Anthropogenic Effects on Invasive Species in Six Lakes in Kenosha County, Wisconsin
By
Marie C. Pichler
An Undergraduate Thesis
Submitted in Partial Fulfillment for the Requirements of
Bachelor of Arts
in
Geography and Earth Science
Research Mentors: Dr. Tracy Gartner and Dr. Scott Hegrenes
Carthage College
Kenosha, WI
April 2011
Anthropogenic Effects on Invasive Species in Six Lakes in Kenosha County, Wisconsin
By
Marie C. Pichler
Abstract
The United States Office of Technology reported that between 1906 and 1991 over $97 billion
was spent in damages caused by invasive species and since then millions more have been spent.
Besides causing economic problems, invasives can have negative impacts on the environment by
reducing biodiversity. Humans have been known transporters of invasive species, and in the case
of aquatic species the movement of boats between waterways has been identified as a major
component of new introductions. Also, runoff from development around lakes is thought to
allow invasives to thrive further. In this study, submerged aquatic vegetation was sampled in six
lakes of differing boat traffic and sizes. Plants were sorted, identified, and recorded by percent of
sample. In addition to field sampling, land cover/use analysis was conducted for the land
surrounding the sites using GIS to determine if there were correlations between land use types
and percent of invasive species in the lakes. One non-native species (curly-leaf pondweed) was
found in lakes with little or no boat traffic. However, invasive species were most dense at lakes
with more boat traffic. The presence of invasives at lakes with less anthropogenic disturbance
suggests that species may be spreading “naturally”, but increased densities at lakes with higher
boat traffic suggest that humans may be playing a significant role in the dispersal of these plants.
In the land use analysis, a strong positive correlation was found between the percent of
developed land (open space, low, medium, and high intensity) and the percent of aquatic
invasive species in the lake. These results are important to help determine how invasive species
are being spread and what factors are favoring their growth.
2
Table of Contents
Abstract………………………………………………………………………………….…2
List of Figures……………………………………………………………………………...4
Lists of Tables……………………………………………………………………………...4
Literature Review…………………………………………………………………............5
Invasive Species General Information..............................................................…....5
Aquatic Invasive Plant Species…………………………………………………….6
Species Descriptions……………………………………………………………….7
Natural Aspects Effecting Invasives……………………………………………...12
Anthropogenic Aspects Effecting Invasives……………………………………...13
Hypotheses………………………………………………………………………………..15
Methods…………………………………………………………………………………..15
Site Selection........................……………………………………………………...15
Field Sampling and Statistical Analysis……….……………………………….....19
Land Use/Cover Analysis………………………………………………………....20
Results………………………………………………………………………………........21
2009 Field Data…………………………………………………………...........…21
2010 Field Data…………………………………………………………………...23
Invasive Species and Species Richness 2009 and 2010..........................................25
Land Use Analysis…………………………………………………………...........25
Discussion…………………………………………………………………………...........30
Field Data…………………………………………………………………………30
Field Data Compared to Land Use …………………………………………….....32
Conclusion..........................................................................................................................34
Literature Cited.................................................................................................................35
3
List of Figures
Figure 1
Lake Sampling Sites
18
Figure 2
Mean Species Richness for 2009
22
Figure 3
Mean Percent of Invasives per Lake 2009
22
Figure 4
Mean Species Richness for 2010
24
Figure 5
Mean Percent Invasives per Lake 2010
24
Figure 6
Correlation between Percent Invasive Species and Species Richness
25
Figure 7
High Land Use Percent around Lake Sites
26
Figure 8
Correlation between amount of Developed Land and Invasive Species
27
Figure 9
Correlation between High Land Use and Species Richness
27
Figure 10 Kull Lake, KD Lake, Mud Lake
28
Figure 11 Rock Lake, George Lake, Silver Lake
29
List of Tables
Table 1
Percentages of Land Use Types
26
4
Literature Review
Invasive Species General Information:
There have been approximately 50,000 non-native species introduced to the United States
(Pimentel 2005). Some of these species have been introduced here on purpose for food. Food
crops such as corn, wheat, and rice, and animals such as cattle and poultry account for more than
98% of the food used in the U.S. (Pimentel 2005). These species have been helpful for food
production in North America and are valued at $800 billion dollars a year (Pimentel 2005).
However, many other species have been brought here for pest control, landscaping, sport, and in
the pet trade and many of these have become problematic. After being introduced, these species
lack their native predators and diseases that keep their population in check in their place of origin
(Czarapata 2005).
When exotic species are released into the wild, they can cause harm to the native
environment by reducing the biodiversity of native species as a result of competition for space,
food, nutrients and other elements needed for survival. For example, 400 of the 958 species listed
as either threatened or endangered under the Endangered Species Act are considered to be in
danger primarily because of the competition with or predation by non-native species (Pimentel
2005). These changes in the numbers of different species in an ecosystem alter the community
structure of that ecosystem.
Invasive species not only have a major effect on ecosystems, but they also have large
impacts on humans. Many non-native plant species are used for landscaping and making yards
and other public areas look nice. But when these species escape into a natural ecosystem, they
can have a negative effect on the aesthetics of the ecosystem by pushing out native plants and
creating homogeneous plant scenery. However, the main issue that many people worry about is
5
how much it costs to remediate an area or make repairs to infrastructure because of invasive
species. Between 1906 and 1991, the United States Office of Technology reported that over $97
billion was spent in damages caused by non-native species (OTA 1993). Since 1991, millions of
dollars more have been spent on invasive species.
