Distribution and Abundance of Marine Plants
In essence ecologists seek answers to the questions of where organisms are found, how
many occur there, and why. Thus, the discipline of Ecology can generally be defined as the
scientific study of the biological and physical/chemical; interactions that determine the
distribution and abundance of species. The study of distribution and abundance is also vitally
important to the applied ecologist, since one good way to asses the impact of human activities on
an ecosystem is to compare the distribution and abundance of a species within a given area to
similar, but undisturbed sites.
The purpose of this week’s lab is to describe and analyze the distribution and abundance
of two sympatric marine plants, the angiosperm Halophila hawaiiana and the Rhodophyte
Acanthophora spicifera, and provide an answer to the question – Why are they distributed the
way the are? Ecologists are not limited by specific biological or botanical knowledge, but draw
freely from such disciplines as historical geology, evolution, chemistry and physics. By this point
in the semester, you are equipped to make observations in the field and recognize patterns;
formulate testable hypotheses to explain the patterns you have documented; design experiments
or measurements that will critically test the hypotheses you have formulated and carry out the
work of gathering the necessary data, analyzing it and evaluating your hypothesis in light of the
data you have collected. This lab will give you a great opportunity to show your abilities in all
these aspects of ecological science.
The study site and the study species
The rapid growth of housing and the lack of environmental planning have resulted in
heavy silt deposits in Paiko Lagoon on O‘ahu’s south shore. To some extent the outer, seaward
side of the lagoon also receives silt deposited by runoff and stream discharge. Two marine plants
are common in the shallow (10 cm – 1m) water on the reef flat outside Paiko Lagoon. Halophila
is an angiosperm which is common in shallow water, and sandy lagoons in Hawai‘i (den Hartog
1970). Acanthophora, on the other hand is an alga belonging to a group known as the
Rhodophyta or red algae. It is an introduced species probably arriving on the bottom of barges at
the end of World War II (Doty 1961). You should be familiar with Acanthophora from the
natatorium study, but Halophila is typically found in quite calm water. In recent years a new
invasive alga has become increasingly common at the study site. This is a green alga
(Chlorophyta) Avrainvillea amadelpha.
When we arrive at Paiko Lagoon, your first task will be to identify and learn to recognize
the species of interest. You should spend enough time becoming familiar with the physical layout of the area that you can begin to discern patterns just from general observations from the
surface (i.e. you will not be snorkeling) You should also try to deduce as much as you can about
the biology of the species. Natural history observations are a central part of an ecologist’s initial
study. By observing the general nature of an area, the ecologist begins to get a ‘feel’ for those
factors that may play an important role in influencing the distribution and abundance of a
species. This ‘feeling’ or ‘sizing up of the situation’ is difficult to teach, but with field
experience, the ability to critically observe and evaluate an area is soon acquired. This ability is
vitally important to the field ecologist, for it provides ideas for preliminary hypotheses and a
specific direction for the subsequent investigation, rather than just blindly simple starting to
measure anything and everything. Thus, the first part of this lab, as with the first part of any field
ecological study, should entail overall observations. Record any observations and your
‘fledgling; hypotheses in your notebook. A rough sketch of the area, with points of particular
ecological interest is often helpful; when studying the problem at a later data. Include in your
notebook any observations on the biology of the plant species.
Once you have determined what you think is a pattern to the distribution of the plants,
formulate one or more hypotheses that might explain your observations. Next determine some
measurements you can make that have the potential to disprove the hypothesis you have set out.
Then collect the data necessary fort the test.
Assignment
1. Spend at least 30 minutes observing the area and getting a ‘feel’ for the on-going ecological
processes. Make notes in your field notebook (always include date, time, weather conditions etc.)
2. Investigate the biology of the plants you will study. Observe growth form, habit associations
size, patchiness location etc.
3. How are the plants distributed? Sketch the distribution of the plants in your study area in your
note book. You should be able to come to several alternate hypotheses to explain the distribution
pattern you have observed.
4. After about an hour we will assemble in groups and discuss your hypotheses about the patterns
of distribution and abundance, and decide what factors you will sample and the sampling
procedure.
5. During the sampling divide into groups (each with its own hypothesis to test). All the data
collected by each group will be shared by that group in their lab write up. But, each individual
will present their own analysis of the data.
Brostoff, W.N., 1989. Avrainvillea amadelpha (Codiales, Chlorophyta) from O‘ahu, Hawai‘i.
Pacific Science 43(2):166-169.
den Hartog, C. 1970. The sea grasses of the World. North-Holland Publ. Co. Amsterdam
Herbert, D. A. 1986. The growth dynamics of Halophila hawaiiana. Aquat. Bot. 23: 351-360.
Doty, M. S. 1961. Acanthophora, a possible invader of the marine flora of Hawaii. Pac. Sci. 15:
547-553.