One small crop or two large pastures?

How the impact of land use on forest fragmentation varies with spatial scale 

Pasture land in the Brigalow Belt. (Photo by Leonie Seabrook)

Pasture land in the Brigalow Belt. (Photo by Leonie Seabrook)

The fragmentation of forest ecosystems is a major cause of species extinction. Fragmentation is the process by which forest cover is broken apart into smaller fragments (as opposed to the loss of the total amount of forest). It’s caused by human activities such as agricultural expansion and intensification, logging and urban development. Different land uses can create different patterns of fragmentation at different scales (see the box ‘Scales of fragmentation’). This is important because it has been shown that forest fragmentation at different scales can have different impacts on biodiversity (see the box ‘Fragmentation and biodiversity’).

A key step in developing appropriate conservation strategies to reduce fragmentation involves identifying the drivers of fragmentation at different scales. For instance, better farm management can be employed to reduce fine-scale fragmentation, while the elimination of subsidies for large-scale clearing could be more suited to target coarse-scale fragmentation.

Determining drivers of fragmentation

How do you know which land use is responsible for fragmentation at each scale? To answer this question, we need to determine how the impact of different land uses on fragmentation varies with spatial scale. That is exactly what we did using Queensland as a case study.

If you are focusing on fragmentation across multiple agricultural fields or small properties, to protect wide-ranging species, our study suggests that your top priority should be to revegetate or keep standing trees in land modified by grazing rather than cropping.” 

We first decided to focus on cropping and grazing land uses as they are the major drivers of native vegetation clearing in Queensland. We expected the influence of cropping and grazing on forest fragmentation to be different at different scales.

Second, we measured fragmentation across a range of spatial scales in a sample of 5,309 landscapes (with a total of around 50 km2). To do this, we borrowed a technique from fractal geometry. This allowed us to detect whether different spatial arrangements of forest patches occurred simultaneously at different scales in the same landscape. We discovered that the spatial arrangement of patches in the landscapes changed at approximately the 1 km2 scale. This is comparable to the average size of an agricultural field in Queensland.

Next we quantified how fragmentation at fine scales (below 1 km2) and coarse scales (above 1 km2) varied as a function of the proportion of land in the landscapes occupied by crops and the proportion of land occupied by grazing pasture.

Figure 1. Bar chart showing the average degree of coarse-scale and fine-scale fragmentation (± 1 Standard Error), in landscapes with different dominant land uses, for different amounts of forest cover (p). The term ‘cropping’ indicates landscapes with a higher proportion of cropping land use than grazing land use, and vice versa for ‘grazing’.

Figure 1. Bar chart showing the average degree of coarse-scale and fine-scale fragmentation (± 1 Standard Error), in landscapes with different dominant land uses, for different amounts of forest cover (p). The term ‘cropping’ indicates landscapes with a higher proportion of cropping land use than grazing land use, and vice versa for ‘grazing’.

We found a scale-dependent effect of land use on forest fragmentation. At coarser scales, forest was more fragmented in landscapes with a higher proportion of grazed land than cropping land, while at finer scales the difference was much smaller (Figure 1). This suggests that grazing drives coarse-scale fragmentation more than cropping.

Our finding is consistent with the fact that, across multiple agricultural properties, vegetation tends to be more clumped as a result of clearing being clustered around areas of high soil fertility. On the other hand, vegetation is usually more fragmented across grazing proprieties, where soil fertility is a less important driver of clearing.

Targeting different drivers

Why would you be interested in this finding? Because it tells you which land use you should invest money in to develop conservation measures to reduce fragmentation at particular scales. If you are focusing on fragmentation across multiple agricultural fields or small properties, to protect wide-ranging species, our study suggests that your top priority should be to revegetate or keep standing trees in land modified by grazing rather than cropping. This could be achieved, for example, by targeting Payment for Ecosystem Services (or PES) to grazed land rather than cropped land.

On the other hand, if you are focusing on fragmentation within an individual agricultural field to conserve species that move more locally, the choice of land use to target with conservation actions is less important.

Thanks to our finding, matching the scale at which fragmentation is managed with scales relevant to the ecology of species or land management is going to be considerably easier.

Cropping in the Brigalow Belt. (Photo by David Salt).

Cropping in the Brigalow Belt. (Photo by David Salt).


More info: Lorenzo Cattarino l.cattarino@griffith.edu.au 

Reference 

Cattarino L, CA McAlpine & JR Rhodes (2014). Land-use drivers of forest fragmentation vary with spatial scale. Global Ecology and Biogeography DOI: 10.1111/geb.12187


Scales of fragmentation

The scale at which forest fragmentation occurs can be roughly thought of as the size of the fragments in which the forest is subdivided. For example, urban development tends to break the forest into small patches (fine scale) while clearing of native vegetation for large-scale farming usually fragments the forest in to larger patches of vegetation (coarse scale).

Fine scale:

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Coarse scale:

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Fragmentation and biodiversity

In fragmented landscapes, animals have to spend a lot of energy moving between patches. This raises the chances of them starving or being eaten. But, do animals have to move more when fragmentation occurs at fine or coarser scales? This is critical to determine at which scale we should manage habitat fragmentation. To answer this question, Lorenzo Cattarino and colleagues from UQ conducted computer simulations on artificial landscapes (Cattarino et al 2013). They found that, as habitat declines, fragmentation at fine scales forces animals to move more than fragmentation at coarse scales, particularly in those species that naturally move short distances. Species moving larger distances are more affected by coarse-scale fragmentation, but to a smaller extent. So, at which scale should we manage fragmentation? The answer depends on the species you want to conserve.

Reference 

Cattarino L, CA McAlpine & JR Rhodes. (2013) The consequences of interactions between dispersal distance and resolution of habitat clustering for dispersal success. Landscape Ecology 28:1321-1334.


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