University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln Nebraska Cooperative Fish & Wildlife Research Nebraska Cooperative Fish & Wildlife Research Unit -- Staff Publications Unit 2002 Variability between Scales: Predictors of Nomadism in Birds of an Australian Mediterraneanclimate Ecosystem Craig R. Allen U.S. Geological Survey, [email protected] Denis A. Saunders Clemson University Follow this and additional works at: https://digitalcommons.unl.edu/ncfwrustaff Part of the Other Environmental Sciences Commons Allen, Craig R. and Saunders, Denis A., "Variability between Scales: Predictors of Nomadism in Birds of an Australian Mediterraneanclimate Ecosystem" (2002). Nebraska Cooperative Fish & Wildlife Research Unit -- Staff Publications. 23. https://digitalcommons.unl.edu/ncfwrustaff/23 This Article is brought to you for free and open access by the Nebraska Cooperative Fish & Wildlife Research Unit at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in Nebraska Cooperative Fish & Wildlife Research Unit -- Staff Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Ecosystems (2002) 5: 348–359 DOI: 10.1007/s10021-001-0079-2 ECOSYSTEMS © 2002 Springer-Verlag Variability between Scales: Predictors of Nomadism in Birds of an Australian Mediterranean- climate Ecosystem Craig R. Allen1* and Denis A. Saunders2 1US Geological Survey, Biological Resources Division, South Carolina Cooperative Fish and Wildlife Research Unit, G27 Lehotsky, Clemson University, Clemson, South Carolina 29634, USA; and 2CSIRO Sustainable Ecosystems, GPO Box 284, Canberra ACT 2601, Australia ABSTRACT Nomadism in animals is a response to resource dis- cance of the variables body mass and diet (nectar) tributions that are highly variable in time and space. may reflect the greater energy requirements of Using the avian fauna of the Mediterranean-climate large birds and the inherent variability of nectar as region of southcentral Australia, we tested a num- a food source. The significance of the variable dis- ber of variables to determine if they predicted no- tance to the edge of a body mass aggregation pro- madism. These variables were species body mass, vides further evidence of inherent variability in re- the distance in body mass terms to the edge of a sources between domains of scale. The edges of body mass aggregation, and diet (for example, body mass aggregations are hypothesized to be ar- seeds, invertebrates, nectar, or plants). We utilized eas of increased resource variability that reflect the two different classifications of the avifauna that transition from one scale of landscape pattern to diverged in their definition of nomadic to build two another. different predictive models. Using both classifica- tions, distance to the edge of a body mass aggrega- Key words: Australia; birds; body mass distribu- tion was found to be a significant predictor of no- tions; body mass pattern; landscape patterns; no- madism. There was also evidence that nomads tend mad; Mediterranean climate; scale; scale break; tex- to feed on nectar and tend to be large. The signifi- tural discontinuity hypothesis. INTRODUCTION of the ecological correlates of nomadism. Dean (1997) reported that nomadic species of the South Nomadism is most prevalent in ecosystems with African Karoo tended to be granivores, especially resources that are highly variable in space and time feeding on grass seeds, and further noted that no- (Sinclair 1984). Nomadism is characterized by un- madic species that feed on grass tended to be small. predictable movements in space and time that track Davies (1984) found that nomads tend to feed on unpredictable changes in resource distribution and seeds or nectar, but also stated that some nomads abundance. Thus, nomadic species tend to occur are raptors, tracking rodent outbreaks. It is possible disproportionately in arid or semi-arid ecosystems to imagine that nearly all resources in arid and (Davies 1984). Nomadism in birds has been rela- semi-arid environments are highly variable in space tively little studied, and surprisingly little is known and time and that their exploitation would lead to the evolution of nomadism, yet few species of birds Received 14 November 2000; accepted 27 June 2001. are in fact nomadic, even where nomadism is rela- *Corresponding author; e-mail: [email protected] tively common. 