For example, the state of Florida spends about $14.5 million each year for control of the
aquatic invasive hydrilla (Hydrilla verticillata) (Pimentel 2005). Even though so much is spent
on control, infestations of H. verticillata have prevented the recreational use of two of Florida’s
lakes, which has caused $10 million annually in losses (Pimentel 2005). In the U.S., 100 million
dollars is spent every year on exotic aquatic plant species control (Pimentel 2005). These
expenditures lead into why aquatic invasive species are such a big concern for humans and
ecosystems.
Aquatic Invasive Plant Species:
Aquatic exotic plants have mostly been brought to North America by the aquarium trade
or have been transferred over by the ballast water of ships. They can reproduce rapidly by
budding, segmentation, and seeds. Many of these plants start to re-grow before native species
after the annual ice melt in northern lakes. Because they are the first in the season to start to
grow, non-natives can out-compete natives for sunlight, space, and nutrients (Czarapata 2005).
These species often have a higher tolerance for many environmental conditions such as
alkalinity, depth, and temperature. Aquatic invaders differ from terrestrial species in that (1) their
seeds are often dispersed by water; (2) whole plants and plant fragments can be dispersed by
flotation; and (3) they exhibit rapid nutrient uptake, allowing rapid growth (Zedler 2004). Also
freshwater plant communities are often disturbed by waves, fluctuating water levels, boats, and
6
fish (Capers 2007); disturbances like these allow invasives to spread even further. All of these
factors contribute to a loss of biodiversity in native freshwater plants caused by the spread and
growth of invasive plants, as evidenced by observational and experimental studies that have
found a negative correlation between native and invasive species richness (Kennedy et al. 2002,
Gilbert and Lechowicz 2005).
In addition to the effects that invasive plants have on native biodiversity, these species
degrade aesthetics and affect the use of the water body by humans. An example of how these
plants can greatly affect economics of an area was stated earlier in what the invasive plant
hydrilla has done in Florida. Dense mats formed from invasive plants clog waterways, increase
sedimentation, degrade pastures and crops, and can enhance mosquito breeding (Zedler 2004).
All of these factors a loss of aesthetics and have a huge impact on the economy.
Aquatic invasive plants affect ecosystems and humans in many different ways. It is
important to find out what factors cause these species to spread and thrive. The study reported
here took place in Kenosha County, Wisconsin where there are two locally problematic invasive
plant species; Eurasian water-milfoil and curly-leaf pondweed.
Species Descriptions:
Eurasian Water-milfoil (Myriophyllum spicatum)
Eurasian water-milfoil is a submerged aquatic plant that is native to Eurasia and northern
Africa. The plant was once commonly sold as an aquarium plant. It is believed that it was
accidentally introduced into North America because of the aquarium trade (IN DNR 2009). The
earliest confirmed specimen of M. spicatum in North America was found in 1942 in Washington
D.C. (Buchan 2000). From there the plant expanded its range and was first recorded in
Wisconsin in 1967 (Buchan 2000). Eurasian water-milfoil was first noted in Devil’s Lake,
7
Wisconsin in a 1974 survey but did not become widespread until 1984. The Eurasian watermilfoil had displaced Elodea (Elodea canadensis), which was noted in 1974, while other species
were able to still survive but in lower densities (Lillie 1986).
Eurasian water-milfoil has the ability to stay alive over winter, begin growing rapidly in
spring, and block out sunlight needed by native plants (Czarapata 2005). The plant reproduces
sexually and asexually but dispersal occurs mainly through fragments. These fragments can
occur by autofragmentation after flowering, disturbances from water turbulence and human
related activities (Buchan 2000). Fragments float to new areas or can be unintentionally spread
by humans; fragments can stay alive for weeks if kept moist (Czarapata 2005). Eurasian watermilfoil also reproduces by runners that grow along the lake or river bed. These runners survive
over the winter and store carbohydrates that boost its growth in the spring (Czarapata 2005). The
seeds that Eurasian water-milfoil produces typically germinate poorly under natural conditions
(Czarapata 2005). These abilities of Eurasian water-milfoil lead to the plant’s total domination of
an area.
Prevention of introduction to an area is important in stopping the spread of Eurasian
water-milfoil. Methods for prevention include removing aquatic vegetation from boats and
equipment before leaving any lake or river landing, draining bilges (the area where water collects
at the bottom of a boat) and live wells, avoiding transfer of water between water bodies, and
washing boats thoroughly after each use (Czarapata 2005). After introduction, other actions must
be taken to stop the spread. Manual or mechanical harvesting is an option, but pulling, cutting, or
raking may promote and sustain the plant’s establishment by increasing fragmentation and
encouraging vegetative growth (Buchan 2000). Chemical controls are another option for
controlling Eurasian water-milfoil populations but must be used very carefully. Systems of
8
herbicide control on Eurasian water-milfoil will be different from site to site and aquatic experts
should be consulted before taking any action. Fluridone and 2,4-D are two common herbicides
used against Eurasian water-milfoil (Czarapata 2005). Both of these chemicals have had mixed
results, can harm native plants, and can temporarily restrict water use for humans (Czarapata
2005). The herbicide 2,4-D can be applied in early spring or late fall. Fluridone can be applied in
low doses in the late fall but if applied at this time, the herbicide can affect overwintering plants
(Czarapata 2005).
