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Assessing the nutrient status of elephant dung in the Aberdare National Park, Kenya

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Abstract

The African elephant (Loxodonta africana) is a keystone species that occupies an integral niche within African ecosystems. Elephants serve as ecosystem engineers due to their wide-ranging ecosystem functions, such as seed dispersal, production of manure compost and as agents of habitat modification. Studies assessing the nutrient status of elephant dung have thus far concentrated on semi-arid grasslands. This study presents the first analysis of the composition of elephant dung obtained from elephants located within a forested Afromontane biome, that of the Aberdare National Park, adding to established literature; and expanding on the beneficial role elephants play in promoting forest ecosystem functioning through the deposition of dung which is rich in carbon, nitrogen, phosphorus, and potassium. Dung deposition per elephant resulted in 0.01 kg N/ha, 0.26 kg C/ha, 0.01 kg P/ha, and 0.01 kg K/ha added to the Aberdare forest ecosystem per annum. This study highlights the need to understand both the direct and indirect repercussions of continued elephant decline on ecosystem functioning. © 2018, International Union for Conservation of Nature and Natural Resources. All rights reserved.
86 Pachyderm No. 59 July 2017–June 2018
Assessing the nutrient status of elephant dung in the Aberdare
National Park, Kenya
Roisin Aoife Stanbrook
John Dalton East, Division of Biology and Conservation, School of Science and the Environment,
Manchester Metropolitan University, UK
email: roisin.stanbrook@ucf.edu
Abstract
The African elephant (Loxodonta africana) is a keystone species that occupies an integral niche within
African ecosystems. Elephants serve as ecosystem engineers due to their wide-ranging ecosystem functions,
such as seed dispersal, production of manure compost and as agents of habitat modication. Studies assessing
the nutrient status of elephant dung have thus far concentrated on semi-arid grasslands. This study presents
the rst analysis of the composition of elephant dung obtained from elephants located within a forested
Afromontane biome, that of the Aberdare National Park, adding to established literature; and expanding on
the benecial role elephants play in promoting forest ecosystem functioning through the deposition of dung
which is rich in carbon, nitrogen, phosphorus, and potassium. Dung deposition per elephant resulted in 0.01
kg N/ha, 0.26 kg C/ha, 0.01 kg P/ha, and 0.01 kg K/ha added to the Aberdare forest ecosystem per annum.
This study highlights the need to understand both the direct and indirect repercussions of continued elephant
decline on ecosystem functioning.
Résumé
L'éléphant d'Afrique (Loxodonta africana) est une espèce clé qui occupe une niche intégrante au sein des
écosystèmes africains. Les éléphants servent d'ingénieurs de l'écosystème en raison de leurs vastes fonctions
écosystémiques, telles que la dispersion des graines, la production de compost de fumier et en tant qu’agents
de modication de l'habitat. Les études qui analysent l'état nutritionnel des crottes d'éléphants se sont jusqu'ici
concentrées sur les herbages semi-arides. Cette étude présente la première analyse de la composition des
crottes d’éléphants obtenues à partir des éléphants situés dans un biome forestier de l’Afromontaine, celui
du Parc national Aberdare, ce qui ajoute à la littérature établie et donne plus de détails sur le rôle bénéque
des éléphants dans la promotion du fonctionnement des écosystèmes forestiers par le dépôt de crottes riches
en carbone, azote, phosphore et potassium. Le dépôt de crottes par éléphant a entraîné une augmentation
de 0,01 kg N/ha, 0,26 kg C/ha, 0,01 kg P/ha et 0,01 kg K/ha par année dans la forêt d’Aberdare. Cette
étude met en exergue la nécessité de comprendre les répercussions du déclin continu des éléphants sur le
fonctionnement des écosystèmes.
