Make Them Live Hard and Die Young
Gary A. Diridoni
Oregon State University
Capstone Project FW 506
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 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). 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).
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. 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 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.
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 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 “jug-o-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).
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 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).
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.).
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).
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
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 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.
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 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% 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 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 filter-feeding of phytoplankton and microalgae (Pryor 2003) which suggests red swamp 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.
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 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 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 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 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 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, non-persistent, 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 & 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.
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 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 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.
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 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.
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