Rarity in Australian Desert Lizards

Austral Ecology (2013) ••, ••–•• Rarity in Australian desert lizards ERIC R. PIANKA* Section of Integrative Biology C0990, University of Texas at Austin, Austin, TX 78712, USA (Email: [email protected]) Abstract Most species of Australian desert lizards are uncommon. Possible causes of rarity are examined, including body size as measured by snout–vent length (SVL), fecundity, number of sites occupied, habitat niche breadth, microhabitat niche breadth, dietary niche breadth, and average total niche overlap with other species. Rare species tend to be larger with lower fecundities than abundant species and they occur at fewer sites. Many, but not all, uncommon species are specialists, either in habitat, microhabitat, or diet. The niche breadth hypothesis, which states that abundant species should be generalists whereas specialized species should be rare, is tested, but rejected as a general explanation for rarity. Some uncommon species exhibit high overlap with other species suggesting that they may experience diffuse competition. However, no single cause of rarity can be identified, but rather each species has its own idiosyncratic reasons for being uncommon. Multivariate analyses show distinct ecological differences between abundant and uncommon species. Key words: diffuse competition, fecundity, generalists versus specialists, niche breadth, niche overlap, relative abundance. INTRODUCTION has proven to be exceedingly difficult to study, but, as mentioned above, rare species could well be very Understanding rarity constitutes a major challenge important to community function (Preston 1948, confronting ecologists. Main (1982) asked ‘Are rare 1962; Main 1982; Morton & James 1988; Kunin & species precious or dross? And, are they vital to com- Gaston 1997; Thompson et al. 2003). munity function?’ Do rare species persist in more Several hypotheses, not all of which can be tested stable communities in spite of their rareness, or does here, for the continued existence of rare species spring the presence of rare species enhance the stability of to mind: ecosystems? Main (1982) suggested that one reason H 1. Body size-trophic level hypothesis. Larger so many rare species exist may be that ecosystems species are uncommon either because they are have been ‘over-written many times after imperfect top predators (monitor lizards) or for other erasures’ (incomplete extinctions). Consequently, reasons. current ecosystems contain numerous relicts of their H 2. Fecundity hypothesis. Some species could be predecessors assembled under different ecological uncommon because of their low fecundity. conditions. Main suggested that rare species could H 3. Geographic range hypothesis. Rare species be vital to long-term ecosystem sustainability, provid- could have narrow geographic ranges, occur- ing ‘insurance’ for the delivery of ecosystem functions ring at only a few sites (Rabinowitz et al. 1986). by alternative means in the event of drastic environ- H 4. Niche breadth hypothesis. Rare species are mental changes. Main’s hypotheses could well be of uncommon because they are specialized with great importance in these times of rapid climatic narrow niche requirements. Resources such as change. habitats, microhabitats, or foods might be Most species of Australian desert lizards are un- scarce or limited. These alternatives can be common, making them difficult to study. Some are tested with data on niche breadths (below). extremely rare to the point of vanishing rareness H 5. Diffuse competition hypothesis. Rare (Pianka 2011). Regardless of how rareness is defined, species could be uncommon because of diffuse most ecologists concur that the majority of species competition from many other, more abundant, are indeed uncommon (Gaston 1994; Kunin & species (MacArthur 1969, 1970, 1972a,b). Gaston 1997). Magurran and Henderson (2003) dis- H 6. Physical tolerance hypothesis. Rare spe- tinguished between relatively abundant ‘core species’ cies might have narrow tolerances to physical and uncommon ‘occasional species’. Chronic rarity environments. H 7. Sink versus source population hypothesis. Rare species might be uncommon only locally *Corresponding author. in ‘sink’ populations, but might be more abun- Accepted for publication May 2013. dant in nearby ‘source’ areas. © 2013 The Author doi:10.1111/aec.12061 Austral Ecology © 2013 Ecological Society of Australia 2 E. R. PIANKA H 8. Dispersal hypothesis. Rare species could be Trappability varies from species to species: slow moving rare because they do not have dispersal powers species, especially agamids like bearded dragons and thorny necessary to find and invade suitable habitats. devils, often walk around pits whereas most skinks run Are rare species merely accidentals, dispersing rapidly down drift fences