The graph above refers to effects of the illegal hunting of African elephants. It is taken from a recent paper by Wittemyer et al.
Once you have given a name to a phenomenon and understood its causes, you can use it as a guide to understanding many other things. So, the concept of the "Seneca Cliff" tells us that the overexploitation of natural resources often leads to an abrupt decline that, often, takes people by surprise. In the case of biological resources, such as fisheries, the decline may be so fast and uncontrollable that it leads to the extinction or to the near extinction of the species being exploited. It has happened, for instance, for whales in 19th century and for the Atlantic cod.
If you keep in mind these historical examples, you can examine other cases and identify possible Seneca cliffs in the making. One such case is the ivory trade from the hunting of African elephants. If you look at the plots (from a recent paper), above, you see that the seized ivory mass has shown a considerable increase starting around 2008. It peaked in 2011, then declined. We can probably take these numbers as a "proxy" for the number of African elephants being killed - which is also visible as the red line in the upper box.
This is very worrisome, because if killings decline, it may very well be because there are fewer elephants left to kill - just as the landings of the fishing industry tend to decline when the fish stocks are depleted. Considering how abruptly these things go (the "Seneca effect") then we may well be seeing a similar trend in progress for African elephants: that is, the prelude of an abrupt crash in their numbers. Considering that elephants are big and reproduce slowly, that may very well lead to their extinction.
On this subject, the authors of the paper seem to be very worried, too. The title, by itself, says it all: "Illegal killing for ivory drives global decline in African elephants". In the text, we can read, among other things, that:
The population [of African elephants] was subjected to unsustainable rates of illegal killing between 2009 and 2012, escalating from a mean of 0.6% (SD = 0.4%) between 1998 and 2008 to a high of 8% in 2011 (Fig. 1). Annual illegal killing of elephants in the Samburu population during 2009 to 2012 exceeded those of all previous years of monitoring (1998–2008) with an estimated aggregate of 20.8% of the known elephants illegally killed during that 4-y period. ... Illegal killing rates were strongly correlated with black market ivory prices in the Samburu ecosystem. ... As a result of this illegal killing, the population currently suffers from few prime-aged males, strongly skewed sex ratios, and social disruption in the form of some collapsed families and increased numbers of orphans (immature elephants without a parent)
Are we going to lose the elephants forever? Right now, we can't say for sure; but when it will be clear that it is happening, it will probably be too late to do something about it. Doesn't that sound familiar?