In the Hawaiian dry forest, 45% of all tropical dry forest trees and shrubs are on the federal threatened and
endangered species list. Research is needed to understand the current range of these endangered species, the factors
that affect their current and future distributions, and ultimately, identify areas where the most successful restoration
can be undertaken. This research uses species distribution modelling to predict the potential range of Hibiscus
brackenridgei, the state flower of Hawaii and a federally endangered species found on Oahu. We used presence data
and the modelling algorithm Maxent to model the current potential distribution of H. brackenridgei, identify climate
and environmental variables that influence the species’ distribution, and model the species’ predicted future distribution
based on a range of projected climate change scenarios. Statistical analysis suggests that the Maxent models accurately
predict the species’ distribution, and therefore, may be useful for conservation management. Comparing the current
model with the future models of changes for 2060-2089, changes in the potential niche of H. brackenridgei only range
by -4% to 14%. This suggests that the predicted changes in climate, under both low (B2a) and high (A2a) SRES
(Special Report on Emissions Scenarios) global emissions scenarios, may not significantly impact the future distribution
of H. brackenridgei on Oahu. We identified a total of 115 km2 of very highly (≥ 0.70) and highly (≥ 0.50) suitable habitat
which represents potential areas where restoration projects could be implemented. This research suggests that threats
like habitat loss, fire, invasive species, and grazing may be more important than climate for the future conservation of
Hawaiian dry forest species.
Summary
Understanding the role of environmental change in the decline of endangered species is critical for designing scale‐appropriate restoration plans. For locally endemic rare plants on the brink of extinction, frugivory can drastically reduce local recruitment by dispersing seeds away from geographically isolated populations. Dispersal of seeds away from isolated populations can ultimately lead to population decline. For localized endemic plants, fine‐scale changes in microhabitat can further limit population persistence. Evaluating the individual and combined impact of frugivores and microhabitat heterogeneity on the short‐term (i.e. transient) and long‐term (i.e. asymptotic) dynamics of plants will provide insight into the drivers of species rarity.
In this study, we used 4 years of demographic data to develop matrix projection models for a long‐lived shrub, Cyrtandra dentata (H. St. John & Storey) (Gesneriaceae), which is endemic to the island of O'ahu in Hawai'i. Furthermore, we evaluated the individual and combined influence of a non‐native frugivorous bird, Leiothrix lutea, and microhabitat heterogeneity on the short‐term and long‐term C. dentata population dynamics.
Frugivory by L. lutea decreased the short‐term and long‐term population growth rates. However, under the current level of frugivory at the field site the C. dentata population was projected to persist over time. Conversely, the removal of optimum microhabitat for seedling establishment (i.e. rocky gulch walls and boulders in the gulch bottom) reduced the short‐term and long‐term population growth rates from growing to declining.
Survival of mature C. dentata plants had the greatest influence on long‐term population dynamics, followed by the growth of seedlings and immature plants. The importance of mature plant survival was even greater when we simulated the combined effect of frugivory and the loss of optimal microhabitat, relative to population dynamics based on field conditions. In the short‐term (10 years), however, earlier life stages had the greatest influence on population growth rate.
Synthesis and applications. This study emphasizes how important it is to decouple rare plant management strategies in the short vs. long‐term in order to prioritize restoration actions, particularly when faced with multiple stressors not all of which can be feasibly managed. From an applied conservation perspective, our findings also illustrate that the life stage that, if improved by management, would have the greatest influence on population dynamics is dependent on the timeframe of interest and initial conditions of the population.
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