There is a weevil that is currently being researched for biological control against Eurasian
water-milfoil. The native North American weevil, Euhrychiopsis lecontei, has been found to be
effective at reducing populations of the plant (IN DNR 2009). The weevil seems to only attack
milfoil and causes a high level of damage to the plant. The adult weevils feed on the stems and
leaves of milfoil while the larvae bore into the stem, the combination causes extensive damage to
the plants (IN DNR 2009) Creed and Sheldon observed that in 1986, prior to the introduction of
the weevil, Eurasian water-milfoil covered about 10 to 11 hectares of the littoral zone of
Brownington Pond in Vermont. Three years after introduction of the weevil, less than 0.5
hectares of Eurasian water-milfoil remained. They monitored Eurasian water-milfoil and weevil
populations between 1990 and 1992 by setting up permanent transects to measure Eurasian
water-milfoil biomass as well as the amount of weevil eggs, larvae, pupae, and adults on
individual Eurasian water-milfoil stems. Throughout the experiment Eurasian water-milfoil
populations fluctuated but remained low as weevil populations increased (Creed and Sheldon
1995). In addition to field observations, in aquarium experiments damage from the weevils
reduced the viability of Eurasian water-milfoil fragments in comparison to plants without
damage, the fragments are crucial for the spread of the plant (Creed and Sheldon 1995). Also in
9
the aquarium experiments, the biomass of the stems and roots of weevil-damaged Eurasian
water-milfoil was significantly lower than plants without damage (Creed and Sheldon 1995).
Curly-leaf Pondweed (Potamogeton crispus)
Curly-leaf pondweed is a submerged aquatic plant native to Eurasia, Africa, and
Australia. In the mid 1880’s the plant was accidentally introduced to the United States by
hobbyists who used it as an aquarium plant (WI DNR 2008) and by the 1930’s it was established
in the Midwest (IN DNR 2009). The plant is now present in almost all of the lower 48 states.
Curly-leaf pondweed is considered a deep-water plant but can grow in shallow water as well.
The plant can tolerate extreme conditions such as low light and cold water temperatures; it has
even been found growing under 20 inches of snow covered ice (IN DNR 2009).
This invasive grows actively during the winter months while most plants are dormant. It
reaches its highest densities in late spring and dies back in mid-summer, a time when most plants
are at their peak growth (IN DNR 2009). The mid-summer die-off of the plant can cause low
oxygen conditions in areas of high decomposition and the nutrients released can trigger algal
blooms (IN DNR 2009). The combination of these growth patterns allows curly-leaf pondweed
to shade out natives at times when they need the most light early in the growing season.
Like Eurasian water-milfoil, curly-leaf pondweed can reproduce by seeds but they are not
its main mean of reproduction. Curly-leaf pondweed reproduces mainly through dormant
vegetative propagules called turions (IN DNR 2009). Each plant can produce hundreds of
turions and they can be dispersed by water currents and by birds (Capers 2005). The turions are
produced in the late spring just before the plants die and remain dormant until cooling water
10
temperature triggers germination in the fall (IN DNR 2009). The germination rate is between 60
and 80% and the turions can also remain viable in sediment for years (IN DNR 2009).
A study in Halverson Lake, Wisconsin that focused on re-growth of macrophytes after
mechanical harvesting found that curly-leaf pondweed prospered under certain conditions during
one year of study (Engel 1990). After an increased harvesting effort in the summer of 1980 and
rapid warming in the spring of 1981, clear lake ice and early ice out all favored curly-leaf
pondweed growth. Their June 1981 harvest was four times larger than the year before and twothirds of the harvest was made up of curly-leaf pondweed (Engel 1990).
Preventative measures work best in stopping the spread of curly-leaf pondweed. These
methods include removing plants from boats, anchors, and trailers and avoiding transfer of water
between lakes. Often, though, the species is already present in lakes. In order for any control
method to be effective, the plant must be removed or killed before turion production. Also, these
management treatments must be repeated for several years in order to remove the turion bank in
the sediment. Plants can be mechanically removed using weed harvesters, hand cutting or raking
but plants should be removed as close as possible to the sediment to reduce turion production (IN
DNR 2009). Fragments of the plant must be removed from the water to prevent vegetative
growth. Remnants can be destroyed by composting, burying, burning, or by landfill (IN DNR
2009).
Herbicides diquat and endothall have had positive effects on reducing shoot and root
biomass as well as turion production (IN DNR 2009). Research has shown that diquat or
endothall should be applied in the spring when the water temperature is around 50 or 55 degrees
Fahrenheit have the greatest reduction on turion production (IN DNR 2009). Either of these two
11
chemicals can be applied over isolated beds of curly-leaf pondweed or on a larger scale like an
entire lake (IN DNR 2009). Another herbicide, fluridone, can be applied in early spring to
restrain production of turions. Fluridone should only be used on whole lake or large scale
treatments (IN DNR 2009). As with any treatment of herbicides, aquatic professionals should be
contacted before treatment because the amount and type of herbicide depends on the site and
severity of the invasion.
Natural Aspects Effecting Invasives:
Connectivity between waterways is a major dispersal mechanism for aquatic plants,
including invasive species (Lacoul and Freedman 2006). Floating seeds, floating stems and freefloating plants are ways that aquatic plants use connectivity to disperse across a landscape. Both
native and non-native plants to North America evolved to use these methods of dispersal. When
aquatic invasive plants are introduced to a water body that is connected to another, these plants
can use their own methods of dispersal to move to new sites without the aid of humans. Since
many invasives tend to produce many seeds, fragment easily, and grow rapidly, these
connections between water bodies can play even a bigger role in the spread of invasives. Linton
and Goulder (2000) found that in addition to connectivity, the distance between water bodies
also plays a role on dispersal of aquatic plants. They found a positive correlation between species
richness of aquatic plants and species richness of neighboring water bodies in 57 ponds in
England. The shorter distance between water bodies allows for easier dispersal of aquatic plants
by animals.
