Everglades plant community invasibility and facilitation of invasion by native
plant species.
Richards, Jennifer
Florida International University, Miami, FL
Invasibility, which describes a community’s ability to resist invasion by exotic
species, is an ecosystem property that emerges from the strength of interactions
among species in the ecosystem and between species and the abiotic environment.
W.M. Lonsdale (1999) proposed that the number of exotic species in a region (E)
is a product of the number of exotic species introduced (I) and the survival rate
(S) of these species in their new habitat. Given the large number of exotic species
already in south Florida, understanding the invisibility of native plant
communities requires understanding S. Lonsdale breaks S down into Sv (survival
after competition with native species), Sh (survival after herbivory and pathogens),
Sc (survival after chance events at establishment), and Sm (survival after
extinctions due to maladaptations). Sv is especially affected by disturbance, which
alters the competitive regime of native species. Propagule pressure, or the
number of propagules arriving at a site, is also important in invasion dynamics of
natural areas. Because native species are already adapted to extant Sh, Sc, and Sm,
and because they are pre-dispersed in the landscape, they have an increased
probability of invading under conditions of environmental disturbance. Such
invasions cause additional disturbance. I propose here that exotic invasions can
be facilitated by invasions by native species (Figure 1) and present 2 examples of
community change caused by invasions by natives in south Florida.
In its undisturbed state South Florida is an oligotrophic ecosystem that has both
aquatic and terrestrial communities. South Florida’s aquatic communities should
be among the least invasible community types, because plants living in these
environments have to adapt first to the aquatic habitat, then to a nutrient-poor
environment. Plants living in short- and many long-hydroperiod marshes have the
additional requirement of being able to withstand more or less extended periods of
dry-down. Native plants are able to out-compete invasive plants in this
environment. Thus, the dominant Everglades macrophyte, sawgrass (Cladium
jamaicense), has a high tissue N:P molar ratio ( 56 + 12 for samples collected at
30 sites across the Everglades), indicating that it can build biomass for less P per
g dry weight than most other plants. Thus, Sv for exotics is very low in pristine
environments, and the natives that grow in these environments have special
adaptations that enable them to succeed.
Figure 1. Model for facilitation of exotic species invasions by native invasive
plant species.
Pristine
flora
Environmental disturbance
Invasive native
plants
Facilitation
Invasive exotic
plants
In 1999, as part of their REMAP study, the US Environmental Protection Agency
surveyed plant species presence and abundance across the Everglades ecosystem
from Loxahatchee National Wildlife Refuge to the landward margin of Florida
Bay. One hundred and twenty sites were sampled in May 1999 and an additional
120 in September/October 1999; a stratified sampling scheme was used. Soil and
water samples were collected for nutrient analysis from the same sites within 2
weeks of the plant sampling. Plant species presence and relative abundance was
recorded for 1 or 2 10 M transects per site; 418 transects were sampled.
A total of 161 plant species were recorded from these transects; of these 128
samples were identified to species and 8 to genus, for a total of 136 identified
taxa. Five exotic species, Alternanthera philoxeroides, Ludwigia peruviana,
Lygodium japonicum, Melaleuca quinquinervia and Panicum repens (3% of the
species sampled), were found along the transects. All but L. peruviana are
Category I or II invasives on the Florida Exotic Pest Plant Council’s 1999 list of
invasive species. They occurred at 1 to 3 sites each and were found in single ¼
m2 quadrates in all cases except for Alternanthera. Alternanthera occurred at a
single site where 2 transects were sampled; it was present in all 20 ¼ m2 quadrates
of 1 transect but only 1 ¼ m2 quadrate of the other transect.
In contrast to these invasive exotics, the invasive native cattail, Typha
domingensis, was abundant in parts of the Everglades ecosystem. Cattails
occurred in 55 transects (13%). This species occurred at frequencies greater than
15/20 ¼ m2 quadrates in 22% of these transects. Cattail invasions are associated
with increased P levels in the soil and, often, with deeper water and/or longer
hydroperiod. Thus cattails, which are preadapted to native herbivores and
pathogens (Sh), natural random disturbances (Sc), and the natural environmental
conditions (Sm), and have extant, predispersed seed sources to provide propagule
pressure (I), succeed when environmental disturbance such as soil P enrichment
or increases in water level favor them in competition with sawgrass.
Similarly, native periphyton mats that are common in pristine south Florida
marshes have a characteristic flora of algae and cyanobacteria that are adapted to
oligotrophic, P-poor conditions. A long-term P dosing experiment in Everglades
National Park has shown that levels of P addition as low as 5 g above ambient
cause a change in mat flora to species that are present in low abundance in the
native mat but are not dominant. These new species create populations with very
different structural and functional characterisitics. Essentially, the calcareous mat
disintegrates, creating new habitat through increased light levels throughout the
water column. This new habitat is open to invasion by exotic species.
Exotic plant invasions are successful when the invaders either create new habitat
or effect ecological processes in existing habitat (Schmitz et al.1997). Invasion of
new habitats by native species is an indicator of an environmental change that
favors the invasive species. These species continue to alter the environment.
Cattail populations can reduce oxygen levels in the water, increase soil nutrient
levels through increased detritus, and reduce light levels in dense stands. Such
environmental alterations create new conditions that may, in turn, facilitate
invasions by exotic species. In the case of the periphyton mat, a completely new
habitat with high light levels throughout the water column is created when the old
habitat (the calcareous mat) is destroyed.
Dramatic increases in populations of native invasive species are thus indicators of
environmental disturbance and can create conditions that facilitate invasion by
exotic species. Because of their preadaptation to existing environmental regimes
and predispersion through the landscape, they provide an early warning of
environmental disturbance that will favor invasive exotics.
Jennifer Richards, Department of Biological Sciences, Florida International
University, Miami, FL, 33199,
Phone: 305-348-3102, Fax: 305-348-1986, e-mail: richards@fiu.edu