SPECIES DIVERSITY
Running Head: SPECIES DIVERSITY
Intermediate Disturbance Creating Species Diversity
Jordan Masse
Wayne State University
ENG 3010, Section 001
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SPECIES DIVERSITY
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Abstract
Environments do change gradually, which probably results in changes in species
composition since disturbances frequently interrupt the competitive process. Disturbances are the
primary force in the maintenance of species diversity in many ecosystems. These disturbances
prevent most communities from ever reaching a state of equilibrium. Joseph Connell (1979) was
the first scientist to propose the term intermediate disturbance hypothesis which states that
diversity is higher when disturbances are intermediate with regards to both frequency and
intensity. After decades of research, it is now clear that without perturbation, species
composition cannot persist. In recent years it has become clear that high diversity is a
consequence of constantly changing conditions. In a natural community, equilibrium is seldom
attained. Disruptions are so common that species assemblages seldom reach an ordered state.
Communities of competing species are not highly organized by co-evolution into systems in
which optimal strategies produce highly efficient associations whose species composition is
stabilized. Scientists believe that without perturbations, diversity would not be as rich.
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Concepts of disturbance, colonization and early development or succession have been a
source of considerable interest to ecologists for more than a century. Back in the 1970’s,
researcher J. H Connell began to hypothesize whether species populations can maintain
equilibrium. Connell (1979) proposed many hypotheses such as the intermediate disturbance
hypothesis and the gradual change hypothesis to explain how local diversity is produced or
maintained in tropical environments. Connell’s main argument was that without perturbation,
species composition cannot persist, however, after perturbation it is restored nearly to the
original state. In recent years it has become clear that high diversity is a consequence of
constantly changing conditions. Several researchers have concluded that the frequency of natural
disturbance and the rate of environmental change are often much faster than the rates of recovery
from perturbations. Scientists believe that without perturbations, diversity would not be as rich.
Disturbances are the primary force in the maintenance of species diversity in many
ecosystems. Wayne Sousa (1985), a key researcher of disturbance to biological systems, has
defined disturbance as damage to one or more individuals that directly or indirectly creates an
opportunity for new individuals to become established (Sousa, 1985). Connell (1979) was the
first scientist to propose the term intermediate disturbance hypothesis. The intermediate
disturbance hypothesis states that diversity is higher when disturbances are intermediate with
regards to both frequency and intensity (Connell, 1979). The best evidence comes from studies
of ecological succession. Disturbances often create open patches within a community providing
space for successional growth. Soon after a severe disturbance, seeds of a few species arrive in
the open space. Diversity is low because the time for colonization is short. Only those few
species that happen to be producing seeds and are within dispersal range will colonize. Another
key factor is the frequency of these disturbances. If disturbances persist, the community will
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consist only of those species capable of quickly reaching maturity. Disturbances interrupt and set
back the process of competitive elimination. However, without disturbances, the competitor that
is the most efficient in exploiting limited resources will eliminate competition. Therefore,
communities are maintained within a non-equilibrium state.
Connell (1979) explained how mixed tropical forests represent a non-equilibrium stage in
a succession after a disturbance. Since mixed rain forests are common in the tropics, this
suggests that disturbance is frequent enough to maintain much of the region in the nonequilibrium state. Mixed forests occur in the places most likely to have been disturbed by man,
whereas single-dominant forests occur in areas of little perturbation rather than a result of biotic
factors. Molino and Sabatier (2001) further studied the intermediate disturbance hypothesis with
a sample of 17,000 trees in a Guianan forest. This study was conducted 10 years after multiple
experiments that added to natural disturbance to the forest populations. The reason there is a
great level of diversity within the 17000 trees is because of these natural disturbances 10 years
ago. The population is composed of several different species altering its once single dominant
characteristic to a mixed forest. It is clear that the highest diversity is maintained at intermediate
scales of disturbance.
