Mark Cleverley, of Seatoun, asks :-

Thirty years ago while living on the North Shore (Auckland) the bane of my life was having to mow a large paspalum lawn. The sticky seed heads just laid down flat before the cutting blades could attack them. We shifted to a cooler Wellington without this annoying grass but of late I have noticed paspalum grass is appearing in patches in my area. Could this be indicative of climate warming?

Trevor James, a biologist at AgResearch and the author of the book "Weeds of New Zealand", responded.

Several years ago the summer growing grass paspalum (Paspalum dilatatum), was largely confined to the warmer areas of the upper North Island. Recently however, it has been seen in Wellington and the question was asked, “Is this an indication of climate change?”

Although it could be the result of climate change there are a number of different but equally plausible explanations.

Darwin first mooted the concept of incremental changes within a species due to selection pressure. With any species that has two parents there is potential for variations within the population to occur. If some of these variations are more suited to the extremes of the habitat environment they are more likely to spread to these areas.

This raises two extreme possibilities with any number in between. The first of these is that the species, paspalum (Paspalum dilatatum) in this case, has not changed but the climate has so it is now found in areas previously uninhabitable. The second is that the climate has not changed but the paspalum has so is now able to live in places where it previously could not. And of course it could be due to a combination of the two in an infinite number of ways.

There is also a third possibility and that is that nothing, climate or paspalum, has changed. In this scenario Wellington has always been suitable for paspalum to grow but it simply has never been introduced there. When looking at the topography of the land in between Auckland and Wellington the most direct route is inhospitable to paspalum, too cold, so it is unlikely to move to Wellington by itself (unless it took the longer and more arduous coastal route). Enter human-assisted-distribution into the discussion. Paspalum has a sticky seed and human-assisted-distribution simply states that the seed has stuck to something or someone in Auckland (or anywhere else that is grows) and then to have dropped off or be knocked off in a frost free area of Wellington and there we have it - paspalum in Wellington.

Without a lot of study and experimentation it is impossible to tell which of these three arguments, or which combination, may be the most accurate. Paspalum is an introduced species and other introduced plant species have shown similar movement, a good example is Kikuyu grass (Cenchrus clandestinus). In fact climate matching and distribution studies appear to indicate that very few introduced plant species, shall we call them weeds for want of a better term, have reached the limits of their potential distribution within New Zealand.