Mangrove forests: A Tough System to Invade
From: Exotic Pests of Eastern Forests, Conference Proceedings - April 8-10, 1997, Nashville, TN, Edited by: Kerry O. Britton, USDA Forest Service & TN Exotic Pest Plant Council
Tropical forests are the most species-rich forests in the world. As many
as 225 tree species per hectare have been reported in these ecosystems,
values that are equivalent to almost finding a different tree species every
other tree encountered in the forest. Under some conditions, tree species
richness decreases in tropical forests. For example, Hart et al. (1989)
reported that forests in Africa that were dominated by a single species
had on average 18 tree species per 0.5 ha. Fewer species still are normally
found in freshwater forested wetlands. Values in these systems range from
1 to 23 species per hectare, with averages at 8.3 and 6 species per hectare
for riverine and basin freshwater wetlands, respectively (Lugo et al. 1988).
Mangrove forests are even more species-poor, and, in fact, are among the
most species-poor forest ecosystems in the tropics (Lugo et al. 1988). Mangrove
stands can be found where the plant species list is only one tree species.
In fact, Jansen (1985) asked: "where is the mangrove understory?"
after he observed that mangrove forests contain no understory plants. Several
articles were written trying to answer the questions raised by Jansen (Corlett
1986, Lugo 1986).
Environmental conditions within mangrove forests make it extremely difficult
for plants to grow and reproduce. These include flooding, prolonged hydroperiods,
anoxic conditions, and salinity. Salinity is the major obstacle to species
invasion to mangrove forests because in order to survive in a saline environment,
plants must possess highly specialized metabolism and mechanisms to either
exclude salt or mitigate its effects on living cells. Worldwide, only 34
tree species have been identified as possessing these adaptations (true
mangroves sensu Tomlinson 1986), 20 other species tolerate some salinity
and are considered minor elements of mangroves, and an additional 60 species
are considered mangrove associates (Tomlinson 1986).
Only a small fraction of the world's flora are halophytes (plants that
tolerate salinity) and those taxa with halophytic species have a lower mean
number of genera per family and a lower mean number of species per genera
than non-halophytic taxa (Waisel 1972). This means that when considering
the subject of mangrove invasions by exotic species, one has to realize
that the species pool available to invade these ecosystems is limited on
a global scale. If a tree was to be able to grow under the saline and hydrologic
conditions of mangroves, it would by definition be a mangrove tree species,
and should it be an exotic to Florida, it would not be an exotic to the
mangrove environment. So, the first question one asks when finding an exotic
tree species or any kind of exotic plant species growing inside a mangrove
forest is: is it a halophyte?
I have witnessed non-halophytes (other than epiphytes) inside mangrove
forests in Florida. For example, floating acquatic plants like the water
hyacinth invade mangrove forests. However, their incursions into mangroves
is short-lived and depend on one of two conditions: (1) how quickly the
plant dies if it floated into saline water, or (2) how long the freshwater
on which it floats lasts as freshwater inside the mangrove. Freshwater lenses
occur in mangroves during periods of high rainfall or runoff, and it is
possible for acquatic plants to occupy that space and survive as long as
the freshwater lens maintains its integrity. Once the saline conditions
is re-established, these invaders are doomed. So, the second question one
must ask when finding a species invasion in the mangroves is: what
conditions of the environment is it occupying and how long will those conditions
Mangrove forests usually have sharp ecotones with adjacent ecosystems
because the saline condition of the mangroves is tidally and topographically
determined. Wherever the tide transports saltwater, the mangroves will follow.
But changes in elevation, even if on the order of centimeters, can create
a sharp ecotone where the mangrove conditions end. Usually, these conditions
either do not have salinity, do not flood, flood without salinity, or have
salinity without floods. Depending on them, the adjacent ecosystem can be
a freshwater wetland, a saline flat, a terrestrial ecosystem, or any combination
of these. The transition from mangrove to nonmangrove conditions can be
shaper as indicated or gradual, where mangroves become less and less important
as the conditions change away from those that delimit the mangrove habitat.
In Florida, I have witnessed exotic plant pecies, including trees, invading
the ecotones of mangroves. These trees can be observed growing quite successfully,
but failing to penetrate the mangrove environment. Examples of these are
the Melaleuca quinquenervia, Casuarina equisetifolia, and Schinus terebinthifolius.
