Principles of terrestrial ecosystem ecology.pdf

Table 10.3. Generic metazoan
diversity of land and
oceans.
ecosystems (Gee 1991). Most filter feeders are
about 100-fold larger than the suspended particles
(seston) on which they feed. Filter feeders
are often selective in the size of particles they
ingest but process algae, bacteria, and suspended
particles of organic and inorganic materials
relatively indiscriminately. Much of this
food may therefore be of relatively low quality,
and substantial quantities of water must be processed
to acquire sufficient energy and nutrients
to support growth.
There are many more species on land than in
the oceans but the broad phyletic diversity for
coping with life is greater in the ocean. About
76% of all species occur on land, and most of
these are insects. Of the 15% of the species that
are marine, most are benthic animals of the
ocean sediments. There are only about 20,000
photosynthetic species in aquatic ecosystems in
contrast to the 300,000 species of terrestrial
plants, and there are very few aquatic insects
(Falkowski et al. 1999). At higher taxonomic
levels, however, 80% the multicellular genera
occur in the sea, and only 20% are on land
(Table 10.3) (May 1994). The greater diversity
of genera and phyla in the ocean than on land
could reflect its longer evolutionary history,
giving organisms more time to try out different
fundamental body plans and functional types
(May 1994). The larger number of species on
land than in the ocean could reflect the greater
heterogeneity and potential for spatial isolation
in terrestrial habitats.
Life evolved in the sea. From there, both
plants and multicellular animals moved to fresh
waters and then to land (Moss 1998). Several
groups of plants and animals subsequently
followed the reverse path from land to fresh
water to oceans.These transitions have occurred
many times, indicating that the physiological
adjustments required are not insurmountable
on evolutionary time scales.
Ecosystem Properties 227
Ocean Ocean
Phyla benthic pelagic Fresh-water Symbiotic Terrestrial
Total (33) 27 11 14 15 11
Endemic 10 1 0 4 1
Data from May (1994).
The marine environment is slightly more
saline (salty) than the internal body fluids of
marine organisms, so organisms must minimize
loss of water and gain of salts to maintain ionic
balance. Movement of organisms from marine
to fresh water reverses this osmotic gradient
and intensifies the costs of osmoregulation.
Terrestrial plants and animals confront two
contrasting problems. First, their source of
water is usually fresh, requiring more energy to
maintain osmotic gradients. Second, they are
exposed to an aerial environment that promotes
water loss and dehydration. One great
advantage to life on land is greater availability
of oxygen and the greater energy provided
by aerobic metabolism. Disadvantages include
greater dehydration and lower buoyancy of air
than water (Table 10.1).
The benthic environment of marine sediments
differs radically from terrestrial soils in
its low oxygen availability. Oxygen concentration
in deep waters is much lower than in air,
and oxygen diffusion into water-saturated
sediments is much slower than through the
air-filled pores of terrestrial soils. Mixing of
sediments by benthic organisms plays a critical
role in promoting oxygen flux into sediments
and therefore benthic decomposition. Redox
reactions involving electron acceptors other
than oxygen play a key role in the metabolism
of benthic organisms and therefore in the
patterns of carbon and nutrient cycling (see
Chapter 3) (Valiela 1995).
The wide range in ecosystem structure
among aquatic ecosystems reflects variations in
physical environment. Most nonpelagic aquatic
ecosystems are intermediate in structural properties
between terrestrial and pelagic ecosystems.
In the littoral zone, where the ocean and
land meet, organisms can reduce the probability
of being swept away by attaching to substrates.
This gives rise to a diverse array of