EdGE meeting – positive vs. negative effects of habitat fragmentation

Are the negative effects of habitat fragmentation a zombie idea? Do we see positive or negative effects of habitat fragmentation more often? And even before we do that, can we make broad generalisations about the effects of habitat fragmentations; if we can, should we be? What would be the potential effects on conservation policy and actions?

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Journal club joy

This week we had our first EdGE meeting, under the theme of a Biodiversity Journal Club, and we pondered all of those questions and more in a lovely thought-provoking and stimulating atmosphere! We focused on the following paper:

Fahrig, L. (2017). Ecological responses to habitat fragmentation per se. Annual Review of Ecology, Evolution, and Systematics48(1).

Here is an extract from the abstract of the paper to get an idea of our discussion theme:

Are most significant responses to habitat fragmentation negative or positive? And do particular attributes of species or landscapes lead to a predominance of negative or positive significant responses? I found 118 studies reporting 381 significant responses to habitat fragmentation independent of habitat amount. Of these responses, 76% were positive. Most significant fragmentation effects were positive, irrespective of how the authors controlled for habitat amount, the measure of fragmentation, the taxonomic group, the type of response variable, or the degree of specialization or conservation status of the species or species group. No support was found for predictions that most significant responses to fragmentation should be negative in the tropics, for species with larger movement ranges, or when habitat amount is low; most significant fragmentation effects were positive in all of these cases.

We all found the two main questions (“Are most significant responses to habitat fragmentation negative or positive? And do particular attributes of species or landscapes lead to a predominance of negative or positive significant responses?“) posed by the paper very intriguing, and had many thoughts – some of us were convinced by the main message of the paper, some of us less so, but we all agreed that “overall, it depends”.

We have also identified a sentence we might often come back to in the future, regardless of the occasion: “Overall, it seem likely that all of the mechanisms above, and possibly additional ones, operate in at least some situations.” Ecology in a nutshell – it’s complicated, and how complicated it is depends. Depends on what? Well, it’s complicated. Ecology has every potential to have you go in circles, which is why it’s particularly great when people come together to share their views and questions.

Our discussion can be summarised by a cloudy day in Edinburgh, people from the University of Edinburgh and the Royal Botanic Garden Edinburgh, many questions and the following five points:

  1. Is patchiness the same as habitat fragmentation? What would the results of a habitat fragmentation review be if one accounts for naturally fragmented landscapes vs anthrogenically-induced fragmentation? What is the effect of the medium of fragmentation, e.g. a road vs. a stream? They both fragment the landscape, but in very different ways.
  2. Shouldn’t we think beyond just species richness? Habitat fragmentation leads to more edges, and edge effects predominantly increase species richness. Therefore, to some of us, it was no surprise that most of the effects to habitat fragmentation are positive. This way of thinking, however, is restricted by focusing mostly on species richness, occasionally abundance. As species richness changes, so does community composition, and even more, species richness might stay the same, but the identity of the species present might be very different! What about feeding guilds, functional diversity, and invasive species?
  3. The effects of habitat fragmentation are not static, perhaps then neither should the studies investigating them. Variation across time and space matters – do the effects of habitat fragmentation become stronger or weaker over time? How do they propagate across the landscape? We thought of two possible scenarios – if a landscape is fragmented by logging, the effects might be the strongest at the beginning, and they could weaken as regrowth establishes. On the other side, when hedgerows are planted in grasslands, their effects might be the weakest while the hedges are still small, but they could increase as the hedges grow and potentially stop animal movement.
  4. The scale at which you investigate your question can influence your answer. Furthermore, since many of the studies in the review focused on particular species, perhaps study area should then scale proportionately to species range – what might be a fragmented landscape for a tiger won’t necessarily be a fragmented landscape for a butterfly.
  5. Confirmation bias and dealing with competing ideas in your head. We finished our discussion with pondering how as scientists we should be unbiased, yet we still sometimes make assumptions, even value-laden assumptions like the effects of something being good vs. bad. We all can hold seemingly conflicting ideas in our heads, and we might not even realise or put the two of them together. That’s why it’s great to have discussion groups that make us stop for an hour and ponder something that might not even have crossed our minds before!

By Gergana

2 thoughts on “EdGE meeting – positive vs. negative effects of habitat fragmentation”

  1. Hi there! I stumbled upon this blog while doing research for a paper in my Wildlife Ecology Methods. I love the idea of ‘zombie ideas’ and think it is important that you are challenging common ecological ideas. (By the way, the subject of my paper is species richness in edge habitat!)

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