Isoscapes: a new dimension in community ecology.

The utility of stable isotopes in interpreting and recording aspects of environmental science, biogeochemistry, ecology and organismal physiology has long been established (Peterson and Fry 1987, Dawson et al. 2002). Examples range from the reconstruction of prehistoric human diets (Schoeninger et al. 1983) and tracing components of the hydrological cycle (Gat 1996), to differentiating C3 and C4 photosynthesis (Farquhar et al. 1989) and recording the humidity under which plants grow (Kahmen et al. 2011). Stable isotopes have proved invaluable in elucidating biological processes at multiple levels, and along with this there has been a recognition of the intrinsic information held within the observed patterns of stable isotopes across landscapes. For example, δH of stem water in understorey plants has been used to define areas influenced by hydraulic lift (Dawson 1993), while patterns of δN have been used to calculate the contribution of marine-derived nitrogen to Sitka spruce growing near salmon-spawning streams (Helfield and Naiman 2001). The term isoscape, first coined by Dr Jason B. West and Dr Gabriel J. Bowen (West et al. 2008, Bowen 2010), refers to any spatially explicit prediction of isotopic values across a landscape. They can be derived from interpolating geographically distributed observations or developed by process-level models that aim to capture and predict this observed heterogeneity from an understanding of isotope fractionation. In a comprehensive book, Isoscapes, edited by West et al. (2010a), a framework was established within which researchers may begin to understand movement, patterns and processes on Earth through the use of this isotope mapping. The value of detailed spatial and temporal maps of isotopic composition is best exemplified in the field of hydrologic science. Initiated in 1958, operational by 1961 (Dansgaard 1964) and still continuing today, the International Atomic Energy Agency (IAEA) and World Meteorological Organization’s Global Network of Isotopes in Precipitation (GNIP) consists of 1000+ meteorological stations at which monthly precipitation is collected for δO and δH analysis. This spatially explicit data can subsequently be transformed into a continuous map depicting how moisture isotopes vary across the landscape (West et al. 2008, Terzer et al. 2013). This makes it possible to determine the probable isotopic signature for the water entering terrestrial systems at any point on Earth, providing numerous insights into environmental processes (Aggarwal et al. 2010). Moisture isoscapes have been used to inform our understanding of plant development processes (Kahmen et al. 2013) and by extension to resolve paleo-proxies (West et al. 2010b); they have (in conjunction with other elements) been used at large spatial scales to determine geographical origins of biological materials (Wunder 2010) and to track animal (including human) migration across the landscape (Hobson et al. 2010). However, more recently, there has been a growing use of high-resolution isoscapes over limited spatial extents (i.e. fieldscale studies) to answer more targeted ecological questions, such as the influence of micro-topography on nutrient cycling in pastures (Dixon et al. 2010) or the biophysical impacts of invading N2-fixing exotic species upon native community structure (Rascher et al. 2012, Bai et al. 2013). In this issue, Hellmann et al. (2016b) build upon previous work (Rascher et al. 2012, Hellmann et al. 2016a) by using isoscapes of δN and δC in leaf-tissue to resolve the spatial extent of plant–plant interactions between the invasive tree Acacia longifolia and three native plant species in a sand-dune ecosystem on the west coast of Portugal. Isoscapes were developed by pooling leaves of each native species found