Effect of food
quality and availability on rainforest rodents of Sri Lanka
Pamoda B. Ratnaweera 1 & Mayuri R. Wijesinghe2
1,2 Department of Zoology, University of Colombo, Cumaratunga
Munidasa Mawatha, Colombo 03, Sri Lanka.
Email: 2 mayuri@zoology.cmb.ac.lk (corresponding
author)
Date of publication
(online): 26 December 2009
Date of publication (print): 26
December 2009
ISSN 0974-7907 (online) |
0974-7893 (print)
Editor: Shomen
Mukherjee
Manuscript details:
Ms # o1940
Received 07 February 2008
Final received 23 August 2009
Finally accepted 18 November 2009
Citation: Ratnaweera,
P.B. & M.R. Wijesinghe (2009). Effect of food quality and availability on
rainforest rodents of Sri Lanka. Journal of Threatened Taxa 1(12):
581-588.
Copyright: © Pamoda B.
Ratnaweera and Mayuri R. Wijesinghe 2009. Creative Commons Attribution 3.0
Unported License. JoTT allows unrestricted use of this article in any medium
for non-profit purposes, reproduction and distribution by providing adequate
credit to the authors and the source of publication.
Author
Details: Dr. Mayuri R. Wijesinghe is
a Senior Lecturer in Zoology attached to the University of Colombo, Sri
Lanka. Her expertise lies in the fields
of Conservation Biology and Toxicology. Her research focuses on collecting baseline data on the distribution and
habitat requirements of many rodents and shrews. She also studies the impact of agrochemicals
and heavy metal pollutants on the survival, growth, development and
histopathology of larval stages of amphibians. Pamoda B. Ratnaweera is a
Lecturer in Zoology at Uva Wellassa University, Sri Lanka. She has specialized
in wildlife management during her undergraduate years.
Author
Contribution:Pamoda B. Ratnaweera: Conducting experiments in Kanneliya, contributed towards
writing and analyses in the field component. Mayuri R. Wijesinghe: Formulating initial concept, research design and
methodology. Conducting field and
laboratory work (all preference and tolerance trials in Sinharaja and part of
field work in Kanneliya); analysis of results and writing paper.
Acknowledgments: We
are thankful to the National Science Foundation and the University of Colombo
for funding this project. We are also grateful to the Forest Department and the
Department of Wildlife Conservation for granting us permission to carry out
studies on small mammals in the Sinharaja and Kanneliya forest reserves.
Abstract:Tropical rodent communities are highly diverse species assemblages, yet remain
poorly studied. This investigation was
conducted with the objective of examining the responses of rainforest rodents
to food quality and availability. These
factors were assessed through laboratory and field trials conducted in the
Sinharaja and Kanneliya rainforests in Sri Lanka. The effect of food quality on the foraging
behavior of rodents was examined through feeding experiments using natural
rainforest fruits/seeds. In addition,
the effect of food augmentation on the rodent population was also
investigated. Diet choice experiments
showed that rodents exhibited clear food preferences, with certain fruit types
being preferentially consumed and others rejected. Tolerance tests where animals were provided
with a single fruit type showed that some items that were avoided when offered
with a range of food items were consumed when no alternatives were available.
In the field a positive relationship was found between fruit/seed and rodent
densities; seed addition resulted in marked increases in rodent numbers. These results suggest that tropical rodent
populations are food limited, at least during seasons when fruits/seeds are in
short supply. Food selectivity also
means that populations of rainforest rodents might be adversely affected by
changes in tree species composition resulting from habitat disturbance and
fragmentation.
