J. OF SCI. 64(2):73-88 MAY, 2012 (PUBLISHED JUNE 2015)

FERAL HOG DAMAGE TO ENDANGERED HOUSTON ( HOUSTONENSIS) IN THE LOST PINES OF TEXAS

Donald J. Brown, Melissa C. Jones, Jim Bell, and Michael R.J. Forstner Department of Biology, Texas State University-San Marcos 601 University Drive, San Marcos, TX 78666.

Abstract.−Feral hogs (Sus scrofa) are considered a destructive exotic invasive in the U.S., and their abundance appears to be increasing in the Lost Pines ecoregion of Texas. This is of particular concern due to the status of the Lost Pines as the last remaining stronghold for the federally endangered Houston toad (Bufo [] houstonensis). We documented short-term impacts of feral hogs to pond perimeters, water quality, and aquatic at ponds on one of the primary recovery sites for the Houston toad, the Griffith League Ranch in Bastrop County. We also investigated the efficacy of exclosure fencing for eliminating feral hog use of a pond on the adjacent Welsh tract. We found that structural damage to pond perimeters by feral hog wallowing resulted in increased nutrient concentrations and total suspended solids in the ponds, but this damage did not affect aquatic predatory captures. We found the exclosure fence to be an effective barrier for feral hogs. The ability of feral hogs to rapidly degrade Houston toad breeding habitat warrants consideration of proactive control or deterrent measures, particularly at heavily used breeding ponds. ______

Domestic hogs (Sus scrofa) were introduced to the United States in 1539, and known feral populations were established in South Carolina in the early 1900’s (Hanson & Karstad 1959). By 1959 feral hogs ranged from North Carolina to east Texas, and by 1998 feral hog populations existed across the entire southern and central U. S. (Hanson & Karstad 1959; Gipson et al. 1998). In addition to a rapidly expanding distribution, hog densities within states continue to increase (Tolleson et al. 1995; Waithman et al. 1999). As of 1995 feral hogs were known to be present in 185 of the 254 counties in Texas, with Texas estimated to house up to 50% of the feral hogs in the United States (Wigley 1995).

Feral hogs are one of the most destructive exotic invasive species in the U. S., and are considered one of the 100 worst invasive species in the world (Lowe et al. 2000). In addition to negative economic impacts to farmers and ranchers (Wigley 1995; Adams et 74 THE TEXAS JOURNAL OF SCIENCE-VOL. 64, NO. 2, 2012 al. 2005), disturbance through wallowing and rooting can cause habitat degradation for wildlife, especially for species inhabiting wetlands. Doupe et al. (2009) showed that feral hogs degraded freshwater habitat in lagoons in Australia through aquatic macrophyte consumption and wallowing, which decreased clarity, dissolved oxygen levels, and pH in the water column. Kaller & Kelso (2006) found that feral hogs altered invertebrate community composition dynamics and increased fecal coliform counts in a Louisiana watershed. Rooting disturbance to soil, forest litter, and vegetation resulted in declines of red-backed voles (Clethrionomys gapperi) and short-tailed shrews (Blarina brevicauda) in Great Smoky Mountains National Park (Singer et al. 1984), and potentially to declines of southern dusky salamanders (Desmognathus auriculatus) and spotted dusky salamanders (Desmognathus conanti) in Florida (Means & Travis 2007). Further, although feral hogs primarily consume plant matter (Everitt & Alaniz 1980), they also consume both invertebrates and vertebrates (Wilcox & Vuren 2009; Jolley et al. 2010). Thus, small vertebrate taxa with high densities near wetlands (e.g., ) could be particularly vulnerable to feral hog , as feral hogs routinely visit wet environments to wallow (Wood & Brenneman 1980).

The Lost Pines ecoregion of Texas is a 34,400 ha remnant of pine- dominated forest that was isolated from the East Texas ecoregion during the Pleistocene (Bryant 1977; Al-Rabab’ah & Williams 2004). The Lost Pines is home to a diverse array of wildlife (Taber & Fleenor 2003; White 2003; Rebhorn 2004; Marcum 2005; Ferguson et al. 2008), and it is well known as the last remaining stronghold for the federally endangered Houston toad (Bufo [Anaxyrus] houstonensis; Gottschalk 1970). Until recent years feral hogs were not abundant in the Lost Pines and thus feral hog impacts to Houston toad habitat was not a concern (U.S. Fish and Wildlife Service 1984). However, the abundance of feral hogs in the Lost Pines is considered to be increasing, and consequently their impacts on Houston toad habitat is of growing concern.

