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The
Species Richness and Abundance of Macroinvertebrates Existing Within the
Inflorescence of Heliconia imbricata at Two Different Sites in La Selva
Biological Reserve, Costa Rica
Jennifer Huntsberger, Ken Mukai, Ronald Rushing, Teri
Sheldahl, Jewel Thornton
ABSTRACT
Heliconia imbricata
is a large colorful plant that occurs in disturbed and secondary growth areas
of the rainforest in La Selva Biological Reserve, Costa Rica. The bracts of each inflorescence support a
wide variety of invertebrates in the liquid that is retained there. We decided to study the microecosystem
within each inflorescence to see if a difference exists in abundance and
species richness of macroinvertebrates in separate habitats. Three flowers were collected from separate
plants in two different habitats: a secondary forest along a trail and an area
around a soccer field. Flowers were assayed
for liquid volume and pH of the liquid within the bracts and macroinvertebrates
were separated and counted. There was a
definitive difference in macroinvertebrates between the two sites. There were many more dipterans including
mosquito larvae in those flowers collected near the soccer field than in those
collected from secondary forest. The
average Shannon-Weaver Diversity Index for the secondary forest was much higher
than that of the soccer field area, which may be an indicator of general ecosystem
health.
INTRODUCTION
Heliconia
imbricata is one of twelve species of Heliconia plants existing
within the forest ecosystem of La Selva Biological Reserve. The primary forest along the Sarapiqui plains
have been indiscriminately and rapidly altered due to domestic and foreign
socioeconomic and political pressures (Janzen 1983). Heliconia growth is
linked to the availability of light as well as to areas of second growth
(McDade 1944).
The Heliconia is a
large-leaved monocotyledon occurring in disturbed areas of the tropical
rainforest. It develops a brilliant,
red inflorescence with boat-shaped bracts spirally arranged around a rachis. The bracts serve as a reservoir to collect
water and contain a microecosystem including numerous invertebrates. The microecosystems within the bracts may
have been severely altered by human activity; therefore, we developed a study
to investigate that possibility. Our
study is designed to see if a difference exists in the abundance and the
species richness of the invertebrates present in the flowers of two different
areas of disturbance within the La Selva Reserve.
PROCEDURE
Heliconia imbricata flowers were obtained from
two locations within La Selva Biological Reserve during a downpour of
rain. Three samples were made from the
understory of a 50 year old secondary forest along the Sendero Tres Rios Trail
near markers 250, 750, and 900. Three
other samples were taken along the border of a regularly cleared out soccer
field and pathway that is immediately below the OTS office and cafeteria. The soccer field was cleared away
approximately 10-15 years ago. Flowers that were 1.5 to 2 meters tall and
having an approximate lower bract count of 10 – 14 buds were cut off
immediately below the bottom bract. The
terminal buds for all selected flowers were still closed and intact. Flowers were bagged, cut from their stalk
and turned upside down to collect all water within the floral bracts. Floral samples were taken to the OTS field
laboratory where they were dissected and assayed for macroinvertebrates. Organisms were separated into petri dishes,
identified with a dissecting microscope and inventoried for species richness
and abundance.
