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Running Head: VECTOR CORRUPTION
Vector Corruption
Make Them Live Hard and Die Young
Gary A. Diridoni
Oregon State University
Capstone Project FW 506
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Abstract
Invasive species can threaten the diversity or abundance of native species and the
ecological stability of the ecosystem by outcompeting natives for breeding sites, prey and
other needed resources, disrupting food webs, degrading habitats and altering
biodiversity. Additionally, invasive species may have reproductive adaptations which
allow them to disperse successfully, tolerate and adapt to a variety of environmental
conditions and establish self-sustaining populations that further their expansions.
Beginning in 2006, a modification to the hydroperiod occurred in two manmade wetlands
within the 54.6 ha Paynes Creek Wetland Complex in the Sacramento River Bend Area
of Critical Environmental Concern (Bend) in Northern California to mimic the natural
hydroperiod of a large vernal pool and recreate conditions that would favor native species
over invasive species. In 2010 a physical barrier was constructed within the wetland
water intake structure as an experiment to exclude two aquatic invasive species,
American bullfrog (Lithobates catesbeianus [Rana catesbeiana]) and Louisiana red
swamp crayfish (Procambarus clarkii), from dispersing into the wetlands. Results
indicate that although invasive species continue to occur within the complex, the
modified wetland hydroperiod and infrastructure improvements limits those individuals
able to colonize the wetlands and precludes permanent populations from establishing,
reducing invasive propagule abundance and density, supporting a multitude of native
species associated with freshwater emergent wetlands and vernal pool complexes in
California.
Keywords: Lithobates catesbeianus, Procambarus clarkii, American bullfrog, red
swamp crayfish, wetland management, aquatic invasive species, vector corruption
VECTOR CORRUPTION
Vector Corruption Make Them Live Hard and Die young
Historically, wetland habitat was often seen as only a breeding ground for
disease-carrying mosquitoes to be reclaimed for productive uses such as agriculture and
municipal use (Biebighauser, 2007). As a result of these and other activities the water
that had naturally flooded the wetlands was diverted for other needs. Wetlands that
historically existed in California are estimated to range from 1.2-2.0 million ha. The
current estimate of wetland acreage in California is approximately 182,108 ha; this
represents an 85 to 90 percent reduction-the greatest percentage loss in the nation
(CERES, 2011).
In contrast to the historical view, the contemporary outlook recognizes wetlands
as very important ecosystems, providing diverse habitat communities, for multiple
species of plants, fish and wildlife. In addition to providing habitat for a multitude of
species, wetlands support a nationwide outdoor recreation industry as well as providing
flood control, ground water recharge and helping to absorb and filter pollutants that could
otherwise degrade water quality of rivers, lakes and estuaries.
Impacts to wetlands in California did not solely arise from draining and
conversion. Wetland habitats are very productive and support a large number of species,
including species listed as threatened or endangered by the Federal Endangered Species
Act, unfortunately however the habitat that these species rely on is being altered and
destroyed by nonnative invasive species (invasives). Invasives have been introduced,
accidentally and intentionally, impacting and altering native biological communities
reducing productivity, diversity and leading to species homogenization across
biogrographic realms (Carroll & Dingle 1996, Clavero & Garcı´a-Berthou 2005).
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Aquatic systems and associated biotic communities are very susceptible to introduced
species colonization and structure alterations due to widespread alterations in hydrologic
regime, community composition and other human induced habitat alterations.
Invasives thrive in these new habitats because they generally lack predators and
other natural controls such as disease or parasites (Shea & Chesson 2002, Torchin et al.
2003). Invasives can threaten the diversity or abundance of native species and the
ecological stability of the whole habitat by outcompeting natives for breeding sites, prey
and other needed resources, disrupting food webs, degrading habitats and altering
biodiversity. Native co-occurring species which evolved in absence of the new invaders,
often lack predator avoidance cues and consequently suffer from a competitive
disadvantage to these invasives. Further, invasives may have reproductive adaptations
which allow them to disperse successfully, tolerate and adapt to a variety of
environmental conditions and establish self-sustaining populations that further their
expansions (Crooks 2002).
The North American bullfrog (Lithobates catesbeianus = Rana catesbeiana) has
been implicated in amphibian declines (Kats & Ferrer 2003, Garner et al. 2006, Boone
2004) through generalist predation, interspecific larval competition (Kats & Ferrer 2003,
Garner et al. 2006), by serving as an agent of ecosystem change (Kupferberg 1997, Pryor
2003)and as a reservoir and vector of chytrid fungus (Batrachochytrium dendrobatidis)
and the associated disease chytridiomycosis, an emerging disease of amphibians
responsible in part for anuran population declines (Hanselmann et al. 2004, Sánchez et
al. 2008).
