名波敦 - 東北大学

ななみあつし
氏名・(本籍)
名波敦
学位の種類
博士(理学)
学位記番号
理博第1801号
学位授与年月日
平成12年3月23日
学位授与の要件
学位規則第4条第1項該当
研究科,専攻
東北大学大学院理学研究科(博士課程)生物学専攻
学位論文題目
Effectofhabitatstructureondynamicsofcoralreemshassemblages
(サンゴ礁魚類群集の動態に及ぼす生息場所構造の影響)
論文審査委員
(主査)教授西平守孝
教授菊地永祐
助教授千葉聡
調又
次
目
署春一主
Summary
Ac㎞owledgments
Chapter1.Generalintroduction
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Chapter2.Thestructureanddynamicsoffishassemblagesinan
Okinawancoralreef:theeffectofcoral-basedhabiしat
structuresatrockyandsandyseabottom
l.Introduction
2.MaterialsandMethods
3.Results
4.Discussion
5.Su㎜ary
Chapter3.Populationdynamicsandsettlementpattemsinpomacentridsandapogonids
1.Introduction
2.MaterialsandMethods
3.Results
4.Discussion
5.Su㎜ary
Chapter4.Differentsurvivalofjuvenilecoralreeffishesbetween
rockyreefflatands&ndyseabottom:doeshabitatstructureaf旧ectpredationpressure?
1.Introduction
2.Materials&ndMethods
3.Results
4.Discussion
5。Summary
Chapter5.Theeffectsofthehabitatchangesonthecoralreeffish
assemblages
5-1.Effectsofexperimentalh&bitatmanipulationonfishassemblages:comparisonbetweencontinuoushabitat
andisoIatedhabitat
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l. Introduction
2. Materials and Methods
3. Results
l:
4. Discussion
5. Summary
5-2. The effect of catastrophic disturbance by typhoon attack
1. Introduction
2. Materials and Methods
3. Results
4. Discussion
5. Summary
Chapter 6. General discussion
References
Appendix l. The list of corals
Appendix 2. Photographs of representative corals
Appendix 3. The list of fishes
Appendix 4. Photographs of representative fishes
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=A
l=r
l
Chapter I . General introduction
Coral reef fish assemblages are recognized for their high diversity, and it remains unclear how so many species
are able to coexist in a limited area. There is, at present, a noticeable disagreement on understanding the mechanisms
of the coexistence of reef fishes. The traditional view suggests that these assemblages are stable, characterized by
fine partitioning of limited resources such as food and habitat, and increased specialization through competitive
interactions. Other view suggests that species composition In fish assemblages varies randomly and is unstab]e. So, it
is thought that stochastic process (e. g. spatia] or seasona] changes in juveni]e supply and predation) would organize
the fish assemblages. However, it has not been sufficiently investigated the relative importance of various ecological
factors (e. g., competition, juvenile supply and predation) in relation to habitat structure. Therefore, in present study,
2 habitats were selected such as a rocky reef flat (continuous habitat: site R) and an adjacent sandy bottom (isolated
habitat: site S).
Chapter 2. The structure and dynamics of fish assemblages in an Okinawan
coral reef: the effect of coral-based habitat structures at rocky and
sandy sea bottom
To clarify what ecological factors organize the population density of fishes and structure of fish assemblages, the
structure and dynamics of coral reef fish assemblages were investigated over a 2-year period in 8 m x 8 m permanent
quadrats in an Okinawan coral reef. A comparison was made between the results obtained at 2 sites where the habitat
structure (habitat connectivity) was markedly different. There were marked differences in the structure of fish
assemblages between the 2 sites. The abundance and species richness of fishes such as pomacentrids, Iabrids,
chaetodontids and acanthurids were greater at site R than at site S, whereas those of blenniids, gobiids and mullids
were greater at site S. Seasonal and annual changes in structure of fish assemblages were relatively low at site R ( >
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70% similarity based on C 7r index), but were high at site S ( < 40%). This indicates that the structure of fish
assemblages was relatively stable at site R, but unstable at site S throughout the study period. The spatial difference
in the structure of the fish assemblages among 4 quadrats at site R was sma[1 (most sites > 65% similarity), but it was
large at site S ( < 40%). My fmdings indicated that habitat structure (habitat connectivity) affects both spatial and
seasonal difference in the structure of fish assemblages. I suggest that the spatial and seasonal variations in juvenile
settlements and predation pressure are the main factors organizing fish assemblages at isolated habitat. On the other
hand, other mechanisms such as competition and post-settlement movement appeared to deterrnine the structure of
fish assemblages at continuous habitats.
a pogonids
The structure of fish assemblages was relatively stable at continuous habitat (site R), whereas unstable at isolated
habitat (site S) over 2-year study period. Two hypotheses were presented to explain these phenomena: 1) the
population dynamics of each species show more regular patterns at continuous habitat, but irregular patterns at
isolated habitat; 2) the dominant species shift easily at isolated habitat. To test these hypotheses, the population
dynamics of 6 pomacentrids and 2 apogonids were investigated in the permanent plots established in Sesoko Island.