Keywords:Fruits, rainforest, rodents, seed predation, Sri Lanka
For Figure &
Tables – click here
Introduction
Seeds and
fruits are key food resources of many animals, especially ants, birds and small
mammals (Kelt et al. 2004). Through
their feeding habits small mammals such as rodents promote seed dispersal by
hoarding seeds (Wall 2001), and they influence the structure and composition of
plant communities via seed and seedling predation (Alcantara et al. 2000; Kelt
et al. 2004; Hoshizaki & Miguchi 2005). The availability of food resources is one of the most important factors
that govern animal populations. In temperate environments, where food supplies
are seasonal, animal population fluctuations are said to be primarily driven by
food (Dunning & Brown 1982; Clifford & Anderson 2001). In tropical rainforest systems, because of
higher plant species richness and lower degree of seasonal variation, one would
expect animals to have an abundant food supply at all times. However, in consuming food resources, animals
exhibit preferences in terms of quality and availability. The optimal foraging theory (MacArthur &
Pianka 1966) predicts that when faced with food types of varying quality with
respect to their palatability (e.g. the presence of toxic compounds) and
nutritional value, a forager will select the food types with highest energetic
returns in preference to others (Pyke et al. 1977; Krebs 1978). The potential value of a food item is also
determined by its search and handling times. A food item that requires greater handling time may be less profitable
in terms of the net energy gained by consuming it. In tropical environments rodents may also
face food shortages during certain periods of the year, when their foraging
decisions become crucial. Many
researchers have speculated that food limitation is responsible for the
fluctuation of rodent populations in rainforest habitats (e.g. Alder 1994;
Dunstan & Fox 1996; Shanker & Sukumar 1999; Wijesinghe 2006), although
no study has experimentally manipulated food type and quantity to test this
hypothesis in rainforests of South and Southeast Asia.
In complex and
biodiversity-rich tropical forest ecosystems, assessing the extent of food
selectivity exhibited by rodents and their response to food availability would
be useful to predict their influence on the structure and composition of plant
communities. Such information will also be important to ascertain potential
impacts of forest disturbance on small mammals, and to provide insight into the
patchy distribution of small mammal assemblages. This paper reports the results of two
investigations. First, it studies the
diet selection of three rodents, and second it tests the response of rodent
populations to food availability in a tropical rainforest ecosystem. With regard to the first study of diet
selection, we hypothesized that an individual forager when offered a range of
food items, its choice of a food item should represent its preferred food resource. But when the range of food items is limited,
it should forage on the less preferred food items. In the second study on food availability we
predicted that in populations that are limited by food, food enrichment should
increase the abundance of that species at the augmented sites. This study is
important for two main reasons: (a) to provide us with information about the
diets of a poorly known system, and (b) to allow us to test if tropical forest
rodent communities are food limited.
Methods
The
investigations were carried out in two rainforests in Sri Lanka, the Sinharaja
and the Kanneliya reserves. The natural
vegetation of Kanneliya and Sinharaja is classified as the wet evergreen forest
type, which is a climax forest formation. The annual rainfall of the forests
ranges from 3750 to 5000 mm and the mean monthly temperature ranges from 18 to
27oC. Much of the
precipitation is from the southwest monsoon from May to September and the
northeast monsoon from November to January. Endemism among the flora in the forests is particularly high. The forests have a characteristic
multistoried structure and a high diversity of species, the dipterocarps being
the dominant floral species (IUCN 1993; IUCN & Forest Department 1996).
Study species
Srilankamys
ohiensis (The Ceylon Bi-coloured Rat): Phillips (1980), who
described this species in 1929, closely allied it to Rattus niviventerof the Himalayas. Ellerman (1947)
included it as a member of the subgenus Apomys, of the genus Rattus,
because its skull characteristics were thought to resemble those of the Rattusspecies. Subsequently, after many
taxonomic considerations, it was seen to differ substantially from all other
forms of described Asian murids in its very peculiar cranial features, skin and
teeth. Hence Musser (1981) placed it in
a separate, monotypic genus Srilankamys which is endemic to Sri
Lanka. Musser (1981) considered this
species to be a relict form, because it possesses a number of primitive
characteristics, and speculated that it is possibly related to the stock from
which other Indo-Malayan rat-like genera evolved. S. ohiensis is a
ground dwelling murid confined to the undisturbed rainforests in the island,
being more common in the mountainous areas than in lowland regions. S. ohiensishas been currently listed as a nationally threatened species (IUCN 2007). Average body weights of male and female S.
ohiensis are 126±5.3 g and 125±4.3 g respectively.
