Grow spines and live in harmony

Posted on March 27, 2012

When exposed to predator pressure, many animal prey species are able to modify their behavior or body form to reduce their vulnerability to attack.   These responses are triggered by chemicals released by predators.  The adoption of improved defences  (e.g., longer protective spines) by prey species not only tends to reduce the feeding success and growth rate of their predators, but it may also impair the growth of the prey themselves, because of the added expense of their defence system.  For these reasons, inducible prey defences are expected to create negative feedback loops that prevent strong population oscillations and encourage species persistence.  While there’s plenty of theoretical support for this prediction, empirical validation has been lacking due to experimental challenges, and little is known about how population dynamics are affected when two or more competing prey species have inducible defences.  To help fill these knowledge gaps, two herbivorous plankton species (rotifers of the genus Brachionus) were collected from Chilean lakes and cultured in the laboratory.  A third rotifer species, the predator Asplanchna brightwelli, was collected and cultured in the same way, and water from containers with different concentrations of Asplanchna was extracted and filtered, to produce test media with high or low levels of predator chemical.  An initial experiment showed that the protective spines of both prey species grew by 40-100% after six days’ exposure to high-concentration predator water, but the response to low-concentration water was much weaker.  A coexistence experiment was then set up by placing identical numbers of both prey species into beakers containing Asplanchna and predator-conditioned water.  The experiment manipulated two factors, namely predator chemical (two levels) and prey resource density (the alga Chlorella; four levels), and ran for 16 days.  Predator numbers were kept constant and prey populations were monitored daily.  When the concentration of predator chemical was low and food abundant, one of the two prey species was dominant.  Under the same predator conditions, but with limited food, the other prey species dominated.  In contrast, when the concentration of predator chemical was high, densities of the two prey species were much more similar, less variable and less sensitive to food availability, and both species had a much lower extinction risk.  These results support the idea that induced defences promote coexistence between competing species and help to explain how complex communities manage to persist in nature.

Reference:  Aranguiz-Acuna, A., Ramos-Jilibertto, R. & Bustamante, R.O.  2011.  Experimental evidence that induced defenses promote coexistence of zooplanktonic populations.  Journal of Plankton Research 33 (3), 469-477.