Ellen Cieraad's Research

Quantative plant ecology & physiology

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Double whammy caused by climate change

Invasive species can have detrimental effects on ecosystem services, including food security and indigenous biodiversity. Similarly, climate change has been predicted to affect global food provision and the stability of ecosystems. Many studies assess the direct effect of either of these issues facing today’s world; however, of course they operate in tandem and this can create complex effects. While it is well established that climate change may facilitate the abundance and/or range expansion of invasive species, it is less known that it may also impact the ability to control and manage this invasion. If, for example, climate change results in the increase in abundance of an invertebrate crop pest, and at the same time in a decrease of a natural pest control agent (for example, insectivorous birds), then climate change can accelerate the impacts of invasions, and create a ‘double whammy’ for the receiving ecosystem. Similarly climate change may alter the effectiveness of ways to control invasive species.

(c) wikipedia

(c) wikipedia

In a recent article, we show that increasing air temperatures over >60 years in New Zealand has reduced the window of opportunity to effectively control an invasive mammalian pest (European rabbit Oryctolagus cuniculus – using Central Otago, South Island, as an example).

Latham ADM, Latham MC, Cieraad E, Tompkins DM, Warburton B 2015. Climate change turns up the heat on vertebrate pest control. Biological Invasions 17(10): 2821-2829. doi: 10.1007/s10530-015-0931-2

Anthropogenic control of invasive vertebrate species is most effective in times when natural food is limiting. In the case of rabbits in temperate southern New Zealand, the most effective period of control is during the coldest period in winter, when temperatures are sustained below approximately 5°C and the above-ground palatable vegetation available to rabbits reaches an annual low. Our study found that the milder winters in recent years resulted in the window of control starting later and ending earlier in the year, and that those windows are increasingly punctuated by warm temperatures. Overall this has resulted in suboptimal conditions for poisoning because of the higher availability of natural foods.

While longitudinal records detailing the effectiveness of control operations are not available, this study suggests that the trend towards warmer winters over the past >60 years has significantly reduced the window of time for effective control of rabbits in temperate New Zealand. As winters are likely to continue to warm, alternative methods of management warrant investigation.

Climate change may thus exacerbate the unwanted impacts of invasive species by reducing our ability to manage them effectively.


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Invasive plants can help establish native species in harsh places

New Zealand has as many native as invasive plant species. A number of woody invasive species have been rapidly increasing their distribution in recent decades. In many places they are chemically or mechanically controlled often with the implicit, rarely achieved, aim of advancing the restoration of native woody vegetation. However, there may be another option: leave these exotic shrubs – they may be ‘nurse-crops’ that aid the recovery of native vegetation. This seems to be particularly effective in moist environments and where nitrogen-fixing species are involved, such as gorse (Ulex europeaus). In the drier parts of New Zealand, broom (Cytisus scoparius, another nitrogen-fixer) is a real problem, and here this species was thought to be fairly useless as a nurse-crop, until recently (see this post).

In an article that is now available online, we describe a field experiment that tested how five different management treatments of broom cover affected the germination, survival and growth of native tree and shrub species.

Burrows L, Cieraad E, Head N 2015Scotch broom facilitates indigenous tree and shrub germination and establishment in dryland New Zealand. New Zealand Journal of Ecology 39(1) In Press.

We imitated different management techniques that are applied to broom in the dryland zone: in some plots the broom shrubs were sprayed with weedkiller, elsewhere we had bull-dozers drive over the shrubs and leave the debris, or tractors rake all the shrubs (including their roots) and take the debris away, in other plots the shrubs were mulched and the mulch was left, and in the last type of plot, we just didn’t touch the broom shrubs and just left them standing. In all plots, we sowed seeds and planted seedlings of six native tree and shrub species. There was no evidence of unassisted regeneration of native shrubs from plants nearby during our experiment.

We sowed thousands of seeds, and very few seeds germinated and even fewer germinants survived until the end of the experiment (3.5 years after sowing), but some species had much higher germination than others (particularly the hard seeded Kowhai, Sophora microphylla, germinated well). Plots where the broom cover was mulched, crushed or root-raked had very low seed germination and high mortality of planted seedlings, which was apparently due to the soil disturbance and harsh conditions of the open sites that were created. Under the living broom canopy germination and survival rates were significantly higher. This indicates that the positive (facilitative) effects of the living canopy, such as the provision of shade and a moister cooler environment in the dry summer, outweighed any negative effects (probably particularly the increased competition for moisture by the living broom shrub and the native seedlings).

This study suggests that at this dry site, compared with the chemical and mechanical treatments of this woody weed, retaining a live broom canopy was most beneficial for the germination and establishment of planted native woody seedlings. Importantly, it was also the cheapest management option by far, and it may thus be an important strategy to advance the succession of indigenous woody species in these dryland weed communities.