Abstract
The long history of armed conflict in Manas National Park has resulted in the
decline of wildlife populations. With the attainment of peace, knowledge of extant animal
population sizes became crucial for making informed conservation decisions. The present
study estimated the population of Asian elephants within a distance sampling framework
using dung counts along 92 line transects. The population size was estimated to be 601
(95%, CI 454–797), with the highest density in mixed moist deciduous forest (1.57
elephants/km2). This serves as the baseline information and also discusses the accuracy
of the dung count method and possible shortcomings of our estimates
Abstract
Dung counts are the most commonly used techniques for estimating elephant numbers in forests, yet
there is considerable scepticism concerning their accuracy. Published accounts of dung counts show that
they give estimates similar to those from other methods for vertebrates ranging in size from lizards to
elephants. For ungulates, macropods and elephants there are strong correlations between estimates
from dung counts and other methods. Thus, dung counts are as accurate or inaccurate as other methods
for estimating vertebrate numbers, including elephants. Dung counts for elephants give estimates that
are as precise as, and sometimes more precise than, those from aerial surveys of elephants. This is
because the variance in dung density is usually low and results in a lower than expected variance for the
final elephant estimate when combined with the variances of defecation and decay rates. Dung counts
may be more appropriate than aerial surveys for monitoring small or declining elephant populations.
Background
Dung transects are the most common technique for surveying forest elephant populations. The dense
forest vegetation makes it difficult to count elephants directly, so an indirect method, such as counting
dung, is necessary. Dr. Fiona Maisels of the Wildlife Conservation Society (WCS) has helped to design
and implement survey methods for primates and forest elephants. She and her colleagues gathered data
from dung transect surveys performed in five countries in central Africa to estimate the changing sizes
of the forest elephant population.
Methods
This method relies on estimating the elephant population size by counting dung piles and
knowing how often elephants defecate and how fast dung piles decay. Based on published
studies, researchers determined the average rate that elephants defecate: 19.77 dung piles per
elephant per day. They gathered information on the time dung takes to decay to the point of
disappearance at 14 sites across various habitats and seasons and calculated the average, which
was 81.82 days. The survey areas in each of the five countries were divided into strata, and a
minimum of 15 transect lines per stratum were regularly placed perpendicular to features such as
roads and rivers to avoid sampling bias.
Dung transects were performed in one of two ways: the line transect method or the recce method.
Line transects – Observers walk along transect lines and measure the perpendicular distance
from the transect to the center of each dung pile they observe. Even though the dung piles are
distributed over the landscape, they become harder to spot the farther they are from the transect
line, decreasing the chance of detection. Modeling software is used to compensate for the missed
dung piles and produce a corrected estimate of the total number of dung piles within a maximum
distance from the line.
---------------------------------------------------------------------------------Transect
Recces – Observers walk along lines and count all dung piles they observe within a 2-meter strip
(1 meter on either side of the line). It is assumed that the observers detect all of the dung within
the 2-meter strip.
The density of elephants within the sampled area can be estimated using the following formula:
Elephant Density = (dung density)/(defecation rate x decay time )
The number of elephants in the population was then estimated by modeling. The relationship
between elephant density and a number of variables known to affect elephant abundance was
calculated using multiple models. The models that best fit the data were chosen, and then, using
maps that cover the entire range, the selected models were used to predict elephant density in
areas that were not sampled.
Advantage
When a large enough percentage of the survey area is sampled, dung surveys can be as accurate as
other survey methods. They are relatively easy to implement and don't require specialized equipment,
but intensive and careful training is required. Elephant dung is easier to detect than the elephants
themselves and generates more data than a ground survey of elephants. As dung counts measure dung
deposited over time, this method is less sensitive to elephant movement between sampling areas or the
clumped distribution of elephants.
Disadvantage coffee prospective
Dung surveys are extremely time- and labor-intensive, and can be expensive if the survey area is
large. This method assumes a stable elephant population creating stable rates of dung
accumulation and decay during the study period. Because dung decay times are long, more rapid
changes in the elephant population may lead to inaccurate estimates. The rate of dung decay and
defecation can also vary with habitat and season.
While less clumped than elephants, dung piles are typically not randomly distributed. To avoid
inaccurate estimates due to the clumped distribution, the transects need to be arranged
throughout all vegetation types in a manner that is representative of the different habitats. It is
often advisable to stratify the study area by vegetation type and/or by poaching intensity. A
minimum of 30 transects per stratum is advised to improve the precision of estimates.