Another major way that native and invasive plants can be spread is by animals (zoochory)
(Lacoul and Freedman 2006). Examples of how zoochory is used by species to disperse is by
12
creating seeds that can pass through the digestive system of an animal, seeds or fruits that snag
on fur or feathers, and by tiny plants that can cling onto fur or feathers (Lacoul and Freedman
2006). One study suggests that dispersal by birds is possible over short distances but unlikely
intercontinental movement because of the fast digestion cycle in birds and the likelihood of
plants falling off the animals (Les et al. 2003). Since many aquatic invasives can survive long
distances if kept moist, animal movement could be a major part of the plants’ movement.
Anthropogenic Aspects Effecting Invasives:
In addition to natural means, humans can help move invasive species via boats. Many
believe that boat movement is the primary means of transport for invasive aquatics (Austen
2002, Buchan and Padilla 2000, Johnson et al. 2001). Also, environmental factors stemming
from the surrounding land use has been studied to have an effect on the amount of aquatic
invasives in lakes (Lacoul and Freedman 2006). These two factors will be described further in
the following paragraphs.
Boat Access and Traffic
There are many ways that boats can affect invasive species populations. Boats can be
taken out of one water body, put on a trailer and travel over land, and put in another water body.
Many native and non-native plants can stay viable on or in different parts of the boat for
extended amounts of time. Macrophytes can get caught around motors, water intakes, caught on
boat trailers and even get stuck to the side of the boat. If a boat is not properly cleaned and set to
dry for an extended amount of time, plants in any of these areas could remain viable and
reproduce if the boat is placed in a new location. Entangled macrophytes can also contain and
harbor other non-native organisms such as zebra mussels (Johnson 2001). One survey of public
13
boaters using Lake St. Clair, Michigan found that entangled macrophytes were on 33% to 36%
of trailers leaving the lake (Johnson 2001). Because of all of these ways that macrophytes can
attach to boats, lakes with public boat access are more likely to be invaded by non-native species.
Along with public access, the more public boat traffic a lake has further increases the likelihood
of invasion and even multiple introductions of the same species. These re-introductions create a
greater genetic diversity among the invasives, allowing them to thrive even further (Roman and
Darling 2007).
Land Use and Land Cover
The way that land that surrounds a water body is used has an impact on the water quality
and the biodiversity in the water. The main issue is not with the land itself, but the runoff that
flows into the water. One major source of runoff is caused from impervious surfaces like roads,
sidewalks, parking lots and buildings. The easiest pollution to see is particulates in the water that
create higher turbidity. Higher turbidity reduces light penetration and can have an inverse effect
on both native and non-native plants (Trombulak and Frissell 2000). Heavy metals from gasoline
and deicing salt are also major inputs into the runoff that flows into waterways (Trombulak and
Frissell 2000) and these chemicals have a direct impact on native species, making them more
susceptible to be out-competed by non-natives. One study found that native species richness was
negatively affected by raised copper levels while the richness of non-native species was not
(Crooks et al. 2010). Areas that are largely impervious also tend not to have sufficient riparian
buffers between the surfaces and the water. This lack of riparian buffer can allow for more
nutrients to flow into the water. Residential and agricultural land uses both play a large role on
the runoff of nitrogen and phosphorus from fertilizers into water bodies. The addition of these
14
nutrients and other pollutants create more extreme conditions in water bodies that native species
may struggle in while invasives survive or thrive.
Hypotheses
To determine if patterns found in previous studies continue for other sites, lakes were
sampled in Kenosha County, Wisconsin during the summer of 2009 and 2010. Lakes in the study
with public boat access and more public boat traffic are expected to have higher densities of
invasive species than lakes with less boat traffic. This will likely be the case because the greater
amount of public boat traffic increases the chance of a species being introduced once or even
multiple times. Lakes surrounded by higher anthropogenic land use and less amounts of natural
areas also are expected to be more susceptible to invasion because of increased pollutants and
nutrients due to runoff. The anthropogenic land use may also be an indicator of human
disturbance to the site. Overall, lakes with higher public boat traffic and higher anthropogenic
land use will have higher densities of aquatic invasive plants and lower plant biodiversity and
lakes with less boat traffic and lower anthropogenic land use.
Methods
Site Selection:
Lakes were chosen based on connectivity as well as accessibility to Carthage. For the
first year of the study, the four lake sites selected were George Lake (Bristol, WI), Mud Lake
(Bristol, WI), Silver Lake (Silver Lake, WI), and Kull Lake (Brighton, WI); which are all in
Kenosha County, WI (Figure 1).
George Lake is a medium-sized lake (59 acres; WI DNR) with public boat access
(medium boat traffic) and with residential buildings around about three-fourths of the lake.
15
Eurasian water-milfoil and curly-leaf pondweed were first documented in 1977 (WI DNR). Mud
Lake (22 acres; WI DNR) is a small lake without public boat (low boat traffic from residents)
access and with houses around one-fourth of the lake. George and Mud Lake both drain into the
Dutch Gap Canal.
Silver Lake is a large lake (464 acres; WI DNR) with public boat access (high boat
traffic) and homes and businesses around half the lake. Curly-leaf pondweed was documented in
1977 and Eurasian water-milfoil was documented in 1994 (WI DNR). Kull Lake is a small lake
(15 acres; WI DNR) without boat access. The lake has no residential development and little
human impact. Kull Lake is upstream from Silver Lake.
During the second year of study, access was lost to Kull Lake and access was gained to
two lakes. These lakes were Rock Lake (Trevor, WI) and KD Lake (Burlington, WI).
Rock Lake is a medium-sized lake (46 acres; WI DNR) that ultimately flows into Lake
Catherine (Antioch, IL). At one time it was a former mining operation. Today, it is a private lake
that prohibits gas powered watercraft (medium/low boat traffic). Residential development of
Rock lake is extensive and growing, with houses, including new construction, surrounding about
three-fourths of the lake. Eurasian water-milfoil was first documented in 1998 (WI DNR).
