|Tussock grasslands on top of the Lammerlaw Range|
Another consequence of living in a windy environment is that the local habitats dry out and become prone to fires. A couple of summers ago we had a major fire that burnt out of control through the shelter belts near Lincoln, stopping just a couple of kilometres from the University. We are looking at another dry summer this year. It has been interesting watching what has grown in the areas that were burnt. Usually the plants that have returned are not necessarily the same as those that were there before. Sometimes it depends on what seeds/plants survived the fire but often it depends on which seeds have turned up from outside the burned area. One can ask the question of whether there are certain types of traits that allow some species to colonise first. Being able to be moved by wind could be an important trait. In addition to the plants, we also might want to know about the animals and how they respond to these crises, especially the invertebrates.
|Spiders are important predators in most habitats.|
Fires are not particularly common in New Zealand, but one area where they can occur in the montane grassland zone. Tussock grasslands tend to be in dry parts of New Zealand where fires can be natural (lightning), accidental (camp fires) or deliberate (set by farmers managing pasture). As more of these areas are transferred into conservation land, it is important that we understand the effects of fire. A great opportunity to study this has been available in the Deep Stream area of east Otago (about an hour inland from Dunedin). This area has never been cultivated and had not been burned for at least 30 years before our study began. Areas were selected to be either burnt in spring or summer as well as control areas with no burning. Invertebrate diversity was then measured in these areas for the three years before burning and four years after burning to examine the effect of fire and the timing of the fire on these populations. We were particularly interested in what happened to the spider communities. Spiders are top predators in these systems and can tell us a lot about the health of these ecosystems. If there are a lot of spiders about then there must be a lot of spider food species.
|Jagoba sampling spiders in a cool and windy habitat.|
My former PhD student, Jagoba Malumbres Olarte, worked closely with researchers from AgResearch, especially Barbara Barratt, in collecting spiders from pitfall traps and by digging out whole tussock plants and extracting spiders in a Tullgren funnel. The outcome of this work has been published in the journal Biological Invasions. Jagoba found over 4500 spiders in the seven years of samples and sorted them into 66 species and 22 families. Ten of these species were exotic (not native to New Zealand) and increased markedly in the burned samples while native species had significantly less resilience. This is a real concern as it suggests that one reason that introduced species can take over a habitat is because they are better at colonising and establishing after a crisis. Jagoba also looked at some of the traits that might explain the advantage of exotic species. Whether a species could balloon was important for colonising after summer burns. Exotic species tend to be good ballooners and can easily waft into burnt habitats from elsewhere. Jagoba also found that large spiders were able to colonise burnt habitats more easily after both summer and spring burns. This may because larger spiders find it safer to walk through the landscape compared to smaller species. Many of the exotic species, especially the Linyphiids, are large. So the wind does help species move about and gives exotic spider species an advantage over natives. One unintended outcome of burning native grasslands is that this provides exotic species with a way into these areas to colonise and then dominate, reducing the local native diversity. More worryingly, any crisis that impacts on an area, say landslides, floods, human modifications, will result in the same benefits for these ballooning, large, exotic spiders. The answer to why introduced species do so well in New Zealand really is blowing in the wind.