18 July 2012 | By Dr Cang Hui and Dr Berthouly-Salazar
Following Charles Elton’s pioneering work, invasion ecology has grown into a mainstream research field focusing on the patterns and processes of human-mediated translocation of alien organisms. As probably the most iconic taxa in conservation, birds provide an ideal natural experiment to test many theories and hypotheses in invasion ecology. This is partially because most exotic birds were introduced deliberately with detailed records. For instance, in 1981 John L. Long recorded more than 1000 introductions of about 400 species during the European Diaspora (18th to 20th centuries). This, however, adds difficulties when we try to analyze and interpret the highly human selective records, especially when we try to identify the environmental determinants and biological traits that are responsible for species’ invasiveness and the invasibility of recipient ecosystems.
Biological invasion is a complicated process. For avian invasion, this invasion pathway or process can be better described as a sequence of stages, resembling a stochastic Markov chain. This stage view of invasion pathway classifies species into (i) those belonging to the regional native species assemblage; (ii) those transported, mainly intentionally, for different purposes; (iii) those that have established viable feral populations; and (iv) those that start to spread and expand their ranges into non-native areas. Knowledge gained so far concentrates on the early stages of introduction and naturalisation.
Moving to the last stage of invasion (i.e. spread), we are facing increasing knowledge gaps. Spreading is traditionally a stronghold of dispersal ecology and biogeography. As such, research emphasizes more the spread dynamics, rather than invasion-related questions, such as invasiveness (i.e. the environmental, demographic and biological characteristics of spreading species). The fact that only few established bird species experience a clear range expansion also hinders a robust statistical inference. In an ongoing collaborative research project, C·I·B research team and colleagues aim to document the dispersal and range expansion of two invasive species, the Common Myna (Acridotheres tristis) and the European Starling (Stunus vulgaris) across their distributions in southern Africa, in order to better understand the mechanisms of dispersal and spread behind successful avian invaders. They presented part of their results in two recent publications.
Common Myna (Acridotheres tristis). [ Photo: J.M. Garg ]In the first publication (Berthouly-Salazar et al. 2012), the research team explain the accelerating range expansion by spatial sorting of individuals with stronger dispersal abilities at the range front. Specifically, they test whether the process of spatial sorting is at work on the range expansion of A. tristis in South Africa. Specifically, they sampled individuals across its invasive range and compared morphometric measurements relevant and non-relevant to the dispersal ability. Besides testing for signals of spatial sorting, they further examined the effect of environmental factors on morphological variations. The paper results show that dispersal-relevant traits are significantly correlated with distance from the range core, with strong sexual dimorphism, indicative of sex-biased dispersal. Morphological variations are significant in wing and head traits of females, suggesting females as the primary dispersing sex. In contrast, traits not related to dispersal such as those associated with foraging show no signs of spatial sorting but are significantly affected by environmental variables such as vegetation and intensity of urbanisation. When taken together, these results support the role of spatial sorting in facilitating the expansion of Common Myna in South Africa despite its low propensity to disperse in the native range.
European starling (Sturnus vulgaris). [ Photo: Petr Kratochvil ]In the second publication (Hui et al. 2012), the research team tested three inter-related hypotheses derived from demographic and ecological models: (H1) short-distance dispersal strategies arise at native range margins due to their demographic advantage; (H2) in non-native areas a high diffusion rate is favoured at the advancing range front for niche filling; (H3) environmental deterioration can increase dispersal and lead to a ‘good-stay, bad-disperse’ strategy. Spatially and temporally explicit rates of spread and dispersal kernels of the European Starling (Sturnus vulgaris) were generated for its native range (Britain) using ringing records from 1909 to 2008, and for a non-native area (South Africa) using ringing data and distributional records since its introduction in 1897. There was a marked spatial and temporal variation in the rate of spread within both native and non-native ranges. In the native range the rate of spread declined with increasing distance from the species’ European distribution (contradicting H1). In the non-native range the rate of spread increased with distance from the introduction locality (supporting H2). The annual rate of spread in the native range also increased significantly when environmental conditions were deteriorating as indicated by marked population declines and relatively low abundance (H3), providing clear evidence for flexible dispersal strategies based on a ‘good-stay, bad-disperse’ rule. Starlings’ dispersal kernel follows an inverse power law and showed strong anisotropy and significant divergence between native and invaded populations, suggesting a flexible strategy comprising a dynamic response to spatial and temporal environmental variation with implications for predicting dispersal and range dynamics arising from environmental change.