Introduction
Animal manure as a fertiliser has been recognized
for centuries as an economical, nutrient-rich
method of replenishing soil organic matter. In
forests, plant growth and habitat distribution
is largely regulated by nutrient availability
(Treseder and Vitousek 2001). The deposition of
herbivore dung forms the rst of many steps in
the process of nutrient cycling. Most mammals
use only a small proportion of the nutrients they
ingest, with 60–99% of the ingested nutrients returned
to the soil in the form of dung and urine (Williams and
Haynes 1990) and this dung provides a considerable
source of nutrients essential for plant productivity in
the form of available elements. Many tropical soils are
poor in nutrients and rely on the recycling of nutrients
from organic matter to maintain soil health and plant
productivity. Elephant dung is a copious resource
with over 150 kg (wet weight) of dung deposited per
elephant per day (Coe 1972). Although widely used
87Pachyderm No. 59 July 2017–June 2018
Assessing the nutrient status of elephant dung in the Aberdare National Park, Kenya
in the tropics as a fertilizer (Sannigrahi 2015), its
nutrient status has not been described in terms of
its potential in nutrient cycling. This is signicant
as many of the important minerals for plant
growth, such as N, P and K, which occur within
elephant dung are present at higher concentrations
within the dung than in the ingested food material
(Anderson and Coe 1974; Greyling 2004).
There is a general paucity of scientic literature
on the nutrient status of faeces of free-ranging
mammals in African biomes, which are dominated
principally by large herbivores. The largest of
these herbivores, the African elephant (Loxodonta
africana) is a keystone species that occupies
an integral niche within African ecosystems.
Elephants serve as ecosystem engineers due their
wide-ranging ecosystem functions, for example
seed dispersal (e.g. Campos-Arceiz and Blake
2011), and as agents of habitat modication (de
Boer et al. 2015). Similar studies assessing the
nutrient status of elephant dung have concentrated
on semi-arid grasslands (Masunga et al. 2006)
and bushland and woodlands (Dougall 1962;
Weir 1972; Anderson and Coe 1974). Here we
present the rst analysis of the composition of
elephant dung obtained from elephants located
within a forested Afromontane biome. The
Aberdare National Park (ANP) occupies an area
of 756 km2 and is located in the central Kenyan
highlands. The entire ANP has recently been
ring-fenced to protect wildlife and reduce illegal
timber extraction; the latter vital to protect the
water catchment of the Afromontane ecosystem,
and coordinated by the charitable trust Rhino Ark.
It is estimated there are around 2,000 elephants
living in the park at a moderately high density
but the actual population status remains unclear
as recent surveys are based on an informed guess
rather than direct counts (Kenya Wildlife Service
2017). Field work was undertaken by the author
together with Geofrey Wafula, a KWS ranger.
We assess the potential of elephant dung in
nutrient cycling, describe its composition in terms
of macronutrients essential for plant growth,
analyse dung composition between sexes, and
compare the results with those of studies in other
biomes. We expect greater concentrations of
essential macronutrients in elephant dung from
Afromontane forest due to a higher availability of
nutrients found in forest vegetation compared to
grassland and bushland biomes.
Dung collection and analysis
An observation post was established adjacent to the
Treetops Lodge in the Salient sector of the Aberdare
National Park (ANP), Kenya from 28 May to 1 June
2015. A herd of elephants (n = 10) were observed
defecating. Their dung collected immediately after
the event and their sex was recorded. Seven samples
of 40 g freshly defecated dung was obtained from the
adult elephants present (3 females and 4 males) and
then frozen at –20°C. Faecal samples were collected
by removing the top section of boli, leaving behind
the dung that made contact with the ground to avoid
soil contamination; similarly, dung tainted by urine
was ignored. Dung samples were dried at 80°C and
then pulverized in a ceramic mortar to pass through
a 2 mm sieve prior to analysis. Phosphorous (P) and
potassium (K) concentrations were determined using
the Mehlich-3 (Mehlich 1984) extraction procedure as
follows: 5 g of dung was added to 20 ml of 0.05 M
HCl in 0.025 M H2SO4, and the ltrate was analysed
by inductively coupled plasma Atomic emission
spectrometry (ICP-AES). Then, 0.5 g of dried and
weighed dung were decarbonized with 1 M solution
of HCl before being analysed for total carbon (C) and
nitrogen (N) concentrations with a LECO TruSpec
analyser using the combustion (Dumas) method.
Results and discussion
Carbon/nitrogen ratio
Carbon and nitrogen are the most important of the
many elements required for microbial decomposition
of organic matter to produce compost (Melillo et al.
1989). The C/N ratio is an important factor determining
how easily bacteria are able to decompose organic
material and indicative of the decomposition rate of
organic matter (Taylor et al. 1989). The presence of
woody material, while taking a longer time to complete
the composting process, encourages fungal activity that
results in compost that is excellent as a soil conditioner.