Brown and Gurevitch (2004) explain how ecological perturbations can either be
necessary for maintaining tropical forest diversity or responsible for its decline, depending on the
nature of the disturbance. Anthropogenic disturbances such as logging and subsistence
agriculture promote the establishment of nonnative, invasive plant species. This potentially
affects forest structure and diversity even long after the perturbation has ceased. Temporal and
spatial scales of disturbance affect forests greatly. Disturbances over a range of scales are critical
determinants of tropical forest composition, and the disturbance regime of tropical forests can be
SPECIES DIVERSITY
5
essential in maintaining community structure. The effect of selective logging on native tree
diversity in tropical forests acts as an intermediate disturbance and allows invasive species to
grow. These disturbances facilitate colonization and establishment of invasive species because
the natural populations are quickly being removed. Diversity within the tropical rain forests is
increasing drastically as a result of anthropogenic disturbances.
Sanford et al. (1985) described how forest fires within the Amazon are subject to
diversity because of local disturbances. The primary topic of this article is how natural and
human-caused fires have altered the Holocene vegetation within the Amazon (Sanford et al.
1985). Sanford et al. (1985) proposed that fire can be considered moderate level disturbances
within the tropical rain forests which maintain high diversity. Grassland and savannahs are areas
with lengthy dry seasons that burn most often. Under these drier climatic regimes, wildfires have
destroyed large areas of forest which resulted in many large successional forests. Large amounts
of charcoal were found in the forest soils. The abundance of charcoal of commonly found in
these rain forest soils suggests that fire has for a long time been a disturbance factor in these
tropical forests. These perturbations have modified the forests during the Holocene as a result of
different climatic circumstances. Therefore, without these wild forest fires natural succession
would not ensue. Wild fires reduce single-species forest and are the result of must diversity
found within the savannahs.
The relation between disturbance and diversity to the tropical forests is similar to the
coral reef. Coral reefs are the largest biological structures made by living organisms (Connell,
1979). They are similar to tropical rain forests and are characterized as one of the most diverse
marine ecosystems. Hongo and Kayanne (2009) focus their research on spatial and temporal
variations with regards to relationships between species diversity and reef growth in the
SPECIES DIVERSITY
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Holocene at Ishigaki Island, Pacific Ocean. Species richness in areas of high tropical diversity
can be a result of speciation, extinction, and dispersal. Upper portions of most coral reefs
actually lie in the intertidal zone and extend to depths far below the intertidal zone. There is a
wide range of species diversity possible over this region because of disturbance and productivity
gradients.
The nature of the physical environment according to different zones in the coral reef
provides a heterogeneous environment. Several methods and equations have been used to test
species diversity such as the Simpson's Index (1949) and the Shannon and Weaver's Index
(1948). In ecology, these methods are often used to quantify the biodiversity of a habitat. They
take into account the number of species present, as well as the relative abundance of each species.
These indexes represents the probability that two randomly selected individuals in the habitat
will not belong to the same species. The purpose of this experiment was to investigate the
relationship between species diversity and the patterns of reef growth during the Holocene. The
western Pacific has the highest species diversity in the world with more than 70 genera of corals
(Hongo and Kayanne 2009). Recovered drilling cores were recovered to examine spatial and
temporal changes in the coral community. Coral reefs are generally exposed to moderate-sized
waves originating from the east Pacific swell causing distinct biological zonation. Hongo and
Kayanne (2009) stated that the region with highest species diversity is found on the crests and
outer slopes that are exposed to damaging storms. In this region hurricanes destroyed much coral
found within the outer slopes but failed to damage the shallow lagoon protected by an adjacent
reef. The disturbed areas have been recolonized by many species after each hurricane.
Disturbances prevented competitive exclusion which maintains species diversity. Disturbances
SPECIES DIVERSITY
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caused by the natural disasters and by predation remove corals and then recolonization by many
species follows.
The gradual change hypothesis states that gradual environmental changes alter the
ranking of competitive abilities and the process of competitive elimination is rarely ever
completed. Connell (1979) postulates that no species has time to eliminate others because of
changes in the physical environment. Over hundreds of years, gradual changes in climate cause
variation within communities, especially to sessile organisms such as trees or corals. Species
composition seen in present forests and reefs depends on the rate of competitive elimination as
well as the rate of environmental change. Species would not co-exist if the time required for one
species to eliminate another in competition is much shorter than the time taken for an
environmental change (Connell, 1979). Slower changes in the physical environment would not
maintain diversity while greater rates would increase species diversity.
