Once upon a time in the far south: Influence of local drivers and functional traits on plant invasion in the harsh sub‐Antarctic islands
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Abstract
Aim
Here, we aim to: (a) investigate the local effect of environmental and anthropogenic factors on alien plant invasion in sub‐Antarctic islands; and (b) explore whether and how functional traits affect alien species dependence on anthropogenic factors in these environments.
Location
Possession Island, Crozet archipelago (French sub‐Antarctic islands).
Methods
Single‐species distribution models were used to explore the effect of high‐resolution topoclimatic and human‐related variables on the occurrence of six alien plants colonizing French sub‐Antarctic islands. Furthermore, plant responses to human‐related variables and the effect of those variables in interaction with plant traits were analysed by means of a multi‐species distribution model. This allowed identifying functional features mediating the influence of human activities on the occurrence probability of alien plant species.
Results
We observed two main invasion patterns: (a) species predicted to occur close to the introduction sites, whose occurrence probability appeared to be strongly affected by anthropogenic factors; and (b) species predicted to occur nearly everywhere on Possession Island, except in areas featuring particularly harsh climatic conditions. Differences in the influence of human‐related variables on the occurrence of the alien species were mostly related to their life history, plant height and residence time, with perennial and low‐statured species introduced earlier appearing less dependent on human‐induced dispersal and disturbance.
Conclusions
We conclude that both topoclimatic and anthropogenic factors affect plant invasion on sub‐Antarctic islands. Specifically, species predicted to occur close to their introduction sites appear much more dependent on human presence and activity, potentially due to the lack of key functional traits allowing them to spread successfully across Possession Island under the harsh sub‐Antarctic climate. Yet, particularly severe abiotic conditions are a major constraint which equally limits the occurrence of all alien plants, irrespective of their dependence on anthropogenic factors.
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Reproductive succes, as a surrogate of individual fitness, depends both on extrinsic and intrinsic factors [1]. Among the intrinsic factors, resource level or health are considered important potential drivers of fitness but exceedingly difficult to measure directly. Thus, a host of proxies have been suggested, known as condition indices [2]. The question arises whether all condition indices consistently measure the same "inner state" of individuals and whether all of them similarly correlate to individual fitness. In this preregistration, Berens and colleagues aim to answer this question for two common condition indices, fat score and scaled mass index (Fig. 1), using great-tailed grackles as a model system. Although this question is not new, it has not been satisfactorily solved and both reviewers found merit in the attempt to clarify …
Reproductive succes, as a surrogate of individual fitness, depends both on extrinsic and intrinsic factors [1]. Among the intrinsic factors, resource level or health are considered important potential drivers of fitness but exceedingly difficult to measure directly. Thus, a host of proxies have been suggested, known as condition indices [2]. The question arises whether all condition indices consistently measure the same "inner state" of individuals and whether all of them similarly correlate to individual fitness. In this preregistration, Berens and colleagues aim to answer this question for two common condition indices, fat score and scaled mass index (Fig. 1), using great-tailed grackles as a model system. Although this question is not new, it has not been satisfactorily solved and both reviewers found merit in the attempt to clarify this matter.  Figure 1. Hypothesized relationships between two condition indices and reproductive success. Single arrow heads indicate causal relationships; double arrow heads indicate only correlation. In a best case scenario, all relationships should be positive and linear. A problem in adressing this question with grackles is limited population, ergo sample, size and limited possibilites of recapture individuals. Some relationships can be missed due to low statistical power. Unfortunately, existing tools for power analysis fall behind complex designs and the one planned for this study. Thus, any potentially non significant relationship has to be taken cautiously. Nevertheless, even if grackles will not provide a definitive answer (they never meant to do it), this preregistration can inspire broader explorations of matches and mismatches across condition indices and species, as well as uncover non-linear relationships with reproductive success. **References** [1] Roff, D. A. (2001). Life history evolution. Oxford University Press, Oxford. [2] Labocha, M. K.; Hayes, J. P. (2012). Morphometric indices of body condition in birds: a review. Journal of Ornithology 153: 1–22. doi: [10.1007/s10336-011-0706-1](https://dx.doi.org/10.1007/s10336-011-0706-1)
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