The C/N ratio was calculated as the ratio of dry weights,
based on average values from the dung collected from
individual elephants in the ANP and is displayed in
Table 1. The ratio in the forested habit of ANP is lower
than in semi-arid grasslands but consistent with those
found with bush and woodland habitats (Table 1) and
still falls within the optimum range outlined by (Fong
et al. 1999) for eective microbial decomposition of
plant matter.
88 Pachyderm No. 59 July 2017–June 2018
Stanbrook
Sex differences in the chemical
composition of elephant dung
Previous studies note that values obtained from
elephant faecal samples are correlated with those
found in the diet (Woolley et al. 2009) and, in
elephants, dierences in diet selection between
sexes have been observed when resources become
limited. According to (Stokke and du Toit 2000),
during dry season browsing adult male elephants
ingest a less diverse range of plant species and
feed on a greater proportion of woody material.
A Mann-Whitney U test was used to compare
the median nutrient concentration between male
and female elephants. The results indicate that
there was no statistically signicant dierence
in dung composition between sexes across
all nutrients: (C: U = 4, p = 0.629; N: U = 7,
p = 0.877; K: U = 5, p = 0.857; P: U = 8, p =
0.629). We hypothesized that the lack of seasonal
variation in vegetative composition and stability
in the diversity of vegetation found within the
study site is reected in homogeneity of the
dung composition and accounts for the general
similarity in dung composition between sexes.
Nutrient availability for plant growth
When our results are extrapolated to assess the
total deposition of dung per elephant per year it
Sample Sex N% %C C/N P (mg/kg) K (mg/kg)
1 m 1.909 38.64 20.24096 1.90000 2.71
2 m 1.839 35.77 19.45079 6.00000 1.86
3 m 1.572 34.89 22.19466 1.21970 1.31
4 m 1.778 36.84 20.71991 0.31454 1.38
5 f 1.812 37.22 20.54084 0.86506 2.10
6 f 1.561 35.91 23.00448 1.34610 2.0
7 f 1.899 38.01 20.01580 0.78853 1.65
*mean 1.76 36.75 - 1.77 1.82
†sd± 0.14 1.32 - 1.92 0.59
Table 1. Levels of nitrogen (N), carbon (C), phosphorus (P), potassium (K), and C/N
ratio within the faeces of individual adult female (f) and adult male (m) elephants (n
= 7). %C and %N values were determined through combustion; available P (mg/kg)
and K (mg/kg) were determined using Mehlich-3 extraction. All values are expressed
as proportions of dry matter. *Mean values for all samples; † Standard Deviation for
all samples.
may be inferred that the elephants within the ANP are
responsible for the addition of total N equivalent to
963.6 kg/yr per elephant; (Ntotal=1.76 mg/kg ×150 kg ×
365); total C equivalent to 20,120.6 kg/yr (Ctotal=36.75
mg/kg ×150 kg ×365); available P equivalent to 969
kg/yr (Pavail.=1.77 mg/kg ×150 kg × 365); and available
K equivalent to 892.4 kg/yr (Kavail.=1.63 mg/kg ×150
kg × 365). When these results are expressed per hectare
an individual elephant inputs a remarkable 0.01 kg N/
ha, 0.26 kg C/ha, 0.01 kg P/ha, and 0.01 kg K/ha into
the Aberdare forest ecosystem every year. Using the
atomic weight of the N, P, K elements found in the ANP
(Table 2) we can convert the macronutrients measured
in mg/kg to percentage concentration by weight.
This conversion allows comparison with elephant
dung to those found in commercial and other organic
fertilisers. Elephant dung does not contain as much
N,P,K percentage concentration by weight (1.76 N;1.09
P; 1.89 K per kg) as most commercial agricultural
fertilisers (15 N; 6.54; P 12.45 per kg) (www.fao.
org) but does compare favourably against other types
of dung used as manure especially commercially
available bat guano (5.5,N;4,P;1.5,K) (Penhallegon
2003).Bat guano has been known to increase plant
growth and has been shown to induce greater growth
when compared to chemical fertilizer despite its lower
NPK content (Sothearen, Furey, and Jurgens 2014).
These results are attributed to the fact that in addition
to elevated concentrations of N, P, K, guano also
contains organic matter, other
important micronutrients and
microora which chemical
fertiliser lacks.