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The feeding and burrowing habits of the Louisiana red swamp crayfish with its
ability to change the physical structure of ecosystem itself (Hereafter, I refer to these
species respectively as bullfrog and red swamp crayfish.) has led to changes in food webs
and the disappearance of native species (Barbaresi & Gherardi 2000) with cascading
ecological effects (Rodriguez et al. 2005, Kats et al. 2006). It is an active, generalist
omnivore, capable of reaching high densities and can thus function as keystone consumer
that both directly and indirectly impacts multiple trophic levels. Additionally, red swamp
crayfish are a vector for crayfish fungus plague, (Aphanomyces astaci), which in
combination with competition has contributed to the decline of European populations of
crayfish (Gherardi & Panov 2006).
The project area lies within the northern Sacramento Valley, 12.9 km N-NE of
Red Bluff, California within the Bend. Red Bluff is approximately 48km south of
Redding, and 201 km north of Sacramento. Within the Bend, (Figure 1 and Figure 2;
10U E568810 N4458876) two invasive species, bullfrog and crayfish have been targeted
for experimental control in two man-made freshwater emergent wetlands. Although each
species has a potential for terrestrial dispersal, ecological or physiological life history
requirements of these species requires a permanent aquatic environment from which to
return.
The goal of this project was to establish a management regime in two wetlands
within the wetland complex designed to mimic the natural hydroperiod of a large vernal
pool and recreate conditions that would favor native species over invasives. Additionally,
through the addition of a settling basin and screen within the irrigation system, I
investigated the effectiveness of a physical barrier to crayfish and bullfrog dispersal.
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Finally, to evaluate the effectiveness of hydroperiod modification and barrier
establishment, I compared the species assemblages of the modified wetlands to those
wetlands where permanent populations of bullfrog and red swamp crayfish are
established and to co-occurring vernal pools to evaluate community composition.
Study Design and Methods
Component 1: Establishment and Spread
Goal: To understand the roles humans played in the range expansion and establishment
of bullfrog and crayfish and their life histories.
The basic methodology involved a literature review collecting and organizing
information on the roles humans played in the range expansion and establishment of the
species and on the life history of bullfrog and crayfish.
Component 2: Impacts on Native Communities
Goal: To investigate the impacts of bullfrog and crayfish on native communities.
The basic methodology involved a literature review of the species and their
impact upon native communities. Additionally, I examine the literature for potential
synergistic (Invasional meltdown) effects when the two species co-occur.
Component 3: Control and Recovery of Wetland Communities
Goal: 1) Establish a management regime within the wetlands designed to mimic the
natural hydroperiod of a large vernal pool and recreate conditions that would favor
native species over invasives. 2) Develop a physical barrier to exclude or impede
bullfrog crayfish from dispersing into the wetlands. 3) To document community
composition following control of bullfrog and crayfish in the modified wetlands to
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determine if it is feasible to recreate natural vernal pool characteristics suitable for
native species within a manmade wetland system.
I addressed the above issues by actively manipulating the hydroperiods of the two
wetlands to mimic that of a large vernal pool. Modification of the hydroperiod removed
access to permanent water, an ecological or physiological requirement for both species of
concern. Barriers were established at water intake structures, to exclude or impede
species from dispersing into the wetlands.
I conducted field surveys and used baited traps within the wetlands throughout the
flood stage and during drawdown to assess species occurrence and density within the
wetlands to determine if infrastructure modifications made to exclude or impede bullfrog
and crayfish dispersal was successful.
I compared the species assemblages of the modified wetlands to co-occurring
vernal pools to evaluate community composition to determine if it is feasible to recreate
natural vernal pool characteristics suitable for native species within a manmade wetland
system.
Preliminary Results
Component 1
Introductions of these two species can be traced to multiple mechanisms, release
from the aquarium trade, introductions for game improvement, pest control, aquaculture
escapes, and dispersal from naturalized populations resulting in each species now being
found spread across the world. Rapid growth, high fecundity, polytrophism, resistance to
extreme environmental conditions and resistance to disease are traits which make a
species suitable for commercial exploitation and it is these same traits which make a
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species a highly successful invader (Barbaresi & Gherardi (2000), traits both bullfrog and
red swamp crayfish possess.
Both species are native to North America from Mexico into the south and eastern
United States however the native range of bullfrog extends into Canada (Santos-Barrera
et al. 2009). Bullfrogs have been introduced for food around the world and can be found
in Hawaii, Mexico, Africa, Europe, Asia, South America, the Caribbean Islands, Brazil,
and Cuba and South Korea (Santos-Barrera et al. 2009). The red swamp crayfish has
become the most cosmopolitan freshwater crayfish species in the world as a result of
numerous intentional and accidental introductions (Treguier et al. 2011).
Component 1.1 Bullfrog
The North American bullfrog (Figure 3) is the largest frog in North America.
Bullfrog reach up to 20cm in snout-to-vent length (SVL) and up to 800g in weight
(Flores, 2005, Leonard et al. 1993). They have a robust body with a wide flat head,
smooth skin and lacking dorsolateral ridges. Dorsal color can range from brown to green
while the ventral surface is white or yellow. American bullfrogs have conspicuous
tympanic membranes (eardrums). In males, the ear drum is larger than the eye, while in
females it is about the same size or smaller. Bullfrogs bred in spring and summer in
deep areas with dense cover (Cook & Jennings, 2007). Tadpoles are greenish yellow
with small spots, growing up to 15 cm (Flores 2005). The bullfrog’s call is a deep “jugo-rum” or “br-wum” bellow, made day and night, and can be heard up to 0.40km away.