Santelices (2007) explains how kelp forests alter the physical environment by modifying
light penetration which influences species diversity. Kelp forests are regarded as highly
productive and highly diverse marine ecosystems. There are hundreds of associated species; 35%
of 275 common taxa found in a given forest are associated with the kelp. Kelps also provide
shelter, food, and nursery grounds for many benthic organisms. Kelps require a minimum annual
penetration of sunlight to support photosynthesis and their growth can be severely limited by the
availability of dissolved nitrogen. Santelices (2007) explains how surface kelp canopies may
deteriorate in summer when dissolved nitrogen reaches levels below 1 micromolar. Kelp
canopies reduce light which create favourable conditions to species found in the understory.
Several species are adapted to low light intensity. By altering light penetration throughout the
kelp communities conditions quickly change which affects competitive interactions among
SPECIES DIVERSITY
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various species. Without these natural disturbances, organisms found within associated kelp
forests would not be able to flourish. Only those adapted to darker conditions will be favoured
which promotes competitive elimination.
Jackson et al. (1989) explains how on 27 April 1986, more than 8 million litters of crude
oil spilled into several region of mangroves, seagrasses, and coral reefs just east of the Caribbean
entrance to the Panama Canal. Back in 1986, this was the largest spill into coastal habitats in the
tropical Americas. Jackson et al. (1989) studied many populations of plants and animals in both
oiled and unoiled sites which provided documentation of the spread of oil and its biological
effects. The intermediate disturbance hypothesis states that diversity is higher when disturbances
are intermediate with regards to both frequency and intensity (Connell, 1979). An oil spill
however, can be considered a large disturbance in which nearly all plants and animals died
wherever they came in contact with oil. By September 1986, a band of dead or dying trees
marked the zone where oil washed ashore between Punta Galeta and Islas Naranjos. The
organisms that came in contact with the oil and survived are able to successfully pass their traits
to future progeny. A disturbance presents new physical conditions within the environment in
which species with different advantageous traits can prosper (Darwin, 1859). Adaptation is the
key factor in survival because the physical environment houses thousands of species before and
after disturbance. Quickly after a perturbation, communities quickly adapt and return to a near
stable equilibrium. Natural selection ensues which diversifies species found within the
community. Jackson et al. (1989) explains that within similar habitats found at nearly the same
distance from the oil spill, it caused considerable variation within species. These successful
organisms could then pass the beneficial genes to their offspring. The spill harmed prominent
organisms and its biological effects were usually devastating to the natural environments.
SPECIES DIVERSITY
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There are several tests that need to be conducted in order to properly evaluate
intermediate levels of disturbance. Researchers must first hypothesize new ideas corresponding
to their data. Before a natural disturbance alters the species richness within the community,
researchers must count all viable organisms found in their test site. This is essential because now
researchers can compare data from before the intermediate disturbance and evaluate how it
altered species composition. The environment plays a major role in species diversity; therefore,
several test sites are needed to eliminate bias within the test group. Furthermore, it is important
to conduct the experiment at multiple test dates. Researchers must also conduct research at
various times after the intermediate level disturbance in order to properly understand the
biological effects imposed by the natural disaster. These various methods can all be found in the
following experiment conducted by Losos et al. (1998).
In October of 1996, Losos et al. (1998) had just finished a census of lizard and spider
populations on 19 islands surrounding the Bahamas. The purpose of their experiment was to
study the long-term ecological effects of various lizard and spider species. During the morning of
19 October, Hurricane Lili passed directly over their study site with sustained winds of 90 knots.
As soon as the storm passed, Losos et al. (1998) recounted their lizard and spider populations on
all the islands. Hurricane Lili allowed researchers to evaluate several propositions concerning the
impact of disturbances on different types of organisms. Their data concluded that larger
organisms may be more resistant than smaller ones to the immediate impact of a moderate
disturbance. For Losos et al. (1998), the data show that the larger lizards were more resistant
than the smaller spiders to the immediate impact of the moderate disturbance on the protected
side. Lizards may have been able to withstand the physical forces of the disturbance or to find
protective cover better than spiders. Furthermore, surviving smaller species may recover faster
SPECIES DIVERSITY
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because their reproductive rate is higher than that of larger species. Spiders are generally more
fecund and have shorter generation time which allows a quicker recovery than lizards. Losos et
al. (1998) also found that for moderate disturbances, the risk of extinction is a function of
population size, whereas no such relationship exists for catastrophic disturbances. Lastly, these
researchers noted that when all populations are exterminated by a catastrophic disturbance, the
recovery rates of different species will be largely determined by their dispersal abilities.