348 Nomadism in Birds 349 It is clear that nomadism is most common in ecosystems with high variation in resource abun- dance, often associated with low, variable, and un- predictable rainfall patterns (Davies 1984; Wiens 1991). For example, Keast (1968) documented a negative correlation between nomadism and rain- fall in honeyeaters; Wiens (1991) suggested that the higher prevalence of nomadism in Australia as compared to North America was due to higher vari- ability in rainfall; and nomadic South African larks have been reported to breed where rains have re- cently fallen (Maclean 1996). Nomadism in hu- mans occurs in arid regions at the periphery of settled heartlands; the reversion of sedentary peo- ples to nomadic lifestyles may be a response to environmental changes leading to increased aridity (Rosen 1988). However, it is not known exactly what resources, and what other traits may be asso- ciated with and predictive of nomadism. Our pur- pose was to determine which—if any—of a number Figure 1. Australia and the Lofty Block region near the of variables suggested or theoretically predictive of city of Adelaide in relation to surrounding biogeographic nomadism, do so in the Mediterranean-climate regions (from Thackway and Cresswell 1995). NCP, Nara- region of southcentral Australia. Schodde (1981) coorte Coastal Plain; GAW, Gawler; EYB, Eyre and Yorke reviewed the birds of the Mallee ecosystems of Block. Australia, which includes the Lofty Block of the Interim Biogeographic Regionalization for Aus- tralia (Thackway and Cresswell 1995). He pro- This region is characterized by cool, wet winters and vided a list of species inhabiting this region and a hot, dry summers. It is an area of nearly 24,000 km2 categorization of species diet, and then classified that includes the southern Flinders Ranges, the species as nomadic or sedentary/migratory. Mount Lofty Ranges, and Kangaroo Island. The A number of variables may theoretically predis- description of the Lofty Block given in the Interim pose a species toward nomadism. Specializing-in Biogeographic Regionalization of Australia (Thack- certain food sources that tend to be more ephem- way and Cresswell 1995) is “eucalypt open forests eral and unpredictable than others may lead to the and woodlands and heaths on mottled yellow and evolution of a nomadic lifestyle. Thus, one set of ironstone gravelly duplex soils in the wetter areas variables (four diet types) focused upon the type of and red duplex soils in drier areas; now largely food resource utilized by a species. Body size corre- cleared for agriculture and urban development.” lates with a number of traits and ecological at- The region has been extensively cleared of native tributes of species (Eisenberg 1981), so we incorpo- vegetation, so that less than 5% of the former veg- rated body size (mass) into our model. A final etation cover remains (Paton and others 2000). variable focused upon the link between the scale- We utilized published sources to determine our specific nature of ecological pattern and structure species list (Schodde 1981) and average body and discontinuous body mass distributions (Holling masses (Dunning 1993). Where published body 1992). The independent variables, and their basis mass estimates were not available, we used unpub- for inclusion in our models, are further discussed lished data provided by R. Schodde (n ϭ 23). Spe- below. cies for which no body mass estimates were avail- able were omitted from analyses (n ϭ 6). We used METHODS two interpretations of nomadism. Our first inter- pretation utilized the published scheme of Schodde Our area of interest was the Mediterranean-climate (1981), which provided a categorization of species region of southcentral Australia, near the city of as nomadic or sedentary. However, these classifica- Adelaide, the capital of the state of South Australia, tions are general and may not always differentiate an area largely encompassed by the Lofty Block of the situation where a species may be resident over the Interim Biogeographic Regionalization of Aus- part of its range but migrant, vagrant, or nomadic tralia (Thackway and Cresswell 1995) (Figure 1). over other parts of its range. Therefore, we also 350 C. R. Allen and D. A. Saunders used local expertise (that of D. Saunders and J. location of body mass aggregations (“lumps”)(sensu Reid, CSIRO Sustainable Ecosystems, Canberra, Holling 1992) and gaps in the body mass distribu- Australia) to categorize the birds of the Lofty Block tion of the birds of our study area—we used com- and developed an independent list of nomadic spe- puter simulations to locate significant discontinui- cies. ties in the body mass distribution. Simulations We used logistic regression (SAS Institute 1989) compared actual body mass data with a null distri- to determine significant predictors of nomadism, bution established by estimating a continuous uni- building two different models based upon the two modal kernel distribution of the log-transformed different categorizations of nomadism. The binary data (Silverman 1981). Gaps were defined as areas response variable was whether or