and Estimation
Field techniques refer to the standardized methods employed to select, count, measure, capture, mark, and observe individuals sampled from the target pop-ulation for the purpose of collecting data required to achieve study objectives.
The term also includes methods used to collect voucher specimens, tissue samples, and habitat data. The choice of field techniques to use for a particular species or population is influenced by five major factors:
1. Data needed to achieve inventory and monitoring objectives 2. Spatial extent and duration of the project
3. Life history and population characteristics 4. Terrain and vegetation in the study area 5. Budget constraints
DATA reQuireMeNTS
The types of data required to achieve inventory or monitoring objectives should be the primary consideration in selecting field techniques. Four categories of data collection are discussed below along with some suggestions for electing appropriate field techniques for each.
occurrEncEand diStribution data
For some population studies, simply determining whether a species is present in an area is sufficient for conducting the planned data analysis. For example, biologists attempting to conserve a threatened salamander may need to monitor the extent of the species’ range and degree of population fragmentation on a land ownership. One hypothetical approach is to map all streams in which the salamander is known to be present, as well as additional streams that may qualify as the habitat type for the species in the region. To monitor changes in salamander distribution, data collection could consist of a survey along randomly selected reaches in each of the streams to determine if at least one individual (or some alternative characteristic such as an egg mass) is present. Using only a list that includes the stream reach (i.e., the unique iden-tifier), the survey year, and an occupancy indicator variable, a biologist could prepare a time series of maps displaying all of the streams by year and distinguish the subset
of streams that were known to be occupied by the salamander. Such an approach could support a qualitative assessment of changes in the species distribution pattern, thereby attaining the program’s objectives, and generate new hypotheses as to the cause of the observed changes.
It is far easier to determine if there is at least one individual of the target species on a sampling unit than it is to count all of the individuals. Determining with con-fidence that a species is not present on a sampling unit also requires more intensive sampling than collecting count or frequency data because it is so difficult to dismiss the possibility that an individual eluded detection. Probability of occurrence can be estimated using approaches such as those described by MacKenzie and Royale (2005). MacKenzie (2005) offered an excellent overview for managers of the trade-off between number of units sampled per year and the number of years (or other unit of time) for which the study is to be conducted. The variation in the estimated trend in occupancy decreases as the number of years of data collection increases (Figure 8.1).
A similar level of precision can be achieved by surveying more units over fewer years versus surveying fewer units over a longer period.
PoPulation SizEand dEnSity
National policy on threatened and endangered species is ultimately directed toward efforts to increase or maintain the total number of individuals of the species within their natural geographic range (Suckling and Taylor 2006). Total population size and effective population size (i.e., the number of breeding individuals in a population;
Lande and Barrowclough 1987) most directly indicate the degree of species endan-germent and effectiveness of conservation policies and practices. Population size or
50 100 200
0 4 8
4
Coefficient of Variation
2
Number of Seasons
Figure 8.1 Simulation-based coefficient of variation for estimated trend in occupancy (on the logistic scale) where 50, 100, or 200 landscapes are each surveyed 3 times per season, for multiple seasons. (Redrafted from MacKenzie, D.I., and J.A. Royle. 2005. Journal of Applied Ecology 42:1105–1114.) Estimates of occupancy can be facilitated by use of computer programs such as PRESENCE (MacKenzie et al. 2003).
Field Techniques for Population Sampling and Estimation 133 more accurately density per unit area is usually used as the basis for trend analyses because changes in density integrate changes in natural mortality, exploitation, and habitat quality. In some circumstances, it may be feasible to conduct a census of all individuals of a particular species in an area to determine the population density.