KD Lake is an old quarry that has been filled with incoming stream water and rainwater.
It is a small lake (about 40 acres) that eventually flows into the Fox River. There are no houses
or buildings around the lake yet; a park is being planned and constructed at the site.
Also, in-between 2009 and 2010 sampling, some changes occurred in George and Mud
Lake. According to local residents, herbicides had been used in George Lake that greatly reduced
16
the densities of aquatic plants there. At Mud Lake, beavers came to the site and built a dam along
the lake’s outlet stream, increasing the lake water level.
17
Figure 1. Map of lake sites for 2009 and 2010 (darker blue)
18
Field Sampling and Statistical Analysis:
Several different sampling techniques were used to gather data about the invasive species
at each site. The first sampling method was the rake technique. The rake technique was a method
used by the UW Extension Lakes Program to find presence or absence and densities of
submerged aquatic vegetation. For this technique, a double sided rake was thrown from shore,
pier, or boat at a random point, chosen by rolling a set of dice, and pulled in to acquire a sample.
The macrophytes in the sample were then separated, identified, and measured as a percent of the
total sample. The percent of total sample were estimates based on coverage of each macrophyte
in the sample. These total sample percents were made for every plant in the sample, including
both native and non-native plants. Presence or absence of zebra mussels was measured by
making visual observations at the shore. Visual observations included inspection of hard surfaces
around the shore as well as inspection of aquatic plants for mussels that may be attached.
Mean species richness was calculated for each lake by taking the mean of every sample’s
species richness at that site. Also, mean species richness without invasive species was calculated
for each site, this was done the same way as total species richness but invasive species were
removed from the calculation. The mean percent of invasive species per site was also calculated.
This was computed by taking the mean of the percents of Eurasian Water Milfoil and Curly-leaf
Pondweed per sample. ANOVAs with Tukey post-hoc analysis were used to test for statistical
significance between the species richness and percent of invasive species between each of the
lake sites.
19
Land Use/Cover Analysis:
To determine the different amounts and types of land use around the lakes, the 2001
National Land Cover Data (NLCD) from the Multi-resolution Land Characteristics Consortium
(MRLC)(US EPA) was used. First ¼ mile doughnut shaped buffers around each lake site were
created using ArcGIS. Then using Spatial Analyst the Cell Statistics tool was used to count the
number of pixels of each land use type in each lake sites’ buffer. The land cover data is divided
into 16 different classes. For this study, classes that are considered as high anthropogenic land
use are: developed-open space, -low intensity, -medium intensity, -high intensity, cultivated
crops, pasture/hay. To find the percent of high land use around each lake, the number of high
land use pixels was divided by the total number of pixels in the buffer. Also, farmland and
developed space were each analyzed separately as well. Farmland was defined as cultivated
crops and pasture/hay and made into a total percent of the buffer. Developed space was defined
as developed-open space, -low intensity, -medium intensity, and -high intensity and was made
into a total percent of the buffer. Linear regressions were used to model the relationships of land
use types and the percent of invasive species and species richness at each lake site.
The NLCD 2001 divides development into four categories as previously stated.
Developed-open space is defined as less than 20 percent of cover is impervious surfaces and
most vegetation is lawn grass (U.S. Department of the Interior). Low intensity and medium
intensity areas have 20-49 percent and 50-79 percent impervious surface cover respectively; both
areas contain a mixture of constructed materials and vegetation (U.S. Department of the Interior).
In high intensity areas impervious surfaces cover 80-100 percent of the total area (U.S.
Department of the Interior).
20
Results
2009 Field Data:
At every lake sampled in 2009, the density of native species was higher than non-native
species. Mud Lake had the lowest species richness (2.3±0.3 st. error) and Kull Lake had the
highest mean species richness (4.5±0.3 st. error) (Figure 2). The difference between the two
means of these lakes was statistically significant at a significance level of 0.05. Differences
between other lakes were not significant. Curly-leaf pondweed was found at every site and
Eurasian water-milfoil was found at every site except Mildred Lake. Eurasian water-milfoil was
found at its highest mean density at George Lake (20.6%±8.31 st. error). George Lake also had
the highest mean density of curly-leaf pondweed (10.8%±4.75 st. error)(Figure 3).
Curly-leaf pondweed was found at every site and Eurasian water-milfoil was found at
every site except Mildred Lake. Coontail (Ceratophyllum demersum), a native plant, was found
at every site. Native plants, wild celery (Vallisnera Americana), naiad (Najas minor),
bladderwort (Utricularia macrorhiza), and the macro-algae chara (Chara spp.) were only found
at Silver Lake. Other native plants such as northern milfoil (Myriophyllum sibericum) and white
water-crowfoot (Ranunculus longirostris) were only found at Mildred Lake.
21
Mean Species Richness for 2009
Number of Species
6
5
a
4
ab
ab
b
3
2
1
0
Kull
Mud
George
Silver
Lake
Figure 2. Total species richness of lake sites ranked from smallest to largest in 2009. Kull Lake
had a significantly higher species richness than Mud Lake.
Mean Percent of Invasives per Lake 2009
35
30
Percent
25
20
15
10
5
0
Kull
Mud
George
Silver
Lake Site
Eurasian Water Milfoil
Curly-leaf Pondweed
Figure 3. Mean percents of Eurasian water-milfoil and curly-leaf pondweed per site. Lakes
ordered from smallest to largest. George Lake had a marginally significant higher amount of
invasive species than Kull Lake (.063 at 0.05 significance level)
22
2010 Field Data:
At every lake, the mean percent of native species outnumbered the invasive species. Mud
Lake had the lowest species richness (1.5±0.5 st. error), while Silver Lake had the highest
(4.5±1.0 st. error). Overall, mean total species richness increased as lake size increased. Silver
Lake had a significantly higher species richness than KD and Mud Lake (Figure 4). George Lake
had the highest mean percent of Eurasian water-milfoil per sample (31.5%±5.02% st. error).
