Since my part of the presentation about loss of plant biodiversity (Friday 11/11) was a bit bad because I had too many things to say and was hiding behind my messy notes, it might have been a bit difficult to understand (and frankly, to be interested in) what I was trying to say. That is why I would like to try and explain it here.
According to a research team of the University of Exeter, England, ‘any extinction can create a ripple effect across a food web, with far-reaching consequences for many other animals.’
To say it in a different way, since every species in an ecosystem holds a particular place in the food chain, the extinction of a species can be equal to the disappearance of the natural predator or the preferred prey of other species, and then cause other extinctions. This phenomenon is also called ‘domino effect’.
I chose two papers to illustrate these points: Dunne et al. 2002 and Petchey et al. 2008.
Dunne et al. 2002 talks about it as secondary extinction, when ‘a non-basal species loses all of its prey items, and also when a cannibalistic species loses all of its prey items except itself’. Basically, it means that when your food disappear, you disappear as well. The mean point of the paper is that community stability will increase as the number of links in a food web increases. It is due to an increasing number of paths through a species, dampening the impacts of its population ﬂuctuations, and results show that the magnitude of secondary extinctions decrease in this case.
Robustness is defined as the proportion of primary species removals that lead to more than 50% total species loss; highly connected communities will tend to be more robust to species losses, and random species losses will tend to have fewer effects on food webs than losses of species with many trophic connections.
But are actual extinctions random? No. It appears that humans have historically tended to impact higher trophic levels with associated cascading trophic and non-trophic effects. Real world extinctions are triggering trophic-related secondary extinctions at greater levels than expected from random species losses, as explained in the next article.
Petchey et al. 2008 examines the effect of these secondary extinctions on trophic diversity, meaning the range of trophic roles (places in the food chain) played by the species in a community. Secondary extinctions cause loss of trophic diversity greater than that expected from chance, and more trophically unique species are more vulnerable to secondary extinctions because do not feed on many species, sometimes even are specialists, so if this single species goes extinct, they do not have anything else to feed on.
For instance, on the left side of this figure, species from 1 to 6 are eaten by species from 7 to 10 that are eaten by 11 and 12. We can see that species 1 is trophically unique because has a unique set of consumers, is essentially preyed by species 7 and 8 (bold arrows meaning strong relationships), so if species 1 disappears, species 7 and 8 will be seriously endangered. In contrast, species 2 and 6 are not unique because are both eaten by species 10 and only this one, and at a same amount so have the same trophic roles. On the right side, we can see a dendogram showing the connectance of species to each other. The densogram shows that indeed the trophically unique species 1 is very poorly connected to the other species, while species 2 and 6 are sister taxa and close to the others.We can understand that trophically unique species have a lower connectance to other species, so like we saw with Dunne et al. 2002, are more vulnerable to extinction. Their relative uniqueness may also mean that loss of these species will have disproportionate effects on rates of ecosystem processes.
Intermediate species (herbivores) tend to be more trophically unique than basal (plants) and top species (carnivores) so are more likely to suffer secondary extinction. The mechanisms involved in the secondary extinctions of intermediate species are direct bottom-up effects (intermediate species become extinct when their resources are deleted) and indirect top-down effects (break down of predator-mediated coexistence when top species are deleted).
If you want to have a look to the two articles here are the PDF: