I thought I might as well post it.
Lizard Population Assessment: No Simple Task!
There are a number of considerations that need to be taken into account before attempting to assess a population of lizards. Differences in trap type design can lead to different levels of detectability for species due to differences in the way they react to any given situation. Similarly, the way in which lizards react to their overall ecosystem and habitat can also play a significant role. Budget and time constraints will also be necessary to factor in. Successful accounting of such factors will ultimately lead to more accurate assessments of any population of lizards.
The need to accurately measure lizard fauna is nothing new to lizard conservation (Clark 2006), but the process is seldom as simple as going out and recording head counts. There are a number of factors that can influence the success of the variety of methods available (Chavel et al. 2012), however, factors that must be accounted for in order to produce reliable population sizes and densities. (Wilson et al. 2011) These can include environmental considerations such as the location of nearby rocky exposures (Blevins and K.A. 2011), as well as behavioural elements like shyness (Clark 2006), the impact of the traps themselves (Lettink 2007), or even practical consideration such as time and money. (Lettink 2007)(Clark 2006)
There Are Many Detection Methods With Unique Strengths And Weaknesses
There are many methods of sampling lizards (such as manual sampling by hand) (Kacoliris et al. 2009), but increasingly standard, especially in New Zealand, are the use of pitfall traps (Lettink 2007)(Clark 2006) and artificial retreats. (Lettink 2007)(Clark 2006) Pitfall traps are essentially holes dug in the ground (Clark 2006), with variations such as the usage of buckets, diameter, and agents such as formaldehyde. (Ribeiro-Junior et al. 2011)(Greenslade et al. 2012) Artificial retreats are essentially assemblages designed to provide shelter similar to its natural environment and these too can also be constructed in a number of different ways. (Lettink 2007)(Clark 2006)(Lettink and Cree 2007) Both types are noted for their tendency to catch different species, but this is not always consistent. For example, pitfall traps sometimes tend to catch skinks, with artificial retreats catching geckos.(Lettink 2007)(Lettink and Cree 2007) Other studies give the opposite. (Clark 2006) There is mounting evidence to suggest that comprehensive schedules consisting of multiple detection methods are best suited to fully representing herpetofauna such as lizards as their capacity to well-measure different species act complementarily to one another. (Reading 1997)(Ribeiro Jr. et al. 2008)
Environmental Factors Can Influence Detection Rates
There are a variety of environmental factors that can influence the rate at which detection methods detect reptiles. In a study done on collared lizards, abundance was positively influenced by local factors such as the complexity of local flora, large rocky ledges (especially with crevices), and places high up on hillslopes (Blevins and K.A. 2011), as well as larger-scale factors such as regular fires (Blevins and K.A. 2011), even after detection biases were accounted for. Common geckos local to the central Otago region tend to be found near rocky locales, where the sun-baked tops play a key role in their thermoregulation. (Clark 2006) Similarly, pitfall traps rely on sunny weather in order to invigorate lizards and other reptiles into activity so that they can actually encounter and fall into them, both tying in environmental factors into that of behavioural ones discussed later. (Clark 2006)
Vegetation height can play a role by altering the microhabitat around traps and retreats. (Jörn 2011) A study on the use of artificial retreats for catching skinks, vegetation height was positively correlated with detection rate, quite possibly due to either improvements to skink carrying capacity (eg insect abundance, concealment) or increased basking opportunities afforded by retreats relative to the higher vegetation compared to areas with low vegetation height. (Chavel et al. 2012) Another such study found no correlation an important insight that opens up the possibility of comparing populations measured with the same techniques between areas with differing levels of grass height. (Schlesinger 2007)
Behavioral Factors Can Also Influence Detection Rates
There are a variety of behavioural considerations that can significantly influence detection rates as well. Two such examples are trap shyness and happiness. (Clark 2006)(Wilson et al. 2011) These can occur when the organisms become familiarized with the traps and learn to avoid them such as with skinks in New Zealand with pitfall traps (Clark 2006), snakes becoming attracted to traps that had accumulated bait over time (Wilson et al. 2011), or geckos and their apparently indifference either way. (Clark 2006) Reptiles are also generally more active during summer months, as indicated by studies on not just lizards but other reptiles as well. (Clark 2006)(Wilson et al. 2011) Detection methods that employ the usage of cover objects can also impact rates, once again due to the resultant shades use in thermoregulation. (Clark 2006) Lastly, one should consider individual characteristics as well, such as geckos and their infamous capacity for escaping pitfall traps (Lettink 2007)(Clark 2006).
Detection Methods Can Be A Factor Themselves
The actual methods used to detect the organisms in question can be a significant factor as well. (Chavel et al. 2012) Pitfall trap construction creates ecological disturbance when dirt is inevitably shifted around to crate the holes, and the physical presence of other trap elements such as the plastic contains put therein create habitat alterations as well. (Clark 2006) Artificial retreats are thought to create less disturbance (Lettink 2007), but their presence is still thought to affect the spatial distribution and movement patterns in a study on geckos. (Lettink 2007) Certain kinds of traps (such as pitfall traps) can inadvertently result in death, especially through overheating and predation. (Lettink 2007)
Biases In Detection Methods Must Be Accounted For
Considering all factors, biological, environmental, and methodological, is important when using data gathered from any method of detection. Effects such as trap shyness (Clark 2006), transient migrations (Wilson et al. 2011), and animal usage of surfaces for thermoregulation (Blevins and K.A. 2011)(Hoare et al. 2009) can be used to optimise data collection (Hoare et al. 2009) and subsequent analyses to provide better and more accurate results. (Wilson et al. 2011) Inaccurate estimates of population sizes or densities can arise when assorted behavioural and environmental factors are not accounted for (Clark 2006)(Blevins and K.A. 2011)(Wilson et al. 2011), such as with mark-recapture models (Wilson et al. 2011)(Kacoliris et al. 2009), notorious for losing their power at low recapture rates. (Wilson et al. 2011) Such models can, however, incorporate such discrepancies in detectability. (Wilson et al. 2011)
Practical Considerations Can Dictate Method Usage
In any experiment, practical limitations such as funding and time constraints can affect the utility of various detection methods. (Clark 2006) Given the rate at which ecosystems across the globe are being impacted by human activity (Wilson et al. 2011), especially in New Zealand (Clark 2006), various attributes such as low cost, disturbance, maintenance, and expertise required for use have become increasingly important when choosing detection methods (Clark 2006), as the money and manpower required to perform less cost-efficient methods can slow or weaken conservation efforts. (Clark 2006) Artificial retreats have been proven to be simple in regards to setup and maintenance (as opposed to pitfall traps), requiring comparatively little skill. (Lettink 2007)(Clark 2006)(Chavel et al. 2012) A study done in rainforests showed that smaller, cheaper pitfall traps can be just as effective at collecting reptiles in general as the bigger ones. (Ribeiro-Junior et al. 2011)
Ultimately, there are going to be many factors and considerations to keep in mind when selecting which methods to use and how to use them. Even the seemingly innocuous and trivial affair of ensuring that numbers gained from detection methods can be a multi-study affair through the myriad of environmental, behavioural, and budget considerations, all of which must be taken into account in order to ensure any assessment of lizard populations remain both accurate and feasible.
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