates are biased, the biomass density estimate has approximately the same bias as the size estimates. The work is illustrated in the context of annual aerial surveys for juvenile southern bluefin tuna in the Great Australian Bight.     

Laboratory sources of error for algal community attributes during sample preparation and counting

Applied algal studies typically require enumeration of preserved cells. As applications of algal assessments proliferate, understanding sources of variability inherent in the methods by which abundance and species composition data are obtained becomes even more important for precision of measurements. We performed replicate counts of diatoms on permanently fixed coverglasses and all algae in Palmer–Maloney chambers to assess precision and accuracy of measurements derived from common counting methods. We counted diatoms and all algae with transects and random fields. Variability estimates (precision) of diatom density, species diversity, and species composition on permanent coverglasses were low between replicate subsamples and between replicate transects. However, average density estimates of diatoms settled on coverglasses determined with transect methods were 42–52% greater than density estimates made with random fields. This bias was due to a predictable, nonrandom distribution of diatoms on the coverglass with few diatoms near edges. Despite bias in density when counting diatoms along coverglass transects, no bias was observed in estimates of species composition. Estimates of density and taxa richness of all-algae in Palmer–Maloney chambers also had low variability among multiple transects and high similarity in species composition between transects. In addition, counting method in Palmer–Maloney chambers did not affect estimates of algal cell density, taxa richness, and species composition, which suggested that counting units were distributed randomly in the chambers. Thus, most sources of variability in sample preparation and analysis are small; however, transect counts should not be used to estimate cell density, and sufficient numbers of random fields must be counted to account for edge effects on cell distribution with material settled on permanently fixed coverglasses.     

Plant community establishment following drawdown of a reservoir in southern Arkansas, USA

Wetland area, function and wildlife habitat value are extensively altered by the construction of freshwater reservoirs. We studied the effects of a temporary drawdown on shoreline vegetation communities of Felsenthal Navigation Pool (“the pool”), an impoundment at Felsenthal National Wildlife Refuge in southern Arkansas that is managed as a greentree reservoir. The pool was permanently flooded from 1985 until the summer of 1995 when the water level was dropped 0.3 m for about 16 weeks, exposing about 1,591 ha of soil. To document plant succession on the sediments exposed, we recorded plant species composition and cover at 14 transects along the pool margin prior to the drawdown, during the drawdown, and in the following summer. A soil disturbance treatment was applied near five transects following the drawdown, and soil was collected at each transect for seed bank and soil analyses. Plants colonized the drawdown zone quickly and high vegetation cover was present at some transects 4 weeks after the drawdown was initiated. Plants included species that are high quality food sources for waterfowl, including Cyperus erythrorhizos and Leptochloa fascicularis var. fascicularis. Vegetation response, measured by species richness, total cover, and cover of Cyperus species, was often greater at low compared to high elevations in the drawdown zone; this effect was probably intensified by low summer rainfall. Response on the disturbed transects was lower than that on the undisturbed transects. This effect was attributed to two factors: (1) removal of the existing seed bank by the disturbance applied and (2) reduced incorporation of seeds recruited during the drawdown because of unusually low rainfall. Seed bank studies demonstrated that several species persisted despite 10 years of continual flooding, and that seed bank species richness increased during the drawdown. Although conclusions are limited by the 1-year time frame of the study, it is unlikely that permanent change to plant community structure in the drawdown zone resulted from the lowered water level.     

A comparison of video and point intercept transect methods for monitoring subtropical coral communities

This study evaluated the use of video transects obtained from SCUBA divers or remote operated vehicle (ROV) and point intercept transect (PIT) method from divers for monitoring subtropical coral communities. Comparisons were made between the datasets obtained by the ROV and SCUBA diver video transect (‘Diver’) and the PIT method on three nearby coral sites with different hydrographies, scleractinian coral composition, dominant species and percentage cover. There was no significant difference between the ROV and ‘Diver’ datasets whereas the PIT method tended to over-estimate percentage cover at sites where corals are not extensive. Power analysis showed that the minimum detectable change in coral percentage cover, δ, had low mean values between 0.39% and 1.65% for the ROV dataset, 0.66% for the “Diver’ dataset, and 12.11% for the PIT dataset. This implied that the ROV and SCUBA survey methods can produce higher precision in terms of detecting temporal changes in coral communities and are thus more suitable for scientific research and management purposes than the PIT method. Other advantages of using video transects by SCUBA divers or ROV include provision of permanent records for subsequent studies and public information, less field time incurred and wider survey areas.     

War of the weeds: Competition hierarchies in invasive species

Invasive plant species succeed because they are able to propagate and disperse into unoccupied habitat, outcompete and suppress other plant species or use a combination of these two strategies. Ecosystems are often invaded by multiple species and it is important to determine which species are likely to dominate plant communities through competition and which species will succeed by other means. We assessed frequency and abundance of 65 invasive species over 1700 km of riparian zone in the Burdekin catchment of North Queensland, Australia and established a dominance hierarchy for those species. The cover of every invasive and one native shrub known to have increased in abundance in historical times, was surveyed in quadrats as part of a nested hierarchical design consisting of 8030 quadrats, within 803 transects, within 90 sites. Total species cover and species frequency were derived from the results of this survey. We estimated the pairwise interactions between individual species and used a non‐parametric David Score to rank species and construct a competition hierarchy of invasive species at the quadrat and transect levels. Species frequency and cover were highly correlated (r² = 0.81). The competition hierarchy of species at the quadrat level was moderately related to species cover, but poorly related to species frequency (r² = 0.24 and 0.02). The competition rank of many species, including Urochloa mosambicensis and Parthenium hysterophorus, changed markedly with scale when assessed at the quadrat and transect levels. This suggests different processes influenced their competitive success at different scales. This technique enabled us to explore and accept the hypothesis that abundant species are often the most competitive. However, some exceptions were identified in this study and these species may in time become more abundant in the catchment.     

Field and simulation analyses of visual methods for sampling coral cover

The consensus view that coral reefs are declining urges the need to test the short-term effects of conservation. Visual sampling methods are a low cost, technically simple, and potentially highly accurate means of monitoring reef status or calibrating larger scale aerial and satellite surveys. We studied the recent literature and found that of the three most commonly used visual methods, the belt transect (BT) method was used twice as much as the line transect (LT) method and 2.5 times as often as the linear point intercept (LPI) method. We tested the repeatability, cost-efficiency, precision, and accuracy of these three methods in the field and in computer simulations. In the field, the observer repeatability was uniformly high among the three methods (R>0.9). Surprisingly, our study indicates that the least used method (LPI) was the most cost-effective means of measuring coral cover while being at least as precise and accurate as the other two methods. Our simulation study indicated that both the accuracy and precision of the three methods decrease sharply with decreasing coral cover, and that using between 5 and 10 transects over a homogeneous area is an appropriate sample size at either low or high coral cover.