Summer success for ballooners: spiders take the high road

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High altitude tussock grassland in Otago. Image from Adrian Paterson.

New Zealand is renowned for the changes that have been wrought on it by humans.  First by Maori, then by Europeans.  One of the long term changes created by Maori have been the tussock grasslands of Canterbury, Southland and Otago.  The local fauna have adapted and created their own ecosystem.  Over the last 150 years with pastoral farming the farmers have been trying to maintain enough grass for their stock, limit tussock regrowth and remove unwanted plants whether exotic or not.  As part of this management system tussock grasslands have been burnt creating a problem for local native species, including spiders. One of the main benefactors of the burning are exotic spiders, many of which balloon in.  Native spiders in the area receive less benefits, especially if the burning is done over summer. PhD student Jagoba Malumbres Olarte with supervisors Adrian Paterson and Rob Cruickshank examined the response of spider communities in high altitude tussock grasslands making use of an existing long term study started by AgResearch ( Malumbres-Olarte et al 2014).

Spiders are often the first invertebrates to arrive at land that has been disturbed, either through ballooning or pedestrian means.  The seasonal timing of this invasion does affect the ability of the different spider families to invade.  The main family invading is the Linyphiidae, which has natives running into the area as well as exotics ballooning in after the summer burns. This ballooning is seasonally based and linked to the size of the spiders.  Some spiders are able to balloon at different stages of their lives due to their small size, others  are limited to  their juvenile stage as beyond that they are too heavy to successfully balloon (ballooning and impacts on humans).

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Some of the many tussock and turf samples collected for the study. Image from Adrian Paterson.

In the  study by Malumbres-Olarte et al (2014) they trialed different burning regimes, with control plots in high altitude tussock grasslands.  These were done in spring and summer to see which spiders arrived and how they were able to get there.  The areas had been studied three years before the burns to assess the spider community.  The plots were then assessed over the next four years, after burning.  Tussock turf was collected and the spiders extracted using Tullgren extraction funnels.  Adult spiders were identified using morphological sources available to species level.

The spring burns had less exotic invaders although they still arrived in greater numbers than the native spiders. This may be due to differences in birth rates or the changeability of the weather reducing the success of ballooning.  The ability of the exotics to invade does not lead to  long term survival.  The exotic populations start to reduce after a peak at three years after the burn offs.

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Jagoba in action measuring a plot. Image from Adrian Paterson.

The native spiders, after the spring  burn offs, return to  similar levels as the control plots after the three year peak.  Part of the ability of the Linyphiidae  and others to survive in this burnt habitat is the use of remaining vegetation and burnt structures as props for their webs which then catch flying and/or wind blown insects or invertebrates.

Currently it appears that the use of burning has short term negative effects, for spiders. A change is possible in longer terms, yet to be studied, for species diversity after summer burn-offs than after a spring burn-off.   If the native spider community is healthy and habitats are filled appropriately there is less likelihood of any ‘visitors’ being more than that.

What does this mean for future management?  Is the real reason for the burn-offs to reduce exotic plant invasion?  Are there ways to reduce exotic spiders changing the communities?  These questions are not discussed in the paper. I will dicuss them  from my point of view.

Burn-offs are simple, not time-consuming (unless they get out of hand) and have immediate  results.  Are they actually needed or is this a way of reducing tussock areas and increasing grazing areas?  If stocking rates are appropriate and the area is regularly grazed it is unlikely the unwanted plants will establish unless the plant is unpalatable to the stock.  If stock are given access within the first two years of germination when most plants are still palatable (for example wilding pines) they will be eaten, reducing costs.

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Native wolf spiders tend to walk back into burned areas. Image from Adrian Paterson.

Manual labour and use of direct application poisons, although time expensive, is one way of reducing older invasive plants and is specific to the plants that are required to be removed.  Manual labour is best in areas that are not grazed, due to ability to identify plants and remove the unwanted without affecting the wanted.  The  better knowledge of the unwanted plants can be gained from the observing the number of plants and their growth rates.  Measures can then be taken to implement management plans for regular observation to consider future removal operations.  These regular observations will also create useful monitoring systems for a variety of aspects about the flora involved and can be used to monitor the fauna of the area as well.

Another aspect of using fire to manage unwanted plants is the destruction of uncommon ecosystems that may be small in size but very important.  These are often unnoticed by the land owner who is interested in the returns of the property and not ecosystems on their land.  Many high country farmers who have been through or are going through the tenure review process are finding out that these parts of their land have a value.  They are willing to acknowledge this value, so they can farm the blocks that are easier on their stock.  Within the uncommon ecosystems are both plants and animals that may have limited abilities to survive in different areas or ecosystems.

Thus it is important for the spiders, the tussock and the relevant ecosystems that regular burning is reduced or eliminated as a means of removing unwanted plants from the tussock grasslands, whether for conservation or farming practices.

The author Paula Greer is a postgraduate student in the Master of Applied Science taught at Lincoln University. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.

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