Nectar in oak savannas: implications for butterfly conservation

Abstract

Introduction

Oak savanna habitats historically covered at least 11 million acres of the Midwestern United States (Abella et al., 2020). Oak savanna habitats are characterized by sparsely dispersed mature oak trees with an understory of grasses and forbs, achieving a semi-open canopy through fire and grazing disturbances (Olson, 1996; Sankaran et al., 2004; Anderson et al., 2007). Since European settlement, oak savannas have become highly fragmented due to fire suppression, agriculture, and urbanization (Nuzzo, 1986; Grossmann & Mladenoff, 2007) causing a severe decline in the diversity and abundance of native wildlife populations (Swengel & Swengel, 1999; Kocher & Williams, 2000; Meehan et al., 2013; Archer et al., 2014). With c. 0.02% of the historic range intact (Nuzzo, 1986) Midwest oak savannas are classified as a critically imperiled habitat in the United States (Noss et al., 1995). Oak savannas naturally preserve high levels of biodiversity relative to neighboring habitats (Leach & Givnish, 1999), making them an important focus for conservation efforts.

Habitat conservation can be guided by historical records (Landres et al., 1999; Swetnam et al., 1999), reference sites with minimal disturbance, or by the living requirements of an indicator species that requires high-quality habitat to survive. A previously used indicator species for oak savanna habitats is the federally endangered Karner blue butterfly (Plebejus melissa samuelis) (U.S. Fish and Wildlife Service, 1992; Shuey, 1997; Chan & Packer, 2006). Due to the severe fragmentation of oak savannas and the intermediate flight ability of this small butterfly (King, 2003), conservationists have worked to improve the quality of remaining habitats and reintroduce populations. The success of Karner blue butterfly reintroduction depends on the suitability of local habitat, the characteristics of which are not yet fully understood (Pickens & Root, 2008; Walsh, 2017). Studies often relate butterfly abundance with host-plant abundance (Fred & Brommer, 2003) or spatial distributions that limit search time (Crone & Schultz, 2022), nectar species abundance (Holl, 1995; Schultz & Dlugosch, 1999), and the area of the habitat (Moilanen & Hanski, 1998; Bergman & Kindvall, 2004).

Vegetation surveys evaluating the flowering plants available to pollinators commonly measure the density of flowering stems within a management unit (Williams, 1988; Chan & Packer, 2006; Walsh, 2017). Estimating resource availability by plant density, or even individual flower numbers, can lead to errors by not considering the nectar content each plant and/or flower can provide. For example, Schultz & Dlugosch (1999) determined that the abundance of Fender's blue butterfly (Icaricia icarioides fender) was not predicted by the abundance of individual flowers but by the total volume of nectar sugar from all native species present. Karner blue butterflies thrive in disturbed and semi-open oak savannas that contain their exclusive larval host plant wild lupine (Lupinus perennis) (Opler & Malilul, 1998). First-generation Karner blue larvae hatch in April and emerge as adult butterflies in early May and June. These adults will feed on nectar, mate, and lay eggs that produce adults in July and August (Grundel et al., 2000). While larval nutritional requirements are fairly well studied, adult nectar requirements of the Karner blue butterfly are less understood.

Nectar is mostly composed of water (35–85%; Seeley, 2009) and thus likely helps pollinators maintain hydration (Nicolson, 2009). However, it also contains nutritional compounds, such as sugars, lipids, and amino acids (Nicolson & Thornburg, 2007; Willmer, 2011; Cahenzli & Erhardt, 2012a). Different plant species can vary in the concentration and composition of nectar sugars (Baker & Baker, 1975; Dafni, 1992; Kearns & Inouye, 1993; Nicolson & Thornburg, 2007). Nectar is derived from photosynthesis, and therefore, nectar composition varies depending on a plant's exposure to light, water, and temperature (Freeman & Head, 1990; Pacini et al., 2003; Gallagher & Campbell, 2017). However, even with variations in nectar production due to environmental differences or plant age, relative uniformity of nectar composition within a species is expected (Nicolson & Thornburg, 2007).

The aim of this study is to provide a better understanding of how nectar resource distribution in space and time may influence habitat quality for the Karner blue butterfly and other nectar-consuming pollinators within oak savanna habitats. Our approach was to couple nectar quality characteristics and floral abundance data for 22 oak savanna forbs with existing nectar plant density data in oak savanna habitats to examine whether sites categorized by Karner blue butterfly occupancy differ in nectar resource quality due to species-specific variation in relative abundance and floral nectar traits. Specifically, we focused on the following questions. (1) How do environmental factors affect nectar composition, and can nectar quality be reliably associated with species identity? (2) Which species contributes the most sugar and hydration potential when considering variation in floral abundance, nectar volume, and sugar availability per stem? (3) How does the quality of habitat nectar resources (sugar density and total per site) vary among sites that differ in Karner blue occupancy or change between spring and summer seasons? To answer these questions, we quantified species nectar and floral characteristics, which were subsequently combined with previously determined flowering stem density estimates established by Walsh (2017) in oak savanna habitats associated with Karner blue butterfly conservation. Understanding these characteristics and seasonal variations in nectar resources will aid habitat restoration planning and benefit conservation efforts for nectar-feeding pollinators of this critically imperiled habitat.