Curly-leaf pondweed also had the highest mean percent per sample at George Lake (2%±4.90 st.
error). George Lake had significantly more invasive species than KD and Mud Lake at a
significance level of 0.05 (Figure 5).
Curly-leaf pondweed was found in samples at Silver and George Lake. The pondweed
was also personally observed at Rock and KD Lake while assessing the site. Eurasian watermilfoil was found at every site except KD Lake. Coontail (Ceratophyllum demersum), a native
plant, was found at every site except KD. Silver Lake had three species not found at any other
site; bladderwort (Utricularia macrorhiza), wild celery (Vallisnera americana), and big-leaf
pondweed (Potamogeton amplifolius). KD Lake had two species not found at any other site;
slender waternymph (Najas gracillima) and quillwort (Isoetes spp.).
23
Mean Species Richness for 2010
6
a
Number of Species
5
4
3
2
ab
ab
b
b
1
0
Mud
KD
Rock
George
Silver
Lake
Figure 4: Total species richness of lake sites ranked from smallest to largest for 2010. Kull Lake
had a significantly higher species richness than Mud Lake.
Mean Percent Invasives per Lake 2010
40
35
Percent
30
25
20
15
10
5
*
0
Mud
KD
*
Rock
Lake
Eurasian Water Milfoil
George
Silver
Curly-leaf Pondweed
Figure 5. Mean percents of Eurasian water-milfoil and curly-leaf pondweed per site. Lakes
ordered from smallest to largest. George Lake had significantly more invasive species than Mud
and KD Lakes (0.05 significance level) *curly-leaf pondweed observed but not in samples
24
Invasive Species and Species Richness 2009 and 2010:
When comparing the combined 2009 and 2010 data, no correlation was found between
the percent of invasive species in the lake and species richness (Figure 6).
Correlation between Percent Invasive
Species and Species Richness
Species Richness
5
4
3
R² = 0.0011
2
1
0
0%
5%
10%
15%
20%
25%
30%
35%
Percent Invasive
Figure 6. The average percent of invasive species in the lakes sites in 2009 and 2010 correlated
to the average species richness in 2009 and 2010.
Land Use Analysis:
Kull Lake had the highest percentage of high land use (developed and farmland) in the ¼
mile buffer around the site (Figure 7). However, the farmland percentage was far greater than the
developed percentage at this site. KD Lake had the lowest percentage of high land use in its
buffer (Figure 7). George Lake had the highest percent of developed land while KD Lake had the
lowest (Table 1). Kull Lake had the highest percent of farmland and Silver Lake had the lowest
percent (Table 1).
A weak negative correlation was found between the high land use percent and the
average percent of invasive species (R² = 0.108). However, when developed land was compared
25
to percent of invasive species there was a strong positive correlation (R² = 0.797, Figure 8). The
correlation between farmland and invasive species percent was weak (negative, R² = 0.275).
There was a positive correlation between high land use and species richness (R² =
0.568)(Figure 9). No correlation was found between the percent of developed land and species
richness (R² = 0.009). A positive correlation was observed between the percent of farmland and
species richness (R² = 0.568).
High Land Use Percent around Lake Sites
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Farmland
Developed
Kull
Mud
KD
Rock
George
Silver
Lake Site
Figure 7. Percent of high land use in each ¼ mile buffer around each site. High land use is
defined as the sum of the percentage of farmland and developed land. The lakes are listed from
smallest to largest.
Kull
Mud
KD
Rock
George
Silver
%
%
%
%
% Developed Farmland
Forest
Grassland
% Wetlands Other
3.9%
82.6%
11.1%
1.1%
0.0%
1.3%
16.7%
27.7%
42.9%
8.8%
0.8%
3.1%
2.6%
39.9%
41.8%
10.0%
1.6%
4.2%
36.6%
17.7%
32.1%
13.0%
0.2%
0.3%
38.7%
31.3%
16.7%
6.8%
5.8%
0.7%
36.4%
12.7%
38.5%
7.7%
0.8%
3.9%
Table 1. Percentages of each land use type from the ¼ mile buffer around each site. %Other
includes barren land and open water. The lakes are listed from smallest to largest.
26
Invasive Species
Correlation between amount of
Developed Land and Invasive Species
35%
30%
25%
20%
15%
10%
5%
0%
R² = 0.797
0%
10%
20%
30%
40%
50%
Developed Land
Figure 8. The correlation between the percent of developed land around the lake sites and the
average percent of invasive species in 2009 and 2010 of the lake sites
Correlation between High Land Use and
Species Richness
Species Richness
5
R² = 0.5676
4
3
2
1
0
0%
20%
40%
60%
80%
100%
High Land Use
Figure 9. The correlation between the percent of high land use (developed and farmland) and
average species richness in 2009 and 2010
27
Figure 10: Quarter-mile land use buffers around the three smallest lake sites. *The scale for each
lake is unique.
28
Figure 11: Quarter-mile land use buffers around the three smallest lake sites. *The scale for each
lake is unique.
