Using Transfer Function Analysis to develop biologically and economically efficient restoration strategies

Abstract

Rare species across taxonomic groups and biomes commonly suffer from multiple threats and require intensive restoration, including population reintroduction and threat control. Following reintroduction, it is necessary to identify what level of threat control is needed for species to persist over time. Population reintroduction and threat control are time intensive and costly. Thus, it is pragmatic to develop economically efficient restoration strategies. We combined transfer function analysis and economic cost analysis to evaluate the effects of biologically meaningful increases in demographic processes on the persistence of a reintroduced population of a Hawaii endemic long-lived shrub, Delissea waianaeensis. We show that an increase in fertility by 0.419 following the suppression of non-native rodents or an increase by 0.098 in seedling growth following the suppression of invasive molluscs would stabilize the population (i.e., λ = 1). Though a greater increase in fertility than seedling growth was needed for the reintroduced population to persist over time, increasing fertility by suppressing rodents was the most cost effective restoration strategy. Our study emphasizes the importance of considering the effects of large increases in plant vital rates in population projections and incorporating the economic cost of management actions in demographic models when developing restoration plans for endangered species.

The Disconnect Between Short- and Long-Term Population Projections for Plant Reintroductions

“…We also found that a three-fold increase in the probability of years with high seedling recruitment would be required for the D. waianaeensis reintroduction to persist over time (Figure 1B). A potential restoration strategy to increase the frequency of years with high seedling recruitment (i.e., 3 seedlings per reproductively mature plant) would be to suppress non-native frugivores and seedling herbivores (Bialic-Murphy et al, 2018). While our simulations provide insight into the population-level responses of increased seedling recruitment via pest control, many other exogenous factors can influence temporal variability in seedling recruitment.…”

“…The density of R. rattus can be highly variable from year to year and is a primary driver of temporal variability in seedling recruitment (Innes et al, 2001;Meyer and Butaud, 2009;Franklin, 2014). At our field site, R. rattus consume, on average, 83% of the mature fruits (Bialic-Murphy et al, 2018). Similarly, non-native molluscs decrease seedling density of endemic plants in Hawai'i by up to 33% (Kawelo et al, 2012).…”

“…Two of the main stressors implicated in the decline of D. waianaeensis are frugivory by Rattus rattus (black ship rat) and seedling herbivory by non-native molluscs (Mollusca: Gastropoda) (Joe and Daehler, 2007;Kawelo et al, 2012;Shiels et al, 2014;Bialic-Murphy et al, 2018). Frugivory by Rattus rattus and herbivory by non-native molluscs have pronounced negative effects on seedling recruitment of rare island endemics, including D. waianaeensis (Joe and Daehler, 2007;Shiels and Drake, 2011;Shiels et al, 2014;Bialic-Murphy et al, 2018). The density of R. rattus can be highly variable from year to year and is a primary driver of temporal variability in seedling recruitment (Innes et al, 2001;Meyer and Butaud, 2009;Franklin, 2014).…”

“…Non-native pests influence the temporal variability of seedling recruitment (Innes et al, 2001;Meyer and Butaud, 2009) and can drive population decline (Bialic-Murphy et al, 2018). In Hawai'i natural areas, the density of rats (Shiels, 2010) and slugs (Stephanie Joe, personal communication) fluctuate from year-toyear and the intensity of seasonal pest control management can be highly variable due to budget constraints.…”