Stage‐structured harvesting and its effects: an empirical investigation using soil mites

Abstract

Summary 1.Population dynamics results from an interplay between the environmental state and population density. With many organisms there is structure to the life history, and this structure has important consequences for the population's density dependence and its interaction with environmental noise, and therefore its population dynamics. Perturbing population structure, such as through harvesting, may therefore affect the way that the populations respond to stochastic environmental variation. 2. We conducted three experiments on populations of soil mites kept under controlled conditions and harvested a constant proportion of eggs, juveniles or adults. The experiments (a) minimized environmental variability, (b) created environmental variability by randomizing food supplies, and (c) provided excess food and repeatedly subdivided the populations to maintain them below carrying capacity. 3. We find that harvesting different stages has marked effects on stage structure, which differ between constant and variable environments. For example, harvesting adults decreases the number of adults, harvesting eggs increases the number of adults and harvesting juveniles has no effect under near constant conditions, whereas in a variable environment harvesting adults and juveniles reduces adult numbers, but harvesting eggs has no effect. 4. As well as changing the mean age structure, harvesting can change the variance of the different stages. For example, in a constant environment harvesting juveniles does not change the variability in juvenile numbers or population size, but in a variable environment harvesting juveniles increases the variability in the size of the juvenile class and hence the total population variability. 5. For populations that are kept at one-third to one-half carrying capacity (approximately where the maximum sustainable yield should result) harvesting of the different age classes has marked positive effects on the population growth rates. Harvesting different age classes causes changes in the density dependence, which explains the way in which the population parameters respond. 6. In conclusion, the population response to harvesting depends on the stage/age structure and the way it changes with harvesting and environmental conditions. Managing economically important populations, subject to harvesting, should consider structured life histories.

Symmetry breaking in ecological systems through different energy efficiencies of juveniles and adults

“…Note that adult biomass decreases with harvesting (no overcompensation) in panel (d) and increases with harvesting (overcompensation) in panel (e). Data were generously provided by T. Cameron and J. Ohlberger (Cameron andBenton 2004, Ohlberger et al 2011). increases in mass-specific rates do not compensate for the decreases in juvenile and adult biomasses. Overall, the monotonic decrease in biomass with increased mortality corresponds to well-known patterns observed in unstructured consumer-resource models.…”

Stage-specific predator species help each other to persist while competing for a single prey

“…We reviewed the literature for three types of experimental evidence that can reveal the likely occurrence of emergent facilitation (Table 1). First, biomass increases in a particular population stage, in response to increased mortality imposed on another stage, have been shown to occur in laboratory populations of blowflies (21), soil mites (22), and fish (A. Schröder, L.P., and A.M.D.R., unpublished data). Second, increases in stage-specific prey density, because of variation in predation pressure, have been shown to occur in cladocerans in both laboratory (24) and field enclosures (25).…”

Stage-specific biomass overcompensation by juveniles in response to increased adult mortality in a wild fish population

“…Such indirect effects of mortality may result in increased stage-specific biomass production thereby compensating for the removal of individuals from the population (Werner andGilliam 1984, De Roos et al 2007). Experimental studies on laboratory populations have shown that stage-specific biomass remains unchanged or increases if not all individuals are subjected to increased mortality rates, for instance in water fleas (Slobodkin and Richman 1956), blowflies (Nicholson 1957), soil mites (Cameron and Benton 2004), and fish (Schro¨der et al 2009). Differences in mortality rates between size classes or life stages of natural populations are commonly observed in nature and may arise from size-specific predation (Brooks andDodson 1965, Werner andGilliam 1984), harvesting by humans (Law 2000, Fenberg andRoy 2008), or parasite infections (Ohlberger et al 2011b).…”