Rattus rattus
kelaarti (The Ceylon Highland Rat) and R. r. kandianus(The Common Ceylon House Rat): The species Rattus rattus has an
extensive distribution. Although native
to the Indian Peninsula, it has managed, with humans as the dispersal agent, to
colonize all six continents and thousands of islands, invading a variety of
habitats from tropical rainforests to arid ecosystems. Consequently, this species shows immense
morphological variation. In Southeast
Asia it has been conventionally divided into many subspecies. Five subspecies
are identified in Sri Lanka, among them R. r. kelaarti and R. r.
kandianus are peculiar to the island while others are introduced. R. r.
kandianus (average weight male = 133±4.4 g and female = 125±4.1 g) is a
commensal of man, being wide spread in the low country and lower hill zones,
while R. r. kelaarti (average weight male = 130±6.6 g and female
=123±7.0 g) is confined to the lowland and montane zone forests in the island
(Phillips 1980). Both these species are
ground dwelling murids.
Mus mayori (The Spiny Rat): This is an endemic ground dwelling species that is
restricted to the moist rainforests of the lowlands and the hill country. Mus mayori has been placed in the sub
genus Coelomys; this species is believed to be a relict form (Crusz
1986). It is represented in the island
by two well marked subspecies: the highland (M. m. mayori) and the
lowland (M. m. pococki) forms (Phillips 1980). M. mayori has been currently listed as
a nationally threatened species (IUCN 2007). Average weights of male and femaleM. mayori are 41±0.66 g and 39±0.6 g respectively.
Investigating
food selectivity
The food
habits of the rainforest rodents were examined using two methods. The cafeteria experiments were used to
investigate food selectivity, and prolonged feeding trials with individual
fruit types were used to assess tolerance levels, both of which would indicate
differences in quality of food items. These experiments were carried out on
three rat taxa, the endemic Srilankamys ohiensis, Rattus rattus
kelaarti and R. r. kandianus captured from within the Sinharaja rainforest
in 2000 and 2001. These are the three
predominant rat species found in this forest. Comparing food habits of these species would reveal information about
food preference since these rats are of similar size and hence have similar
energy requirements. For instance, the mean weights of females of S.
ohiensis, R. r. kelarti and R. r. kandianus captured from the forest were
between 125-123 g. In rainforest
habitats these species are predominantly fruigivorous, but R. r. kandianusin particular may feed on insects.
Two captive
experiments were designed to test the food preference and food tolerance of the
three species of rats. The first set of
tests, the cafeteria experiments, provided information on the food preferences
while the second set, which involved four-day feeding trials using selected
food items, provided information on the degree of food tolerance of each
species. The feeding experiments were conducted in cages made of wire mesh with
wooden frames of size 1 x 0.3 x 0.3 m.
Fruits of 25
tree species from the floor of the Sinharaja forest were used for the food
preference tests (Table 1). The months
during which the experiments were conducted coincided with the flowering and
fruiting seasons of many of the forest tree species. The selection of the fruit types for the
feeding trials was based on availability.
Before
commencing trials, preliminary tests were carried out to obtain information on
handling time of the wild caught rodents. Time spent on handling and ingestion of a food item by each rat was
recorded by offering a single fruit and observing the animal for a period of
two hours. A given fruit type was tested
only once on a selected individual. In
the cafeteria experiments, fruits from 25 tree species, 24 from the forest and
one from forest areas around the periphery of the reserve were used. Because of the difficulty of simultaneously
obtaining all 25 fruit types tested, ten fruit types were presented at a time
(two fruits of each type) to each individual rodent, and recordings made of
whether the fruits were eaten either fully, partially, or shunned. Additionally, the animals were observed to
ascertain which item was selected first from among the given fruits. The experiment commenced on the night
subsequent to the night of capture. Trials were conducted at night as this was the active period of the
rodents. The experiments commenced at
1900 hr and the results were obtained the following morning at 0700 hr. Each fruit type was tested on 15-20
individuals of both sexes from the three rat taxa. A given fruit type was tested on an equal
number of animals of each taxon. However, because of restrictions in availability the number of animals
tested per seed/fruit type varied from 15 to 20. For instance, when fruits were less abundant
only 15 animals of each taxon were tested on that particular item, while 20
animals were tested if fruits were highly abundant.