Typically, however, population size and density parameters are estimated using sta-tistical analyses based on only a sample of population members. Population densities of plants and sessile animals can be estimated from counts taken on plots or data describing the spacing between individuals (i.e., distance methods) and are relatively straightforward. Population analyses for many animal species must account for animal response to capture or observation, observer biases, and different detection probabilities among subpopulations. Pilot studies are usually required to collect the data necessary to address these factors in the analysis. Furthermore, mark-recapture studies, catch-per-unit effort surveys, and other estimation methods require multiple visits to sampling units (Pradel 1996). These considerations increase the complexity and cost of studies designed for population parameter estimation.
abundancE indicES
The goals and objectives of some biological inventories and monitoring studies can be met with indices of population density or abundance, rather than population esti-mators. The difference between estimators and indices is that the former yield abso-lute values of population density while the latter provide relative measures of density that can be used to compare indices to populations among places or times. Indices are founded on the assumption that index values are closely associated with values of a population parameter, although the precise relationship between the index and parameter usually is not quantified. Examples of abundance or density indices are plant canopy cover, numbers of individuals captured per 1,000 trap nights, and counts of individuals observed during a standardized unit of time, among many others.
From a data collection perspective, density indices often require less sampling intensity and complexity than population estimation procedures. However, popula-tion indices are not comparable among different studies unless field techniques are strictly standardized. Furthermore, the assumption that an abundance index closely approximates population density is rarely tested (Seber 1982).
fitnESS data
For rare or declining populations, estimates of survival in each life stage as well as reproductive rates are required. These data not only provide useful trigger points for estimating rates of decline (lambda) they also allow trigger points for removal of a species from a threatened or other legal status. Collecting these sorts of data is often labor intensive and expensive. In a study on northern spotted owls, for instance, millions of dollars have been spent collecting these types of data (Lint 2001). This is not particularly surprising as the types of data that would be necessary to under-stand the population dynamics of a bird are numerous and complicated to generate.
Nest densities, clutch sizes, hatching rates, fledging rates, and survival rates to matu-rity and survival rates as reproductive adults would be a minimum data set. New
approaches to estimating individual contributions to population growth and changes in distributions of quantitative traits and alleles include genetic analyses, which can lead to even more detailed understanding of the potential for a population to adapt to variations in environmental factors (Pelletier et al. 2009).
rESEarch StudiES
Studies of habitat relationships or cause-and-effect responses require coordinated sampling of the target population and environmental measurements or stressors to which the population may respond. Data collection efforts tend to be complex, requiring multiple sampling protocols for the target population, study site attributes, and landscape pattern metrics. The funding required to conduct research studies typically limits their application to species or populations in greatest need of man-agement planning, such as those listed as threatened or endangered. Manipulative studies are often carried out to generate the necessary data, but when these focus on a threatened species, ethical questions regarding the conduct of the experiment plac-ing the species at even greater risk, at least locally, often emerge. Hence, it is often monitoring of both environmental conditions and aspects of population density or fitness that is used to assess associations in trends between population parameters and environmental parameters.
SPATiAL eXTeNT
Clearly the scope of inference will influence the type of sampling technique used.
Breeding bird atlas techniques commonly use large grids placed over entire states to assess the occurrence of species in a grid cell. Such approaches and those of the Breeding Bird Survey (Sauer et al. 2008) can be conducted through volunteer efforts.
On the other hand, monitoring the trends in reproductive rates of northern spotted owls, northern goshawks, or grizzly bears over their geographic ranges requires a huge budget to collect the level of population data over large areas needed to under-stand trends. Great care must be taken when deciding what technique to use because both budgets and sample size requirements enter into logistics. Indeed, it is often the trade-off between more detailed data and the cost of producing those data that drives decisions regarding monitoring designs for species at risk.
FreQueNTLY uSeD TeCHNiQueS FOr SAMPLiNg ANiMALS
The array of techniques available to sample animals is vast and summarized else-where in techniques manuals (e.g., Bookhout 1994). We summarize a few examples of commonly used techniques, but strongly suggest that those of you developing monitoring plans do a more complete literature search on sampling of the species that are of most concern in your monitoring program. We first provide a brief over-view of techniques used to sample vertebrates and then point out which techniques are commonly used among various taxonomic groups.
Field Techniques for Population Sampling and Estimation 135
aQuatic organiSmS
Some aquatic organisms can and have been monitored using techniques that are essentially identical to those used for terrestrial vertebrates. For instance, in Brazil, arapaima have been monitored using a point count technique that counts individuals as they surface for aerial breathing (Castello et al. 2009). Point counts were more logistically and economically feasible, were determined to more accurately represent population changes over time, and led to more effective management, but a con-ventional mark-recapture technique was also attempted with the same fish species (Castello et al. 2009).
