Bioenergetics, overcompensation, and the source–sink status of marine reserves

Abstract

One of the hypothesized functions of marine protected areas (MPAs) is to serve as sources of biomass, with biomass spilling over from the reserve into neighbouring, harvested areas. We argue that the net larval flow (from or to the marine reserve) depends on between-area differences in the population-level biomass production rate, whereas the direction of adult flow depends on differences in the biomass standing stock. Hence, an important question is whether population-level biomass production increases (overcompensation) or decreases (undercompensation) with increased per capita mortality. We show that in a consumer–resource context, the source–sink status of an MPA may depend on the details of the individual-level bioenergetics, as well as on the dispersal rates of larvae and adults. We compare two classic bioenergetic models (net-production vs. gross-production allocation). The net-production model predicts that population-level reproduction may increase with mortality (overcompensation), whereas gross-production allocation always results in undercompensation. We show that models often implicitly assume gross-production allocation, thus potentially overestimating the capacity of MPAs to source unprotected areas. We briefly discuss results of two other models (a simplified, logistic model and a size-structured model), suggesting that the relation between overcompensation and the larval sink status of MPAs is general.

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

“…Our study suggests that natural populations may respond to increased mortality rates caused by infectious diseases or harvesting by stage-specific biomass overcompensation. This has management implications for at least two reasons: biomass overcompensation may result in counterintuitive measures necessary to restore fish stocks , and it may render marine protected areas less efficient than expected based on models without overcompensation (Claessen et al 2009). The populationlevel response depends on the increase in mortality rates, the competitive interactions within and between life stages, and the environmental conditions that determine resource availability.…”

Effect of habitat productivity and exploitation on populations with complex life cycles

“…Changing basic assumptions of our model on resource growth, consumer foraging function, and energy allocation do not affect the presence of alternative stable states (Schreiber & Rudolf 2008, Claessen et al 2009, Guill 2009). Moreover, Guill (2009 demonstrated that the existence of alternative stable states only depends on the ability of both stages to have overcompensation at high biomass densities, a requirement fulfilled when there is only within-stage competition.…”

“…This suggests that management actions may have very different effects on exploited fish with stage-specific habitat shifts than on fish with habitat overlap throughout ontogeny. For example, the effectiveness of Marine Protected Areas (MPAs) in increasing overall abundance of harvestable life stages has been shown to depend on density-dependent interactions within other stages not of direct interest for management (Dugan & Davis 1993, St. Mary et al 2000, Claessen et al 2009). Processes within stages without commercial interest are thus also likely to influence fishing mortality effects in exploited stages.…”

“…This pivotal role of habitat productivity demonstrates the need for integrated approaches to rebuilding overexploited fish stocks. In addition, it shows that management focusing purely on the exploited stage of fish stocks (typically adults) may only be effective when juvenile habitats are more productive than adult habitats.Changing basic assumptions of our model on resource growth, consumer foraging function, and energy allocation do not affect the presence of alternative stable states (Schreiber & Rudolf 2008, Claessen et al 2009, Guill 2009). Moreover, Guill (2009 demonstrated that the existence of alternative stable states only depends on the ability of both stages to have overcompensation at high biomass densities, a requirement fulfilled when there is only within-stage competition.…”

Population and Life-History Consequences of Within-Cohort Individual Variation

“…B in the online edition of the American Naturalist). Understanding how assimilated energy is actually channeled in an organism is complicated, and numerous energy allocation rules have been proposed (Kooijman 2000;Claessen et al 2009). We assume that individuals follow a "net allocation model" (Kooijman 2000) before first reproduction and a "gross-production allocation model' (Kooijman 2000) after the first reproduction event (see app.…”