In my third year of Undergrad I took a course on the global struggle for food and food security and have since been really interested in organic vs. Conventional farming. We studied that topic alone for about 6 lectures and even had guest lectures from some local organic farmers. When I saw a study that compared organic farming and land sparing with respect to optimal yields and butterflies I couldn’t resist blogging about it. I mean, come on, who doesn’t love butterflies. And ... it’s a natural field study to boot!
As biology students I am sure we all know about the advantages and disadvantages to organic and conventional farming. Organic farming often yields fewer products but is generally better for the environment and uses no fertilizers or pesticides. Conventional farming uses large amounts of these pesticides but has yields that can realistically address the global struggle for food--something organic farming cannot do.
Extinctions and declines in many plant and animal species are a result of pesticide use on farms, monocultures, habitat fragmentation, etc. Efforts are being put forth all over the world to help solve this problem, by introducing more and larger organic farms. For example, as is highlighted in today’s focus paper, in the UK in 2008, 435m pounds was spent on agri-environment schemes. According to the government of Canada, hundreds of thousands of dollars are being directed towards organic initiatives. The organic retail market is now valued at 2 billion dollars in this country.  |
| Number of Certified Organic Farms in Canada |
Hodgson and colleagues designed an interesting study that you can find in the November 2010 issue of Ecology Letters. In this study they aim to provide a quantitative assessment of the landsharing/land sparing (I’ll explain what this is) trade-off for Lepidoptera (butterflies).
Biodiversity is usually higher on organic farms, right? If you were to ask anyone that, most would probably say yes. And they would be correct in saying yes. However, the benefit of organic farming also depends on the surrounding landscape—this is supported by a wide body of literature—and this makes it difficult to measure the benefits of organic farming. If one farm has an enormously high level of biodiversity, it may be simply because that farm was developed in an area that had a pretty high level of biodiversity to begin with. Contrasting this, a successful implementation of an organic farm could mean wildlife spilling over into the adjacent landscapes and having a bigger effect then can be measured.
Land-sparing occurs when you farm as intensively as possible on a portion of the land and leave the rest untouched for wildlife. Land-sharing, as I am sure we know, makes all land better for biodiversity maintenance. So which one is better? Or is there a trade-off?
Hodgson et. al, developed a natural, field survey in which sixteen 10 x 10km landscapes in England were selected. Within each landscape they surveyed one organic farm, one conventional farm, and one grassland. The 16 landscapes were 8 matched pairs with similar environmental conditions. Butterflies were recorded in the fields along the centre and margins with standard 15 min walks (butterflies within 2.5m of either side of the transects were recorded). The grasslands were sampled using a zig zag pattern since there is no identifiable margin or centre. These surveys were done in optimal butterfly conditions with certain temperatures, wind speeds and times throughout the day. Yield was estimated from all fields.
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| Relationship between species richness and number of individual butterflies |
After analyzing their data, they found that density of butterflies is higher on organic than on conventional farms and is the highest in the grasslands. The number of butterflies seen was the single most important determinant of butterfly species richness. It explains 59% of the variation alone (see figure below). It is important to note though that there are several variables that affect the slope of the line below including higher number of species in reserves and also in margins compared to centres.
The main idea here is that organic farms support more butterflies than conventional farms. This means that if there was no difference in yield it would always be better to farm organically. But, as we know, and as data from this study shows as well, the lower organic: conventional yield ratio suggests a land sparing strategy would be more advantageous. There is a great example of hypothetical critical value calculations with respect to this study’s data I encourage everyone to look at in the actual paper.
Mathematical models were also used throughout the study to address ideas such as what would happen with an increase in organic farms in these landscapes, or an increase in grasslands. They found using these models that the average butterfly density of a landscape could increase by increasing either organic reserves or grasslands. Reserves have a greater impact relative to their area and so a landscape with 2.5% reserves and no organic farms will have the same butterfly density as one with no reserves and 20% organic farms.
I won’t even get into my feelings about mathematical models. They are generally focused on ideals that really aren’t reproducible and are hard to test. Who knows if this particular model’s conclusions will hold strong. On the other hand, as I have stressed in previous posts, it is important to use multiple approaches to studying a topic and the authors did a great job with that.
There are other strengths in this study, but many weaknesses as well. The use of closely paired farms allowed them to control for environmental context which is hard to do—even harder to do naturally, so good for them. But, at the same time this meant only comparing 2 field types (cereal and pasture) and only on mixed farms. Many conventional farms are specialists and have massive monocultures which present a whole different slew of variables to consider. The tendency of organic farms to grow mixtures instead of monocultures also means that this might by default cause an increase in biodiversity on organic farms. I appreciated the focus of this study on such a popular and important creature the butterfly, but different taxa will undoubtedly respond differently to these farming methods which limit the conclusions. The conclusion of this paper seems like a no-brainer: the best strategy for managing biodiversity while maintaining yield is context-dependent. The best strategy for anything is context-dependent. Duh.
Overall this paper had a very relevant and important basis. The methods were great and I appreciated the simple and naturalist design-approach. The model was confusing, but models easily confuse me so I won’t comment on that. The conclusions seemed obvious but they did suggest some good opportunities for future research.
Farms themselves are not natural, so some may argue that this is not a natural study, but I beg to differ. Farming and agriculture have evolved with humans, and are necessary for our survival, so designing a natural study around them is about as close to real and natural that a study can get. Hats off to the authors for a great natural field survey—Eat organic and save the butterflies!
http://www.meattradenewsdaily.co.uk/news/220310/canada___government_money_for_organic_farming_.aspx
Photos:
http://americangatewaycenter.com/Home_Page.php
http://www4.agr.gc.ca/AAFC-AAC/display-afficher.do?id=1276033208187&lang=eng
http://www.cornwall-butterfly-conservation.org.uk/about_us.php
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