29
Discussion
Field Data:
Contrary to the hypothesis, species richness increased with boat traffic (with the
exception of Kull Lake). It was hypothesized that lakes with more access by people and boats
would have lower species richness than lakes with less access. The larger lakes in this study had
more access than the smaller lakes. So, certain sizes of lakes may have more damage caused by
non-natives than others. When an invasive plant enters a small lake, it is easier for the species to
devastate the lake because of the lakes size and that smaller lakes have fewer different species,
and sometimes densities, than larger lakes. Lakes with a high species richness before invasion
can prevent invasive species from taking over the entire lake. This possible threshold pattern was
only seen in macrophytes, not all species. Also, lake age may be a factor. It is known that KD
and Rock lakes are relatively younger than Silver or George because they were a quarry and a
mine. Since these younger lakes did not have as much time to mature before invasion, it is also
possible that invasive species may have more of an impact.
However, lakes with higher boat traffic had higher densities of invasive species than
lakes with less boat traffic. This finding followed the hypothesis but as stated earlier, species
richness tended to be higher at lakes with more boat traffic. The lack of correlation between of
the percent invasive species and species richness in the lake sites was a surprising find. It is
possible that not enough data has been collected to show a correlation. Two years of data was
collected with a total of six lakes, which is not generally seen as enough data to perform statistics
to a high degree of confidence. Perhaps with the addition of more lakes and more years of data a
correlation may appear. A main reason for lack of correlation is that the larger lakes generally
had a higher species richness than smaller lakes, this observation follows the biogeographic
30
theory of a species-area curve. The theory is that larger areas tend to have greater diversity of
habitats and more resources, which leads to these areas having a higher species richness than
smaller areas. Since the larger lakes in the study tended to have more species and higher
densities of invasive species, no correlation was found between species richness and percent of
invasive species. Another reason why there may be no correlation is that there may be some
“empty” biodiversity at the lake sites. This refers to the increase of biodiversity due to non-native
species but possible lower densities of the native species. So, the species richness may be high
but the diversity of species in the lake may be low.
Curly-leaf pondweed was found at every site, except for Mud Lake, in 2010. Even the
KD lake site had curly-leaf pondweed; a site with no boat traffic and very few visitors. This
observation suggests that curly-leaf pondweed is moving around on its own without much aid
from humans. However, the pondweed was not found at high densities at any of the sites. So, it
appears that it is moving on its own but is not having a great impact on biodiversity in this
region’s lakes at this time. It seems as though there is some factor that is controlling its growth in
this area. Factors that could be controlling the growth of curly-leaf pondweed include the
average temperatures of this area and competition from the native plants at the sites. Another
possible issue with the curly-leaf pondweed data is that the sampling period may have been just
past the peak growth time. In 2009 sampling took place between June 25th and July 15th and in
2010 sampling occurred between June 15th and July 20th. Since the die-back of curly-leaf
pondweed is mid-summer, some of the data may have been collected after many of the plants
started dying. To solve that problem, sampling in following years should be taken earlier in the
summer rather than later.
31
In addition to aquatic invasive plants, zebra mussels were observed at Silver Lake in
both 2009 and 2010. This is probably because Silver Lake is such a large lake and that boaters
who use it may also be using their boats on Lake Michigan. While the boats are in Lake
Michigan, they can pick up the veligers (larval stage of the mussel) or the zebra mussels
themselves and inadvertently transfer them to inland lakes like Silver Lake.
George Lake had the highest mean percent of both Eurasian water milfoil and curly-leaf
pondweed; however in 2010 these densities were very low because the lake had been treated
with an herbicide in front of some of the homes on the lake. It will be interesting to see what
species grow back after this treatment or, if the treatment continues, what species will continue
to survive. At all the lakes sampled, native species out-numbered non-natives. This result could
be because the invasive species have not been at the sites very long or it could be that the native
species are holding off the non-natives. The following years of field data should be able to show
whether natives become more or less abundant than the non-natives. Hopefully the native species
will continue to be able to compete with the invasives and further research will be able to reveal
if this trend continues.
Field Data Compared to Land Use:
The hypothesis about high land use, developed and farmland areas together, was not
supported. However, when the percent of developed land was compared to the percent of
invasive species there was a strong positive correlation. Both developed areas and farmland
produce a lot of runoff but the type of runoff pollution may be what is effecting invasive species
growth. Perhaps runoff from farms (nitrogen, phosphorus, and pesticides) affects both natives
and non-natives similarly; while runoff from developed areas (salts, heavy metals, petroleum
32
products, lawn fertilizers and pesticides) may have a heavier impact on natives while promoting
growth of non-natives. Crooks et al. found that elevated copper levels in water negatively
impacted native species richness and did not have an impact on the richness of non-native
species (2010).
Although there is a strong correlation, there was some variability between the three lakes
with the highest percents of developed land in regard to the percent of invasive species. George
Lake had the highest percent of developed land and the highest percent of invasive species
(32.23%) while Silver Lake had only 2% less developed land than George Lake with a much
lower density of invasive species (13.67%). This difference may be attributed to the large
forested area around about half of Silver Lake. Silver Lake has most of its developed area
concentrated on one half of the lake while George Lake’s development is spread more evenly
around the lake (Figure 11). The majority of samples at Silver Lake were collected near the
forested areas due to accessibility issues. It is possible that invasive species at Silver Lake may
be in denser populations closer to the developed side of the lake. Lake size is another main
difference between these two lakes; Silver Lake is almost eight times larger than George Lake.
Pollution may be diluted much more easily in Silver Lake than in George Lake which could also
lower the effect of pollution on native and non-native plants.
The positive correlation between high land use and species richness was unexpected. This
correlation may have been thrown off slightly by Kull Lake as a possible outlier. Kull Lake had
the highest high land use, most of the high land use was farmland, and the highest species
richness. If Kull Lake was removed from the calculation the correlation may be less significant
or there may not be one at all.