BROWN, ET AL. 75

The purpose of this study was to document short-term impacts of feral hogs to Houston toad habitat, and to assess how the impacts might affect the Houston toad in the Lost Pines. Because this is a recent problem, there are no data linking feral hog activity to Houston toad population trends. However, it is clear from previously published research that feral hogs pose a major threat to populations, and this paper seeks to increase awareness of this threat to the Houston toad.

METHODS Study Area.−This study was conducted on the 1,948 ha Griffith League Ranch (GLR) and the adjacent 157 ha Welsh tract in Bastrop County, Texas. The majority of documented Houston in the wild are located on two properties, the GLR and (Swannack et al. 2009), which are separated by a straight-line distance of 2.2 km. The property is underlain by deep sandy soils of the Patilo-Demona-Silstid Association (Baker et al. 1979). The forest overstory is dominated by loblolly pine (), post oak (), blackjack oak (Quercus marilandica), and eastern red cedar (Juniperus virginiana). The understory is dominated by yaupon holly (), American beautyberry (Callicarpa americana), and farkleberry (Vaccinium arboreum).

The GLR contains 17 ponds, which range from highly ephemeral (n = 2) to permanent (n = 3). We have observed Houston toads at 15 of these ponds since we began monitoring reproduction on the property in 2000, and 11 are known breeding ponds. The Welsh tract contains one semi-permanent and one ephemeral pond. We have documented Houston toads and successful reproduction at the semi- permanent pond.

Monitoring efforts on the GLR and the adjacent Welsh property have been extensive since 2000 (Forstner & Ahlbrandt 2003; Jackson et al. 2006; Swannack et al. 2009). In 2001 we observed feral hog wallows at the Houston toad breeding pond on Welsh, with continual 76 THE TEXAS JOURNAL OF SCIENCE-VOL. 64, NO. 2, 2012 hog use apparent in subsequent years. In 2008, a hog exclosure fence was installed at this pond, with the goal of excluding hogs from the pond and 8 ha of surrounding upland habitat. In 2001 we also observed feral hogs in upland habitat on the GLR. However, feral hog wallowing at Houston toad breeding ponds on the GLR did not occur until 2005 (pers. obs.) and remained restricted to a single pond (Pond 11) until 2011. In February and March of 2011 we documented wallows at an additional nine ponds. By April of 2011 substantial structural damage to shorelines had occurred at seven of the ponds.

Data Collection.−We monitored Houston toad use of the two ponds on the Welsh tract before and after exclosure fence installation. Houston toad surveys are generally conducted during warm humid nights from January to May of each year. The primary purpose of these surveys is to document male chorusing, but we also visit the ponds to collect data on individual males or females. Following chorusing we inspect the ponds for Houston toad egg strands, and subsequently tadpoles and toadlets. In each year since 2001 we have completed more than 12 audio surveys and 10 daylight surveys for the Welsh ponds, and 20 audio surveys and 10 daylight surveys for the GLR ponds. In addition to monitoring Houston toads on Welsh, we investigated the value of the fence for hog exclusion through monitoring of fence-penetration by feral hogs or other large .

On the GLR, we assessed feral hog wallow impacts to pond perimeters, aquatic arthropods, and water quality using habitat monitoring data collected prior to and after the presence of hog wallows. We documented structural damage to pond perimeters through photographs. We assessed aquatic arthropod changes using samples collected approximately two months prior to, and one month after, initial detection of hog wallows. We sampled aquatic arthropods at eight ponds using a standard dip net (900 micron netting). At each pond we sampled three points along the perimeter, maintaining approximately even spacing between points, and performed three dip net sweeps per point. We stored samples in BROWN, ET AL. 77 plastic collection tubes containing 95% ethanol, and later identified insects to family and other arthropods to class or order. Because this study was concerned with potential impacts to an amphibian, we were particularly interested in insect families that are known to prey upon larval amphibians (Toledo 2005; Wells 2007).

We assessed water quality at all ponds holding water during the study period using water samples collected approximately one week prior to, and one month after, initial detection of hog wallows. We sampled water within 1 m of pond edges using 1 L Nalgene® collection bottles. Within 24 hours of collection we estimated pH using a SympHony 5B70P pH meter, filtered pond water through Gelman A/E glass-fiber filters (1 μm pore size), and preserved water samples with 85% sulfuric acid. We extracted chlorophyll a (Chl a) from filters with acetone, and analyzed Chl a using a Turner Designs Trilogy fluorometer. For quantification of total suspended solids (TSS) in the water column, we dried pre-combusted and pre-weighed filters at 60 °C for 48 to 72 hours, re-weighed them, and combusted the filters at 550 °C for four hours (Heiri et al. 2001). We obtained filter mass using a Mettler Tolido MX5 microbalance.

