chapter - 4 population study of...
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CHAPTER - 4
POPULATION STUDY OF GRASSHOPPERS
4.1 INTRODUCTION :
The term "population" has been defined by Richards (1961) as,
"all those in d iv idua ls o f one species whose lives are suffic ien tly
integrated to have an influence on one another". Nothing is well-
established in Zoogeography than the existence of life zones that
differ in their fauna and flora. The ecological divisions of life zones
a ttr ib u te d w ith the te rm "ecosys tem " is o f a la te r o rig in from
Zoogeographical divisions. Of the many types of ecosystems, such
as. aquatic, fo res t and te rres tria l w ith subd iv is ions in each one,
the grassland ecosystem has been maximally explored by a number
of ecologists and biologists all over the world. This is because the
insect eco log ists have conducted many find ings about the grass
hoppers in the g rass land . In fact, grasslands are very closely
associated with the well-being of the grasshoppers. These insects,
constituting one of the leading categories of destructive insects, have
attracted attention of entom ologists and animal ecologists from the
beginning of the twentieth century. They inhabit mainly grasslands
but do not rest to this habitat, and often cause heavy damage to crops
and forest vegetation. Their attack to our fields, forests and crops have
dragged interested scientists to move out of laboratories and rush to
the field. It is from this spot that their ecological study started in the
real sense. Huntsman (1948) aptly stated that, "Ecology is the study of
organisms made in the places where they live". However, if a persistent
in te llectua l e ffo rt fo r a s low and thorough eco log ica l analysis is
continued, it is bound to deal the most satisfying results, both intellectually
and aesthetically, even in a very complex case.
The early studies of the populations of animals by population
ecologists was basically limited in the laboratories, but today's dedicated
field ecologists stubbornly refuse to accept any relationship demonstrated
in a controlled and continued environment. As a result, these days
the sampling study of population is finding diversion more towards the
field than the laboratory all over the world.
Study of the population structure was an essential base of the
present study. For this part of the study, grasshopper sampling was
done by direct estimation of number per unit area, in the forest flore,
intermittently studded with the matrix of grasses. Of the three most
prevalent methods, i.e. (A) sweeping, (B) direct counting, and (C) capture
m arking-release-recapture, the direct counting method was preferred
because of the following drawbacks of the other two methods ;
Sweeping by net collects only those grasshoppers which are
sitting on the upper part of vegetation. Further, during later part of season,
when the vegetation grows taller and denser, the capture by sweeping
becomes quite ineffective. Similarly, capture-marking-release-recapture
method also is not suitable for grasshoppers as during the sampling
period hoppers may undergo moulting, thus their marking is lost. This
may very likely lead to an erroneous result of an honest work. Direct
counting of individuals was, thus, the only safer method left to be
adopted.
4.2 OBSERVATION AND RESULTS :
In the study site censusing of grasshoppers was carried out in the
protected area of the forest-floor of Guru Ghasidas University campus for
the whole year, starting from June uptill May, the following year. The censusing
primarily aimed to find out the species richness and species dominance in
the grasshoppers community. Secondly, the population structure, biomass
structure and secondary productiv ity was also determ ined on two
grasshopper popula tions, namely, Catantops pinguis innotabilis and
Spathosternum prasiniferum prasiniferum.
The prevalent conviction among ecologists that natural community
represents important and meaningful assemblage of organisms, has
promoted a divergence of analysis. The structure and nature of any animal
community is determined by the species contents and their ecological
amplitude. Structure of the grasshopper population with special reference
to species richness, average density, re la tive abundance, relative
frequency, relative density and association indices of the aforesaid two
grasshopper populations is presented in this chapter. The interaction of
abiotic factors upon the population dynamics have also been statistically
analysed. Partial corre lation and significance tests based on Karl
Pearson's formula have also been performed to confirm the theoretical
findings about the impact of certain physical factors upon the population
density of grasshoppers.
Tabl
e 4.
i Sp
ecie
s ric
hnes
s an
d di
vers
ity
of gr
assh
oppe
rs
in th
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udy
site
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4.2 (i) SPECIES RICHNESS :
Tab le 4 .i sh o w s the g ra ssh o p p e r spec ies com pos ition ,
richness and diversity index in the study site. A total of twenty three
grasshopper species were collected from the study site in different
seasons of the year. These belonged to two suborders and six
families. Out of the whole, twenty one species belonged to suborder
Caelifera which were the short-horned grasshoppers. Another suborder,
Ensifera comprised of only two members from two families. These were
the long-horned grasshoppers. The Caelifera suborder was composed of
two superfamilies (Dirsh, 1962, 1963), the Acridoidea and Tetrigoidea. All
the twenty one species of grasshoppers of this group were distributed in
four fam ilies, viz., Pyrgom orphidae, Acrid id iae, H em iacrid idae and
C atantop idae. Super fam ily Tetrigo idea was not represented by
any speices in the study site. Table 4.i also shows that two families,
namely, Acrididae and Catantopidae were further composed of three
and two families respectively. The three subfamilies of Acrididae contained
nine d iffe re n t species; w hereas five sub fam ilies of C atantop idae
contained only eight species of grasshoppers.
As exhibited by Table 4.i, of the total collected speices of grass
hoppers (23), 91 percent belonged to suborder Caelifera while 9 percent
belonged to suborder Ensifera. The data were further computed to analyse
the proportion of the different species in the suborders. It is seen from
Table 4,i that family Acrididae constitued the highest proportion both in the
order (39%) and in the suborder (43%). Family Catantopidae occupied the
second place in the order. Its members constituted the second highest
proportion of 34.8 percent in the order and 38 percent in the suborder.