For 6 pomacentrids, seasonal changes in spatial patterns and species composition were also investigated. Population
dynamics (pattems of changes in population density) of 5 pomacentrids showed irregular patterns, whereas it showed
more regular patterns for I pomacentrids. Seasonal fluctuation of population density of I apogonids Apogon
properuptus was more prominent at site S. The degrees of aggregation of juveniles of 3 pomacentrids were greater at
i.1
site S than at site R. Species composition of 3 pomacentrids did not change greatly at site R, but It changed markedly
at site S. Present study suggests that spatial distribution of juvenile settlement of pomacentrids is affected greatly by
the structure of coral assemblages (coral coverage and coral morphology) at continuous habitat. Therefore, coral
zonation has an important role to determine the species composition of pomacentrids at continuous habitat. On the
other hand, species composition of pomacentrids might be determined by the temporal variation in juvenile supply
and predation at isolated habitat. So, species composition of pomacentrids would changed randomly. Furthermore,
seasonal fluctuation of population density (such as apogonids) might cause the obvious seasonal change in structure
of fish assemblages at isolated habitat.
Chapter 4. Different survival of juvenile coral reef fishes between rocky reef
flat and sandy sea bottom: does habitat structure affect predation
;
pressure?
It has been thought that predation is a major factor structuring coral reef fish assemblages. I hypothesized that
variation in predation pressure is caused by the difference in habitaL structure (habitat connectivity), i.e., a
continuous distribution or isolated distribution of corals. I conducted field experiments to determine whether 1)
survival rates of juvenile varies between sites with different habitat structure, 2) predation press. ure is a density-
dependent on juvenile mortality, and 3) small-si/,ed juveniles are more susceptible to predation. In the field
experiments, juveniles pomacentrid Dascyllus aruanus were set into artificial model colonies, covered or not covered
with a cage at 2 sites on a coral reef in Okinawa. The density of predators was 1.37-times greater around at isolated
habitat (sandy sea bottom: site S) than at continuous habitat (a rocky reef flat: site R). Just after the start of the
experiment, some juveniles moved out from the model colonies at site R, while few of them were observed oulside
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l
Chapter 3. Population dynamics and settlement patterns in pomacentrids and
[,
model co]onies at site S. The survival rate in caged model colonies was significantly higher than that in non-caged
colonies. So. I conc]uded that predation pressure has an important role in regulating the population density of
juveniles. The survival rate in non-caged model colonies was significantly higher at site R than at site S (3.33-times
greater at the end of study period). Although predation was not significantly density-dependent on juvenile mortality,
the difference in survival rates between 2 density groups was larger at site S that at site R. This suggests that density-
dependent effect of predation occurs frequently at isolated habitat. Survival rates of large-sized juveniles was not
significantly higher than that of small-sized juveniles. This suggests that intraspecific competition have few
importance on the juveniles mortality caused by predation. My findings suggest that predation pressure is affected by
the habitat structure.
Chapter 5. The effects of the habitat changes on the coral reef fish assemblages
5-1 . Eftects of experimental habitat manipulation on fish assemblages:
comparison between continuous habitat and Isolated habitat
Several studies have demonstrated the effects of the habitat changes on fish assemblages. However, few studies
have been conducted for the comparison of the effects at different habitat structures. I tested the following 3
hypotheses: 1) the attractiveness of corals for fishes is larger at isolated habitat, 2) the effect of cora] disturbance is
Iower at continuous habitat, and 3) habitat connectivity is an important factor determining the dynamics of fish
assemblages. To test the hypotheses, field manipulations, such as transplantation or removal of corals, were
conducted at continuous habitat (site R) and isolated habitat (site S). Although 4 treatments were conducted, the
dynamics of fish assemblages were relatively stable at site R (> 70% in 3 quadrats), whereas unstable at site S (<
40% in 4 quadrats). These findings suggest that different dynamics in structure of fish assemb]ages between 2 sites
was caused by the spatial arrangement of corals. The branching corals were proved to provide attractive habitat
space for residents, because population density and species richness increased at quadrats where branching corals
were transp]anted. However, relative attractiveness was greater at site S than at site R. Especially, 3 pomacentrids of
the genus Dascyllus and I apogonid Cheilodiptcrus quinquelineatus markedly increased at site S after manipulations.
Since the average density of residents did not markedly change in the quadrats where massive corals were
transplanted, it was suggested that sma]1 massive corals (40 cm x 40 cm x 40cm) might not provide enough habitat
spaces for residents. At site R, the population density and species richness of visitors decreased in the quadrat where
corals were removed. This might be related to the decrease of small invertebrates (prey), because visitors included
small invertebrates feeders. It was demonstrated that the effects of habitat changes on fish assemblages was different
between 2 sites with different habitat structure.