We used a Varian Cary 50 Ultraviolet-Visible light spectrophotometer for the remaining water quality analyses. We measured soluble reactive phosphorus (SRP) and total phosphorus (TP) using the molybdenum blue method (Wetzel & Likens 2000). To estimate TP we digested unfiltered samples with potassium - persulfate then quantified SRP. We measured nitrate (NO3 ) and total nitrogen (TN) using second-derivative UV spectroscopy (Crumpton et al. 1992). To estimate TN we digested unfiltered samples with - alkaline potassium persulfate then quantified NO3 . We analyzed + ammonium (NH4 ) using the phenol-hypochlorite method (Wetzel & Likens 2000). For all water quality analyses we collected two water samples per pond, and used the average of the replicates for this study.

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We considered ponds as having intermediate damage if less than 30% of the pond perimeter had been affected by feral hogs. For this investigation we grouped the GLR ponds into three categories based on structural damage to pond perimeters at the time of the study: no damage (ponds 1, 3, 4, 8, 14, 16, 17), intermediate damage (ponds 6, 9, 10), and substantial damage (ponds 2, 5, 7, 11, 12, 13, 15). The water quality investigation included two, two, and four ponds in the no damage, intermediate damage, and substantial damage categories, respectively; the remaining ponds were dry during one or both sampling events and thus were not sampled. The aquatic arthropod investigation included one, two, and four ponds in the no damage, intermediate damage, and substantial damage categories, respectively. We did not use data from Pond 11 because wallows were present prior to this study, and thus the pond was not suitable for before-after comparisons. The small sample sizes within feral hog damage categories precluded our ability to conduct formal statistical comparisons on these data. Thus, we used simple descriptive statistics to draw conclusions about short-term effects of hog wallowing activity on Houston toad habitat in the Lost Pines.

RESULTS Structural damage to ponds included soil upturning and creation of temporary pools on shallow slopes at pond edges (Fig. 1). Rooting activity near ponds disturbed the leaf litter and duff substrate layers. Water quality changes after wallowing activity included decreases in - + pH, and increases in SRP, TP, NO3 , NH4 , TN, Chl a, and TSS (Table 1). Based on proportional changes of parameter values between sampling periods, wallowing activity had a much larger influence on water quality in ponds with substantial damage, while proportional changes were smaller between ponds with intermediate damage and ponds with no damage (Table 1). Ponds with substantial damage had lower pH, higher nutrient levels BROWN, ET AL. 79

Figure 1. Photographs showing feral hog damage in spring 2011 to endangered Houston toad (Bufo houstonensis) habitat on the Griffith League Ranch (GLR) in Bastrop County, Texas: (A) upland rooting near Pond 12; (B) aerial image displaying hog wallows around the perimeter of Pond 2, taken with a gyrocopter at ca. 45 m; (C) a hog wallow at Pond 2; (D) a hog wallow at Pond 7. and chlorophyll a, and more suspended solids in the water column. Alternately, water quality at ponds with intermediate damage was similar to ponds without hog wallows (Table 1). There was no indication that wallowing activity affected aquatic predatory or non- predatory arthropod captures (Table 2).

On the Welsh tract we have not observed penetration of the exclosure fence by feral hogs in the years since installation. There have also not been any new hog wallows at the ponds within the exclosure, and we have not detected hogs at the Houston toad breeding pond using camera trapping since the fence was installed. Prior to 2005 we documented chorusing and toadlet emergence at the Welsh breeding pond. In 2005 feral hog damage to the pond,

BROWN, ET AL. 81

Table 2. Counts of aquatic invertebrate families before and after the presence of feral hog wallows at ponds 5 and 7 on the Griffith League Ranch (GLR) in Bastrop County, Texas. We included Pond 16, which had not been utilized by feral hogs, as a control. Families with bold lettering are known amphibian predators.

