105 ARTICLE Partial support for food availability and thermal quality as drivers of density and area used in Yarrow’s Spiny Lizards (Sceloporus jarrovii) L.D. Patterson and G. Blouin-Demers Abstract: Contrary to traditional models, habitat selection in ectotherms may be chiefly based on a habitat’s thermal properties rather than its food availability, due to their physiological dependence on environmental temperature. We tested two hypoth- eses: that microhabitat use in ectotherms is driven by food availability and that it is driven by thermoregulatory requirements. We predicted that the density of lizards would increase and the mean area used would decrease with the natural arthropod (food) availability (or thermal quality) of a plot, as well as after experimentally increasing plot arthropod availability (or thermal quality). We established two plots in each of four treatments (food-supplemented, shaded, food-supplemented and shaded, and control) on a talus slope in Arizona, USA. We measured the density and area used in Yarrow’s Spiny Lizards (Sceloporus jarrovii Cope in Yarrow, 1875) before and after manipulations, and determined whether lizard density and area used were related to natural arthropod availability or thermal quality at the surface and in retreat sites. Density and area used were unaffected by the manipulations, but both increased with natural arthropod availability and decreased with higher thermal quality in retreat sites. These results provide partial support for both food availability and thermal quality as drivers of density and microhabitat use in S. jarrovii. Key words: Yarrow’s Spiny Lizard, Sceloporus jarrovii, arthropod availability, temperature, area used, population density. Résumé : Contrairement aux modèles traditionnels, la sélection d’habitats chez les ectothermes pourrait être basée principale- ment sur les propriétés thermiques de l’habitat, plutôt que sur sa disponibilité de nourriture, en raison de la dépendance physiologique de ces organismes aux températures ambiantes. Nous avons vérifié deux hypothèses, à savoir que l’utilisation de microhabitats chez les ectothermes est contrôlée par la disponibilité de nourriture et qu’elle est contrôlée par des exigences associées à la thermorégulation. Nous avons prédit que la densité de lézards augmenterait et que la superficie utilisée moyenne diminuerait avec l’augmentation de la disponibilité naturelle d’arthropodes (nourriture) (ou de la qualité thermique) d’une parcelle, ainsi qu’après une augmentation expérimentale de la disponibilité d’arthropodes (ou de la qualité thermique) dans la For personal use only. parcelle. Nous avons établi deux parcelles pour chacun des quatre traitements (supplémentation de nourriture, ombre, supplé- mentation de nourriture et ombre, et témoin) sur un talus d’éboulis en Arizona (États-Unis). Nous avons mesuré la densité de lézards épineux de Yarrow (Sceloporus jarrovii Cope dans Yarrow, 1875) et la superficie utilisée avant et après les manipulations, et déterminé si la densité de lézards et la superficie utilisée étaient reliées à la disponibilité naturelle d’arthropodes ou à la qualité thermique à la surface et dans des sites de retrait. La densité et la superficie utilisée n’étaient pas influencées par les manipu- lations, mais ces deux paramètres augmentaient parallèlement à la disponibilité naturelle d’arthropodes et diminuaient avec l’augmentation de la qualité thermique dans les sites de retrait. Ces résultats appuient partiellement les hypothèses selon lesquelles la disponibilité de nourriture et la qualité thermique contrôleraient la densité de S. jarrovii et leur utilisation de microhabitats. [Traduit par la Rédaction] Mots-clés : lézard épineux de Yarrow, Sceloporus jarrovii, disponibilité d’arthropodes, température, superficie utilisée, densité de population. Introduction physiological processes (Huey 1991; Blouin-Demers and Weatherhead Determining the factors that govern the abundance and distri- 2008), population dynamics (Holt 1987; Kristan 2003), and community- Can. J. Zool. Downloaded from www.nrcresearchpress.com by University of Ottawa on 01/30/20 bution of organisms is central to ecology (Krebs 2001). Species level processes (e.g., Resetarits et al. 2005), and is how individuals abundance varies both spatially and temporally (Brown et al. 1995; distribute themselves on a landscape. Ysebaert and Herman 2002), and is driven both by abiotic factors According to traditional habitat selection models, habitat selec- such as temperature and precipitation and by biotic interactions tion and population density depend primarily on the capacity of such as competition and predation (Hubbell 2001; Houlahan et al. individuals to acquire resources, especially