Nutrient Polution

Stan Harpole talking about how using fertilisers can reduce biodiversity by destroying the niches that allow complex inter-species competition
22 April 2007

Interview with 

Stan Harpole, University of California, Irvine


Helen -   You describe yourself as a Community Ecologist, what does that actually mean?Stan -   I study how different species, different organisms, coexist.  How they interact, what controls biodiversity and what contributes to loss of biodiversity.Crop planeHelen -   Cool, sounds great.  In your latest paper which came out in Nature, you looked at crop fertilisers and the number of species, or biodiversity, that we find in grasslands in California, why did you expect there to be a link between fertilisers and biodiversity?Stan -   There's a long-standing theoretical prediction that the diversity of niches, and by niche I mean the different ways that species compete and specialise in nature, that the more complex the niches, the greater the number of niches, the more species there should be.  If you do something that reduces the number of niches, there should be fewer species.  For plants, they're competing for things like nitrogen, phosphorus, potassium, water, light and all these different things and we're finding that natural systems are really quite complex.  Plants are competing for many, many different things simultaneously.  As we add nitrogen or phosphorus etc lose the niche, and as we add more and more things to make them superabundant, so the plants no longer compete for them, we see a loss of species.  Helen -   It seems to be a little counter-intuitive, I would possible expect that you would get more species if there were more nutrients to go around.  It seems lightly odd that this is the other way around.Stan -   It does, in one way it's been described as having too many good things.Helen -   I see...Stan -   When you add too much nitrogen or too much phosphorus etc, the system becomes limited by just one thing; possibly just light, perhaps magnesium or another resource, so there is only one way for the species to differentiate and compete, and fewer opportunities for them to co-exist.  That's what we see in aquatic systems too, as we add more and more fertilisers, it becomes very dark.  The algae grow very quickly and it becomes so dark that there's only one species best adapted to those low light conditions that can survive.Helen -   So we have these theories about the interplay between fertilisers and biodiversity, how did you go about testing these theories to see what's actually happening in the real world?Stan -   I work in grassland communities, these are natural, prairie type systems with many native species and many exotic species but there's fairly high grassland diversity.  We added combinations of nitrogen, phosphorus, potassium and water and found that if we added just one thing there was little impact on diversity.  As we added more things, up to all four things at once, we saw the biggest loss of diversity.  This is evidence for the loss of niche dimension. As we take away each one of these specialities that plants have, there are fewer opportunities for them to co-exist. We end up with, as in aquatic systems, the species that's the best competitor for really high nutrients, but really low light conditions.  So just one species.Helen -   How long did it take for these changes to take place? How long did you keep watching?Stan -   This was within two years, so it was very quick.  We've also looked at really long-term data sets and this loss of diversity can persist for 150 years.  There was a great experiment in the UK, the Park Grass Experiment, they've been applying fertiliser since 1858, so a long time ago, and there's no recovery of species diversity even after all that time, so it can be a very long term effect.Helen -   That's amazing.  Surely a reduction in biodiversity in terms of field crops, and I'm assuming the fertilisers we're talking about are the sort of things we use for crops...Stan -   The problem is that they don't stay on just the agricultural land; the nitrogen can volatilise and go up into the atmosphere then back down as precipitation; phosphorus can wash off into streams and rivers and then into gulfs and coasts and so on.  There are very large 'dead zones', huge areas where there has been a lot of fertiliser input, tens of thousands of square kilometres.  All this fertiliser causes high algae growth, which causes a really strong decrease in light. One way that marine systems differ from the grassland systems I work on is that as all these algae fall to the bottom of the ocean and die they decompose, and in the process, oxygen is depleted, and this then kills the fish, which creates this 'Dead Zone'.Helen -   I'm pretty ignorant about just how much fertiliser we use globally every year, is it really huge amounts that we're still using?  I would imagine that organic farming is still a tiny proportion.Stan -   Yes, and nitrogen in particular.  Humans have become as big an input of nitrogen to natural systems as natural processes are.  We are one of the drivers of the nitrogen cycle now, globally.Helen -   And you think that also, possibly, there's a link to our burning of fossil fuels and this input of nutrients into systems, is that right?Stan -   Absolutely, a lot of nitrogen comes from fossil fuel combustion.  These nitric acids then eventually come back down, and usually in places where nitrogen is not so abundant, so we're increasing nitrogen in places which were typically pretty limited in nitrogen.Helen -   You mentioned that long term studies don't show much recovery.  If we aren't seeing much recovery, is there anything we can do about this in terms of the impact we're having on biodiversity?Stan -   I think the main thing is to reduce the inputs of nutrient pollution to natural systems, and that could be changing our fertilisation techniques in managed agricultural systems, reducing fossil fuel combustion...  We could also try to manage natural systems to find ways to remove excess nutrients, if possible.  There have been some studies trying to restore natural systems by decreasing the amount of nitrogen that's available, but its still early stages.Helen -   I would like to bring Annelise Hagan, coral reef specialist, into the conversation now.  Stan you've already mentioned how, in aquatic systems certain organisms tend to grow much more quickly under high nutrient conditions and cause 'dead zones'.  If we add nutrients to coral reef systems, can we cause this imbalance of algae growth?Annelise -   Definitely. Nutrient impact will greatly increase Macroalgae on coral reefs, that's fleshy algae like seaweed.  Coral reefs have to have light in order to live so they're very restricted by depth, they only thrive in the upper 30 meters of the oceans.  There are lots of animals that want to live within this zone and so competition for space is a huge problem.  With nutrient enrichment, the macroalgae can out-compete the corals.  It will take over any bare surfaces so coral larvae can't settle and the reef cannot develop any more.Helen -   We're seeing that quite a lot, aren't we?  We're seeing reefs which are becoming dominated by algae, but then it's very difficult to go back to a situation where it's dominated by coral.Annelise -   It is very difficult, and one big problem as well is human impact of overfishing.  If the fishermen are removing the herbivorous fish, which can control the macroalgal population, that allows more macroalgae to grow, which will further out compete the corals.


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