No Damage Intermediate Damage Substantial Damage

Order/Class Pond 16 Pond 9 Pond 10 Pond 2 Pond 5 Pond 7 Pond 12 Family Pre Post Pre Post Pre Post Pre Post Pre Post Pre Post Pre Post Amphipoda 0 2 1 0 0 0 62 123 0 0 0 0 1 0 Coleoptera Curculionidae 00 0010 00000000 Dytiscidae 00 3507 10282012 Haliplidae 02 0000 13000000 Hydrophilidae 00 3400 00600000 Diptera Ceratopogonidae 0 0 17 0 0 0 0 0 0 0 13 0 0 0 Chironomidae 1 5 44 4 0 0 31 129 60 43 32 84 38 19 Culicidae 01 0000 01000000 Tabanidae 00 0000 00010000 Tipulidae 00 1000 00002000 Ephemeroptera Caenidae 01 0000 00000021 Hemiptera Belostomatidae 00 0001 00000000 Corixidae 00 0100 00011000 Gerridae 00 0000 00020000 Notonectidae 0 0 20 1 0 2 9 1 4 43 6 5 0 0 Gastropoda Physidae 3 8 164 28 2 0 5 7 13 1 40 10 2 17 Planorbidae 10 5400 00000000 Hirudinea Erpobdellidae 0 0 10 214 01000000 Odonata Anisoptera 2 1 2 3 8 19 1 15 8 5 8 5 0 0 Calopterygidae 00 0000 00100000 Zygoptera 00 0011 00001000 Oligochaeta 00 2000 00000000

Total (Predators) 2 1 5 8 9 28 2 15 11 14 11 5 1 2 Total (Other) 5 19 267 44 4 6 108 265 83 90 94 99 43 37 particularly around the Houston toad emergence area, became substantial. Between 2006 and 2008 reproduction of Houston toads was not detected at the pond, and we noted chorusing of only one or a few males. Following the exclosure fence installation we documented an increase in Houston toad chorus size in 2009 and 2010, with reproduction confirmed in 2010. 82 THE TEXAS JOURNAL OF SCIENCE-VOL. 64, NO. 2, 2012

DISCUSSION The results of this investigation agree with previous studies that found increased nutrient concentrations in aquatic environments following hog wallowing and rooting activity (Singer et al. 1984; Doupe et al. 2009). Our results were in contrast to Kaller & Kelso (2006), who reported positive associations between hog disturbance and captures of several aquatic predatory arthropods. However, our study addressed only short-term changes, and longer-term invertebrate community composition and abundance effects are possible (Kaller & Kelso 2006). The data we have from excluding hogs from a single breeding site are intriguing because chorusing and reproduction increased after hogs were excluded. Below we list several concerns about impacts of feral hogs to Houston toad populations in the Lost Pines.

Concern 1: Reduction in reproductive success due to structural damage to pond perimeters.−Structural changes to ponds could affect egg strand and tadpole survivorship, as well as breeding site selection. The creation of temporary pools at pond perimeters is problematic if Houston toads utilize the wallows for reproduction. Houston toad eggs hatch in two to seven days and tadpoles typically metamorphose in approximately two months in the wild (Hillis et al. 1984; Quinn & Mengden 1984). We received very little rainfall on the GLR in March and April of 2011 (0.78 cm), and with minimal precipitation input most of the perimeter wallows dried within three weeks of creation (pers. obs.). Thus, there is strong potential for wallows to trap tadpoles. Although we did not detect Houston toad egg strands on the GLR in 2011, we did observe gray tree (Hyla versicolor) eggs in hog wallows.

Adult male Houston toads aggregate at locations within ponds containing shallow bank slopes and shallow water, and Houston toad tadpoles aggregate in the shallows during the day for feeding (Forstner & Ahlbrandt 2003). Thus, the heterogeneous slopes created by hogs could influence Houston toad calling site selection within BROWN, ET AL. 83 ponds and potentially use of ponds for breeding when shallow banks are limited. Although we did not detect Houston toads calling on the GLR in 2011, we did observe gulf coast toad (Bufo [] nebulifer) activity. These toads did not avoid areas with structural damage. Rather, they used the raised soil around wallows as calling sites. The implications of this behavior are unknown.

Concern 2: Effects on tadpole growth and survivorship.−Feral hog wallowing resulted in nutrient and productivity increases in Houston toad breeding ponds. Increased productivity is not necessarily detrimental to Houston toads, as the tadpoles feed on algae (Hillis et al. 1984). However, at high levels ammonium and nitrate can be toxic to anurans (Boyer & Grue 1995; Rouse et al. 1999; Schuytema & Nebeker 1999). We also found that hog activity resulted in lowered pH levels and substantial increases in TSS. Many anurans are sensitive to low pH, which negatively impacts osmoregulation at the tadpole life-stage (Rowe et al. 2003). Increases in TSS are associated with increases in water temperature and decreases in dissolved oxygen (Brönmark & Hansson 2005; Waite 2010). Water temperature is positively associated with timing of metamorphosis in anurans (Alvarez & Nicieza 2002; Schiesari 2002). Thus, increases in water temperature could potentially benefit the Houston toad by reducing time spent in the tadpole life-stage, when vulnerability to predators is high (Boone et al. 2007; Adams et al. 2011). In contrast, reduced dissolved oxygen is a concern. Anuran larvae primarily respire through their skin (Hillman et al. 2009), and low dissolved oxygen levels are known to negatively impact amphibian survivorship during the larval stages (Tattersall & Ultsch 2008; Sacerdote & King 2009).