Unlike other frogs, it spends most of its time in water from where it also does most of its
hunting (Murphy 2003).
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In concert with human assisted dispersal, multiple life history traits have allowed
the bullfrog to become established outside its native range. The species is a large, gape
limited, i.e. the body size and width of the mouth determine how large a prey item it can
consume, generalist carnivore, willing to cannibalize its own young. They are a highly
adaptable species that can easily adapt to environments modified by human.
Bullfrogs are often located in warmer, lentic habitats such as sluggish backwaters
and oxbow lakes, farm ponds, reservoirs, and marshes however; bullfrogs occupy a wide
range of aquatic habitats including lakes, ponds, swamps, reservoirs, marshes, streams,
rivers, irrigation ponds and ditches. It has been suggested that bullfrogs may have a
preference for highly artificial and highly modified habitats, such as millponds, livestock
grazing ponds and reservoirs (Doubledee et al. 2003, Ficetola et al. 2007).
Human-driven habitat modification, such as changes in hydrology from seasonal
to permanent water, removal of emergent vegetative cover, increased water temperatures
and resulting aquatic vegetation, which are common factors in lakes polluted by humans,
favor bullfrogs by providing suitable habitats for growth, reproduction and escape from
predators (Murphy 2003, Cook & Jennings 2007). Such habitats are typically
characterized by a decrease or complete lack of habitat complexity which probably
enhances habitats for bullfrogs by providing optimum conditions for bullfrogs to find and
devour their prey (Doubledee et al. 2003). Li et al. (2011) concluded from their study
that the ease with which bullfrogs have invaded islands of the Zhoushan archipelago
relative to the mainland has little to do with biotic resistance but results from variation in
factors under human control. Fuller et al. (2011) conclude that extensive human
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modifications of the Trinity River system in California have caused it to support
American bullfrogs to the detriment of native amphibian species.
Although bullfrog metamorphosis can occur in less than one year, in most of the
range, the tadpoles of bullfrogs require more than 1 year for metamorphosis. This species
therefore requires water permanence for reproduction (Ficetola et al. 2007). For this
reason bullfrog are often excluded from temporary ponds because they have larval
periods exceeding one year. Most fish appear to be averse to eating bullfrog tadpoles
because of their undesirable taste, so the bullfrog has been able to thrive in freshwater
lakes and rivers without these fish predators. Bullfrogs also have a longer breeding
season and a higher rate of pre-metamorphic survivorship, which also allows them to be
more successful than other frogs. One possible reason the bullfrog has thrived in
California is that, in eastern North America bullfrogs evolved with a diverse predatory
fish fauna, so that it has evolved mechanisms that enable it to avoid predation by fish in
various environments (Murphy 2003). 790 species of fish are native to the United States
and Warren and Burr (1994) believe that at least 375 species occur in the Mississippi
basin alone, while California possess a total of 66 native freshwater, estuarine, or
anadromous species within its huge area (Moyle 2002).
1.2 Crayfish
Red swamp crayfish (Figure 4) are dark red in color with raised bright red spots
covering the body and claws and a black wedge-shaped stripe on the top of the abdomen.
They may vary in length from 5 to 13 cm. Occasionally, a genetic mutation may turn the
body and/or claws blue, however all other features including the red raised spots remain
the same (“Red Swamp Crayfish” n.d.).
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Red swamp crayfish may inhabit a wide variety of freshwater habitats including
rivers, lakes, ponds, streams, canals, and seasonally flooded swamps, marshes rice fields
and irrigation ditches. It is very tolerant and adaptable to a wide range of aquatic
conditions including moderate salinity, low oxygen levels, extreme temperatures, and
pollution (Cruz & Rebelo 2007, Gherardi & Panov 2006, Global Invasive Species
Database 2011).
Red swamp crayfish employ an r-strategy, exhibiting a short life cycle and high
fecundity (Global Invasive Species Database 2011). In their native range, red swamp
crayfish mate in autumn and lay eggs in spring to early summer however in places with a
long flooding period, greater than 6 months, there may be at least two reproductive
periods in autumn and spring. The females burrow into soft sediment and lay eggs. The
number of eggs varies with the size of the female, with large crayfish laying as many as
650 eggs at a time. They are tolerant of fluctuating water levels and can survive long dry
spells by remaining in burrows or crawling over land to other water sources. Red swamp
crayfish are omnivorous, feeding on aquatic plants, snails, insects and fish and amphibian
eggs and young (“Red Swamp Crayfish” n.d., Global Invasive Species Database 2011).
Component 2
Ilheu et al. (2007) characterizes successful invaders as possessing a tolerance to
wide environmental conditions, omnivory, rapid growth, dispersal, breeding in ephemeral
habitats, and other traits associated with opportunism. High predation efficiency and the
lack of predators frequently make them the originators of important changes to the
original biota. These are traits both bullfrog and red swamp crayfish possess.
