Changes in the species and functional diversity of birds in tropical secondary forests

It’s been a while since I’ve posted anything on here and I’ve been feeling a bit bad about it. My excuses (and I think they’re good ones) are that I’ve been busy buying and fixing up a house and preparing for impending fatherhood (my partner is pregnant with twins and is due later on this month). I’ll try to post on here a bit more regularly over the next year, but I imagine that I’ll be quite busy*.

Anyway, a few months ago my master’s student Catherine Sayer had a paper we worked on during my PhD (and for a good while afterwards) published in Biological Conservation. She did an awesome job – I’m really happy to get this paper out there and feel like it produced some novel and interesting results.

We started the work with the idea of looking at how various aspects of bird biodiversity change during recovery of secondary tropical forests, as a kind-of companion piece to my paper on changes in plant biodiversity and biomass in secondary forests. Although people had previously looked at how bird biodiversity changes during secondary forest succession these studies almost exclusively looked used species richness as a metric, which as I’ve said previously isn’t necessarily that useful for telling us what is going on in an ecological community or for conservation. As a result, we decided to look at how species diversity and functional diversity – diversity defined by what species do in an ecosystem rather than their taxonomy – changed during recovery of tropical forests from agricultural clearance and how this compared to diversity in nearby primary forests.

After a systematic search, Catherine found 44 studies from which she could access raw data on species counts in secondary forests and nearby primary forests. As you can see from the map below, these were largely in South and Central America, with a number of other studies in Asia and relatively few in Africa. These secondary forests ranged from ones only a few years old to ones that were nearly a century old. We combined this with a great, global data set on bird traits to calculate a number of metrics of functional diversity.


Our most striking results were that, relative to primary forests, the species richness of forest specialist species in secondary forests increased over time since last disturbance and that the standardised effect size of functional diversity declined over time.


Broadly speaking this suggests that forest specialists increase in richness over time, that secondary forests resemble more mature forests as they age and that they can potentially support populations of specialists that may be threatened by deforestation. Regarding changes in the standardised effect size of functional diversity, what this means is that functional diversity was lower than expected relative to the number of species in bird communities for older secondary forests. This means that there were more species that were functionally similar in older forests, meaning that these species were likely to be carrying out similar processes. This is important because it means that the effects of population declines of one species on ecosystem process could be buffered by another similar species. As a result, ecosystem processes in secondary forests, and ultimately the ecosystem services they provide, may become more resilient as these forests age.

Our assertion that ecosystem processes in secondary and primary forests might be similar was also supported by our finding that there was relatively little difference between secondary and primary tropical forests when comparing other metrics of functional diversity.


These metrics did not appear to respond as strongly to forest age as the metrics mentioned earlier. However, even though we didn’t find a relationship with forest age I’d be surprised if they didn’t change during succession – I just think that there was too much noise in our data to pick up a signal. Factors such as the length and intensity of previous disturbances, connectivity, and size of forest patches are all likely to be important in determining how secondary forests change over time but unfortunately, most studies didn’t collect this data and so we couldn’t address these issues in our analysis.

From the results of our paper, it seems that primary forest is particularly important to conserve forest specialists, and if this is a priority it is clear that primary forests must be protected (as previous papers have also suggested). However, it also appears that secondary forests have the potential to provide similar ecosystem processes, and possibly services when compared to primary forests. Even if this is the case, older secondary forests are more likely to be resilient to disturbances than young secondary forest and so any regrowth should be protected wherever possible, particularly in areas where little primary forest remains.

*This is an understatement, I’m led to believe that I should expect around three hours sleep a night for the first few months…

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