Concern 3: Potential direct mortality.−Feral hogs primarily feed on herbaceous vegetation, roots, and tubers (Everitt & Alaniz 1980; Adkins & Harveson 2006). However, they are opportunistic foragers that are known to consume amphibians (Jolley et al. 2010). Rooting near ponds is problematic because, at least during the breeding season, adult Houston toads typically remain within 75 m of ponds 84 THE TEXAS JOURNAL OF SCIENCE-VOL. 64, NO. 2, 2012

(Swannack 2007). Thus, because densities are high near ponds, feral hog predation could become a major source of adult mortality for Houston toads.

The abundance of feral hogs in the Lost Pines appears to be increasing, which is a concern for management and recovery of the Houston toad. Although we do not yet have data linking effects of feral hog activity to Houston toad population dynamics, the data we presented indicates how rooting and wallowing is likely detrimental to the Houston toad. In addition to the Welsh tract exclosure fence data, we have ancillary data concerning cattle usage of ponds on the GLR. Effects of cattle are similar in terms of water quality effects, and to a lesser degree, structural effects (Schmutzer et al. 2008). Houston toads did not call at a cattle pond (Pond 5) in 2000. Cattle were excluded from the pond following the breeding season and Houston toads chorused at the pond in 2001 and reproduced at the pond in 2002. In 2007 cattle were again present at Pond 5 due to a problem with the fence and cattle damage to the pond’s edges and surrounding vegetation occurred again. Houston toads did not call at the pond that year. Again, cattle were excluded from the pond following the breeding season and Houston toads chorused at the pond in 2008 and reproduced at the pond in 2010. A similar result was obtained when cattle were excluded from Pond 9 in 2000. This pond went from having minimal calling activity to its current position as the second most utilized pond for calling by Houston toads on the GLR by 2007 (pers. obs.). Although these data do not provide us with the evidence necessary to infer causation, they do indicate that cattle and feral hog usage of ponds during the breeding and emergence season negatively impacts the Houston toad.

Although feral hogs do not currently appear to be present in Bastrop State Park (McClanahan pers. comm.), they are likely to disperse into the park in the near future. Because Bastrop State Park has the largest Houston toad breeding population and is a critical site for the long-term persistence of the species (Hatfield et al. 2004), we suggest implementing proactive hog-deterrent measures (e.g., BROWN, ET AL. 85 exclosure fences) at heavily used breeding ponds. If management proceeds reactively, we recommend rapid response to the first signs of hog presence, as this study documented the ability of feral hogs to cause widespread damage in a short period of time.

In April and May of 2010 collaborating hunters captured and removed hogs from the GLR. Hogs were captured using snare traps tied to logs at heavily used ponds. This trapping method was optimal for the property because it ensured human safety, was inexpensive, and proved effective at capturing hogs. Indeed, we did not detect new feral hog wallows at ponds in May and June. However, we documented fresh hog wallows at Pond 5 in July and Pond 2 in August. Thus, it appears that short-term trapping was only temporarily effective. Campbell & Long (2009) concluded that using a variety of methods was necessary for feral hog population reduction because individuals learn to evade traps. Unfortunately, management of feral hogs in the Lost Pines will likely require continuous efforts, as it has proven very difficult to extirpate established populations elsewhere (Hone & Stone 1989; Wigley 1995; Waithman et al. 1999; Kaller et al. 2007).

ACKNOWLEDGEMENTS The Capitol Area Council of the Boy Scouts of America provided access to the Griffith League Ranch, and we are appreciative for their continuing support of our research. We thank the Nowlin Lab at Texas State University-San Marcos for providing laboratory equipment and supplies and assisting with water quality analyses. M. Ray assisted with aquatic arthropod collection. S. McCracken supplied the aerial image used in Figure 1. We thank T. McClanahan, Texas Parks and Wildlife Department, for informing us of the status of feral hog activity in Bastrop State Park. We thank Tim Schumann, U.S. Fish and Wildlife Service, and Milton Urban, Texas Fence LLC, for their assistance designing and installing the feral hog exclosure fence on the Welsh tract. This research was funded in part by the 86 THE TEXAS JOURNAL OF SCIENCE-VOL. 64, NO. 2, 2012

Texas Parks and Wildlife Department and U. S. Fish and Wildlife Service through a traditional Section 6 grant.

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