The results on the exposed islands 1 year after the hurricane corresponded with the
organism’s ability to disperse and colonize new habitats. Although the number of spider
individuals averaged about one-third of the prehurricane value, Losos et al. (1998) explained
how the mean number of spider species rebounded to exactly the same value as before the
hurricane. Many small islands were quickly recolonized where spiders can multiply in number.
This study illustrates that after a catastrophic disturbance, the recovery rate among different
types of organism’s increases strongly with dispersal ability.
Methods
Participants
The Caribbean Sea is home to about 10% of the world's coral reefs covering about
18,000 square miles, most of which are located off the Caribbean Islands (Curran, 2009). The
purpose of this experiment is to evaluate species diversity before and after a natural disaster. The
animal participants through this experiment are various types of coral. Found within the
Caribbean Sea are more than 65 different corals. Within those 65 different corals, 4 were
selected to be used for this experiment: Euphyllia divisa, Leptoseris kalayaanensis, Astreopora
myriophthalma, Australogyra zelli (Fig.1.). These corals were selected from the 65 because they
have fast growth rates and they reproduce quickly.
SPECIES DIVERSITY
Fig. 1.
Euphyllia divisa
Leptoseris kalayaanensis
Astreopora myriophthalma
Australogyra zelli
Within the sampling area on both northern and southern sides, twenty different adult
corals were selected of the four corals tested. Sex plays no important role in species diversity.
When selecting corals, it can either be male or female.
Ecological analyses
Several methods and equations have been used to test species diversity. The index used
primarily throughout this experiment was the Shannon and Index of diversity (H') which often
quantifies the biodiversity of a habitat (Fig. 2.). The purpose
of calculating H' was to investigate the relationship between
species diversity and the patterns of destruction due to
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hurricanes. In order to analyze the species diversity, researchers must understand each variable
within the equation. Pi is the proportion of a particular species to the total number of individuals
found within the population. S is the total number of species within the area. The larger the H'
value, the more diverse the coral community.
Materials
For this experiment there were very few materials needed. In order to identify various
types of corals, Scuba gear along with a flash light are needed. Research must dive in the
sampling area and count the various corals found. From the 65 different corals in the Caribbean
Sea, researchers must be familiar with Euphyllia divisa, Leptoseris kalayaanensis, Astreopora
myriophthalma, Australogyra zelli. The ability to identify the selected corals is essential in order
to properly quantity species diversity at the U.S Virgin Islands.
Procedure
The United States Virgin Islands are located in the western Atlantic (18° 21" North, 64°
56" West), approximately 64km east of Puerto Rico (Fig. 3.). The Caribbean Sea, specifically
near the U.S Virgin Islands, is flourished with various types coral species. The Eastern Atlantic
has some of the highest species diversity in the world. There are more than 65 different corals
found within that region of the Caribbean Sea. The average sea-surface temperature is 23 °C in
winter and 27 °C in summer. The tide is semidiurnal, with a spring range of 2.0 m, and the mean
low-water level is 1.0 m below mean sea level. These ecological conditions help promote fast
growing coral reefs. The Caribbean Sea located at the U.S Virgin Islands is also a hot spot for
hurricanes during the months of May through June. The northern side of the island is protected
from hurricanes due to cooler water conditions, while the southern side is struck by a hurricane
every third year (Fig. 3.). Hurricanes can be considered a natural disaster which can help
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promote species diversity found in the sea. There were two sampling area of 50 m2 found on
both the northern and southern sides of the U.S Virgin Islands.
The preferred sampling time is one year after a hurricane strikes the Caribbean Sea. Once
the hurricane hits the southern side of the U.S Virgin Islands, time is needed for corals to
reproduce and flourish in their nature habitat. The northern protected side acts as a control to
compare species diversity found after a natural disaster. One year after the hurricane has struck
the sampling area, researchers must then count the first 20 corals of the 4 selected species. Proper
identification must be done in order to properly compare species diversity found within both the
northern protected side and the southern hurricane hot zone.