food, in a given area (e.g., 2007; Mutshinda et al. 2009). Habitat selection is a major mech- Rosenzweig 1991; Mobæk et al. 2009). Although food availability anism driving patterns of abundance of a species within its is a crucial environmental variable for any organism, affecting distribution (e.g., Resetarits 2005). Habitat selection influences growth, reproduction, and higher level processes such as food- Received 8 July 2019. Accepted 15 October 2019. L.D. Patterson* and G. Blouin-Demers. Department of Biology, University of Ottawa, 30 Marie-Curie Private, Ottawa, ON K1N 6N5, Canada. Corresponding author: L.D. Patterson (email: [email protected]). *Present address: Pukaskwa National Park, P.O. Box 212, Heron Bay, ON P0T 1R0, Canada. Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink. Can. J. Zool. 98: 105–116 (2020) dx.doi.org/10.1139/cjz-2019-0166 Published at www.nrcresearchpress.com/cjz on 26 November 2019. 106 Can. J. Zool. Vol. 98, 2020 web dynamics and spatial structure of communities (Taylor et al. If microhabitat use was based on food availability, then we 2005), temperature may be equally if not more important than predicted (i) that the density of lizards would correlate positively food availability for ectotherms (e.g., Halliday and Blouin-Demers and the mean area used would correlate negatively with the nat- 2014). The body temperature (Tb) of ectotherms is strongly influ- ural food availability of the plot, and (ii) that following food sup- enced by environmental temperature (Bennett 1980). Thus, envi- plementation, lizard density would increase and the mean area ronmental temperature, through its effect on Tb, affects most used would decrease relative to control plots. Conversely, if micro- physiological, developmental, and behavioural processes, includ- habitat use in S. jarrovii was driven by thermoregulatory require- ing the rate of food processing (reviewed in Angilletta 2009). If an ments, then we predicted (i) that the density of lizards would ectotherm cannot maintain appropriate Tbs in a given habitat, correlate positively and the mean area used would correlate neg- then its energy gain will be constrained even if food is abundant atively with the thermal quality of the plot, and (ii) that after (Sinervo and Adolph 1989). Thus, the rate of food consumption in experimentally increasing plot thermal quality, lizard density ectotherms may be more constrained by temperature, which in- would increase and the mean area used would decrease relative to fluences the rate of food processing, than by food availability, control plots. We predicted an increase in density and a decrease which affects the rate of food acquisition. in area used following improved plot quality because S. jarrovii The primary factor driving habitat selection in ectotherms may decrease their territory size following food supplementation be the habitat’s thermal quality. Reptiles typically strive to main- (Simon 1975), and we expected other lizards in the population to tain their Tb within a narrow optimal range (Tset, here defined as immigrate to high-quality areas (e.g., Boutin 1984). Given that the central 50% of selected Tbs) that maximizes performance and neonate S. jarrovii exhibit territorial behaviour as early as 13 days fitness (Huey and Kingsolver 1989; Huey 1991). To maintain their of age (Simon and Middendorf 1980) and that all lizards in this Tb within Tset, many ectotherms must thermoregulate behav- study were at least 1 year old, it is unlikely that changes would iourally (Angilletta 2009). However, the ease with which ecto- occur due to juvenile lizards establishing new territories. therms maintain their Tb within Tset depends on the habitat’s To test these predictions, we established eight plots on a large thermal quality. From an ectotherm’s perspective, a habitat is of rocky talus slope, divided among four treatments: food-supplemented, high thermal quality when the difference between environmental shaded, food-supplemented and shaded, and control. We mea- temperatures and its Tset is minimal (Hertz et al. 1993). Indeed, sured the density and area used by lizards, natural arthropod various reptiles preferentially use habitats of higher thermal qual- (food) availability, and thermal quality of each plot before and ity (e.g., snakes: Row and Blouin-Demers 2006; turtles: Dubois after experimental manipulations. Thus, we determined how et al. 2009; lizards: Sabo 2003; Ortega and Pérez-Mellado 2016; both lizard density and mean area used were affected by the nat- Paterson and Blouin-Demers 2018). Furthermore, areas with desir- ural food availability and thermal quality of the plot, as well as able thermal properties, such as basking sites, are actively de- how these variables