Other four families were proportionately much lower both in the richness
4.2 (ii) AVERAGE DENSITY :
Abundance (Chauvin, 1967; Kendeigh, 1974) or average density
(Ananthakrishnan, 1979) of the grasshopper populations has been
calculated as the average number of individuals of a species per unit
area of the ecosystem. Chauvin (1967) has stated that the terms mean
density and abundance carry the same sense.
The results of individuals species density of the two grass
hoppers, e.g. C. pinguis innotabilis and S. prasiniferum prasiniferum are
exhibited by Table 4,ii and figure 4.1. The results revealed that hatching
in both the species started in July, after about 15 to 20 days of break of
monsoon. The result of individual species density shows that the
average density of C. pinguis innotabilis was highest (5.8 ind./m2) during
m id-August, which increased from its in itia l value of 2.3 ind./m 2.
Average density of S. prasiniferum prasiniferum was only 1.2 ind./m2
at the time of emergence of popualtion. It increased to its highest value
of 1.7 ind./m2 between latter part of August and earlier part of September.
The data also show that in the two grasshoppers the average density
of C. pinguis innotabilis remained much higher compared to that of
S. prasniferum prasiniferum.
Fig. 4.1 further shows that both the grasshopper types completed
their two life cycles in the study site in an year. They, thus, showed the
bivoltanity in their reproductive behaviour. It was also noted that rainy season
generation of C. pinguis innotabilis lived in the aboveground part for over
PERIODC. pinguis innotabilis
(no./m2)
S. prasiniferum prasiniferum
(no./m2)
Joint Maximum & Minimum of values
July 15, ’96 2.3 0.2
July 31 4.8 1.2
Aug. 15 5.8 1.3 7.1
Aug. 31 4.5 1.7
Sept. 15 3.4 1.7
Sept. 30 2.8 0.7
Oct. 15 2.2 0.6
Oct. 31 1.6 0.5
Nov. 15 1.3 0.3 1.6
Nov. 30 1.2 0.4 1.6
Dec. 15 0.8 -
Dec. 31 - —
Jan. 15 '97 - -
Jan. 31 - —
Feb. 14 -0.3
Feb. 28 - 1.1
Mar. 15 — 0.7
Mar. 31 1.4 0.7
Apr. 15 2.9 0.5 3.4
Apr. 30 2.9 0.4
May 15 1.9 0.2
May 31 1.3 0.3
June 15 1.0 0.30.9
June 30 0.7 0.2
C. pin
guis
inno
tabi
lis
Fig.
4.1
Aver
age
dens
ity
of C
atan
tops
pi
ngui
s in
nota
bilis
an
d Sp
atho
ster
num
pr
asin
iferu
mpr
asin
iferu
m
in the
fo
rest
flo
or d
urin
g 19
96-9
7
five and a half months; while this generation of S. prasiniferum prasini-
ferum lived for about four and a half months. However, the second or the
summer season generation of C. pinguis innotabilis lived for a shorter period
(of about 3.5 months). The population density of both the grass-hoppers
rem ained low in sum m er season than in ra iny season. In
summer season also, the average density of C. pinguis innotabilis
remained higher than that of S. prasiniferum prasiniferum.
4.2 (Mi) PERCENTAGE FREQUENCY : (Table 4.iii)
Population density provides the idea about the populations at any
given time, i.e. the structure of a population at any particular moment. The
percentage frequency, on the other hand, is one parameter which
enables us to get an insight about the percentage of unit areas
(quadrats) occupied by the speices in subject out of the total areas
(quadrats) studied (Odum, 1971). Percentage frequency of a community
also provides an insight of the spatial distribution of its component
species in the ecosystem. Odum, 1971 has used the term 'Relative
abundance' instead of percentage frequency to analyse the data for
working out the spatial distribution of grasshoppers and other insects
studied by him. The term 'relative abundance' and 'percentage frequency
have both been conveniently used here as synonyms to make the
expression more explicit.
The result of percentage frequency of two grasshopper types has
been exhibited in table 4.iii. It is seen that at the time of first capture
C. pinguis innotabilis was available in 80% of the quadrats. Its frequency
of occurrence increased in the captures of successive periods when in
mid-August its individuals frequented in all (100%) of the quadrats
Period C. pinguis innotabilis
(%)
S. prasiniferum prasiniferum
(%)
July 15, '96 80 20
July 31 90 40
Aug. 15 100 40
Aug. 31 80 50
Sept. 15 70 50
Sept. 30 70 50
Oct. 15 60 30
Oct. 31 60 30
Nov. 15 50 30
Nov. 30 50 30
Dec. 15 40 -
Dec. 31 —
Jan. 15 ’97 - -
Jan. 31 -
Feb. 14 - 20
Feb. 28 - 70
Mar. 15 - 40
Mar. 31 50 30
Apr. 15 80 30
Apr. 30 80 30
May 15 50 20
May 31 50 20
June 15 40 20
June 30 40 10
sampled. A decrease in relative abundance was recorded in the very
next capture at the end of August when the species frequented only
80% of quadrats. This decrease in relative abundance of grasshoppers
continued in successive fortnights. Thus, the rainy season generations of
C. pinguis innotabilis was obtained last in only 40% of the quadrats around
mid-December.
However, after the total disappearance from the aboveground
part between second half of December to middle of March, the young
stages of C. pinguis innotabilis were again seen and captured in the forest
floor at the end of March. As exhibited in table 4.iii, the population was
recorded only in 50% of the quadrats. However, the highest percentage
frequency reached to only 80% in both the fortnights of April. A fluctuation
was regularly found in the relative abundance of this species throughout
its aboveground existence in summer season. The lowest percentage
frequency of 40% was recorded in the month of June when only adult
individuals were captured in quadrats.