Fig. 3.
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Results
Northern Community
 Yellow - Euphyllia divisa.
 Orange - Leptoseris kalayaanensis.
 Purple - Astreopora myriophthalma.
 Brown - Australogyra zelli.
Table 1
Northern Community
Species
Euphyllia divisa
Leptoseris kalayaanensis
Astreopora myriophthalma
Australogyra zelli
Total
Abundance
17
1
1
1
20
Proportion (pi)
0.85
0.05
0.05
0.05
1.00
ln (pi)
-0.163
-2.996
-2.996
-2.996
pi ln (pi)
-0.139
-0.150
-0.150
-0.150
-0.586
Southern Community
 Yellow - Euphyllia divisa.
 Orange - Leptoseris kalayaanensis.
 Purple - Astreopora myriophthalma.
 Brown - Australogyra zelli.
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Table 2
Southern Community
Species
Euphyllia divisa
Leptoseris kalayaanensis
Astreopora myriophthalma
Australogyra zelli
Total
Abundance
5
5
5
5
20
Proportion (pi)
0.25
0.25
0.25
0.25
1.00
ln (pi)
-1.386
-1.386
-1.386
-1.386
pi ln (pi)
-0.347
-0.347
-0.347
-0.347
-1.388
Discussion
The intermediate disturbance hypothesis states that diversity is higher when disturbances
are intermediate with regards to both frequency and intensity (Connell, 1979). The results
indicate that the hypothesis was correct. It has become clear that high diversity in the Caribbean
Sea surrounding the U.S Virgin Islands is a consequence of constantly changing conditions due
to the hurricanes. Within the northern community, researchers found a single dominate species.
Seventeen Euphyllia divisa corals were found on the northern side while only one Leptoseris
kalayaanensis, Astreopora myriophthalma, Australogyra zelli were found. Furthermore, the H'
value for the northern community was 0.586. This is lower than that of the southern community.
From these findings, it is clear that without perturbations, diversity would not be as rich. The
most efficient competitor is exploiting limited resources which will then eliminate the
competition.
Hurricanes create open patches within a community providing space for successional
growth. Disturbances interrupt and set back the process of competitive elimination. This can be
seen within the southern community. Unlike the northern community, the abundance of the four
SPECIES DIVERSITY
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species was even. Euphyllia divisa , Leptoseris kalayaanensis, Astreopora myriophthalma,
Australogyra zelli were all counted 5 times in the sampling area. The H' value for the southern
community was 1.388. This value is much higher than the northern community which proves that
disturbances caused by the natural disasters remove corals and then recolonization by many
species follows (Hongo and Kayanne, 2009). Disturbances prevented competitive exclusion
which maintains species diversity.
Environments do change gradually, which probably results in changes in species
composition since disturbances frequently interrupt the competitive process. These disturbances
prevent most communities from ever reaching a state of equilibrium. The changes maintain
diversity by preventing the elimination of lesser competitors. Natural selection fits and adjusts
species into an ordered system (Connell, 1979). Therefore, ecological communities are highly
organized and developed in order for species to maximized efficiency. Tropical rain forests and
coral reefs are generally regarded as very ordered systems. Within a local area, there are usually
enough variations in habitats and resources to enable several species to coexist at equilibrium as
a result of niche differentiation. In the contrasting view, equilibrium is seldom attained.
Disruptions are so common that species assemblages seldom reach an ordered state.
Communities of competing species are not highly organized by co-evolution into systems in
which optimal strategies produce highly efficient associations whose species composition is
stabilized (Hongo and Kayanne 2009).
References
SPECIES DIVERSITY
17
Brown, K. A., Gurevitch, J. (2004). Long-term impacts of logging on forest diversity
in Madagascar. National Academy of Sciences of the USA, 101, 6045–6049.
Connell, H. J. (1979). Diversity in tropical rain forests and coral reefs. Science, 199(1), 14-32.
Curran, M., Hosein, N. (2009). Caribbean student environmental alliance.
http://www.caribbean-sea.org/home.htm
Darwin, C. (1859). The Origin of Species. London: John Murray.
