Understanding how many deer are within a given area of land is key to successful management, no matter what the management goal may be. Without knowledge of animal populations, mangers have a significantly reduced capacity to make evidence-based decisions relating to their management goals. Having an awareness of the ways in which deer density can be quantified is critical for intelligent conservation and management of deer.
What is ‘abundance’?
The term ‘abundance’ simply refers to the number of organisms within a defined area. The absolute abundance is the actual number of animals in a given area – this is virtually impossible to determine unless the population size is sufficiently small to permit each individual to be located and counted. Additionally, populations of free ranging animals change constantly in response to mortality, births and animal movements, so even if absolute abundance was able to be obtained, it would only be accurate at the time the count was made. Fortunately, a variety of other methods exist which can provide valuable information relating to population trends, helping inform us if the population is increasing, decreasing or stable.
Indices of Abundance
Tracks left by animals, quantity of faecal piles, trail camera data, spotlight counts, hunter harvest effort etc. can all be used to ‘index’ deer populations. Each of these factors is assumed to hold some relation to the actual population size (which is obviously not known), and therefore, relative changes in the chosen factor can be used as in an indication of population trends. Indices of abundance generally do not provide an estimate of the numbers of individuals in the population, but are used to identify relative changes in abundance (e.g. % growth). Anderson (2001) highlights some of the pitfalls relating to index values, highlighting many of the ‘assumptions’ that are often made in their use, which inevitably lead to uncertainty in the accuracy in the final figures obtained. Unfortunately, animals and ecosystems are complicated, a perfect tool to assess populations does not exist!
Faecal Counts
Indexing population changes using faecal pellet counts has been utilised extensively in New Zealand, America and Europe for decades (Forsyth, Barker, Morriss, & Scroggie, 2007), making it one of the main methods to assess populations changes in wild herds across the globe. The method is quite simple in theory, with full details provided in Forsyth (2005), developed for Department of Conservation in NZ. Essentially, the area of interest is mapped and classified according to habitat type (each habitat type is assessed separately, since deer use habitats differently). Starting points for transects (straight lines of defined distance to undertake sampling along) are randomly allocated within the study area, ensuring the data obtained is representative of the whole area. For this particular method, a minimum of thirty 150m transects are utilised. Every five metres along the transect, a plot with 1m radius is searched for ‘intact pellets’, defined as pellets where “no recognisable loss of material” has occurred. The number of intact pellets within each pellet group is counted and recorded. Calculations are then made using the data obtained, to build a picture of the quantity of pellets within the study area as a whole.
In order for this method to provide any measure of population change, a second set of data would need to be obtained, usually in 12 or 24 months’ time. Changes in the number of pellets from one survey to another could indicate a change of deer density at that particular location. If the same trend was determined in multiple study areas across a broad area, the change may then be attributed to an overall change in deer density. The size of any change is quantified using statistics. Any person with a basic understanding of deer behaviour will realise more pellets does not necessarily mean more deer! For this reason, having alot of data from multiple locations is essential to gaining a reliable measure of population changes. Other indices may also be used to determine if a similar pattern exists. For example, hunter harvest success may be declining, along with trends in fecal counts, thus increasing our certainty of the overall changes to the deer population.
Estimating the number of individuals deer using faecal counts
Can faecal counts be used to estimate the number of individual deer in a population? The short answer is yes, in some special circumstances. In order for this to be achieved, there must first be a known linear (direct) correlation between faecal pellet counts and the absolute abundance of deer. This link has been made in relation to deer in New Zealand (Forsyth et al., 2007), though not without limitations. In this study, the method for faecal counts as described above, was undertaken in a series of enclosures containing a known quantity of deer. This enabled the results of the counts to be correlated with known deer density. It was determined that the relationship was positive and “approximately linear”, making it of potential use for quantifying wild deer herds. Considerations still need to be made in relation to differences between enclosed and non-enclosed areas, different types of terrain, feed quality/quantity etc. all of which introduce further uncertainty as to how well this correlation could apply to other study areas. The authors made clear recommendation that if faecal counts were to be used as an index of abundance in another location, the relationship should first be determined in that geographic area in the same way. Naturally, this would be at considerable cost, and therefore, faecal counts are likely to remain useful only as a tool to determine trends in populations over time.
References
Anderson, D. R. (2001). The Need to Get the Basics Right in Wildlife Field Studies. Wildlife Society Bulletin, 29(4), 1294–1297.
Forsyth, D. M. (2005). Protocol for estimating changes in the relative abundance of deer in New Zealand forests using the Faecal Pellet Index (FPI), (October), 24. Retrieved from http://www.conservation.govt.nz/upload/documents/conservation/threats-and-impacts/animal-pests/fpi-protocol.pdf
Forsyth, D. M., Barker, R. J., Morriss, G., & Scroggie, M. P. (2007). Modeling the Relationship Between Fecal Pellet Indices and Deer Density. Journal of Wildlife Management, 71(3), 964–970. https://doi.org/10.2193/2005-695
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