Wednesday, March 8, 2017

Habitat relationships in complex landscapes

I'm working on some old data from Columbus, GA that I should have published years ago. So it goes.  

This is something that has been bugging me for over a decade.  If you look at the relationship below which shows how native bird diversity changes with urbanization. 

Urbanization is quantified as the amount of impervious surface a km around where they were counted. The data are transformed but the whole scale goes from 0% to 100% (just take the sine of any of those numbers to transform back into a percent). 

Note that the relationship is positive from 0 to 20% and negative >20% urbanization. The post 20% is easy to understand: as urbanization comes to dominate a landscape, native birds find fewer habitats. 

Less than 20% there is little urbanization. That's easy to get. But what is there? In a real complex landscape, where there is not city there can be anything. It can be completely forested, agriculture, suburban, a mix of all the above. I think that explains all the noise on the low-urbanization side. 

I think what's lacking - in any statistical approach that I'm aware of - is how to model with factors whose effect can vary with their values. So in this case, I bet species richness increases with urbanization at first because a little bit of urbanization adds some birds associated with urban habitats but there's still enough natural habitat to have those birds persist but as urbanization increases the natural habitat gives way and birds are lost. 

So I need to invent a statistical method. 


  1. You may be overthinking the concept. Just about any small-scale disruption to a local environment - whether it is cutting down an acre of forest to build a home - or an acre of forest burning down - will increase the local biodiversity. The point being is that the statistical method may already be there in papers which note how non-anthropogenic environment change influences local biodiversity.

  2. Or for anthropogenic change, there have been reams written on the comparative impact of clear-cutting.

  3. But I think it's more complicated in complex landscapes. For clearcutting, I'm familiar with a few papers from Maine where it's forest or clearcut so it's relatively simple: there's only two habitats - forest and not forest. But in the landscape where I was working there was seven major land uses (I can't call them habitats) . So locally increasing local habitat diversity should increase bird diversity if that diversity includes the natural types (pine, hardwoods, shrub) but the context is also important - the larger scale and that seems to kick in as the context becomes more dominant in one habitat type or another. So if you take the local environment and move in different larger environments the importance of the local environment will change. For instance, if there isn't much urbanization on a larger then diverse habitats on a small scale will have lots of birds but as you move into a urbanized area, even locally diverse habitats will be overwhelmed by the urbanization effect. So a park in an urban area will have fewer birds than a park in a forested landscape.

  4. Clearcutting studies have included the further variable of shelterwood ( for example) - but there is one issue that is very problematic - in terms of scale clearcuts fall on the first part of the graph.

    As an aside, the example of Central Park shows that in extreme cases parks in urban areas become magnets for far more bird species than one would expect for a similar environment in a non-urban area.