Mislaid. We’ve all done it. We all have things that we know that we had once but can no longer find. From a stray sock to our hopes and dreams, life is full of lost things. Sometimes things are just gone. The broken vase, the wrecked lamp, the old decrepit chair – all off to the recycling station. Sometimes it’s not entirely our fault. The book we lent to a friend a decade ago, the coat they borrowed on a rainy day, the hammer that the neighbour needed – did we ever get them back? Sometimes we’re not sure. Then there are the things that are mislaid. We think they are still around somewhere but we just can’t find them. These are the worst to deal with.

About a decade ago my wife received a couple of booklets of tickets to the local movie theatre. Twenty free movies. What joy, how would we spread those out over time? What would we see? Unfortunately, nothing, as it turned out, because they were mislaid. How this happened we can’t say. Did they get bundled up in a pile of junk mail and thrown out? Put carefully into some container for safe keeping? Fall out of a handbag as it was knocked over in a cafe? We’ll never know. Every few weeks after they went missing I would have a new idea for where to look and go to it. Nothing. We have moved out and back into our house to effect earthquake repairs but they didn’t turn up during that packing. Even now when I do a bit of spring cleaning I expect them to turn up somewhere. They are long expired but it would be good to know what happened to them.

Given the amount of searching over the years I am reasonably confident that they are not in our house. But how can I be sure? When I do a spring clean I often find things that have been mislaid, gift vouchers in birthday cards, artwork from younger phases of our kids, or, as found last week, a box of dress-up wigs (timely for Halloween). Many of these things have also been looked for unsuccessfully over the years. So I know that it is possible for something which appears to be absent to still be present but not found. So how long should I expect to keep looking before I am convinced that the movie tickets are not still somewhere in the house?

We ask similar questions in ecology and conservation all of the time. New Zealand has set a target of being predator-free by 2015. How do we know that an area is predator free? An obvious thing to do is to search a lot and not find predators. But how much searching is required? Finding one predator tells us an area in not predator free. Finding no predators may tell us that an area is predators free or that we just haven’t found them.

For any population with small numbers this is an issue that we have to deal with. We could call it the Nessie effect. The Loch Ness Monster has been searched for over many years with no luck and we are very confident that there is no pterosaur-like beastie lurking in the depths of the Scottish lake. And yet there are still people searching and documentaries being made. Why? Well you never know, like an old odd sock it might be found down some little hidey hole.

How can we be sure that there are no more members of an endangered species in an area? Or that we got the last weed? Or that there are no more pests to trap? We we could sample a lot. And that should help. Simon Hodge (Lincoln University) and Cor Vink (Canterbury Museum) asked that question about a threatened New Zealand spider species, the katipo (a relative of the Aussie redback and the black widow, and our only home grown poisonous species).

The katipo is found on sandy beaches around New Zealand but has declined in range and population due to competition with introduced species as well as habitat change and fragmentation. They are now difficult to find in most areas and we would like to know when they are truly absent from an area.

Simon and Cor visited New Brighton Beach in Christchurch and sites around nearby Banks Peninsula where katipo had been found in the past. They made a lot of visits. In a paper in the New Zealand Journal of Zoology they report how they went to New Brighton beach 382 times over three years. Each time they searched the strand line and upper shore debris for 30 minutes. They did the same thing around Banks Peninsula 153 times. Katipo were found at four beaches (out of 35) on Banks Peninsula. On the beaches where they were found they were then found 70% of the time on repeat visits. This suggests that when katipo are present they are relatively easy to find. At New Brighton katipo were never found. At all! Over twice the search effort and no sign of our red-striped friends. Clearly they must be absent from New Brighton Beach.

Simon and Cor also did six surveys of the dunes at New Brighton on the fringe of the vegetation and on four occasions found katipo. So katipo seem to be happy to frolic on the beaches around Banks Peninsula but not at the nearby New Brighton Beach, even though they are found lounging in the dunes. No amount of absence from sampling of the strand line would have given the correct answer here. If you look for something hundreds of times without finding it then you might feel justified in saying a species is truly absent. But this is not the case in a world where animals can have population level preferences or local conditions can play a role.

So how do we ever know if a species is truly absent from an area. Quite simply you can never know for sure. However, by searching a lot we can at least be more and more confident in their absence. So maybe this is good news for Nessie? Perhaps I should keep looking for those movie tickets.