RESULTS
Data
Class Insecta
Order Coleoptera
|
|
Unknown
A
|
Unknown
B
|
Unknown
C
|
Unknown
D
|
Unknown
E
|
Unknown
F
|
|
Disturbed 1
|
3
|
2
|
0
|
5
|
0
|
2
|
|
Disturbed 2
|
4
|
3
|
0
|
5
|
0
|
4
|
|
Disturbed 3
|
13
|
6
|
0
|
18
|
0
|
1
|
|
Secondary Forest 1
|
8
|
1
|
4
|
0
|
0
|
0
|
|
Secondary Forest 2
|
9
|
7
|
0
|
0
|
1
|
0
|
|
Secondary Forest 3
|
14
|
2
|
2
|
0
|
1
|
0
|
Order Diptera
|
|
Unknown
A
|
Unknown
B
|
Unknown
C
|
Mosquito
Larva
|
|
Disturbed 1
|
26
|
0
|
2
|
82
|
|
Disturbed 2
|
48
|
13
|
1
|
0
|
|
Disturbed 3
|
94
|
0
|
2
|
206
|
|
Secondary Forest 1
|
28
|
5
|
0
|
2
|
|
Secondary Forest 2
|
2
|
7
|
0
|
1
|
|
Secondary Forest 3
|
16
|
1
|
0
|
6
|
DATA ANALYSIS
There
is a variety of life supported by Heliconia floral bracts. Individuals
of three different orders of Insecta were found and other invertebrates
including two different spiders, a snail, and an annelid. The volume of water in each inflorescence
probably varies with the number of bracts present and amount of rain. In our samples the water collected from
similar sized inflorescences was between 130 mL and 306 mL and they were
collected during a large downpour. The
pH is generally around 7.0 with a notable exception in the Disturbed 2 area,
which had a pH of 6.0. The volume of this sample was the lowest collected (131
mL).
We did find a difference in macroinvertebrates present in
the two sites. There was a much larger
number of dipterans including mosquito larvae in those flowers collected from
around the soccer field than in those collected from the secondary forest
trail. One notable exception is the
secondary forest sample 2, which was
collected close to the river, with a pH of 6,
had no mosquito larvae. The pH may have inhibited the dipteran growth
and development and would certainly be worth another look.
A
Shannon-Weaver Diversity Index was carried out to give us some idea of the species
richness and evenness within each inflorescence. The closer this index number is to 1, the more diverse and even
the species abundance, which are indicators of ecosystem health. We did find a large difference in species
richness and abundance between the two areas.
The disturbed area around the soccer field had an average value of .48
while the secondary forest treatment area along the trail had an average index
of .77. These numbers indicate that the
secondary forest Heliconia floral bract microecosystems are ecologically
healthier with a greater species evenness and diversity than those within the
floral bracts from the disturbed area.
Biodiversity
refers to genetic, species, and ecosystem diversity. Biodiversity stabilizes
ecosystems and strengthens their ability to recover from damage. The different
diversity indices of the Heliconia bract microecosytems could reflect a
greater stability of the secondary forest ecosystem versus that of the
disturbed area around the soccer field.
While
we are confident that our results represent a definite trend, there are some
consideration that would help researchers in the future conduct a better study.
- If time had permitted,
more flowers could have been collected to give us a larger sample size to
work with.
- There may have been
errors made in classifying individuals, particularly different stages of
the same species.
- The inflorescences
collected had a range of bracts numbering from ten to fourteen. We could have had cleaner results,
perhaps, if all flowers had the same number of bracts.
- As we were counting
insects, a few with wings were able to escape being counted.
CLASSROOM APPLICATIONS
Conduct similar surveys of plant species that are
reservoirs of invertebrates to:
·
Model
ecosystem dynamics such as food webs and biodiversity
·
Develop
taxonomy skills
·
Study
metamorphosis and invertebrate life cycles
·
Compare
macroinvertebrates within the same plant species in disturbed and undisturbed
areas as indicators of global environmental change
·
Model
inquiry processes in a cooperative learning environment
·
Conduct
data analysis
ENDNOTES
Janzen,
Daniel H. 1983. Costa Rica Natural History. University of Chicago Press.
McDade, Lucinda A,
Kamaljit S. Bawa, et.al. 1994.
LaSelva:Ecology and Natural History of Newtropical rain Forest; University of Chicago Press, Chicago.
Zar, Jerrolod H.
1999. Biostatistical
Analysis. 4th Edition. Prentice Hall, New Jersey.
ACKNOWLEDGEMENTS
We
would like to thank Evan Notman and Senor OrlandoVargas for their help with the
biology and location of our Heliconia
samples.
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