Hongo, C., Kayanne, H. (2010). Relationship between species diversity and reed growth in the
Holocene at Ishigaki Island, Pacific Ocean. Sedimentary Geology, 223(1), 86-99.
Jackson, J. B. C., Cubit, J. D., Keller, B. D., Batista, V. (1989). Ecological effects of a major oil
spill on Panamanian coastal marine communities. Science, 243(1), 37-44.
Losos, J. B., Schoener, T. W., Spiller, D. A. (1998). Impact of a catastrophic hurricane on island
populations. Science, 281(1), 696-697.
Molino, J. F., Sabatier, D. (2001) Tree diversity in tropical rain forests: A validation of the
intermediate disturbance hypothesis. Science, 294(1), 1702-1703.
Sanford, L. R., Saldarriaga, J., Clark, E. K. (1985). Amazon rain-forest fires. Science, 273(1),
167-173.
Santelices, B. (2007). The discovery of kelp forests in deep-water habitats of tropical regions.
National Academy of Sciences of the USA, 104, 42-43
Sousa, W. P. (1985). Disturbance and patch dynamics on rocky intertidal shores. Ecology of
natural disturbance and patch dynamics, 164(1), 101–124.
Jordan Masse
SPECIES DIVERSITY
18
Jule Wallis
ENG 3010
07/18/2010
Annotated Bibliography for Project Three
Brown, K. A., Gurevitch, J. (2004). Long-term impacts of logging on forest diversity
in Madagascar. National Academy of Sciences of the USA, 101, 6045–6049.
Brown and Gurevitch (2004) explain how ecological perturbations can either be
necessary for maintaining tropical forest diversity or responsible for its decline, depending on the
scale, nature, and frequency of the disturbance. Anthropogenic disturbances such as logging and
subsistence agriculture may promote the establishment of nonnative, invasive plant species
potentially affecting forest structure and diversity even long after the perturbation has ceased.
Temporal and spatial scales of disturbance affect forests greatly. Disturbances over a range of
scales are critical determinants of tropical forest composition, and the disturbance regime of
tropical forests can be essential in maintaining community structure. The effect of selective
logging on native tree diversity in tropical forests acts as an intermediate disturbance and allows
invasive species to grow. These disturbances facilitate colonization and establishment of invasive
species because the natural populations are quickly being removed. Diversity within the tropical
rain forests is increasing drastically as a result of anthropogenic disturbances.
Connell, H. J. (1979). Diversity in Tropical Rain Forests and Coral Reefs. Science, 199(1), 14-32.
Back in the 1970’s, researcher J. H Connell began to hypothesize whether species
populations can maintain equilibrium. Connell proposed many hypotheses to explain how local
diversity is produced or maintained in tropical environments. Connell’s main argument was that
without perturbation, species composition cannot persist, however, after perturbation it is
restored nearly to the original state. In recent years it has become clear that high diversity is a
consequence of constantly changing conditions. The frequency of natural disturbance and the
rate of environmental change are often much faster than the rates of recovery from perturbations.
Connell believes that without perturbations, diversity would not be as rich. Connell was the first
scientist to propose the term intermediate disturbance hypothesis. The intermediate disturbance
hypothesis states that diversity is higher when disturbances are intermediate with regards to both
frequency and intensity (Connell, 1979). The best evidence comes from studies of ecological
succession. Soon after a severe disturbance, seeds of a few species arrive in the open space.
Diversity is low because the time for colonization is short. Only those few species that happen to
be producing seeds and are within dispersal range will colonize. If disturbances persist, the
community will consist only of those species capable of quickly reaching maturity. Disturbances
interrupt and set back the process of competitive elimination. However, without disturbances, the
competitor that is the most efficient in exploiting limited resources will eliminate competition
(Connell, 1979). Therefore, communities are maintained within a non-equilibrium state.
Darwin, C. (1859). The Origin of Species. London: John Murray.
SPECIES DIVERSITY
19
Individuals having any advantage over others due to a slight variation can potentially
have a better chance of surviving and passing their traits to next generation. This preservation of
favorable variation and the rejection of non-favorable traits are known as natural selection.
Variability occurs in different condition of life, specially the one acting on the reproductive
system increases or cause variability. If profitable, these variations are going to keep occurring
by natural selection.
