Aquatic Plants
Hydrilla: the Jekyll and
Hyde of aquatic plants
Exotic hydrilla restricts boater access, but according to
a new study, it doesn’t diminish ecological diversity. Photo courtesy of David J. Moorhead, University of Georgia,
Bugwood.org
The exotic plant hydrilla was established
in Florida waters during the 1960s and has
since spread across much of the United States.
However, it has proved especially preva­
lent in the state where it all began. In Flori­
da’s shallow lakes, hydrilla can cover nearly
the entire surface area in a short time.
Exotic species have the potential to replace
native plants and animals, rightly causing
concern for biologists and the public. Over the
past 25 years, the state of Florida has spent ap­
proximately $174 million to control hydrilla.
Given the cost of control measures, we need to
Mike McClelland
Gerald Swindle
fully understand any negative — and positive
— impacts of exotic species like hydrilla.
Hydrilla can have beneficial effects for
fish and wildlife. Substantial research has
shown that moderate hydrilla coverage (say,
10 to 60 percent) of lake surface area pro­
vides quality habitat for bass and high catch
rates for anglers. This is especially true for
large Florida lakes, including some of the
state’s most high profile fisheries like
Tohopekaliga and Istokpoga.
Waterfowl populations in Florida also
benefit from hydrilla, with duck hunters tar­
Gary Klein
Shaw Grigsby
Cliff Pace

Cliff Pace – our top finisher in the Bassmaster Classic
gested that infestation of hydrilla (for more
geting water bodies where hydrilla occurs.
than 20 years) did not reduce native aquatic
For these and other reasons, any negative
plant and fish species. Diversity measures also
ecological and economic impacts of hydrilla
should be weighed against potential benefits to did not decline.
The study showed that presence of hydrilla,
fish and wildlife.
even for long periods, did not reduce native
At the University of Florida, we recently
conducted a study to evaluate whether hydrilla plant and animal measures for Florida lakes.
This finding has important implications if a
infestation reduced the number of native
goal of hydrilla control programs is to “restore”
plants, fish and bird species for Florida lakes.
native plant and animal communities. Clearly,
The first data we used were from the Florida
hydrilla control efforts (herbicides or grass carp)
Lakewatch database, which had one year of
sampling for aquatic plants, fish and bird com­ cannot be assumed to improve native plant, bird
and fish communities in Florida lakes.
munities from central Florida lakes. We com­
However, the ecological health of a lake is not
pared the total number of species for lakes
the only reason to control hydrilla. Extremely
with hydrilla and those without.
high hydrilla coverage limits angler, boater and
We also evaluated the diversity of species,
homeowner access to the lake, which has eco­
which measures how evenly the numbers of
each species were distributed throughout each nomic implications that cannot be ignored. Hy­
drilla control methods will continue to be an
ecosystem. Diversity is commonly used as an
important lake management tool.
indicator of ecologic health. If ecologic harm
Our study was observational and was not an
were occurring, we would expect diversity to
be lower in hydrilla lakes than in those without experiment to measure the effect of hydrilla on
native plants. More research is needed to fully
it. The total number of species for plants, fish
understand the effects of hydrilla on native
and birds increases with lake size, so we were
careful to compare only lakes of approximately plants, birds and fish, and our study doesn’t
prove that hydrilla causes no negative impacts
the same size and nutrient concentrations.
to some species. However, it does indicate that
The results showed that lakes with hydrilla
the presence of hydrilla in these lakes for
did not have fewer species of native plants,
birds and fish (see chart).
Lakes with hydrilla con­
tained, on average, 14 plant
species (not counting hy­
drilla), 24 bird species and
17 fish species. Lakes with­
out hydrilla averaged 12
plant species, 19 bird species
and 14 fish species. Similar­
ly, diversity of birds and fish
did not differ between
hydrilla and non-hydrilla
lakes, suggesting that hy­
drilla infestation did not
cause any individual species
to dominate the community
relative to non-hydrilla con­
ditions. The average values
were not statistically differ­
ent between hydrilla and
non-hydrilla lakes for any of
the metrics we evaluated.
In a recent study, the number
These results showed no evi­
of species for aquatic plants,
MEDIAN
RANGE
dence that hydrilla infesta­
bird and fish was higher in
VALUE
tion harmed native plant,
lakes with hydrilla than in lakes
without hydrilla.
bird and fish communities
in Florida lakes.
In case long-term infestation of hydrilla had periods of up to 20 years did not cause signifi­
a different impact over time, we used a second cant losses of native species from the lakes.
It’s our experience that some fish and wildlife
data set that compared six large lakes that had
hydrilla for at least 20 years to six lakes of sim­ biologists and members of the public may be­
ilar size with no hydrilla for the same period of lieve that presence of exotic species will always
reduce the number of native species. We hope
time. Bird species information was not avail­
able for these 12 lakes, but results for plant and the study will lead to further evaluations of the
effects of non-native species on freshwater lakes.
fish species was similar to the previous data.
We found an average of 38 plant species in hy­ The implications of these studies will be impor­
tant for fisheries management in the future.
drilla lakes and 30 for non-hydrilla lakes, and
24 fish species in hydrilla lakes and 23 for
lakes not infested with the plant. These results
MIKE s. ALLEN and mark v. hoyer
were similar to the previous analysis and sug­
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22 BASS T IMES April 2010