Yet cases such as the arapaima are uncommon because this species is detected when surfacing for aerial breathing, has a low enough population density in a small enough area to be counted effectively, and possesses certain subtle visual and acous-tic characterisacous-tics that allow for the identification of individuals (Castello et al.
2009). Most techniques used to sample aquatic organisms are conceptually similar to those used to sample terrestrial organisms. But constraints placed on observers of dealing with sampling in or on the water and at various water depths require that many techniques be more specialized. There are a variety of techniques commonly used to sample fish and aquatic amphibians as well as aquatic invertebrates (Slack et al. 1973). A systematic assessment of stream reaches using either snorkel surveys (Hankin and Reeves 1988) or electrofishing equipment is commonly used in shallow streams and rivers (Cunjak et al. 1988).
In estuaries and large rivers, quantitative studies are often confounded by the high variability of fish populations and the high efficiency of fish sampling gear (Poizat and Baran 1997). In light of this, Poizat and Baran (1997) undertook a study assess-ing the efficacy of surveyassess-ing fishermen, compared with a scientist-managed gill-net sampling approach, and determined that combining both approaches is the best way to increase confidence that observed trends are real. In other words, if both sets of survey data suggest the same trend, it is safer to infer that the trends are real than if the data sets suggest different trends or there exists but one type of data.
Manta tows, which are comparable to line-transect methods but must account for uniquely marine conditions such as turbidity of water, tides, and sea-condition characteristics, are often utilized to monitor general characteristics of coral reefs and their associated populations. The technique, which has been employed in both scientist-run and community-based programs, consists of towing a snorkeler trained to observe certain variables behind a boat at constant speed along a predetermined stretch of reef (Bass and Miller 1996; Uychiaoco et al. 2005). In one study along the Great Barrier Reef, where manta tows have been employed since the 1970s, the sam-pled line is broken up into zones that take 2 minutes to sample, and every 2 minutes the boat stops for the observer to record data on an aquatic data sheet (Bass and Miller 1996). In these surveys, data often include counts of conspicuous species, such as giant clams, or of entire assemblages, such as carnivorous and herbivorous fish, but the technique is also used for monitoring habitat (Bass and Miller 1996;
Uychiaoco et al. 2005). Indeed, observations of suites of variables designed to inform practitioners about the state of coral reefs over time—such as reef slope, dominant
benthic form, dominant hard coral genus, and structural complexity of coral—are also commonly recorded (Bass and Miller 1996; Uychiaoco et al. 2005).
Welsh (1987) proposed a habitat-based approach for amphibians in small headwa-ter streams, and time-constrained and area-constrained approaches have also been used for headwater species (Hossack et al. 2006). Pond-breeding species or spe-cies that inhabit deeper water are often sampled using minnow traps, nets, or call counts of vocalizing frogs and toads (Kolozsvary and Swihart 1999; Crouch and Paton 2002).
Tracking of individual animals through tags, passive integrated transponders (PIT tags), and similar techniques is expensive but provides information on animal movements and estimates of population size and survival. Such approaches have been used with species of high interest such as coho salmon in the Pacific Northwest (Wigington et al. 2006).
tErrEStrialand SEmi-aQuatic organiSmS
The diversity of forms, sizes, and life histories among terrestrial vertebrates has led to the development of hundreds of field techniques designed for different spe-cies and survey conditions. Table 8.1 lists the most widely used field techniques for collecting wildlife data, but it is by no means an exhaustive list of all inventory and monitoring methods. Techniques are separated into observational, capture, and marking methods and by the mode by which data are collected. A comprehensive review of all the different field techniques for terrestrial and semiaquatic organisms is a separate book in itself (see Bookhout 1994). Here we provide a brief overview of some of the commonly used techniques.
For certain species and conditions, it may be feasible to determine a count of individual members of the population on quadrats (sample plots) randomly or systematically positioned in the study area. Searches can be conducted on foot, all-terrain vehicles, or airplane depending on the scale and circumstances of the survey. Quadrat sampling is commonly used for plants and habitat elements, but with animals quadrat sampling poses some challenges. If animals are mobile during the sampling period, then there needs to be some reasonable assurance that an individual is not double-counted at multiple quadrats as it moves. Size, spacing, and mobility of the organisms must all be considered.