Hongo, C., Kayanne, H. (2010). Relationship between species diversity and reed growth in the
Holocene at Ishigaki Island, Pacific Ocean. Sedimentary Geology, 223(1), 86-99.
Hongo and Kayanne (2009) described the relationship between species diversity and reef
growth in the Holocene at Ishigaki Island, Pacific Ocean. Their research focused on spatial and
temporal variations with regards to relationships between species diversity and reef growth.
Species richness in areas of high tropical diversity can be a result of speciation, extinction, and
dispersal. Upper portions of most coral reefs actually lie in the intertidal zone and extend to
depths far below the intertidal zone. There is a wide range of species diversity possible over this
region because of disturbance and productivity gradients.The nature of the physical environment
according to different zones in the coral reef provides a heterogeneous environment. Several
methods and equations have been used to test species diversity such as the Simpson's Index and
the Shannon and Weaver's Index. These indexes represents the probability that two randomly
selected individuals in the habitat will not belong to the same species. The purpose of this
experiment was to investigate the relationship between species diversity and the patterns of reef
growth during the Holocene. The western Pacific has the highest species diversity in the world
with more than 70 genera of corals (Hongo and Kayanne 2009). Recovered drilling cores were
recovered to examine spatial and temporal changes in the coral community. Coral reefs are
generally exposed to moderate-sized waves originating from the east Pacific swell causing
distinct biological zonation. Hongo and Kayanne (2009) stated that the region with highest
species diversity is found on the crests and outer slopes that are exposed to damaging storms. In
this region hurricanes destroyed much coral found within the outer slopes but failed to damage
the shallow lagoon protected by an adjacent reef. The disturbed areas have been recolonized by
many species after each hurricane. Disturbances prevented competitive exclusion which
maintains species diversity. Disturbances caused by the natural disasters and by predation
remove corals and then recolonization by many species follows.
Jackson, J. B. C., Cubit, J. D., Keller, B. D., Batista, V. (1989). Ecological Effects of a Major Oil
Spill on Panamanian Coastal Marine Communities. Science, 243(1), 37-44.
Jackson et al. (1989) explains how on 27 April 1986, more than 8 million litters of crude
oil spilled into several region of mangroves, seagrasses, and coral reefs just east of the Caribbean
entrance to the Panama Canal. Back in 1986, this was the largest spill into coastal habitats in the
tropical Americas. Jackson et al. (1989) studied many populations of plants and animals in both
oiled and unoiled sites which provided documentation of the spread of oil and its biological
effects. The intermediate disturbance hypothesis states that diversity is higher when disturbances
are intermediate with regards to both frequency and intensity (Connell, 1979). An oil spill
however, can be considered a large disturbance in which nearly all plants and animals died
SPECIES DIVERSITY
20
wherever they came in contact with oil. By September 1986, a band of dead or dying trees
marked the zone where oil washed ashore between Punta Galeta and Islas Naranjos. The
organisms that came in contact with the oil and survived are able to successfully pass their traits
to future progeny. Natural selection ensues while competitive elimination diversifies species
found within the community. Jackson et al. (1989) explains that within similar habitats found at
nearly the same distance from the oil spill, it caused considerable variation within species. Only
those well adapted to resist and deter the oil would survive. These successful organisms could
then pass the beneficial genes to their offspring. The spill harmed prominent organisms and its
biological effects were usually devastating to the natural environments.
Losos, J. B., Schoener, T. W., Spiller, D. A. (1998). Impact of a Catastrophic Hurricane on
Island Populations. Science, 281(1), 696-697.
Losos et al. (1998) studied both lizards and spiders throughout the Bahamas after
intermediate levels of disturbance. In October of 1996, Losos et al. had just finished a census of
lizard and spider populations on 19 islands surrounding the Bahamas. The purpose of their
experiment was to study the long-term ecological effects of various lizard and spider species.