Loope et al. (1994) discusses these and other examples. These trees form
dense and vigorous stands at mangrove ecotones, but fail to invade the saline
soils of mangroves because they are not halophytes. Therefore, a third question
one needs to ask when considering the invasion of exotic species into mangroves
is: What is the geographic location of the invasion, is it only
at the ecotone or does it penetrate the forest?
There is a quick action mechanism that promotes species invasions into
forest ecosystems. That mechanism is disturbance. Disturbance events disrupt
ecosystem structure and functioning, and can create conditions for the invasion
of species. There are two ways in which disturbances can create conditions
for species invasions. First, the disturbance alters microsite conditions
on a temporal basis. For example, after a canopy opening, light energy and
air temperatures increase in the resulting gap. Through succession, the
gap is repaired and original stand conditions return. Invading species have
a window of opportunity to enter the system during the time its repair and
succession are taking place. The invasion after a frost of frost-intolerant
mangroves by first tolerant Spartina marshes, is an example of how a disturbance
can determine the dominance of species at a site (Lugo and Patterson Zucca
1977, Kangas and Lugo 1990).
A second way in which a disturbance can affect site conditions is through
a radical modification of the environment such that succession is not likely
to return to original conditions. Instead, succession may proceed through
an alternative pathway into a different ecosystem state. An example would
be if a disturbance changes the course of a river, or impounds a mangrove,
or removes the mangrove substrate, i.e., the peat. Succession after these
changes is likely to proceed to different states because hydrological, edaphic,
topographic, or even salinity conditions have been modified. Invading species
have an opportunity toexploit the new environment and gain an advantage
over the original species composition at the site. Species invasion of mangroves
after a disturbance raise a fourth and fifth question. Is the invasion
a shortterm response to changes in microsite conditions? or Is the invasion
the result of a longterm shift in the mangrove habitat?
My experience in Florida and elsewhere, suggests that exotic species
fail to invade mangrove forests after disturbances, such as hurricanes,
as long as the hurricane fails to change salinity and hydrological conditions.
However, it is conceivable that native or exotic species could invade mangrove
habitats in locations where the disturbance has changed the salinity and
the hydroperiod of the stand. Smith et al. (1994) reported both native and
exotic grasses and sedges growing on the tip-up mounds inside mangroves
in the moths after passage of a hurricane. These elevated mounds lose their
soil salt by leaching and become a different environment than the soil below.
Human activities such as the construction of canals, diversion of water
flows, construction of roads, dredging, and filling, greatly modify mangrove
wetland conditions, and could facilitate the introduction of native or exotics
species into impacted mangrove habitats. In these instances, it is necessary
to carefully access the environmental change, the nature of the species,
and the spatial and temporal distribution of the species before one can
conclude that a mangrove habitat is being invaded.
The observations of Pimm et al. (1994), Loope et al. (1994), and Smith
et al.(1994) after Hurricane Andrew, impacted south Florida mangroves are
consitent with the discussion above. The description by Loope et al. (1994)
of the invasion of Schinus into "higher (less wet and less saline)
areas within the mangrove zone" deserves further analysis and an ecophysiological
determination on whether this species is a halophyte or not. Pimm et al.
(1994) suggest that Schinus can outgrow mangroves in open areas, but this
broad generalization is not supported by the description of the phenomena
in Smith et al. (1994). Smith et al. (1994) qualify their observation to
"along the upstream mangrove marsh interface" from the Shark River
to the Chatham River where Schinus leafed out faster than the surviving
mangroves. Clearly, the "invasion" of Schinus is at the ecotone
and it is not clear if this species has the capacity to invade mangrove
Mangrove forests are a tough ecosystem to invade because there is a small
species pool that can survive its salinity, hydroperiod, and anaerobic soil
conditions. Even species that survive one of the conditions may not be able
to survive all three. For example, Conocarpus erectus, listed erroneously
as a mangrove, can tolerate salt but not flooding. The same is true of Casuarina,
while Melaleuca tolerates flooding, but not salinity. Before one can conclude
that a species has invaded a mangrove forest, one has to answer five questions
that lead one to rule out the following: if the species has adaptation to
salinity or not, if the species is just taking advantage of a temporary
environmental condition, if the species is located at a particular geographic
zone avoiding the stressors of the mangrove environment, if the species
is temporarily taking advantage of a disruption of the forest by a disturbance,
or if the disturbance has sochanged the habitat that it is no longer a mangrove
environment. Reports of mangrove invasions by exotic species in south Florida
may be premature.
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