S. prasiniferum prasiniferum also completed its two generations
in one year and the bivoltanity was apparent in this species too. The rainy
season individuals appered as first instar nymph in the second half of
July. The percen tage frequency of th is period was only 40%.
Hereafter the population showed an increase (table 4.ii) in the percentage
frequency in later periods and it increased from its initial value to the
peak value of 50% in the second half of August. At this time, the individuals
were obtained in 50% of the quadrats sampled. This value remained
consistent till the end of September. Decrease in percentage frequency
was recorded in the month of October. It is clear from the table 4.iii that
the percentage frequency of S. prasiniferum prasiniferum remained
unchanged and the individuals of the species frequented 30% of the
quadrats for the rest period of the occurrence of the rainy season
generation.
The population of S. prasiniferum prasiniferum was not available
in the aboveground part in the months of December and January.
However, young hoppers were again observed in aboveground part in
mid-February. The initial percentage frequency at this period was 20%.
By the end of the same month the percentage frequency increased to an
all time high of 70% when only the young stages of the species frequented
the habitat. Decrease in the value of percentage frequency started from
the beginning of March when the species could be captured in only 4 0 /o
of the quadrats sampled. This decline continued in the ensuing periods
also and the lowest percentage frequency of 10% was recorded during
the end of June when only the adults of this species were captured last.
Observation into the percentage frequency of the populations of
the two grasshoppers showed that C. pinguis innotabilis frequented into
the aboveground part for 9 months while S. prasiniferum prasiniferum
frequented for 10 months. But the individuals of the former species were
distributed in larger area of the study site as is evident from the fact
that individuals of this species were captured in all the quadrats sampled
in the mid-August. The latter species remained confined to limited part of
the study site and its individuals could not be obtained in more than 50%
of the quadrats in ra iny season. However, it was only at one
occasion (February end) that the individuals were obtained in 70% of the
quadrats sampled. One apparent cause of such a difference in the
values of percentage frequency of the two grasshoppers is that the
population density of C. pinguis innotabilis remained always higher than
that of the S. prasiniferum prasiniferum.
It was noted that the percentage frequency of C. pinguis innotabilis
showed a faster increase compared to the S. prasiniferum prasiniferum
Moreover, in August and September, when the vegetation structure
generally remains at its bloom, frequency index also increased to its
highest in both the grasshoppers.
October is a month which registered evident meteorological
changes (table 1 ii) and had its consequent effects upon the ecosystem
structure. It is apparent from the present observations that a decline in
the percentage frequency of both the grasshopper populations occurred in
this month. November is the month which heralds the approach of
winter and generally shows a gradual fall in the meteorological conditions.
In this month, the temperture of the winter starts falling, the percentage of
relative humidity shows downward trend and the availability of the green
food and also its texture changed. This affects the biological activities of
most of the insect species, that cease in general. The subject grass
hoppers were no exception to it. The mature ind iv iduals quickly
completed their biological purpose and started becoming less frequent
during this month. December and January are the peak winter months.
They caused complete disappearance of the rainy season population of
both the species from the aboveground parts.
The second or the summer season generation of S. prasiniferum
prasiniferum appeared about 45 days earlier than that of C. pinguis
innotabilis Besides the fact that all the diapausing eggs did not hatch
due to dry and hot atm ospheric conditions, d is tribu tion of green
vegetation in limited number and restricted patches together with the
hot soil surface also played their role in restricting the percentage
frequency that remained low in summer season populations.
4.2 (iv) RELATIVE FREQUENCY : (Table 4 iv)
Relative frequency is another parameter pertaining to the popula
tion structure. It enables to derive a relation between two or more species
of a community and is expressed in terms of the percentage value.
Hariston (1959) used the term 'species frequency' instead of 'relative
frequency’. Fie stated that analysis of species frequency gives a picture
of the regular arrangement of species in a community. In the present
work, relative frequency of the subject grasshoppers is analysed and the
result is given in table 4.iv. It is seen that at the time of emergence of
hoppers around the first half of July, C. pinguis innotabilis showed
the highest relative frequency (80%) in the study site. S. prasiniferum
prasiniferum was recorded at comparatively lesser occasions (20%)
during this period. In the latter parts of the rainy season, a gradual decline
was recorded in the values of relative frequency of C. pinguis innotabilis
and contrastingly a gradual increase was found in the relative frequency
of S. prasiniferum prasiniferum. Flowever, during the last month of rainy
season (October) an increase in relative frequency of C. pinguis innotabilis
and decrease in this value in case of S. prasiniferum prasiniferum was
again recorded. During last days of this season, value of relative frequency
shifted again to lower side in C. pinguis innotabilis and higher side in
S. prasiniferum prasiniferum (table 4.iv).
Relative frequency of both the grasshoppers showed a similar
trend in the second generation too, during summer season. In this
generation, the highest value of relative frequency (72.7%) was recorded
Period C. pinguis innotabilis
(%)
S. prasiniferum prasiniferum
(%)
July 15, '96 80.0 20.0
July 31 69.2 30.7
Aug. 15 71.4 28.5
Aug. 31 61.5 38.4
Sept. 15 58.3 41.6
Sept. 30 58.3 41.6
Oct. 15 66.6 33.3
Oct. 31 66.6 33.3
Nov. 15 62.5 37.5
Nov. 30 62.5 37.5
Dec. 15 - -
Dec. 31 -
Jan. 15 ’97 - -
Jan. 31 -
Feb. 14 - -
Feb. 28 -
Mar. 15 — -
Mar. 31 62.5 37.5
Apr. 15 72.7 27.2
Apr. 30 72.7 27.2
May 15 71.4 28.5
May 31 71.4 28.5
June 15 66.6 33.3
June 30 80.0 20.0-------------------------- J
in April for C. pinguis innotabilis population while the highest relative
frequency of 37.5% was recorded in the second half of March for
S. prasiniferum prasiniferum. In general, the relative frequency was
recorded always h ighe r fo r C. p ingu is innotabilis and lower for
S. prasiniferum prasin iferum also during the second generation of
these grasshoppers that lived in summer season.