During the morning of 19 October, Hurricane Lili passed directly over their study site with
sustained winds of 90 knots. As soon as the storm passed, Losos et al. (1998) recounted their
lizard and spider populations on all the islands. Hurricane Lili allow researchers to evaluate
several propositions concerning the impact of disturbances on different types of organisms. Their
data concluded that larger organisms may be more resistant than smaller ones to the immediate
impact of a moderate disturbance. For Losos et al. (1998), the data show that the larger lizards
were more resistant than the smaller spiders to the immediate impact of the moderate disturbance
on the protected side. Lizards may have been able to withstand the physical forces of the
disturbance or to find protective cover better than spiders. Furthermore, surviving smaller species
may recover faster because their reproductive rate is higher than that of larger species. Spiders
are generally more fecund and have shorter generation time which allows a quicker recovery
than lizards. Losos et al. also found that for moderate disturbances, the risk of extinction is a
function of population size, whereas no such relationship exists for catastrophic disturbances.
Lastly, these researchers noted that when all populations are exterminated by a catastrophic
disturbance, the recovery rates of different species will be largely determined by their dispersal
abilities.
Molino, J. F., Sabatier, D. (2001) Tree Diversity in Tropical Rain Forests: A Validation of the
Intermediate Disturbance Hypothesis. Science, 294(1), 1702-1703.
Molino and Sabatier (2001) further studied the intermediate disturbance hypothesis with
a sample of 17,000 trees in a Guianan forest. This study was conducted 10 years after multiple
experiments that added to natural disturbance intensities. The reason there is a great level of
diversity within the 17000 trees is because of these natural disturbances 10 years ago. The
population is composed of several different species altering its once single dominant
characteristic to a mixed forest. It is clear that the highest diversity is maintained at intermediate
scales of disturbance.
SPECIES DIVERSITY
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Sanford, L. R., Saldarriaga, J., Clark, E. K. (1985). Amazon Rain-Forest Fires. Science, 273(1),
167-173.
Sanford et al. (1985) described how forest fires within the Amazon are subject to
diversity because of local disturbances. The primary topic of this article is how natural and
human-caused fires have altered the Holocene vegetation within the Amazon (Sanford et al.
1985). Sanford et al. proposed that fire be considered moderate level disturbances for tropical
rain forests which maintain high diversity in tropical forests arise. Grassland and savannahs are
areas with lengthy dry seasons that burn most often. Under these drier climatic regimes, wildfires
have destroyed large areas of forest which resulted in many large successional forests. Large
amounts of charcoal were found in the forest soils. The abundance of charcoal of commonly
found in these rain forest soils of Rio Negro suggests that fire has for a long time been a
disturbance factor in these tropical forests. These perturbations have modified the forests during
the Holocene as a result of different climatic circumstances. Therefore, without these wild forest
fires natural succession would not be able to ensue. Wild fires reduce single-species forest and
are the result of must diversity found within the savannahs.
Santelices, B. (2007). The discovery of kelp forests in deep-water habitats of tropical regions.
National Academy of Sciences of the USA, 104, 42-43
Santelices (2007) explains how kelp forests alter the physical environment by modifying
light penetration which influences species diversity. Kelp forests are regarded as highly
productive and highly diverse marine ecosystems. There are hundreds of associated species; 35%
of 275 common taxa found in a given forest are associated with the kelp. Kelps also provide
shelter, food, and nursery grounds for many benthic organisms. Kelps require a minimum annual
penetration of sunlight to support photosynthesis and their growth can be severely limited by the
availability of dissolved nitrogen. Santelices (2007) explains how surface kelp canopies may
deteriorate in summer when dissolved nitrogen reaches levels below 1 micromolar. Kelp
canopies reduce light which create favourable conditions to species found in the understory.
Several species are adapted to low light intensity. By altering light penetration throughout the
kelp communities conditions quickly change which affects competitive interactions among
various species. Without these natural disturbances, organisms found within associated kelp
Forests would not be able to flourish. Only those adapted to darker conditions will be favoured
which promotes competitive elimination.
Sousa, W. P. (1985). Disturbance and patch dynamics on rocky intertidal shores. Ecology of
natural disturbance and patch dynamics, 164(1), 101–124.
Disturbances often create open patches within a community providing space for
successional growth. Wayne Sousa, a key researcher of disturbance to biological systems, has
defined disturbance as a discrete, punctuated killing, displacement, or damaging of one or more
individuals that directly or indirectly creates an opportunity for new individuals to become
established.