Component 2.1 Bullfrog
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Bullfrogs impact native communities through competition and predation, serving
as an agent of ecosystem modification through modification of nutrient regimes and
through altering the biomass, structure and composition of algal communities. Finally,
bullfrogs serve as a disease vector and reservoir for chytrid fungus.
Bullfrog tadpoles are large giving them a competitive size advantage when
compared to other amphibians that co-occur within their range. Larger size in
combination with a higher pre-metamorphic survivorship and predator avoidance
strategies which permits the species to coexist with predatory fish provides the species a
higher fecundity rate than native species. The use of modified environments, provides
the opportunity to occupy habitats unavailable to other species, potentially provides
bullfrog a numerical advantage in habitats.
Boone (2004) suggests that bullfrogs may eliminate native amphibians directly
through predation or interference competition, or indirectly through exploitative
competition, behavior modification, habitat alteration, or introduction of disease or
parasites. Li et al. (2011) suggest that frog community responses to recent American
bullfrog invasions in the Zhoushan Archipelago, China suggest that post-metamorphosis
bullfrog impacts on native frog communities are proportional to post-metamorphosis
bullfrog density.
Where bullfrog co-occur with California red-legged frog (Rana draytonii),
D’Amore et al. (2009) documented male R. draytonii in amplexus with juvenile bullfrog
and has proposed that this causes reproductive interference by the invasive species which
could cause a reduction in the population growth rate of R. draytonii since these males
no longer call to attract females which causes fewer females to attempt to breed at the
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site, and because males engaged in amplexus are at greater risk of predation.
Additionally it raises the possibility that male R. draytonii will find the smaller female R.
draytonii unattractive, leaving them without mates (D’Amore et al. 2009).
As a large generalist carnivore, bullfrog will eat anything that can fit in their
mouth. Their diet includes small mammals, birds, other amphibians, invertebrates, and
reptiles. In laboratory conditions, tadpoles feed upon eggs and larvae of fish and their
densities in artificial habitats can depress fish larvae recruitment (Kraus, 2009). Pryor
(2003) suggests that tadpoles have considerable impact on nutrient cycling and primary
production in freshwater ecosystems due to high food intake and high population
densities (up to thousands of individuals per m²). Pryor (2003) notes that bullfrog
tadpoles alter the biomass, structure and composition of algal communities and that
several studies portray tadpoles as ‘‘ecosystem engineers’’ (Dickman, 1968; Seale, 1980;
Osborne & McLachlan, 1985; Kupferberg, 1997; Flecker et al., 1999; Peterson &
Boulton, 1999 in Pryor, 2003).
Chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis, is an
emerging disease of amphibians responsible for population declines and even extinctions
globally (Hanselmann et al., 2004). Introduced populations of bullfrog can harbor
reservoirs of the fungal agent without suffering population declines and local extinction
events which has occurred to other amphibian species.
2.2 Crayfish
Barbaresi and Gherardi (2000) identified the red swamp crayfish as a large,
prolific, aggressive species that is well adapted to life in areas with drastic, seasonal
fluctuations in water levels, where it survives by digging deep burrows. Red swamp
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crayfish are a highly adaptable, tolerant, and fecund freshwater crayfish that may inhabit
a wide range of aquatic environments. Impacts include predation on and competition
with a variety of aquatic species, reduction of macrophyte assemblages and alterations in
community compositions, alteration of water quality, and introduction of the crayfish
plague.
Red swamp crayfish are omnivorous, feeding on aquatic plants, snails, insects and
fish and amphibian eggs and young. Intense herbivory by red swamp crayfish often
causes the reduction of macrophyte mass and biodiversity (Cruz & Rebelo 2007) (Figure
5) and may lead to changes in food webs and even disappearance of some species
(Barbaresi & Gherardi 2000).
The species is known to compete with, prey on, and
reduce populations of a wide variety of aquatic species where they have been introduced
including amphibians (Diamond 1996, Cruz & Rebelo 2005, Ilheu et al. 2007) mollusks
(Cruz & Rebelo 2007), macroinvertebrates (Correia et al. 2008), and fish (Mueller et al.
2006). Part of the strong impact of crayfish might come from their potentially large per
capita effects, but the overall effect appears to be due to their capacity to become
abundant (Barbaresi & Gherardi 2000, Rodriguez et al. 2005).
Another effect of the feeding, as well as burrowing, behavior of red swamp
crayfish is altered water quality, increased bioturbation, and increased nutrient release
from sediment (Angeler et al. 2001) which may cause additional indirect impacts and
cascading ecological changes. Rodriguez et al. (2005) determined the introduction on
red swamp crayfish to Lake Chozas, Spain caused a reduction of macrophyte plant
coverage by 99% which in turn caused a 71% loss in macroinvertebrate genera, 83%
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reduction in amphibian species, a 75 loss in duck species, and a 52% reduction in
waterfowl.
In addition to competition, red swamp crayfish have also led to declines in native
crayfish species in Europe through transmission of the crayfish fungus plague
Aphanomyces astaci (“Red Swamp Crayfish” n.d.).