and readily fall into pit traps. from one habitat to another? Fossorial Lerista bipes are exceptionally prone to fall into pit traps. Arboreal species are less likely to be trapped than H 9. Predation hypothesis. Predators could hold terrestrial species. Nevertheless, pit trapping provides a population densities of uncommon prey species standardized collecting method that allows relative abun- at low levels. dances to be compared across space and time.This technique Some related questions that can be asked about rare also allows informative estimates of point diversity, which can species include: be exploited to infer habitat requirements. How can rare species find mates and continue to To maximize sample sizes, I combine all data here and exist? classify species into three broad natural abundance catego- Is rarity an illusion because of cryptic behaviour ries: common, rare, and species of intermediate abundance making putative rare species difficult to find? (see Fig. 1). A species was considered common if more than Merely being in an alien habitat is not necessarily 500 individuals were captured over of the course of my a death sentence, as these habitats offer shelter and surveys, rare if 100 or fewer were caught, and intermediate if from 100 to 500 were collected. food – a migrant that succeeds in reaching its correct Niche breadths were computed for each species based on habitat could also reap the benefits of sweepstakes proportional representation of each resource category using reproductive success. Which of these factor(s) is/are 2 the inverse of Simpson’s (1949) index of diversity, 1/Σ pi crucial determinants of commonness or rarity needs to where pi is the proportion in a given category i. Standard be determined for each species. multivariate statistical methods including principal com- ponent analysis (PCA) and discriminant function analysis were employed. METHODS RESULTS Field collections Over the last 42 years, I have witnessed metapopulation-like local extinctions and colonization In the past few decades, ecology has gradually changed with in a few species. I have captured migrants of a number greater emphasis now being placed on conservation biology, of species (Ctenophorus fordi, Ctenophorus scutulatus, rare species, and phylogenetically corrected analyses of evo- Ctenotus greeri, Ctenotus leae, Ctenotus leonhardii, lution using modern comparative methods. Whereas data collected before 1979 were intended for studying niche par- Lophognathus longirostris, Nephrurus vertebralis, and titioning and community structure, data collected since 1989 Tiliqua occipitalis) dispersing through habitats that they using pit traps were designed to study rare species, fire suc- do not normally occupy. Until recently, inadequate cession, long-term change, habitat and microhabitat require- sample sizes have prevented me from doing much with ments, adaptive radiations, and phylogenetic constraints. uncommon species, but I have now finally managed to New data thus complement older data but are qualitatively acquire large enough samples to attempt to under- different than those collected earlier. stand the ecologies of most of them – no one else has On 11 separate research expeditions from 1966 to 2008, ever managed to collect such large samples of uncom- 41 months and 1256 days were spent in the field (Pianka mon Australian desert lizards. 1969a,b, 1982, 1986, 1994, 1996; Pianka & Goodyear One species, the Great Desert Skink Liolophis (for- 2012). Extensive data were gathered at ten major study sites merly Egernia) kintorei merits special comment. One in the Great Victoria Desert of Western Australia (for exact locations, see Pianka 1986 and Pianka & Goodyear 2012). juvenile specimen of this very rare nocturnal lizard was Lizards were collected by any means possible using a wide collected on my L-area in 1967. This represents the variety of different techniques including exhuming, grab- southernmost known locality for the species and its bing, noosing, shooting, tracking, whomping (smashing a identity has been confirmed by experts. Despite con- spinifex tussock with a shovel), locating lizards at night by siderable effort, no others were found either at the eyeshine and/or body shine, and by pit trapping. L-area or on any other study area. This singleton may Early on (1966–1968, 1978–1979), lizards were hunted have been among the last of its kind, now considered and captured during their normal daily course of activity, endangered in Western Australia and possibly on its providing data on time of activity, microhabitat, ambient air way to local extinction (Pearson et al. 2001). temperature, and active body temperature. Data acquired up Dates, numbers of sites visited, numbers of species until 1979 were summarized

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