4.2 (v) RELATIVE DENSITY : (Table 4 v)
Relative density is a parameter which expresses the percentage
of a particular species in the capture of total number of species
(Chauvin, 1967; Odum, 1971 and Atwal, 1974). Table 4.v shows the
results of fornightly variations in the relative density of the two bivoltine
grasshopper populations. It is seen that at the time of emergence of the
population in the rainy season an all time highest value of realtive density,
being 92% was recorded for C. pinguis innotabilis. Its lowest value of
8% was recorded for S. prasiniferum prasiniferum. This was a period
when the populations contained not only the newly emerged hoppers
but also some adult individuals of the previous season generation.
From here onward, a gradual decrease was recorded in the relative
density of C. pinguis innotabilis which continued till mid-September
when its lowest value was obtained to be 68%. In the periods after
m id-September the re lative density of the species increased once
again and it fluctuated between 81% to 75%. During rainy season, relative
density of S. prasiniferum prasiniferum, which was only 8% in the
mid-July, increased to its highest value of 32% in the middle of
September. The relative numbers decreased hereafter and the relative
density fell to 20% in the very next fortnight. The values of relative density
Period C. pinguis innotabilis
(%)
S. prasiniferum prasiniferum
(%)
July 15, ’96 92.0 8.0
July 31 81.3 18.7
Aug. 15 82.8 17.2
Aug. 31 73.7 26.3
Sept. 15 68.0 32.0
Sept. 30 80.0 20.0
Oct. 15 78.5 21.5
Oct. 31 76.1 23.9
Nov. 15 81.2 18.2
Nov. 30 75.0 25.0
Dec. 15 - —
Dec. 31 -
Jan. 15 '97 - —
Jan. 31 -
Feb. 14 - —
Feb. 28 —
Mar. 15 — -
Mar. 31 66.6 33.4
Apr. 15 85.2 14.8
Apr. 30 87.8 12.2
May 15 90.4 9.6
May 31 81.2 18.8
June 15 76.9 23.1
June 30 77.7 22.3
C. p
ingu
is in
nota
bilis
S. p
rasin
iferu
m
pras
inife
rum
(% ) Ajisuaa 3A!iB|aa
Fig.
4.2
Rela
tive
dens
ity
of C
atan
tops
pi
ngui
s in
nota
bilis
an
d Sp
atho
ster
num
pr
asin
iferu
mpr
asin
iferu
m
in the
fo
rest
flo
or d
urin
g 19
96-9
7
for this species fluctua ted between 18.2 to 25% in the remaining
period of the rainy season.
When the populations emerged for the second time in summer
season the relative density of C. pinguis innotabilis was found 66.6%
and that of S. prasiniferum prasiniferum 33.4% . In this generation, the
relative density of C. pinguis innotabilis increased from its initial value of
66.6% in March end to its highest value of 90.4% recorded in mid-May.
The realtive density of this species showed decline through-out the
remaining period of its aboveground existence. The relative density of
S. prasiniferum prasiniferum was, although at its highest (33.4%) in
March end, but it decreased to less than half of its initial value in the
very next fortnight (14.8%). It further decreased to as low as 9.6% in
mid-May which was also the lowest relative density for this species of
summer generation. A gradual increase in the relative density values
of the species was further recorded. The last value for summer
generation of S. prasiniferum prasiniferum grasshoppers was found to
be 22.3% at the end of June when the population comprised of only the
adult insects of both the species.
Fig. 4.2 shows the trends of the relative desnities of the two
grasshopper populations in subject. The figure suggests that the relative
density of C. pinguis innotabilis species was always higher than that of
S. prasiniferum prasiniferum and this trend remained persistent both, for
the rainy season as well as for summer season generations, of the two
grasshoppers.
4.3 ASSOCIATION BETWEEN THE SPECIES :
Generally in the synecological study, it is important to compare
the association of two or more species living in the same habitat
(Southwood, 1975). The two main approaches to measure the degree of
association include the Index of Affinity and the Index of Association.
Such analysis of the natural populations was suggested long back by
Fager (1957) and Davis (1963). The procedure of analysis has been
elaborately discussed by Southwood (1975).
4.3 (i) Index of Affinity :
Index of Affinity is actually a paramter to show the frequency of
occurring together of the individuals of different species of the same group
in the same habitat. This index is chiefly concerned with the assessment
of joint occurrences of the two or more species in the sum of occurrences.
Table 4.vi shows the results of the frequency of occurring together
of the individuals of the two populations of grasshoppers in the study site.
The data show that the indivduais of the two species, namely, C. pinguis
innotabilis and S. prasiniferum prasiniferum came together in 40% of the
catches in the mid-July. An increase in the value of Index of Affinity was
recorded to its all time peak of 61.5% at the end of July. The same value
was also found at the end of August (table 4.vi). Hereafter, the affinity
index decreased to an all time low of 22.2% in the captures of the October
month.