For food
tolerance trials, captive rats were tested for their ability to maintain weight
over three days on a diet of fruit supplied in excess, one fruit type at a
time. Each animal was provided with 100g
of a particular fruit type, with water provided ad libitum. The weight of the animal was recorded to the
nearest 0.5g using a Pesola balance before and after the experiment. The uneaten fruits were removed from the cage
and a fresh supply was provided to the animals on each successive day. Five individuals from each rat taxon were
tested on a given fruit type.
Assessing the
effect of food availability
The effect of
availability of preferred fruit/seed types was assessed by estimating their
natural availability and rodent abundance in selected areas of the forest, and
through seed enrichment trials. The
fruit/seed estimations and the augmentation experiments were conducted in the
Kanneliya rainforest during October to February (year 2006 and 2007) which is
outside the main fruiting season. The
sequence of the study was as follows: a four day trapping, followed by
fruit/seed counts, food addition on the same day after the seed/fruit count,
and finally another four day trapping session after two weeks.
Six plots of
40x90 m were randomly selected. Live
trapping of small mammals was conducted using 50 Sherman’s traps per plot, laid
in a grid configuration with 10m spacing between each trap. Trapping was conducted for four consecutive
nights, yielding a total of 1200 trap nights. Fruit/seed searches were conducted within 2x2 m quadrates by clearing the
litter to a depth of about 3cm. Five
such quadrates placed at the four corners and middle of each study plot was
examined for fruits/seeds consumed in the cafeteria experiments. These were categorized by species and the
density of each was calculated.
The influence
of food abundance on rodent populations outside the main fruiting season, when
food resources could be a limiting factor, was examined through seed enrichment
experiments. Seed enrichment was carried
out in each of the six study plots. Preliminary
investigations revealed that rodents eagerly fed on peanuts, which were
therefore selected for the food enrichment trials. Five trays of 20x30x1 cm size, each
containing 500g of peanuts were placed on the ground at the four corners and
middle of each plot, and they were replenished after a week. After two consecutive weeks of seed
enrichment a second trapping session was conducted.
Further
confirmation of seed predation by the rodents was sought by using specially
designed food trays (Image 1). To allow
seed predation by rodents while preventing the activity of ants and birds, the
method of Herrera et al. (1994) and Alcantara et al. (2000) were used with a
few modifications. The peanuts were
pasted using ant proof glue on the tray, evenly spread in a grid fashion. Pasting the seeds in this manner prevented
the removal of seeds by ants. The entire
tray was then covered by a mesh box to prevent seed predation by birds. Openings of 10x25 cm on the two sides of the
mesh box allowed free access to rodents but prevented entry of large
granivores. At the entry point leaf litter
was piled so that the entrance was not obvious. The mesh box was also covered by a hard, non-transparent polythene sheet
to prevent rainwater seeping into the tray and to be less visible to the birds. Faecal matter found on food trays was
identified and used as evidence for the presence of rodents.
Results
Food selectivity
The diet
choice experiments showed that rodents exhibited food selectivity, in that some
fruits were eaten fully while others were avoided without being consumed. From among the seeds provided, certain seed
types were preferred whilst others were not consumed (Table 1). Observations revealed that all three rat taxa
selected Elaeocarpus glandulifer, Syzigium rubicundum and Clidemia
hirta as their first choices in the fruit combinations provided. Items avoided included Chaetocarpus
castanocarpus, Mussaenda frondosa, Calamus ovoideus, Fagrea ceilanica and Semecarpus
moonii.
