Modelling species distributions without using species distributions: the cane toad in Australia under current and future climates

Abstract

Accurate predictions of the potential distribution of range-shifting species are required for effective management of invasive species, and for assessments of the impact of climate change on native species. Range-shifting species pose a challenge for traditional correlative approaches to range prediction, often requiring the extrapolation of complex statistical associations into novel environmental space. Here we take an alternative approach that does not use species occurrence data, but instead captures the fundamental niche of a species by mechanistically linking key organismal traits with spatial data using biophysical models. We demonstrate this approach with a major invasive species, the cane toad Bufo marinus in Australia, assessing the direct climatic constraints on its ability to move, survive, and reproduce. We show that the current range can be explained by thermal constraints on the locomotor potential of the adult stage together with limitations on the availability of water for the larval stage. Our analysis provides a framework for biologically grounded predictions of the potential for cane toads to expand their range under current and future climate scenarios. More generally, by quantifying spatial variation in physiological constraints on an organism, trait-based approaches can be used to investigate the range-limits of any species. Assessments of spatial variation in the physiological constraints on an organism may also provide a mechanistic basis for forecasting the rate of range expansion and for understanding a species' potential to evolve at range-edges. Mechanistic approaches thus have broad application to process-based ecological and evolutionary models of range-shift.

Adrenocortical stress responses influence an invasive vertebrate's fitness in an extreme environment

“…Thus, cane toads living in arid environments face strong seasonal selection from temperature and desiccation stressors. Their ability to regulate physiological traits that balance thermal and water homeostasis is essential for survival and range occupancy [25].…”

“…Body mass and body length were measured to estimate each individual's hydric state using the residual scores derived from a log mass against log length linear regression model. Because toads can store more than 50% of their mass as water in their bladder, evaporative water loss leads to large changes in daily body mass and is highly indicative of toad hydric state and hence desiccation stress [25][26][27][28]33]. We collected 2 ml of blood following decapitation (less than 1 min).…”

High-resolution niche models via a correlative approach: Comparing and combining correlative and process-based information

“…These behaviors may occur at a shorter temporal scale than can be modeled effectively over long durations, and thermal suitability may not be easily represented in correlative procedures. Additionally, the spatial scale at which thermal mechanisms seem to constrain distributions is likely larger than the scope of our study (Kearney et al, 2008;Kearney et al, 2009). The importance of thermal and nutritional constraints on mate-seeking and nesting strategies should not be downplayed, however; rather, at a local spatial scale, our data suggest that these factors may not be easily expressed in a correlative modeling approach or combined with temporally coarser habitat GIS layers .…”

“…Some proponents of process-based models have suggested 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 needed to clearly identify and model axes of the fundamental niche (Kearney and Porter, 2004), whereas others have suggested that focusing solely on the fundamental niche does not provide a complete picture of a species' spatial distribution (Godsoe, 2010) or the underlying biotic interactions that define it. Correlative approaches are not as powerful, explicit, or transferable to novel areas as mechanistic ones (Kearney et al, 2008;Kearney et al, 2009;Bartelt et al, 2010), but for many species they offer a quick, easy, and often robust estimate of occupancy (Tsoar et al, 2007;Barrows et al, 2008;Kharouba et al, 2009;Buckley et al, 2010). Morin and Thuiller (2009) suggested that a more robust estimate of occupancy may be achieved by combining correlative and process-based models, which is the primary aim of this paper.…”

Potenciais efeitos das mudanças climáticas futuras sobre a distribuiçãode um anuro da Caatinga Rhinella granulosa (Anura, Bufonidae)

“…Há fortes evidências que determinadas espécies tenham mudado o período de seus ciclos de vida durante o ano, bem como suas distribuições espaciais (Kearney et al, 2008;Beaumont et al, 2009;Thomas, 2010;Loyola & Martins, 2011). Isto provavelmente esteja ligado às variações anuais e de longo prazo na temperatura (Sinervo et al, 2010;Chen et al, 2011).…”