Component 2.3 Synergistic (Invasional Meltdown) Effects
Invasional meltdown is the process by which a group of non native species act in
concert, facilitating one another’s invasion in various ways, increasing the likelihood of
survival and potentially the ecological impact (Simberloff & Von Holle 1999). For
example, Adams, Pearl and Bury (2003) noted that the bullfrog invasion in Oregon, USA,
is facilitated by the presence of non-native fish, which increase tadpole survival by
reducing predatory macroinvertebrate densities. Red swamp crayfish has been found to
promote and maintain other invasive species populations including largemouth bass,
(Micropterus salmoides) and pike, (Esox lucius) by serving as a primary food source
(Hickley et al. 1994, Elvira et al. 1996).
Although no studies or meta-analysis has been conducted to document or analyze
the interactions and potential facilitation between bullfrog and red swamp crayfish, per
se, I conducted a literature search and constructed a logic path which documents how the
direct and indirect impacts and cascading ecological changes each species may cause
facilitates one another’s invasion, increasing the survival and ecological impacts of each
species. This interaction between these species may eventually create a predator-prey
feedback loop of unknown stability. Elvira et al. (1996) noted that when pike eliminated
their prey base, they eventually made a shift to cannibalism and despite the presence of
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red swamp crayfish as an available prey item; pike became extinct in Daimiel National
Park, Spain.
The cascading ecological changes caused by crayfish documented by Rodriguez
et al. (2005) and the resulting reduction and simplification of community composition
and structure provide bullfrog a predatory advantage (Doubledee et al. 2003). Bullfrog
are known to feed frequently on aquatic prey such as crayfish, dragonfly larvae, aquatic
hemipterans and dysticid beetles (Tyler & Hoestenbach 1979, Werner et al. 1995, in
Hirai 2004) which can be effective predators of early juvenile stages of the crayfish
(Correia 2001, Witzig et al. 1986). Although these macroinvertebrate prey upon the
juvenile stages of red swamp crayfish, the adult red swamp crayfish preys upon and
reduces populations and diversity of macroinvertebrates through predation and ecosystem
changes (Correia et al.2008). Predatory behavior by adults of each species has the
potential to correspondingly increase juvenile and larval survival.
In portions of their shared range, red swamp crayfish make up the majority of the
bullfrogs' diets (Wylie et al. 2003, Clarkson & DeVos 1986, Hirai 2004). Additionally,
Hirai et al. (2004) noted that the removal of the bullfrog, together with crayfish, is
strongly recommended for conservation and restoration of the fauna in the Mizorogaike
Pond in Japan due to the crayfish serving as a principal food source for bullfrogs.
In addition to being a prey item, the feeding and burrowing behavior of red
swamp crayfish can lead to a switch from clear water to turbid water with abundant,
bloom forming microalgae and phytoplankton (Gherardi & Acquistapace 2007,
Matsuzaki et al. 2009) (Figure 5). The feeding habits of bullfrog tadpoles include filterfeeding of phytoplankton and microalgae (Pryor 2003) which suggests red swamp
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crayfish may increase the food resources for bullfrog tadpoles while impacting ecosystem
structure and composition.
Finally, the switch from clear water to turbid water with abundant, bloom forming
microalgae and phytoplankton potentially decreases sight based predator efficiency by
providing cover. Fiksen et al. (2002) and Orr (n.d.) documented that the shading effect
of higher phytoplankton concentrations or increased turbidity may reduce predation rates
on fish larvae substantially by predators that detect their prey by sight. It is likely that the
same shading effect of higher planktonic, algal cover and turbidity reduces predation
upon bullfrog larvae and crayfish by predators that detect their prey by sight.
Component 3
Pond hydroperiod is known to regulate some amphibian communities (Semlitsch
et al. 1996 in Boone et al. 2004). In temporary pond habitats, the need to cope with
periodic drying imposes severe constraints on a species' behavior, development, and life
history, and only those species able to deal with drying are successful in these habitats
(Wellborn et al. 1996). Although bullfrogs occur in temporary waterbodies, they are
typically found in permanent ponds, whereas most other amphibians inhabit temporary
ponds. Bullfrogs are often excluded from temporary ponds because they have larval
periods exceeding one year. The pond hydroperiod gradient may be the main means of
separating Bullfrogs from other amphibian species (Boone et al. 2004). Red swamp
crayfish however are tolerant of fluctuating water levels and can survive long dry spells
by remaining in burrows or crawling over land to other water sources.
Prior to 2006, the wetlands within the project area followed a late-summer
flooding period, typically beginning in August and late-spring drawdown, beginning
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approximately May. Records indicate that intentional drawdowns did not occur on an
annual basis and that permanent retention of water to encourage establishment of bulrush
(Schoeneoplectus californicus) and cattails (Typha spp.) occurred. When previous
drawdowns occurred, when they occurred, drawdowns did not mimic natural conditions
of a California vernal system, rather it seems that water was quickly drained from the
wetlands or water was left to evaporate which had the potential to retain water through
the summer in small ponds at the outflow and in deep water portion of the pools. This is
evidenced by cattail established at the outflow structures of each of the two wetlands.