After the population emerged to complete its second generation
at the advent of summer season, the frequency of occurring together
of the two populations was recorded to be 25% in the end of March. An
increase in this frequency was recorded to 54.5% in the very next
capture in mid-April, which was also the peak value for this parameter
Table 4.vi : Association between populations of Catantops pinguis
innotabilis and Spathosternum prasiniferum prasiniferum
in their habitat
Period Index of Affinity(%)
Index of Association
July 15, '96 40.0 -0 .6 0
July 31 61.5 -0 .3 8
Aug. 15 57.1 -0 .4 2
Aug. 31 61.5 -0 .3 8
Sept. 15 50.0 -0 .5 0
Sept. 30 50.0 -0 .5 0
Oct. 15 22.2 -0 .7 7
Oct. 31 22.2 -0 .7 7
Nov. 15 50.0 -0 .5 0
Nov. 30 50.0 -0 .5 0
Dec. 15 - -
Dec. 31 -
Jan. 15 ’97 - -
Jan. 31 -
Feb. 14 - -
Feb. 28 -
Mar. 15 — -
Mar. 31 25.0 -0 .7 5
Apr. 15 54.5 -0 .4 5
Apr. 30 36.3 -0 .6 3
May 15 28.5 -0 .71
May 31 28.5 -0 .71
June 15 33.3 -0 .6 6
June 30 40.0 -0 .6 0
for summer population. A continuous decrease in the affinity value was
recorded in captures of successive periods till May end. A second
increase in the affinity value of population was further recorded to the
extent of 40% at the end of June (table 4.vi).
4.3 (ii) Association Index :
Index of Affinity, although reveals the facts of togetherness of the
individuals of different species of a community in a natural habitat, it takes
no account of the number of the individuals involved. Association Index,
on the other hand, is based on the "Total number of individuals of each
species occurring together in samples as a proportion of the total
individuals'. W hittaker and Fairbanks (1958) formulated a method so
as to ascertain the index of association of individuals of the populations
in the natural habitats.
Table 4.vi also shows the Index of Association among the
individuals of the same two types of grasshopper populations, whose
index of affinity was also exhibited in the same table. The index of
associations always ranges between only +1 and -1. The results in the
table 4.vi show that during the different periods of study, values of Index
of Association fluctuated between these 'plus’ and 'minus’ limits of 1 (one).
However, in the present study, it was found that the association between
the individuals of the two populations remained always negative. The
computed values of the index of association fluctuated between -0.38 to
-0.77 in rainy season; and between -0.45 to -0.75 during summer
season.Thus, the two parameters of associations, such as the Index of
Affinity and the Index of Association, led to state that although
percentage of frequency of jo int occurrences of the individuals was
significantly high, but when the same phenomenon was tested on the
basis of number of individuals involved, it was found that the association
among the indivduals of the two species was not only negative but
also very poor.
4.4 DISPERSION :
Dispersion concerns with the distribution of individuals of an
animal or a plant species on the surface of the earth. For larger
species, particularly of the animals, e.g. birds, dispersion means the
distribution on the continental level, but for smaller species of animals,
e.g. grasshoppers, it means the distribution of the individuals in the area
of their occurrence. Tables 4.vii and 4.viii show the dispersion of the two
experimental grasshoppers in the study site. It is seen (table 4.vii) that
C. pinguis innotabilis was available in all parts of the study site during
its immature stages. But, as the hoppers grew to maturity and adult
stages frequented the study site, their dispersion became restricted to
generally the areas around the middle part of the forest floor. Such a
behaviour in the distribution of the members of this species was noted
in both the generations, i.e. the rainy season and the summer season
generations (table 4.vii).
Table 4.viii shows the result of the dispersion of individuals of
S. prasiniferum prasiniferum in the same site. It is seen that individuals
of this species were not available in the quadrats in about one third of
the study site, adjoining to the road with vehicular traffic during most
part of the diel period. The individuals were largely available in the
Table 4.vii : Dispersion of individuals of Catantops pinguis
innotabilis in the forest floor during 1996-97
Period 1 2 3 4 5 6 7 8 9 10
July 15, ’96 + + + + + + + + -
July 31 + + + + + + + + +
Aug. 15 + + + + + + + + + +
Aug. 31 + + + + + + + - - +
Sept. 15 - + + + + + - - + +
Sept. 30 - + + + + —"
+ + +
Oct. 15 _ — + + - + + - + +
Oct. 31 - - + + + — + — + +
Nov. 15 _ — + + + - + - + -
Nov. 30 - - - + + + + +
Dec. 15 - - - + + - ■ - + -
Dec. 31 - - - — — —
Jan. 15 ’97 - - - - - - - - - —
Jan. 31 - - - — —
Feb. 14 - - - - - - - — —
Feb. 28 - - - — —"
Mar. 15 — - - - - - - —
Mar. 31 + + + - — + +
Apr. 15 ■ + + + + + - - + ■
Apr. 30 + + + + + + + +
May 15 _ — + + • - + + -
May 31 - - + + + ■ +
June 15 — + + ■ + - — — —
June 30 — — + + + +-----------
Table 4.viii : Dispersion of individuals of Spathosternum prasiniferum prasiniferum in the forest floor during 1996-97
