TY  - THES
AB  - Kakamega Forest represents the last remnant of
equatorial rainforest in Kenya. Hence, it is of great
importance as last habitat for species specialized on
these forests and their associated ecosystems and, consequently, for East Africa’s biodiversity. The forest has been divided
into several fragments due to anthropogenic habitat
change, which is still continuing today. Even though
large parts of the forest have been designated protected
areas, they are still used for illegal hunting, logging,
and firewood collection.
This thesis focusses the pollination ecology of two
species of Acanthus. A. eminens occupies clearings
and riversides inside the forest, while A. polystachyus
grows in copses and hedgerows of the surrounding
farmland, as well as at the forest edge. Through forest
fragmentation, A. eminens continues to loose suitable
habitats. In addition, the distance of its populations to
the congener is reduced, and the relative abundance
of the species shifts towards A. polystachyus.
As flowering time and floral morphology of the
species are highly similar, it seems likely that habitat
fragmentation causes changes in pollination and
reproduction of the species. In particular, A. eminens
may loose pollinators to A. polystachyus, and receive
more heterospecific pollen. As A. eminens is a common
species of the natural ecosystem, such effects
would indicate that habitat fragmentation causes longterm
changes in ecosystem processes of Kakamega
Forest, threatening its future existence in its natural
state.
In my thesis, I demonstrate that the species flower in
synchrony during winter dry season between October
and February. There are no negative effects of distance
to or habitat availability of the congener on pollination
and reproduction of the species. However, I find plants
in drier habitats and flowers opening later during dry
season to display higher fruit set.
Both species are pollinated by carpenter bees (Xylocopa).
There is no indication for a partitioning of pollinators
through divergent flower morphology, as the
species display highly similar flowers. A. eminens offers
greater quantities of both nectar and pollen, while
the total amount of floral rewards is greater in the
larger individuals and populations of A. polystachyus.
Both species likely represent important resources for
pollinators in their respective habitats. Differences in
pollinator abundance between populations are likely caused by habitat preferences of the pollinators.
Even though the species share most of their pollinators,
interspecific pollen transfer does not affect
their reproduction. This may be explained by the prevalence
of geitonogamy, as bees commonly visit several
flowers on the same inflorescence, plant, and neighbouring,
likely related individuals. Moderate visitation
rates with low pollination success indicate that carpenter
bees are ineffective pollinators, and that the greatest
proportion of pollen is lost between visits. Consequently,
most pollen is transferred between close,
potentially closely related neighbours, reducing the
incidence of heterospecific pollen transfer.
These findings indicate that reproduction of either
species may be limited by pollen quantity or pollen
quality. Hand-pollination experiments show that both
species are limited by pollen quantity but not pollen
quality, as fruit set nearly doubles when supplementary
pollen, regardless of its source, is provided. As
there is no difference in seed set in seed viability between treatments, I
conclude that pollen quality is generally high.
There is convincing evidence that A. eminens and
A. polystachyus are adapted to flowering during dry
season, which offers favorable conditions for fruit and
seed development as well as for seed dispersal and germination.
Differences in humidity are strongly linked
to reproduction in either species, and these effects are
strong enough to mask all putative effects of competition
for pollination and heterospecific pollen transfer.
DA  - 2012
KW  - Kenya
KW  - pollination
KW  - Africa
KW  - ecology
KW  - tropical ecology
KW  - carpenter bees
KW  - Xylocopa
KW  - Acanthus
KW  - bee pollination
KW  - plant-animal interaction
LA  - eng
PY  - 2012
TI  - So happy together? : Pollination and reproductive strategy of two sympatric species of Acanthus L. in Kenya
UR  - https://nbn-resolving.org/urn:nbn:de:hbz:361-25182700
Y2  - 2024-11-21T22:31:22
ER  -