Quantifying the relative contributions of habitat modification and mammalian predators on landscape-scale declines of a threatened river specialist duck

Abstract

Habitat modification and introduced mammalian predators are linked to global species extinctions and declines, but their relative influences can be uncertain, often making conservation management difficult. Using landscape-scale models, we quantified the relative impacts of habitat modification and mammalian predation on the range contraction of a threatened New Zealand riverine duck. We combined 38 years of whio (Hymenolaimus malacorhynchos) observations with national-scale environmental data to predict relative likelihood of occurrence (RLO) under two scenarios using bootstrapped boosted regression trees (BRT). Our models used training data from contemporary environments to predict the potential contemporary whio distribution across New Zealand riverscapes in the absence of introduced mammalian predators. Then, using estimates of environments prior to human arrival, we used the same models to hindcast potential pre-human whio distribution prior to widespread land clearance. Comparing RLO differences between potential pre-human, potential contemporary and observed contemporary distributions allowed us to assess the relative impacts of the two main drivers of decline; habitat modification and mammalian predation. Whio have undergone widespread catastrophic declines most likely linked to mammalian predation, with smaller declines due to habitat modification (range contractions of 95% and 37%, respectively). We also identified areas of potential contemporary habitat outside their current range that would be suitable for whio conservation if mammalian predator control could be implemented. Our approach presents a practical technique for estimating the relative importance of global change drivers in species declines and extinctions, as well as providing valuable information to improve conservation planning.

Fig 1. Distribution of whio observations in New Zealand from 1979–2016 relative to public conservation land.

Sites where contemporary populations of whio occur show a high diversity of habitats, suggesting their range is not restricted by a lack of suitable habitat. Photos top to bottom: Whirinaki, Ōpārara, Cleddau, and Clinton Rivers [Photo credit–ALW]. Public conservation land republished from https://koordinates.com/layer/754/ under a CC BY license, with permission from Department of Conservation, original copyright 2017.

Fig 2. The approach used to assess the relative impacts of habitat loss and mammalian predation on whio range declines.

a). Whio occurrence and contemporary habitat data were combined in 200 bootstrapped boosted regression tree (BRT) models to identify species-habitat relationships. b). We used these relationships to predict the relative likelihood of occurrence (RLO) of whio under two scenarios: potential pre-human with pre-human land cover and no mammalian predators; and potential contemporary with contemporary land cover and no mammalian predators. The observed contemporary results filtered the potential contemporary RLO to known whio occurrence sites only, assuming mammalian predators occur elsewhere. c). Finally, we estimated the relative impact of habitat loss and mammalian predation on whio range declines by comparing the effective whio habitat (sum of RLO x reach length, km) between scenarios.

Fig 3. Key relationships between habitat and the relative likelihood of occurrence (RLO) for whio.

(A) The four most important predictors in the full boosted regression tree (BRT) model. The dotted line represents the threshold above which whio occurrence is likely. Decile ticks at the bottom of each plot show how the contemporary New Zealand riverine environment is distributed for each predictor, while decile ticks at the top of each plot show how whio are distributed. (B) Partial dependence plots for the two strongest interactions in the full BRT model of whio occurrence, with darker colors indicating environmental space where whio are mostly likely to occur. To produce the plots, predictors except those plotted were held at their means. See Table 1 for predictor descriptions and units.

Fig 4. Predicted relative likelihood of occurrence for whio under three scenarios.

A) Spatial distribution and B) density plots of the mean predicted relative likelihood of occurrence for whio prior to human arrival in New Zealand (potential pre-human), in contemporary habitat with the exclusion of predators (potential contemporary) and the observed distribution (observed contemporary). Potential values represent predictions from 200 bootstrapped simulations of a boosted regression tree model, while observed values are the potential contemporary predictions at reaches where whio were observed from 1979–2016.

Fig 5. Availability of predicted effective whio habitat under three scenarios.

Effective whio habitat is the sum of the predicted relative likelihood of occurrence for each reach x reach length prior to human arrival in New Zealand (potential pre-human), in contemporary habitat with the exclusion of predators (potential contemporary) and the observed distribution (observed contemporary). Potential values represent predictions from 200 bootstrapped simulations of a boosted regression tree model, while observed contemporary values are the potential contemporary predictions at reaches where whio were observed from 1979–2016. Boxplots show the median (solid line) and quartiles with whiskers reaching up to 1.5 times the interquartile range.