Period 1 2 3 4 5 6 7 8 9 10
July 15, ’96 _ — — + - + - - - -
July 31 - - - + + + +
Aug. 15 - - - + + + + - - —
Aug. 31 - - - - + + + + +
Sept. 15 — - - - - + + + + +
Sept. 30 - - - - + + + + +
Oct. 15 — - - - + - + + - —
Oct. 31 - - - - - + + +
Nov. 15 — - + - + - - + -
Nov. 30 - - + - — +“
+
Dec. 15 - - - - - - - - — —
Dec. 31 - - - - —
Jan. 15 ’97 - - - - - - - — — —
Jan. 31 - - - — —
Feb. 14 — — - + - + - - — —
Feb. 28 - - + + + + + +
Mar. 15 — + + + + - - — —
Mar. 31 - - - + + +
Apr. 15 - - - + + + - - — —
Apr. 30 - - - + + +
May 15 - - - + - + - - - —
May 31 - - — + +
June 15 - - - - + - + -
June 30 - — —+
middle part and in the inner part of the forest floor. But in this grass
hopper also, it was noticed that the adult individuals preferred to
migrate to the middle part of the study site, similar to the adults of
C. pinguis innotabilis.
4.5 CLIMATE AND POPULATION STRUCTURE :
Wellington (1957) has stated that, since populations, weather
and climate are all dynamic, a static conceptual frame-work can foster
only inept method of approaching the problem involved. The suitable or
unsuitable rainfall may bring under— optimum or over-temepratures,
thus affecting the population. The change in relative humidity, soil
moisture and vegetation structure, due to change in temeperature, all
exert a notable check upon the limits of population. The optimum diel
temperature (table 1.ii) during which the rainy season population of
both the grasshopper species was recorded at its highest (table 4.ii),
ranged between 25.0 to 28.0°C during the months of August and
September. A change in the average diel temperature of later months
has undoubtedly caused its adverse effect on the population of both
the grasshopper types.
The second important factor, determining the population structure,
is rainfall. In fact, rainfall and temeprature influence the relative
humidity and soil moisture of the terrestrial ecosystem. The fortnightly
average of diel relative humidity during which the individuals were
recorded in the study site, ranged between 77.5 to 85.5% although
the limits of maximum and minimum relative humidity in this period
fluctuated between 95 ± 4.07 to 63 ± 8 .14/o.
EMPIRICAL FINDINGS :
An attempt has been made to calculate the Karl Pearson s
correlation coefficient ( r ) between mean density and climatic factors,
such as, temperature and relative humidity for the periods when the
species were present in the aboveground part of the forest floor. The
result of these findings, along with their values, is illustrated through
tables 4.ix and 4.x. The values have been further matched with critical
values to strengthen the theoretical findings in the following accounts.
4.5 (i) Mean Density and Temperature : (Table 4 ix)
(a) Catantops pinguis innotabilis :
The computed value of ' r ’ for rainy season population was
found to be 0.59 at n-2(9) degree of freedom. Its appended value is
0.60 which suggests that there exists a significant correlation between
mean density and population of the rainy season. The correlation
computed for summer season generation revealed that V value drawn
at n-2(5) degree of freedom was -0.45, which was much less from
its appended value (0.75) at 5% probability level. It indicated a poor and
negative correlation between the temperature and population of summer
season.
(b) Spathosternum prasiniferum prasiniferum
The calculated value of V for rainy season population of the
insect was 0.34 at n -2 or 8 degree of freedom. This value was much
less than its critical value of 0.63 at 5% probability level. The ramy
season population, thus, showed only a poor correalticn with the
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computed value of 'r' was -0.49 at n-2 or 8 degree of freedom. Its critical
value at 5% probability level was 0.63. Thus a poor negative correlation
existed between mean density and temperature.
4.5 (ii) Mean Density and Relative Humidity : (Table 4.x)
(a) Catantops pinguis innotabilis :
The computed value of coefficient of correlation at n-2 degree
of freedom was 0.88 while its critical value at 5% probability level was
0.60. The statistics, thus, showed that a strong correlation existed
between the mean density and relative humidity of the population
during rainy season.
In the summer season, the computed value drawn between the
same two variables was obtained as -0.34. However, its critical value
at 5% probability level is 0.75 which is much higher than the computed
value. The statistics suggests that the summer season population is
only slightly influenced by relative humidity of the period.
(b) Spathosternum prasiniferum prasiniferum
The coefficient of correlation drawn for the rainy season population
of this species gave the computed value of V as 0.79. Its critical value
at 5% probability level has been 0.63. The computed value was, thus,
sufficiently above the critical value. Like Catantops pinguis innotabilis..
population of Spathosternum prasiniferum prasiniferum was also signi-
ficantly and positively influenced by temeperature.
When the same statistics was drawn for summer season popula-
tion of the insect, it was found that computed value of 'r' was -0.19
while the critical value for the same degree of freedom and at the 5%
probability level was 0.63. This suggested that the population of the
summer season is hardly influenced by the relative humidity of that
period.
4.6 DISCUSSION :
The study of population structure and population dynamics was
conducted on the two grasshopper species, namely, C. pinguis innotabilis
and S. prasiniferum prasiniferum . Both these species belong to the
short-horned category of grasshoppers occurring from the superfamily
Acridioidea but the form er belonged to family Catantopidae, while the
latter, to fam ily Hemiacrididae.
4.6 (i) Species Richness :
Appendix Table-1 shows the number of individuals of each
stage of the grasshopper C. pinguis innotabilis and Appendix Table-2
exhibits similar data for S. prasiniferum prasiniferum. The total number
of individuals collected on each turn from individual and all 10 quadrats,
is shown for both the species in respective tables. The data revealed
that population of C. pinguis innotabilis was much richer than
S. prasiniferum prasiniferum in both the generations, as is supported from
the fact that the highest number of individuals captured in ten quadrats
was 58 in case of C. pinguis innotabilis whereas the highest number
of S. prasiniferum prasiniferum in the same number of quadrats was only
17. These were the values of rainy season generation. The richness of
both the species in the forest floor in the summer season generation
decreased to approximately half number, compared to their richness in
rainy season.