Point counts are perhaps the most extensively used technique for measuring bird abundance and diversity in temperate forests and on rangelands but have also been used to estimate abundance of other diurnal species such as squirrels. Variations in the technique have been described for different species and to meet different data needs (Verner and Ritter 1985; Verner 1988; Ralph et al. 1995; Huff et al. 2000).
Ralph et al. (1995) provided a collection of papers examining sample size adequacy, bird detectability, observer bias, and comparisons among techniques.
Spot mapping, also referred to as territory mapping, often is used to estimate avian population densities by locating singing males during a number of visits to the study area and delineating territory boundaries. The technique is further described in Ralph et al. (1995). Nest searches can be used to assess reproductive success in an avian population by monitoring the survival of eggs and nestlings over the course of
Field Techniques for Population Sampling and Estimation 137
TABLe 8.1
Field Techniques for inventory and Monitoring Studies of Terrestrial and Semiaquatic Vertebrates Avian point counts Bird species that sing
or call on territories
Large herbivores, Cook and Jacobson 1979 Ultrasonic detectors Bats Thomas and West 1989
Audio monitoring Frogs Crouch and Paton 2002
Hair traps Small-medium Marine radar Bats, migrating birds Harmata et al. 1999 Harmonic radar Bats, amphibians,
reptiles
Pellet et al. 2006
continued
a breeding season. Both techniques are labor intensive and are not commonly used for inventories, but the information gained from these methods (i.e., territory densi-ties, productivity) may be better indicators of population trends and habitat quality than simply counts of individuals.
Line transect and point transect sampling are specialized plot methods in which a search for the target organism is conducted along a narrow strip having a known area. Rarely can it be assumed that all animals are detected along the transect.
However, if the probability of detection can be predicted from the distance between the animal and the centerline of the transect, then a detection function can be used to estimate population density. The approach can be adapted to surveys conducted by foot, snorkeling, and ground or air vehicles. Buckland et al. (1993) provided a complete, though highly technical, introduction to line transect and point transect methods. The approach has been widely applied to surveys of vertebrates, includ-ing desert tortoise (Anderson et al. 2001), marbled murrelets (Madsen et al. 1999), TABLe 8.1 (continued)
Field Techniques for inventory and Monitoring Studies of Terrestrial and Semiaquatic Vertebrates
Snap traps Small mammals Mengak and Guynn
1987 Leg-hold and snares Large mammals Bookhout 1994
Mist nets Kuenzi and Morrison Hand capture Salamanders Kolozsvary and
Swihart 1999
Marking Tags Birds/mammals Nietfeld et al. 1994
Mutilation Small mammals Wood and Slade 1990
Pigments Small mammals Lemen and Freeman
1985
Collars and bands Birds/mammals Nietfeld et al. 1994
Field Techniques for Population Sampling and Estimation 139
songbirds in oak–pine woodlands (Verner and Ritter 1985), and mule deer (White et al. 1989).
Audio recordings of animal vocalizations have been used to elicit calls and dis-plays from species otherwise difficult to detect. The technique has been applied in studies of blue grouse (Stirling and Bendell 1966), northern spotted owls (Forsman 1988), ground squirrels (Lishak 1977), and others. The number of responses by the target species elicited by the recording is tallied during a prescribed interval and provides a population density index.
Standardized visual searches refer to techniques used to determine species occurrence, species richness, or relative density values, where sampling effort is standardized by space or time. Examples include road counts for large mammals (Rudran et al. 1996), raptor migration counts (Hussell 1981), and visual encounter surveys for terrestrial amphibians (Crump and Scott 1994) (Figure 8.2). Some visual search techniques do not necessarily equalize the amount of survey effort among sampling units. Instead, animal counts or species detected are standardized during analysis by dividing the number of observations by a unit of area or time. Variability among observers and environmental conditions may be significant sources of error unassociated with the sampling technique and should be assessed prior to data col-lection to minimize biases and improve precision.
Under certain circumstances, it may be possible to effectively observe all individuals in the target population. In such cases, population size can be
Under certain circumstances, it may be possible to effectively observe all individuals in the target population. In such cases, population size can be