Phosphorus availability
is a major limiting factor of
plant growth in terrestrial
ecosystems (Agren et al.
2012) and the observations of
Quesada et al. (2012) suggest
that soil P availability may
be the key modulator of the
above-ground net primary
productivity of tropical forests.
The mean available P content
of dung in this study was six
times greater than elephant
dung found within grassland
and bushland biomes (Table
1). This very high in P content
is indicative of (1) a tightly
bound P cycle within the soil,
89Pachyderm No. 59 July 2017–June 2018
Assessing the nutrient status of elephant dung in the Aberdare National Park, Kenya
forest vegetation and organisms, (2) the presence
of strong mechanisms to mobilize and distribute
P among the ecosystem constituents (e.g. Dalling
et al. 2016) and (3) low levels of leakage of P
from the system.
Potassium is important for the productivity
and sustenance of many forests (Tripler et al.
2006). The K content of the dung from the ANP
was found to be within range of values reported
in previous studies (Table 1) and similar to those
in the bushland and woodland habitats described
by Weir (1972) and Masunga et al. (2006). This
suggests that the ingested plant materials contain
a high level of K, and that the K cycle in all three
habitats functions with similar eciency.
Conclusion
To conclude, the dung from elephants that reside
in forested areas may provide an eective and rich
fertilizer that is comparatively higher in C and P
compared with dung from elephants in grasslands
and woodlands. This is pertinent as C and P are
essential macronutrients on which plants are
dependent to maintain ecosystem productivity
and growth. These observations provide further
support for the hypothesis that elephants, as
‘gardeners of the forest’, are not just ecosystem
engineers and eective seed dispersers, but also
Study Sample
size (n)
Habitat / study
area Ntotal (%) Ctotal (%) C/N P (mg/
kg)
K (mg/
kg)
Semi-arid grassland
Dougall (1963) 1 Tsavo NP 1.28 - - 0.25 0.58
Anderson and
Coe (1974)* 4Tsavo NP 1.39 49.82 36:1 0.25 0.58
Bushland/woodland
Weir (1972) 14 Serengeti NP - - - - 0.95
7Manyara NP - - - - 0.64
2Arusha NP - - - - 1.42
Masunga et al.
(2006) 2Chobe NP 1.62 39.3 24:1 0.28 1.66
Afromontane forest
This study 7 Aberdare NP 1.76 ± 0.14 36.75 ±1.32 21:1 1.77 ± 1.92 1.82 ± 0.59
Table 2. Mean levels of nitrogen (N), carbon (C), phosphorus (P), potassium (K), and C/N ratio in dung obtained from
adult elephants in Aberdare National Park (NP), Kenya. All values are expressed as a percentage dry matter. Error values
are standard deviations from the mean.
*Elephant diet supplemented with Medicago sativ.
play a key role in nutrient cycling to support soil health.
East Africa’s elephant population has halved since
2007 (Thouless et al. 2016) and this decline in elephant
abundance continues unabated. Anthropogenic drivers
are responsible for both this decline and the reduction
of habitat in which elephants live. The eects of trophic
cascades and the decline of taxa with a high capacity to
transport nutrients are increasing reported in scientic
literature, highlighting the need to understand both the
direct and indirect repercussions of elephant decline on
ecosystem functioning in general and nutrient cycling
in particular. This study conrms the importance of
elephants to ecosystem functioning and the health and
continuing productivity of Africa’s tropical forests.
Acknowledgements
I thank the Kenya Wildlife Service, especially Corporal
Geofrey Wabomba Wafula and the rangers of the
Aberdare National Park. I also thank David McKendry,
Dr Christopher Fields and Dr Robin Sen at Manchester
Metropolitan University, and the helpful comments of
two anonymous reviewers.
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... K w a n d o L i n y a n t i (Haynes, 2012), bolus dry weight set at 20% (Anderson & Coe, 1974), and C content of dry weight set at 39% (Stanbrook, 2018). ...