4.6 (ii) Average Density :
Observation on the average density of the two species suggested
that during rainy season, population density of both the grasshoppers remained
higher, com pared to tha t in summer season. It was also
noted that individuals of C. pinguis innotabilis were always found in
higher number and those of S. prasiniferum prasiniferum in lesser
number. In winter season, both the populations disappeared from the
aboveground part and underwent in diapause. However, with the advent
of summer season, population of both the species of grasshoppers
again emerged in the aboveground part, first that of S. prasiniferum
prasiniferum (Feb., 14) and then, of C. pinguis innotabilis (March, 31).
Study of the other attributes further suggested that :
(1) Both the species possessed bivoltine character, completing
two generations in the same year. Nzekwu and Akingbohungbe (1998)
have reported that grasshopper Cedaleus niseriunsis appears in two
different seasons of the year, completing its two generations; and thus
shows a bivoltine character. Patel (2001) in his studies on grass
hoppers of a forest floor has recorded not only bivoltine but also many
monovoltine and a few trivoltine grasshoppers.
(2) C. pinguis innotabilis led a shorter epigean period
(9 months) and S. prasiniferum prasiniferum a longer (10 months)
epigean life.
(3) The lower average density and shorter life span of both
the grasshoppers in summer season suggested that all the eggs laid
by rainy season females did not hatch in summer period and some
clutches rem ained in d iapause during entire w inter and summer
season. They hatched only after the break of a monsoon at the end of
summer season.
(4) The shorter life span of summer generation compared to
rainy season generation also suggested that higher atmospheric temp
erature and dry winds influenced the metabolic rates of the individuals
and helped them reach to maturity in lesser time.
4.6 (iii) Percentage Frequency :
The data of percentage frequency (Table 4.iii) suggest that of
the two grasshoppers, members of C. pinguis innotabilis, soon after their
emergence, frequented almost all the quadrats. When its population grew
to maturity, the individuals showed a tendency of movement towards the
middle part of the forest floor. This behaviour was noticed for the species
in its both the generations (Table 4.vii). Population of S. prasiniferum
prasiniferum showed restricted distribution and limited frequency in the
study site. Its members never frequented all the ten quadrats in the
entire study period. Table 4.iii also suggests that individuals of this
grasshopper avoided the marginal areas of the study site and preferred
to live in the inner part of the habitat.
4.6 (iv) Relative Frequency :
Observations into relative frequency (Table 4.iv) revealed that at
the time of the first census, in the hot dry climate of mid-June, only
adults of the experimental species occurred in the aboveground part.
C. pinguis innotabilis showed a frequency of 66.6% and S. prasiniferum
prasiniferum 33.3%. W ith the emergence of new generation in the
month of July a change in the relative frequencies of both the grass
hoppers became apparent and this change continued throughout the
entire period of the first generation. It may be stated that, in this
generation, as the population grew to maturity comprising largely of adult
individuals, the relative frequency of C. pinguis innotabilis gradually
decreased and that of S. prasiniferum prasiniferum increased. Almost a
similar trend was also found in the relative frequencies of these grass
hoppers even in their second generation. It may, further, be stated that
relative frequency of C. pinguis innotabilis remained always higher
compared to that of S. prasiniferum prasiniferum in both the generations.
4.6 (v) Relative Density :
Analysis of the relative density values of the two grasshopper
populations (Table 4.v) revealed that C. pinguis innotabilis formed the
larger proportion in the two experimental species. In rainy season, the
highest proportion of 92 percent of individuals of C. pinguis innotabilis
was found at the time of emergence of population in mid-July. The
highest proportion of S. prasiniferum prasiniferum population was only
32 percent which was recorded in mid-September. In the second
generation (summer season) the highest value of relative density was
90.4 percent recorded again for C. pinguis innotabilis in mid-May. This
value, for S. prasiniferum prasiniferum was only 33.4 percent, recorded
at the end of March. Analysis of relative density suggested that
C. pinguis innotabilis constituted a greater proportion while S. prasiniferum
prasiniferum a lesser proportion among the population of the two
grasshoppers in both the generations.
4.6 (vi) Association between Populations :
One of the most persistent belief in ecology is that, not all
species populations are independently distrbuted in a community. The
populations of plant and animals in an area exist in an organised,
distinct entity. They show significant association, both positive and
negative, between many species of plants and animals due to interaction
between the species, and due to similar responses of species to the
same environmental variables. The observation on association and its
measurement was taken up using two parameters, namely, Index of
Affinity and Association Index. The former indicates the degree of
togetherness between the members of population and the latter shows
the proportion of individuals living together. As exhibited in Table 4.vi,
highest degree of joint occurrence (61.5%) was found at the end of July
and August when the populations comprised of immature individuals.
The degree of joint occurrences between the populations of summer
season was recorded highest (54.5%) at the end of April. The degree
of joint occurrences decreased in the successive periods of the study.
However, degree of the joint occurrences suddenly increased around
the last days of aboveground existence of the populations, in both the
generations. The highest degree of joint occurrences was recorded
when the population comprised of immature individuals. The sudden
increase in the degree of togetherness was recorded when the
population comprised of only adult individuals. The facts of the result
permit to infer that immature individuals show lesser movement in the
habitat and prefer to confine around the area of their emergence. Decrease
in the degree of togetherness in the consecutive periods suggested
that, with their growing maturity the individuals move to other parts of the
habitat. However, the sudden increase in the degree of togetherness
as exhibited by adult individuals suggested that the adult insects try
to conglomerate to the safer areas of their habitat. This, they do with
the inherent purpose of fu lfilling the reproductive needs.