33
Conclusion
Lakes with higher public boat traffic had higher densities of invasive species while still
generally having a higher species richness than lakes with less boat traffic. Features of the lake
such as age, times of invasive introduction and native population before invasion are probably
affecting the species richness. The amount of developed land around lakes seems to play an
important role in the densities of invasive species in those lakes, which is likely due to runoff and
human interaction with the lake. There was an unexpected non-correlation between the percent
of invasive species and species richness possibly due to “empty” biodiversity where the species
richness is misleading about the actual diversity of species. More data needs to be collected at
these and other new sites to determine whether these correlations and trends are supported or
rejected.
34
Literature Cited
Austen, Madeline J.W. et al. “Impacts of Nonindigenous Invasive Species on the Lake Erie
Ecosystem” Proceedings of the 11th International Invasive Species Conference (2002): 117-131.
Blossey, Bernd. “Before, during and after: the need for long-term monitoring in invasive plant
species management.” Biological Invasions 1 (1999): 301-311.
Buchan, Lucy A.J. and Dianna K. Padilla. “Predicting the Likelihood of Eurasian Watermilfoil
Presence in Lakes, a Macrophyte Monitoring Tool.” Ecological Applications 10 (2000): 14421455.
Capers, Robert S. Roslyn Selsky, Gregory J. Bugbee and Jason C. White. “Aquatic Plant
Community Invasibility and Scale-Dependent Patterns in Native and Invasive Species Richness.”
Ecology 88 (2007): 3135-3143.
Creed, Robert P. and Sallie P. Sheldon. “Weevils and watermilfoil: Did a North American
herbivore cause the decline of an exotic plant?” Ecological Applications 5 (1995): 1113-1121.
Crooks, Jeffrey A. Andrew L. Chang and Gregory M. Ruiz. “Aquatic pollution increases the
relative success of invasive species.” Biological Invasions (2010) Accessed October 22 2010.
Doi: 10.1007/s10530-010-9799-3.
Czarapata,Elizabeth J. Invasive Plants of the Upper Midwest. Madison: The University of
Wisconsin Press, 2005.
Engel, Sandy. “Ecological Impacts of Harvesting Macrophytes in Halverson Lake, Wisconsin.”
Journal of Aquatic Plant Management 28 (1990): 41-45.
Gilbert, Benjamin. Martin J. Lechowicz. “Invasibility and Abiotic Gradients: The Positive
Correlation between Native and Exotic Plant Diversity.” Ecology 86 (2005): 1848-1855.
Indiana Department of Natural Resources. “Curlyleaf Pondweed.” Indiana Department of
Natural Resources (2009). http://www.in.gov/dnr/files/CURLYLEAF_PONDWEED.pdf
Johnson, Ladd E. Anthony Ricciardi and James T. Carlton. “Overland Dispersal of Aquatic
Invasive Species: a Risk Assessment of Transient Recreational Boating.” Ecological
Applications 11 (2001): 1789-1799.
Kennedy, Theodore A. Shahid Naeem, Katherine M. Howe, Johannes M. H. Knops, David
Tilman and Peter Reich. “Biodiversity as a barrier to ecological invasion.” Nature 417(2002):
636-638.
Lacoul, Paresh. Bill Freedman. “Environmental influences on aquatic plants in freshwater
ecosystems.” Environ. Rev. 14(2006): 89-136.
35
Les, Donald H. Daniel J. Crawford, Rebecca T. Kimbal, Michael L. Moody, and Elias Landolt.
“Biogeography of discontinuously distributed hydrophytes: a molecular appraisal of
intercontinental disjunctions.” International Journal of Plant Sciences 164(2003): 917-932.
Lillie, Richard A. “The Spread of Eurasian Watermilfoil Myriophyllum spicatum in Devils Lake,
Sauk County, Wisconsin.” Lake and Reservoir Management 2(1986): 64-68.
Linton, S. and R. Goulder. “Botanical conservation value related to origin and management of
ponds.” Aquatic Conservation: Marine and Freshwater Ecosystems 10(2000): 77-91.
OTA. “Harmful Non-Indigenous Species in the United States.” Office of Technology
Assessment, United States Congress, 1993.
Pimentel, David. and Rodolfo Zuniga and Doug Morrison. “Update on the environmental and
economic costs associated with alien-invasive species in the United States.” Ecological
Economics 52 (2005): 273-288.
Roman, Joe. John A Darling. “Paradox lost: genetic diversity and the success of aquatic
invasions.” Trends in Ecology & Evolution 22 (2007): 454-464.
Trebitz, Anett S. and Debra L. Taylor. “Exotic and Invasive Aquatic Plants in Great Lakes
Coastal Wetlands: Distribution and Relation to Watershed Land Use and Plant Richness and
Cover.” Journal of Great Lakes Research 33 (2007): 705-721.
Trombulak, Stephen C. and Christopher A. Frissell. “Review of Ecological Effects of Toads on
Terrestrial and Aquatic Communities” Conservation Biology 14(Feb. 2000): 18-30.
U.S. Department of the Interior. “NLCD 2001 Land Cover Class Definitions.” (2008)
http://www.mrlc.gov/nlcd_definitions.php
Wisconsin Department of Natural Resources. “Aquatic Invasive Species.” (2009)
http://dnr.wi.gov/invasives/aquatic/species
Wisconsin Department of Natural Resources. “Wisconsin Lakes with DNR Lakemaps.” (2009)
http://dnr.wi.gov/lakes/maps
Zedler, Joy B. and Suzanne Kerchner. “Causes and Consequences of Invasive Plants in
Wetlands: Opportunities, Opportunists, and Outcomes.” Critical Reviews in Plant Sciences 23
(2004): 431-452.
36