Chi-square
test of independence was used to ascertain if the responses of a given taxon
for a particular seed type showed significant variations (Table 1). The number of responses for each category,
i.e., for fully eaten, partially eaten and not eaten, was used for this
purpose. For the majority of the fruit
types, the responses of the rat taxa were either identical or not statistically
different from each other. Contrasting
responses were evident for only four of the 25 fruit types. The results thus showed that rodents differed
significantly in their preference for the fruits of Doona congestiflora,
Syzygium neesianum, Gomphia serrata and Melastoma malabathricum (Table
1). While Chaetocarpus
castanocarpus, Calamus ovoideus, Mussaenda frondosa, Fagrea ceilanica andSemecarpus moonii were completely rejected by all three rodents, the fruits
of Syzygium rubicundum and Clidemia hirta were fully eaten by all
taxa (Table 1).
A two way
ANOVA was used to test if handling times differed significantly between the
rodents and seed type. Seed type, rodent species and their interactions were
the independent variables (factors), while handling time was the dependent
variable. The results revealed that handling time significantly differed
between seed types (F22=55.97, P=0.0001). However, there were no differences in
handling times between the three rat taxa (F2 =0.59, P=0.557). The interaction between rat taxa and seed
types was also not significant (F44 =0.45, P = 0.999) indicating
that all 3 rat taxa spent similar times on the various seeds.
The different
taxa displayed varying degrees of food tolerance (Table 3). R. r. kandianus maintained weight on 15
seed types, R. r. kelaarti on 13 and Srilankamys on nine. Weight loss occurred with some fruit types
that were offered. Both S. ohiensis and R. r. kelaarti loss
weight on 11 fruit types (Table 3). Some items that were avoided during the
cafeteria tests were consumed during the food tolerance trials. For example S. ohiensis did not show
any interest in the fruits of Syzygium neesianum, Myristica dactyloides,
Gomphia serrata and Mussaenda frondosa during cafeteria experiments,
but these were ingested when other food types were not available.
Effects of food
augmentation
In the
trapping sessions in the Kanneliya forest, both before and after food addition,
with the exception of one individual of Suncus zeylanicus (Sri Lanka
Jungle Shrew; Phillips 1928), all the captures were of the two murids Mus
mayori and Rattus rattus kandianus. Prior to food enrichment there was a significant difference in the
numbers of rodents captured in each of the six study plots (Chi-square test - c2= 20.39; P = 0.001).
With regard to
the link between rodents and fruit/seeds availability, prior to food addition
it is seen that over 50% of the variation in the rodent population is explained
by fruit/seed densities (Total murids R2 = 66.4 %, F = 7.92, P =
0.04) under natural conditions (Fig. 1). There were significant differences in fruit/seed densities between the
plots (Table 4). The food addition experiments confirmed the favourable impact
of food availability on rodent populations (Fig. 2). The patterns observed in each of the
individual plots are shown in Fig. 2. Paired t-test results suggest a significant difference in murid density
before and after food enrichment (T = 2.73, n=6, P = 0.04). All plots (except plot 2) showed an increase
in rodent density (Fig 2) after addition of food. Taking individual plots, the
plot that had the lowest original density of 4 x 10-4 ha-1increased to 36 x 10-4 ha-1 after food addition. It is noteworthy that this increase in density
was more than double in plot 5 and 6 where the initial densities were low
indicating that the effect of resource augmentation was most significant in the
resource poor plots.
Discussion
This study has
revealed that the abundance and distribution of frugivorous rodents even in
resource rich tropical rainforests are greatly influenced by the availability
of preferred food types, providing direct evidence for the food limitation
hypothesis in these ecosystems. Very few
studies have examined food habits of tropical rodents in captivity or under
natural conditions. The findings of the
present investigation indicate that rainforest rodents actively select certain
fruit types from the wide range that is available to them on the forest
floor. The study also highlights the
fact that the availability of preferred fruits varies throughout a given
rainforest, which in turn causes uneven occurrence patterns of the
rodents. This observation is of special
relevance to the distribution of rodent populations within the Sri Lankan
rainforests where many studies have shown that the occurrence of tree species
is patchy and highly localized (e.g. Gunatilleke & Gunatilleke 1981;
Gunatilleke & Ashton 1987). Although
tropical rainforests harbor a multitude of trees that bear seeds and fruits,
which presumably provide an abundant source of food throughout the year for
frugivores, the present investigation conforms to the hypothesis that rodents
actively consume only selected food items based on the ease of handling
(Goodyear 1992). Although this was not
tested in the present study, observations revealed the trend that the rats
mostly consumed fruit types with a fleshy pulp and a thin, easily chewable
outer peel.
