Multispecies harvest: the trap of opportunistic exploitation

People often wonder how exploiting an economically beneficial species to extinction is even possible. According to economic theories, once a harvest becomes overly costly or time-consuming, shouldn’t harvesters simply switch to another species? Some argue that increasing rarity subsequently increases value, thus compensating for growing harvesting costs, and keeping harvest profitable even at very low densities.

In reality, catching the few remaining individuals or populations indeed becomes too costly in a single-species system maximizing net present value. But such situations are fairly rare in nature, as multispecies systems usually occur, offering several alternative modes of exploitation. Single-species extraction can decimate species very quickly if no control is executed on the harvest, but multispecies extraction also easily leads to species extinctions because such a system allows for the concurrent exploitation of several species, thus leading to several pathways of decreasing populations. Such extraction has been coined opportunistic exploitation.

Opportunistic exploitation can continue even in situations where the process itself is not viable: the only prerequisite for this a multispecies system where other more abundant species are harvested in a similar way. A highly valued species first becomes rare due to overexploitation, and its bioeconomic equilibrium density (the state where the profit gained from its harvest is equal to the profit of harvesting other available species) is surpassed. Even this does not discontinue the extraction. Harvesters do not target the species in question any longer, but continue extracting it when encountered while harvesting other species. Thus the highly valued species will continue to decline, and can even become extinct, despite it no longer being a large-scale harvest focus. In fact its increasing rarity will only add to its value, as its harvest requires no extra expenditure (time or money).

The harvest of increasingly rare species usually becomes regulated, banned or illegal at some point, but monitoring the intake is notoriously difficult. This is particularly true in remote areas or when the harvest target is taken for sustenance. An example of this is the harvest of several water snake species in Asia, where a few of the species are valued for their skins and sold to traders in larger cities, while other species are harvested for food after overexploitation has decimated initial protein sources (local fish populations). Typically for such systems, overexploitation continues even after maximizing the net present value is no longer possible, mainly because the harvesters have no way of evaluating the overall profitability of their actions.

In general, opportunistic exploitation is probably fairly widespread in marine and freshwater environments, and other examples include the harvest of the Antarctic blue whale (Balaenoptera musculus intermedia), several fish species, sea cucumbers, and may also have led to the extinction of some marine mammals, e.g. the Steller’s sea cow (Hydrodamalis gigas). Terrestrial species are also under threat of this phenomenon, with examples of overexploited species including the big-leaf mahogany (Swietenia macrophylla) of the Amazon, several species of wild pigs, and the black rhinoceros (Diceros bicornis).

The black rhinoceros (Diceros bicornis) in Berlin Zoo. © Mia Vehkaoja

The black rhinoceros (Diceros bicornis) in Berlin Zoo. © Mia Vehkaoja

Conservation policies aiming to mitigate opportunistic exploitation should identify the lower-value species occurring in the system, which lead to the continued extraction of the more rare but highly valued species. Implementing and enforcing harvest and trade restrictions is also key.

Identifying the reasons behind a harvest becoming opportunistically exploitive is a baseline requirement for any conservation efforts. It is additionally often not enough to treat such problems as purely ecological or conservational. Often an opportunistically exploitive system is complex, involving both social and ecological aspects. Resolving these issues involves socio-ecological efforts, which are time- and money-consuming and rarely work unless the area’s human communities partaking in the harvest are also involved in the process. Such schemes aim to change the steady state of the entire socio-ecological area, which will subsequently allow harvesters to make larger changes in their extraction targets and methods.

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