Cattails grow along lake margins and in marshes, and require wet soils during its seedling
establishment period. These wet soils or small ponds could have provided habitat and
refugia for bullfrog and crayfish to maintain populations in the wetlands.
The modification of the hydroperiod in the two wetlands was intended to result in
the elimination of the two species by removing the necessary aquatic environment
required for each. The two species were treated together for removal due to the resulting
impacts associated with each species and the potential synergistic effects when the
species co-occur. The two wetlands where the project occurred are referred to as
Roadside North and Roadside South wetland. Roadside North wetland is approximately
4.03 ha in size and Roadside South wetland is approximately 5.36 ha. Both wetlands
have a maximum depth of approximately 1m.
Under the hydroperiod modification, flooding of the wetlands occurs in
September for waterfowl use. Drawdown occurs as a slow process, mimicking a local
large vernal pool, Hog Lake (10 ha), 5.8 km east of the project area. Maximum
inundation occurs during the winter and winter rains maintains water level until
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approximately the 3rd week in April, depending on rainfall patterns. After this period,
drawdown occurs first as a rapid process to reduce initial depth and provide wet soil
characteristics to vernal pool plant associates prior to summer soil desiccation. The
initial fast drawdown of approximately 45cm is followed by a slow drawdown until the
wetland is dry. Although residual soil moisture may remain, removal of standing water
typically occurs prior to July 1st.
Initial impressions from hydrological modification are consistent with findings by
other authors (Semlitsch et al. 1996, in Boone et al. 2004, Wellborn et al. 1996) that
modification favored native species over bullfrog and crayfish. Upon hydroperiod
modification, field surveys were conducted to determine if crayfish burrows would
permit the species to remain persistent in the wetlands. Although crayfish dig burrows,
burrows excavated by field crew during August 2009, 2010 and 2011 did not locate live
individuals. This may be due to the shallow, impermeable hardpan and bedrock layer
which inhibits deep excavation of burrows by crayfish. Bullfrog spawn in the spring and
summer and by drying the ponds during the summer, no survival of juveniles occurs.
Surveys beginning in 2009 and continuing to date, have not locate bullfrog egg masses
and no bullfrog tadpoles have been found within the wetlands. Additionally, drawdown
creates conditions that give raptorial and mammalian predators an advantage in locating
these species, increasing predator efficiency. Juveniles of both species may pass through
the screen, however due to the drying of the wetlands; individuals may mature but not
successfully breed due to the drying of the ponds in the late spring and summer acting as
an ecological trap for these two species. Due to a two year larval period of bullfrog, the
modified hydroperiod does not provide water for the bullfrog larvae to remain therefore
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any individuals that successfully breed in the modified wetlands do not produce
successful metamorphosing individuals. Finally, drying the pools in the summer is not
conducive to overland travel of each species, due to low humidity and high ambient air
temperature, (July’s highest daily mean temperature for July is 36.7oC, daily mean is
27.6; NCDC 2012) leading to potential desiccation and death of individuals in the upland
environment.
Although the modification of the hydroperiod in the two wetlands results in the
annual elimination of the two species, reflooding the wetlands has the potential to
reintroduce each species on an annual basis. As part of an infrastructure improvement
project, a physical barrier was developed to preclude bullfrog and crayfish from entering
the system. In 2010, the installation of a settling basin, screen (Figure 6 and Figure 7)
and the modification of the water intake structure limited the size of bullfrog and crayfish
which were able to pass the screen to less than 2.54cm, a size incapable of breeding
within a single season. The intake pipe was additionally raised from the bottom of the
intake structure requiring bullfrog and crayfish to actively swim in for entrainment into
the irrigation line that supplies water to the wetlands. Previously, the intake structure was
seated on the bottom of an intake weir and required no upward movement for
entrainment. The screen eliminates large, mature individuals of both species, potentially
reducing or eliminating gravid female crayfish carrying young into the wetlands.
To determine the efficacy of the basin and screen, I conducted trapping using
baited traps within the wetlands. Common cylindrical wire minnow traps baited with
pieces of fish were used to catch crayfish (Momot & Gowing, 1972). The disadvantage
of this method is that it samples an unknown area and gives a relative estimate of density
20
VECTOR CORRUPTION
with an unknown standard error. Trapping occurred in the wetlands from March 26 until
May 12 yielding 48 trapping days. From March 26 until April 15, two traps were present
in each wetland and from April 16 until May 12, a single trap was present in each
wetland; 1656 trapping hours spent in each wetland yielding a total of 3312 trapping
hours.
At the time of writing, one adult and one juvenile red swamp crayfish were
removed from the north wetland while 5 juvenile and 2 adult were removed from the
south wetland. An additional 4 dead crayfish were removed from the north wetland and
18 from the south wetland yielding 31 crayfish that made it through the barrier system.