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Differences in feeding patterns of the African elephant were examined by sex and age during the dry season in a dystrophic savanna-woodland ecosystem in northern Botswana. Adult males had the least diverse diet in terms of woody plant species, but they consumed more plant parts than family units. The diameter of stems of food plants broken or bitten off was also greater for adult males than for females and subadult males. Adult males spent more time foraging on each woody plant than did females. The number of woody plant species and individuals present were higher at feeding sites of family units than at feeding sites of adult males, indicating that family units positioned themselves at feeding sites with higher species diversity than those of males. We argue that the most likely explanation for these differences is related to the pronounced sexual size dimorphism exhibited by elephants, resulting in sex differences in browsing patterns due to the allometric relationships that govern the tolerance of herbivores for variation in diet quality. From our results this Body Size Hypothesis is accepted rather than the alternative Scramble Competition Hypothesis, which predicts that adult male elephants consume lower quality browse because they are displaced from preferred browse as an outcome of scramble competition with adult females and their offspring. If the feeding patterns of adult male elephants were affected by intersexual scramble competition, we would expect adult males to browse at a higher level in the canopy than the smaller-bodied females and their offspring. No evidence was found for this, although adult females were found to browse at a higher level in the canopy when feeding in close proximity to subadults and juveniles than when feeding alone. Sex differences in elephant browsing patterns are, we propose, of relevance to understanding and managing elephant impacts on African woodlands.
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Forest structure and dynamics vary across the Amazon Basin in an east-west gradient coincident with variations in soil fertility and geology. This has resulted in the hypothesis that soil fertility may play an important role in explaining Basin-wide variations in forest biomass, growth and stem turnover rates. Soil samples were collected in a total of 59 different forest plots across the Amazon Basin and analysed for exchangeable cations, carbon, nitrogen and pH, with several phosphorus fractions of likely different plant availability also quantified. Physical properties were additionally examined and an index of soil physical quality developed. Bivariate relationships of soil and climatic properties with above-ground wood productivity, stand-level tree turnover rates, above-ground wood biomass and wood density were first examined with multivariate regression models then applied. Both forms of analysis were undertaken with and without considerations regarding the underlying spatial structure of the dataset. Despite the presence of autocorrelated spatial structures complicating many analyses, forest structure and dynamics were found to be strongly and quantitatively related to edaphic as well as climatic conditions. Basin-wide differences in stand-level turnover rates are mostly influenced by soil physical properties with variations in rates of coarse wood production mostly related to soil phosphorus status. Total soil P was a better predictor of wood production rates than any of the fractionated organic- or inorganic-P pools. This suggests that it is not only the immediately available P forms, but probably the entire soil phosphorus pool that is interacting with forest growth on longer timescales. A role for soil potassium in modulating Amazon forest dynamics through its effects on stand-level wood density was also detected. Taking this into account, otherwise enigmatic variations in stand-level biomass across the Basin were then accounted for through the interacting effects of soil physical and chemical properties with climate. A hypothesis of selfmaintaining forest dynamic feedback mechanisms initiated by edaphic conditions is proposed. It is further suggested that this is a major factor determining endogenous disturbance levels, species composition, and forest productivity across the Amazon Basin.
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Growth of plants in terrestrial ecosystems is often limited by the availability of nitrogen (N) or phosphorous (P) Liebig's law of the minimum states that the nutrient in least supply relative to the plant's requirement will limit the plant's growth. An alternative to the law of the minimum is the multiple limitation hypothesis (MLH) which states that plants adjust their growth patterns such that they are limited by several resources simultaneously. We use a simple model of plant growth and nutrient uptake to explore the consequences for the plant's relative growth rate of letting plants invest differentially in N and P uptake. We find a smooth transition between limiting elements, in contrast to the strict transition in Liebig's law of the minimum. At N : P supply ratios where the two elements simultaneously limit growth, an increase in either of the nutrients will increase the growth rate because more resources can be allocated towards the limiting element, as suggested by the multiple limitation hypothesis. However, the further the supply ratio deviates from these supply rates, the more the plants will follow the law of the minimum. Liebig's law of the minimum will in many cases be a useful first-order approximation.
Decomposition of Elephant Dung in an Arid
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Nutrient Availability in Tropical Rain Forests: The Paradigm of Phosphorus Limitation
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Results Of Censuses Of Elephant, Buffalo, Giraffe And Grevy's Zebra Counted In Five Key Ecosystems Conducted In
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An Update Form the African Elephant Database. Tripler CE, Kaushal SS, Likens GE and Walter MT. 2006. Patterns in Potassium Dynamics in Forest Ecosystems
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