Aversion to
particular plants or parts of plants may result from olfactory and gustatory
responses, spines, hard shell, dense husks, or other structural features
(Randolph et al. 1991). It may also be a
reaction to an unpleasant taste resulting from noxious or toxic secondary compounds
(Freeland & Janzen 1974; Bryant & Kuropat 1980). In Brazil, Bergallo & Magnusson (1999)
have shown that terrestrial small mammals such as small forest dwelling
rodents, in particular, select fruits and seeds that have a high energy content
and nutritional value, whilst avoiding those containing toxic compounds. In the present study rodents were low in
areas that had Chaetocarpus castanocarpus which has a spiny outer
covering.
Numerous
examples from captive experiments and field studies in temperate countries have
demonstrated that, as the supply of the more preferred food item declines, a
species becomes less selective (e.g. Krebs et al. 1977; Sinclair et al.
1982). The present investigation
suggests that such a phenomenon may also occur in tropical rainforest
ecosystems. In the feeding trials with selected fruit types, when preferred
alternatives were not available, all three rat taxa fed on fruit types that
they refrained from consuming during the cafeteria trials. In situations when food is scarce, if the
animal continued to refrain from feeding owing to the absence of preferred food
items it would lose weight even faster than if it fed on lower quality food
items, and this in turn would reduce the time the animal can withstand
nutritional stress before dying. Therefore, despite the loss in weight, it is profitable for animals to
consume fair quantities of even the low quality food items.
In New
Zealand, a temperate country, Choquenot & Ruscoe (2000) attributed the
sporadic eruptions in rodents to an increase in food availability. Similar observations have been made by
Angelstam et al. (1987). In the
present study a positive correlation was evident between rodent densities and
the availability of palatable fruits/seeds in the rainforest. The food enrichment trials provided further
evidence that the availability of favourable resources is positively linked to
increases in rodent populations. Both
rodent species, M. mayori and R. r. kandianus, increased in
abundance in all plots except in plot 2, with seed addition. It was particularly noteworthy that the
increase in abundance was of a much higher magnitude (more than double) in the
previously sparsely populated plots than in others. Boutin (1989) reviewing 138 cases that have
used food enrichment experiments under field conditions to investigate its
impact on animal populations have concluded that there has been 2-3 fold
increases in densities in response to food supplementation. However, as pointed out by this reviewer, it
would be preferable to use long term experiments conducted over a large special
scale in order to draw firm conclusions. The findings of the present investigation nevertheless provide some
evidence for the food limitation hypothesis in tropical rainforest ecosystems.
Although Sri Lanka’s wet zone has been described as aseasonal (Ashton &
Gunatilleke 1987) the forest has distinct periods of flowering and
fruiting. The main fruiting season in
lowland wet zone forests is during the months of April to August when rainfall
is heavy. Increased populations occur at
this time, presumably because food is more abundant (Leung 1999; Wijesinghe
2006).
For
predominantly frugivorous rainforest species, the scarcity of good quality food
would be particularly important outside the main fruiting season. Consequently, during such food shortages,
animals could be expected to aggregate in selected seed-rich areas where the
preferred food types are available. The
findings of this study also imply that frugivorous rodents could be adversely
affected if the tree species composition of these ecosystems is altered through
habitat disturbance and fragmentation.
Conclusion
Food
availability, particularly during the non-fruiting season, may at least in part
be responsible for the regulation of rodent populations in tropical
rainforests. The study also shows that the scarcity of food during the
non-fruiting season may be aggravated due to the food selectivity exhibited by
these rodents.
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