At Lake Naivasha, Kenya, red swamp crayfish achieved densities of over 500 m-2
(Harper et al. 2002) and experiments conducted by Gherardi & Acquistapace (2007)
found community impacts at low densities of 4 m2 with more pronounced impacts when
crayfish reached densities of 8 m2. The relative density of crayfish within the Roadside
North wetland is 1.48 x10-4 m2, while the relative density of crayfish within Roadside
South is 4.66 x 10-4 m2 indicating the relative density of crayfish within Roadside North
and Roadside South wetlands are very low. Within the settlement basin, 103 total
individuals were removed from September 201 until May; 14 dead individuals, 89 live
individuals of multiple age classes, including females carrying young, indicate that the
basin and screen function as an effective, yet permeable barrier to invasion, limiting the
recolonization by crayfish.
To determine if it is feasible to manage for and recreate natural vernal pool
characteristics for native species I conducted field surveys documenting multiple species
of plants, invertebrate, and vertebrate species not previously documented as using the
21
VECTOR CORRUPTION
wetlands, or not found in abundance are now found in the wetlands. Additionally, during
surveys in 2010-2012 a federally threatened plant species, slender Orcutt grass (Orcuttia
tenuis), was identified as occurring in North wetland in abundance. Prior to 2010, it did
not occur within either North or South wetland.
In their surveys of vernal pools in the region, Lis and Eggeman (2000) classified
the vegetation mosaic observed into three grades:
Grade I vegetation is dense, persistent, and tall, consists primarily of Eleocharis
and Eryngium, and occurs in regions of the pool that are inundated for 6 or more
months. The soil is deep and the water column is almost completely occluded
with vegetation… Grade II vegetation is dense, non-persistent, and short, consists
mostly of 50-70% Isotes, Orcuttia, Pilularia, and Marsilea, and 50-30%
Eleocharis, Eryngium, and Downingia. These types occur in regions of the pool
that are inundated for 4-6 months, the soil is 3-6 cm deep, and the water column is
deep and mostly open measuring 7-30+cm... Grade III vegetation is sparse, nonpersistent, and short, consists of Isoetes, Orcuttia, Pilularia, Marsilea, and
Downingia in combination with areas of exposed bedrock or hardpan. The soil
depth is 0-3cm and the water column is deep and open measuring 7-30+cm.
Spatially these three grades are distributed across the pool bottom in varying
patterns having either distinct boundaries or fuzzy transition zones (p. 35-37).
Results indicate that the wetlands possess vegetation characteristics and species which
are associated with Grade I, II, and III (Figure 8-Figure 10). Additional field surveys
conducted in March, May and in June documented multiple macroinvertebrate groups
within the wetlands. Predatory aquatic mites, copepods, Crustacea (Conchostraca &
22
VECTOR CORRUPTION
23
Anostraca), 2 species of snails (Gastropoda), planaria, multiple arthropod families,
aquatic worms, and two vertebrates, western toads (Anaxyrus boreas) = (Bufo boreas)
and Pacific chorus frog (Pseudacris regilla). I believe the outcome of five years of
drawdown, to mimic natural vernal pool conditions, and the establishment of barriers to
bullfrog and crayfish colonization and persistence led to the development of a wetland
community composition comparable to a natural vernal pool system while becoming
unsuitable habitat for bullfrog and crayfish.
Discussion
Modifying the hydroperiod of the two wetlands to mimic a native vernal wetland
system, favors native species that evolved in a system of alternating wet-dry periods
(Semlitsch et al. 1996, in Boone et al. 2004, Wellborn et al. 1996). Native species were
able to successfully colonize and establish self sustaining populations in the wetlands
after hydroperiod modification and invasive control.
The presence of bullfrog and crayfish in the ponds indicate that overland travel
occurred and/or the barrier was permeable, permitting invasive propagules to disperse
into the wetlands. During the 2011 irrigation season, structural improvements were made
to the wetlands irrigation system and the barrier screen was removed but not immediately
replaced on multiple occasions. There was the expressed perception by personnel that the
screen was acting as a flow impediment and that bullfrog and crayfish should be in the
wetlands. During this period, the flooding of Roadside South wetland was occurring
which may account for the higher density of crayfish within this wetland. To prevent this
from occurring in the future and to reduce the permeability of the barrier, project
VECTOR CORRUPTION
planning in the fall of 2012 includes the placement of a screen with 0.635cm openings
across the width of the settling basin.
In review of the literature regarding impacts from crayfish, it may be perceived
that the relative density of crayfish within Roadside North and South wetlands may be
below the threshold of community impacts. However, a relative crayfish density of 0.10
m2 was obtained from a 0.15 ha natural, ephemeral wetland (Figure 5) within the wetland
complex in 2011. This ephemeral wetland possesses a persistent population of bullfrog
and crayfish and although it dries in the summer resulting in mortality to an unknown
percentage of the crayfish population, it is deep enough that moist soil is maintained
within burrows constructed by crayfish.
