Stage-specific biomass overcompensation changes with stage-specific resource productivity

Background/Question/Methods

In size-structured populations theory predicts that population structure is determined by stage-specific resource use efficiencies. When adults are more efficient energetically the population is dominated by juveniles (maturation regulation), and when juveniles are more efficient energetically the population is dominated by adults (reproduction regulation).  Differences in energetic efficiency between stages may result either from intrinsic size-specific differences in the ability to use a common resource or from differences in the stage-specific resource productivities.  If juveniles and adults differ in their energetic efficiency, size-structured populations will respond to moderate harvest levels with stage-specific biomass overcompensation in the energetically more efficient stage. So far the first mechanism, differences in the resource use efficiency intrinsic to the organism has been the main target of experimental investigations.  Here we test how stage-specific resource productivities will affect population structure and population regulation. We separated juvenile and adult fish (Heterandria formosa, intrinsically reproduction regulated), creating a juvenile habitat and an adult habitat with distinct resource productivities that was either equal for both stages or twice as high for the adult stage.  Resource treatments were crossed with 3 harvest levels and population dynamics were followed over eleven months.

Results/Conclusions

No difference in stage-specific resource productivities resulted in populations energetically dominated by the adult stage, exhibiting juvenile biomass overcompensation when harvested. These results were in line with our expectation that in the reproduction regulated H. formosa juveniles are the more efficient resource users. An increase in adult-specific resource productivity resulted in increased adult reproduction and juvenile biomass, but did not switch population structure from an adult-dominated system to a juvenile-dominated system. In comparison to populations with equal stage-specific resource productivities, populations with higher resource productivities in the adult habitat had higher total biomass when not harvested and were more negatively affected by harvest. Furthermore, similar maturation rates over all treatments suggest that the increase in reproduction rate with increased adult-specific resource productivity was not sufficient to constrain maturation enough to course a complete switch to maturation regulation. Nevertheless, our results show that changes in stage-specific resource productivities can potentially change population level mechanisms that affect population regulation, population structure, and, consequently, population level responses to harvest. Changes in stage-specific resource productivities can thereby have major effects on overall community structure through affecting stage-specific biomass responses to increased mortality.