When species like this toucanet are lost, the interactions that they are a part of are lost too. So how can we restore them? (Image Credit: Jairmoreirafotografia, CC BY-SA 4.0, Image Cropped)

Estimating interaction credit for trophic rewilding in tropical forests (2018) Marjakangas, E.-L. et al., Philosophical Transactions of the Royal Society of Biology, 373, https://dx.doi/10.1098/rstb.2017.0435

The Crux

We have reviewed more than enough papers on biodiversity loss to entitle us to skip the whole “losing species is bad” spiel (see here, here and here). But what we haven’t talked about is that when some species are lost, specific interactions that those species participate in disappear from an ecosystem. Those interactions range from the minute to the crucial. One such crucial example is that of seed dispersal, whereby specific plants rely on specific animals to disperse their seeds, thus maximising biodiversity in other parts of the forest and creating a positive feedback loop.

Naturally, conservationists will want to reintroduce animals to propagate some of these reactions. But as is always the case in conservation, maximising return is absolutely essential when you’re faced with limited resources and a lot of ground to cover. Today’s authors wanted to develop a system for maximising the effect of species reintroduction.

What They Did

The study utilised 912 sites scattered throughout the Atlantic Forest in Brazil. The researchers needed to know 3 things: how often the dispersers occur in the same area as the species they disperse the seeds of, how likely is an interaction between the species given they are in the same area, and how likely is seed dispersal given an interaction?

To do this, a Joint Species Distribution Model (JSDM, we’ve looked at them before) was used to determine where species were most likely to occur in the same place. They then used documented interactions between certain species to calculate the likelihood of interaction given this co-occurrence. With this knowledge, they then estimated where the dispersed species should occur, and compared this to the actual distribution of the species.

Did You Know: Seed Dispersers as Keystone Species Seed dispersal is integral to any ecosystem, but some plants take this interaction to extremes. There are plenty of plants which rely on one type of frugivore (fruit eater) to disperse their seeds. In the tropical rainforests of Northern Australia, some plants have gone one step further. Many plants in these forests cannot grow unless their seeds are first ingested by cassowaries, which must then excrete them for the seeds to grow. This makes the cassowary a keystone species, as their extinction would lead to the extinction of a larger number of species, drastically changing the affected rainforests.

What They Found

Sites where restoration of ecological interactions had the most potential were in areas where plant species diversity was high but fruit eating species were low. There were 13 species which scored highest at at least one site, 3 primates and 10 birds. However, the species values differed: some species had the potential to restore more interactions, but as their distribution was limited, reintroducing them would have potentially had less effect than other species.

Problems

Coming up with metrics for species reintroductions is great, but if the threats which caused the species removal in the first place are still present, rewilding efforts may well be in vain. Additionally, knowledge of biology is likely scarce for some of the rarer species on the list, which means that the accuracy of predictions regarding their ability to disperse seeds may vary wildly.

So What?

Studies like this are invaluable, as they provide conservationists with tangible results with which to base management policy. Reintroduction of species is inherently costly, and ensuring that these efforts actually pay off is essential. Given that 3 of the species involved are also endangered, this study is also cause for optimism.