INTRODUCTION
Interactions between predators and prey are rarely observed in nature,
making it inherently difficult to understand the effect that predators
have on the population sizes of prey species. Nevertheless, predation
has historically been of interest to biologists and is thought to be one
of the most influential ecological processes governing community
structure (Lima 1998, Sih et al. 1998, Shochat et al. 2006). Top down
effects of predators on their prey are well documented and are thought
to be in part responsible for densities of prey species at lower trophic
levels (Newton 1998, Marzluf 2001). Removal of certain keystone
predators like wolves (Canis lupus ) in Yellowstone National Park,
can even impact entire communities and landscapes (Fortin et al. 2005).
With natural habitats being increasingly altered by human development,
an important conservation question, now and in the future, will be to
determine how predation affects the structure and assemblages of urban
communities.
Urbanization is a major factor in the loss of biodiversity worldwide
(Czech and Krausman 1997, Wilcove et al. 1998, McKinney 2008). Altered
community structure is a hallmark of urban environments; urban
communities can differ greatly from their natural counterparts with some
species able to adapt while others decline (Fischer et al. 2012). The
role that predation plays in structuring urban communities is not well
understood, but has been suggested to be similar to what is found in
natural areas (Shochat et al. 2006). Increasing evidence, however,
indicates that predation may act differently in urban environments,
leading to what authors have termed an urban predation paradox (Fischer
et al. 2012, Eötvös et al. 2018). Studies done largely on urban birds
and mammals have shown that urban environments have high densities of
mesopredators, but paradoxically lower rates of predation (Fischer et
al. 2012, Eötvös et al. 2018). Lower predation rates in urban
environments have been attributed to predators subsisting mainly on
anthropogenic subsidies (i.e. trash and domestic pet food) (Rodewald et
al. 2011). Subsequently, prey species in urban areas may experience an
ecological release and lower predation rates that can allow them to
exist in hyperabundance (Fischer et al. 2012). Alternatively, some
research has shown that predators are more sensitive to urbanization and
are pushed out of urban areas because of a lack of suitable habitat.
Consequently, urban environments may act as refugia for some prey
species due to the lack of predators in those areas (Berger 2007,
Leighton et al. 2010, Muhly et al. 2011, Shannon et al. 2014,
Rebolo-Ifran et al. 2017).
Few studies have been conducted on the impact that predation has on
reptiles living in urban environments, and results from these are often
conflicting (French et al. 2018). Species richness and abundance are
generally negatively correlated with urbanization (Mckinney 2008);
however, some studies show that reptiles thrive in urban environments
and even increase in abundance and diversity under certain conditions
(Schlauch 1978, Moreno-Rueda and Pizaarro 2007, Barret and Guyer 2008).
Due to the difficulty in observing predation events, many researchers
have turned to the use of clay or foam models to measure predation
(Bateman et al. 2016). Of these studies, only Mcmillan and Irshick,
(2010) explicitly tested differences in predation rates between urban
and natural environments. Their results, consistent with the urban
predation paradox, showed significantly lower amounts of predation on
green anole models (Anolis carolinensis ) in the urban area.
Texas horned lizards (Phrynosoma cornutum ) are highly specialized
lizards with unique morphological characteristics and dietary
preferences and exhibit a variety of adaptations for life as
myrmecophagus, sit-and-wait predators, living in arid environments
(Pianka and Parker 1975, Sherbrooke 2003). Many of these behavioral and
morphological adaptations can be attributed to selective forces in
response to predation (Edmunds, 1974, Endler, 1986) and include: cryptic
coloration, cranial horns, blood squirting, and specific behaviors in
response to distinct predators (Pianka and Parker 1975, Sherbrooke 1987,
Middendorf III and Sherbrooke 1992, Sherbrooke 2008). Texas horned
lizards are known to have a multitude of predators including: snakes,
predatory lizards, birds, rodents, canids, and felids (Sherbrooke 2003).
Their low annual survival rate (8.9-54%) is also often attributed to
high predation pressure (Fair and Henke 1999, Endriss et al. 2007,
Miller et al. 2020).
Texas horned lizards, an iconic vertebrate of the American southwest,
have declined, especially in eastern areas of their historic range and
are now a threatened species in the state of Texas
(Donaldson et al. 1994, Texas
Conservation Action Plan – TCAP 2012). Declines in these once common
lizards are attributed to a variety of factors including: urbanization
and habitat conversion, invasive red fire ants (Solenopsis
invicta ), which can prey on the eggs and young of horned lizards, the
loss of harvester ants (Pogonomyrmex spp) due to widespread use
of insecticides and competition with fire ants, and over-collecting for
the pet and curio trades (Donaldson et al. 1994, Henke 2003). Texas
horned lizards are still found in some small Texas towns, including
populations occurring in the towns of Kenedy and Karnes City in southern
Texas. Past research has shown that lizards in these towns occur at
average densities of 52.32 ± 11.2 SE lizards/ha (Ackel 2015), which is
much higher than the reported densities in more natural areas (3-10
lizards/ha) (Whitford and Bryant 1979, Whiting et al. 1993). Lizards in
these towns predominately eat smaller ants (Pheidole spp.) and
termites (Tenuirostritermes cinereus ) rather than their commonly
preferred prey of large harvester ants (Alenius 2018). Foraging on
smaller prey items may increase handling time for Texas horned lizards,
which would put them at higher risk of predation; therefore, we
hypothesized that predation rates are lower in town than in more natural
areas, and that this has facilitated both high densities of lizards and
the exploitation of small prey items.
In this study we created foam models of hatchling, juvenile, and adult
Texas horned lizards and placed them in small towns and a natural
habitat to test the hypothesis that predation rates would be lower in
town. Our goal was to create morphologically accurate models, that when
placed on the landscape, would sample a variety of predators of Texas
horned lizards. Specifically, we asked: 1) do predation attempts on foam
models differ between urban and natural areas?; and 2) do hatchling,
juvenile, and adult Texas horned lizard models differ in their number of
predation attempts?