Within this wetland, no submergent or emergent vegetation is present; water is
turbid due to suspended particles and a microalgae bloom. Surveys indicate a complete
lack of use by native amphibians; no native amphibian egg masses, tadpoles,
metamorphed juveniles or adults were located utilizing the aquatic habitat provided by
this wetland. In comparison to Roadside North and South wetlands, it possessed a meager
macroinvertebrate assemblage consisting of 2 species of snails (Gastropoda), aquatic
worms, and two families of arthropods, Hemiptera and Coleoptera. The lack of use by
native amphibians, the meager macroinvertebrate assemblage present and the absence of
aquatic vegetation in and around the wetland indicates that impacts to native communities
may occur at much lower densities than has been reported in the literature.
The project demonstrated those invasives that require permanent water cannot
establish permanent populations in the Roadside wetlands and although individuals may
colonize on an annual basis, the modified wetland habitat is ultimately an ecological sink
24
VECTOR CORRUPTION
25
for these species and results in death of individuals, interrupting the vector preventing
further satellite populations from developing from which to spread. Interrupting the
vector results in a reduced abundance of the invasives, and provides native species with a
potential competitive advantage over the invasives.
Although simplified, this situation illustrates vector management as described by
Ruiz and Carlton (2003). In Ruiz and Carlton (2003) four components of vector
management are identified; (1) vector analysis, (2) vector strength, (3) vector interruption
and (4) efficacy. In this situation, vector analysis (propagule supply) of the water
delivery system was modified to exclude bullfrog and crayfish into a wetland complex.
A second vector was of overland dispersal of each species was insufficient to maintain
populations.
Management actions were implemented consisting of hydroperiod modification,
and intake modification screening. The efficacy of vector interruption has been
examined through monitoring and modifications to further improve vector interruption
are proposed for implementation. Surviving individuals and individuals within the intake
structure are documented and removed. Successful reproduction efforts are documented
via the observation of larvae in the modified wetlands and the production of dispersing
juveniles which are able to survive drawdown and breed themselves are used to
determine the efficacy of vector interruption. To date, monitoring has indicated that
adults of each species are not able to produce offspring that are able to survive drawdown
and breed themselves, reducing and disrupting the flow of invasive propagules,
corrupting the vector.
Conclusion(s)
VECTOR CORRUPTION
Providing water intakes with a screen and through the use of a settling basin to
trap individuals, invasive species control can benefit native species by reducing invasive
species abundance to wetlands by invasive species. Additionally, wetland design that
incorporates annual drying facilitates the establishment and use of native biota while
precluding those invasive species, which require permanent water.
Doubledee (2003) suggests that pond draining is a successful course of action
when overlap of bullfrog, fish and crayfish occur and Kirby (2005) is emphatic that
barriers are paramount in limiting crayfish distribution. It is anthropogenic changes that
has facilitated the movement of invasive species like bullfrogs and crayfish by increasing
the amount of suitable habitat and speed their distribution throughout the landscape.
Sepulveda et al. (2012) suggests using the cancer treatment model when tackling aquatic
invasive species, prevention, early detection, diagnosis, treatment, and rehabilitation.
Efforts should focus on preventing aquatic invasive species introductions. Early
detection requires diligence, recognizing that new populations or individuals may present
themselves at any time. When discovered, the severity and potential spread of the
invasion must be diagnosed. Use available treatments methods to contain or eradicate the
invasion and finally, employ available tools to increase community resistance to future
invasions or increase native species resilience.
Although bullfrog and crayfish will always be present in the Bend wetland
complex due to the presence of adjacent, permanent waters, reducing invasive propagules
and enhancing the habitat for native species through management actions has been a
success. When complete eradication of aliens is not possible, alternatives should be
considered, including management strategies that attempt to reduce aliens to low numbers
26
VECTOR CORRUPTION
to facilitate the recovery of native species (Kats & Ferrer 2003). The management
actions and infrastructure modifications demonstrate that relatively simple actions, such
as a hydroperiod modification, settling basin, and intake screen installation, can help to
reduce invasive propagule supply and corrupt vector supply.
27
VECTOR CORRUPTION
28
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Figure 1. Locality map of project area.
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Figure 2. Roadside North and Roadside South wetlands within project area.
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Figure 3. (male) American Bullfrog (Lithobates catesbeianus) (= Rana catesbeiana)
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Figure 4. Louisiana Red Swamp Crayfish (Procambarus clarkii)
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Figure 5. Ephemeral wetland (0.15ha) within the Bend with a persistent population of
bullfrog and crayfish. Note turbid water with microalgae bloom and lack of
macrophytes.
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VECTOR CORRUPTION
Figure 6. Settling Basin, dimensions L1.8m x W1.8m x D1.5m
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VECTOR CORRUPTION
Figure 7. Looking down into Settling Basin at 2.54cm screen. Pipes elevated
approximately 30cm from floor of basin and extend approximately 46cm from the
walls of the basin.
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VECTOR CORRUPTION
Figure 8. Grade I vegetation in Roadside South wetland.
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VECTOR CORRUPTION
Figure 9. Grade II vegetation in Roadside South wetland.
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VECTOR CORRUPTION
Figure 10. Grade III vegetation in Roadside North wetland. Glaucous vegetation in the
right portion of photo is the federally threatened species, slender Orcutt grass
(Orcuttia tenuis).
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