5-2. The effect of catastrophic disturbance by typhoon attack
Since fishes associate closely with substrata which are subjected to recurring disturbance, it is of ecological and
managerial interest to ask how reef fishes respond to disturbance of their habitat. Typhoon caused the catastrophic
destruction of who]e coral assemblages at rocky reef flat in Sesoko Island. So, it was used as a natural experiment to
clarify the response of fishes when habitat is destroyed extremely. The structure of fish assemblages at rocky reef
flat (site R) changed after disturbance, which had never been observed in initial condition. In contrast, the structure
of fish assemblages at sandy sea bottom (site S) obviously changed seasonally, which was consistent with initial
condition. At site R, the popu]ation density of residents (especially pomacentrids and apogonids) increased in a
quadrat where coral remained after the typhoon. However, at site R, the popu]ation density and species richness of
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residents decreased in other quadrats where corals were destroyed. These findings suggest that residents were rei
distributed in undisturbed area which had escaped the disturbance. In contrast, population density and species
richness of visitors decreased after the typhoon at site R. At site S, the population density of fishes (residents and
visitors) decreased after the typhoon. However, the susceptivity agalnst disturbance was dlfferent between species, i.
e., the population density of 3 Pomacentrus species recovered after the typhoon, whereas 3 Dascyllus species
disappeared after the typhoon. I suggest that, at continuous habitat (site R), habitat space was sufficient and fishes
were distributed rather uniforrnly before disturbance. When habitats suffered from the disturbance, fishes had to
move and aggregate on the undisturbed habitat which escaped typhoon damage. On the other hand, at isolated
t
habitat (site S), although fishes were removed by the disturbance, the population density recovered. At isolated
i
habitat, it is suggested that the structure of fish assemblages varies seasonally.
Chapter 6. Generai discussion
Present study suggests that various ecological factors (such as coral zonation, inter- and intraspeciric competition,
predation, chance of juvenile settlement and post-settlement movement) interacts with habitat structure. It is
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suggested that the main factor organizing the reef fish assemblages was different al 2 sites.
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論文審査の結果の要旨
サンゴ礁魚類の群集構造の研究は,主に孤立した生息場所で行われ,群集の動態や安定性に幼魚の着
底の偶然性や捕食が関わっていることが示唆されてきた。一方,棲息場所が豊富な岩礁では,サンゴの
被度や形状と魚類の空間分布の関係に関する短期的記載的研究が多く,群集動態に関して未知な部分が
多い。
名波敦提出の論文は,隣接する岩礁礁原と砂礫底の魚類群集の動態を長期間にわたって同時並行的
に追跡し,野外実験やサンゴの生育状況の操作を行って,競争・捕食・幼魚の着底・サンゴ群集の特徴
などが群集の動態へ及ぼす影響を解明したものである。沖縄県瀬底島の岩礁と砂礫底の両地点に設けた
複数の調査区における潜水調査により,魚類群集の安定性がサンゴ群集の強い影響を受けて岩礁で高く,
砂礫底では幼魚の着底の偶然性と捕食の影響を受けて低いことを示唆した。群集の主要な構成種である
スズメダイ類の種構成と個体群変動は岩礁で安定的で調査区間差が小さく,砂礫底では大きいことを示
した。テンジクダイ類の個体数変動は,砂礫底でより大きかった。このように,群集動態にとって,岩
礁では'サンゴ群集の特徴が,砂礫底では幼魚の着底の様相が重要であることが確認された。巧みな野外
実験によって,捕食圧が密度依存的で小サイズ個体ほど捕食されやすい傾向があり,生息場所の連続す
る岩礁では幼魚は着底場所以外に移動できるため生存率が砂礫底より有意に高いことを見出した。サン
ゴの移植や除去操作の結果,複雑な形の群体が付加されれば定住魚の種数や個体数が増加し,個体数増
加は孤立した砂礫底で著しかった。また,台風でサンゴ被度が激減した場所で魚の個体数は減少し,撹
乱されなかった場所で急増した。これは,岩礁ではサンゴの被度が高いときには魚はほぼ均一に分布し
ているが,被度が減少すると移動して集中分布することを示しており,生息場所の連続性が魚類の空間
分布に影響を及ぼすことが明らかになった。
このように,棲息場所の状況によって魚類群集の動態が異なり,群集形成に影響する生態的要因(競
争・捕食・幼魚の着底'サンゴ群集の特徴〉の相対的重要度が場所によって異なることを明らかにした
ことに加え,捕食圧や棲息場所の孤立性の効果を定量的に把握したことも大きな成果の1つである。
これらの研究成果は,名波数が自立して研究活動を行なうに必要な高度の研究能力と学識を有して
いることを示している。よって名波敦提出の論文は,博士(理学)の学位論文として合格と認める。
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