Data of Index of Association, on the other hand when analysed,
were always found in negative (Table 4.vi), in both the generations. When
tested for their significance these values were found significant on most
of the occasions. It suggested that the members of the two populations
of the grasshoppers move independently in the ecosystem and are not
influenced by each other.
4.6 (vii) Dispersion :
The dispersion pattern of the population, i.e. the positions of the
individuals in the environment, at any instant represents the picture of
births, deaths and movements of the individuals. As stated by Poole
(1974), in the field of population it is difficult to define sharply the
population interaction, but sometimes, by observing the dispersion
pattern of the individuals, some insight into the biological characteristics
of the species and reasons behind the changes in density of the
population can be gained. Tables 4.vii and 4.viii exhibit the dispersion
of the two grasshoppers in the study site. It may be inferred from the
dispersion picture of the tables that in the present investigation, population
of S. prasiniferum prasiniferum existed in an aggregated pattern, while
that of C. pinguis innotabilis existed in a nearly regular dispersion
pattern. Further, as inferred in the above para (Association), the
individuals of the two populations of the grasshoppers occurred in
contagious distribution.
4.6 (viii) Climate and Population :
Seasonal climatic records of temperature, humidity and rainfall
(Table l. i i i) have been studied in the perspective of the population
dynamics of the grasshoppers. These records revealed that in rainy
season the averages of maximum and minimum temepratures fluctuated
within a narrow range and atmospheric air moisture (R.H.) remained
consistently high in this period. These conditions changed in the winter
and summer seasons, which are generally characterised with cold-dry
and hot-dry atmosphere respectively. The soil surface temperature first
gradually decreases (in winter), then increases (in summer). The soil
moisture content sharply decreases, converting the surface soil hard and
dry. The herbal strata of vegetation, making the soil surface, look fully
green in rainy season also wither in the post-monsoon months, leaving
only some green patches in the forest floor in its middle region.
Edwards (1964), Hewitt (1979), Joern and Gaines (1990) are of
the view that temperature and precipitation have both direct and indirect
effects on grasshopper development and survival. Skinner and Child
(2000) hold the view that extensive body of earlier researches on the
relationship between the grasshopper abundances and abiotic factors
have not yielded satisfactory models for predatory grasshopper changes.
Relationship between grasshoppers' number and hot-dry weather has
been recognised by many earliest observers (Riley, 1877; USEC, 1878,
1880). However, attempts with little success have been made to
establish the correlation between grasshopper densities and weather
conditions, and the same has been confirmed by numerous late studies,
e.g. Edwards (1960), Gage & Mukherjee (1977), Capinera & Horton
(1989), Lockwood & Lockwood (1991), Belovasky and Slade (1995).
In the present finding, it was seen that population densities of
both the grasshoppers remain high in rainy season. The climatic
conditions of this season perhaps provided most optimal condition for
the growth and development of grasshopper populations. The popu
lation dynamics showed a significant and direct correlation with the
temperature and relative humidity (Tables 4,ix and 4.x). As the abiotic
conditions of winter season progressed towards cool and dry side, the
dryness of soil and wind naturally exerted their resultant effect on the
phenology and texture of the ground vegetation. Withering of large
number of grasses, sedges and herbs started at this time bringing
dryness and death of their several species. Atmospheric conditions of
summer season not only rendered the air further dry but also changed
it into hot dry. Soil surface temperature naturally increased. Heating of
the site-habitat, due to the increased exposure of soil surface further
increased dryness of the forage. Changes in plant cell structure,
consequence of environmental stress, caused a gradient of favourable
forage loss (Schell & Lockwood, 1996). This further rendered the situation
unfavourable. The combined effect of all these abiotic and biotic changes
brought a quickened decline of the present population. It was abundently
evident from the population data of the summer season that both the
species com plete ly vanished from the aboveground part in quick
succession. It may also be inferred that individuals of rainy season
population were stenotherm ic while those of the summer season
population were eurithermic. Stenothermic generation showed strong
and positive correlation with temperature and relative humidity while
eurithermic generation of the same species showed a poor and negative
correlation. The euritherm ic species showed less dependence on
temperature and relative humidity changes of summer season and hence
were able to survive in the study site in this season also. The result also
agrees with the earlier view of Lockwood et. al (1994) who have reported
that grasshopper species are not known to overwinter as nymphs on
the Asian steppes.
However, the author finds weightage in the statement of
Fellaouine & Louveaux (1994), according to whom, drying out of
vegetation caused a directional movement in the grasshopper towards
the green patches of the vegetation under the weathering edge-effect.
The author also feels credible the reports of Skinner and Child (2000),
according to whom, univariate analysis is not adequate to arrive at a final
result, hence multivariate methods must be used to investigate relation
ship which are based on interaction among factors. Suggestions of
Carter & co-workers (1998) which says that eurithermic effects of
weather are more important in the maintenance of high densities of
the insects.In the end, the author agrees with the earlier view of Joshi et. al
(1999) who have told that different forest ecosystems may require
independent faunal surveys, as they support different species of grass
hoppers, and aptly feels that surveys of the natural forest in this
forest necklaced district should be encouraged so as to find a complete
list of the group Orthoptera on the one hand, and their biodiversity,
eco-adaptation, economic role on the other.