Summer Diet Selection by Snowshoe Hares by Pippa Elizabeth Seccombe-Hett B.Sc. University of British Columbia, 1996 THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Botany) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September 1999 © Pippa Elizabeth Seccombe-Hett, 1999 f In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver, Canada DE-6 (2/88) ABSTRACT The primary objective of this study was to identify the plant species included in the diets of male snowshoe hares (Lepus americanus) during the summer months. Snowshoe hare selection for and against each plant species was assessed by comparing the relative use and availability of each plant species. The secondary objective was to examine several hypotheses of what influenced the selection of those species individually and within an integrated modeling framework. Three hypotheses were examined, i) Hares might select plants with high nutritional content (of energy or protein), ii) Hares might select plants to avoid or minimize deleterious plant secondary compounds (i.e. tannins and alkaloids), iii) Hares might select plants that minimize their risk of predation. The hare diet during the summer consists of five main plant species: Betula glandulosa, Festuca altaica, Lupinus arcticus, Salix spp. and Shepherdia canadensis, although a number of other species were occasionally included. Nutritionally, hares select for plant species with high protein content and avoid toxic effects from secondary compounds by ingesting a diverse diet. Hares are not consistently found associated with any particular vegetation types, although they prefer habitats with both a dense understory and an abundance of preferred food species. Although some support was generated for all of the hypotheses of diet selection, no single hypothesis explained all of the observed patterns of diet selection. A linear programming foraging model combined with optimization techniques was thus used to examine the interactions between the variables. Overall, the model is successful in integrating the conflicting hypotheses of hare foraging. It appears that hares change their diet selection response to conflicting goals such as reproductive condition and risk of predation. The model suggests that the interaction between plant protein and chemical defense compounds are the primary determinants of hare diet. Table of Contents Abstract 11 Table of Contents lv List of Tables v List of Figures wn Acknowledgements VIU Chapter 1 1 Chapter 2 4 Methods 8 Results 21 Discussion ^1 Chapter 3 71 Methods 74 Results 80 Discussion ^7 Chapter 4 102 The diet selection model 104 Results 121 Discussion 1^9 Chapter 5 147 References iv List of Tables Table 2.1 The relative proportion of species available in the hare enclosures 22 Table 2.2 Selection ratios for hares at time 1 29 Table 2.3 Comparison of species selection ratios at timet 32 Table 2.4 Selection ratios for hares at time 2 33 Table 2.5 Comparison of species selection ratios at time 2 34 Table 2.6 Mean crude protein content of plant tissues 37 Table 2.7 Comparison of mean consumed versus mean available plant nutritional properties 43 Table 2.8 Mean gross caloric content of plant tissues 46 Table 2.9 Secondary compounds identified in plant tissues 58 Table 2.10 ANOVA results for protein, energy, secondary compounds in diet selection 59 Table 2.11 Comparison of results from studies of summer diet selection 62 Table 3.1 Mean habitat characteristics on three study sites 81 Table 3.2 Results from correlation of fecal pellet number against habitat variables... 82 Table 3.3 Summary of composite variable analysis 86 Table 3.4 Correlation between composite variables 87 Table 3.5 Mean vegetation characteristics within each cluster on each grid 90 Table 3.6 Results from ad hoc logistic regression to predict cluster membership 94 Table 3.7 Significant results from logistic regression on fecal pellet transects 95 Table 3.8 Correlations between tree cover, Betula glandulosa and Festuca altaica 96. Table 4.1 Parameter definitions for model 107 Table 4.2 Mean plant nutritional characters included in the model 109 Table 4.3 Time related parameters included in the model 114 Table 4.4 Comparison of predicted and observed diets using Belovsky's original model 123 Table 4.5 Summary of constraint equations violated by observed hare diets 136 Table 4.6 Results from sensitivity analysis of model constraint equations 137 Table 4.7 Summary of sensitivity analysis on time costs 138 Table 4.8 Assumptions of the diet selection model 146 vi List of Figures Figure 2.1 Schematic diagram of sampling design within hare enclosure 12 Figure 2.2 Comparison of proportion of plant species consumed versus available within hare enclosures time 1 25 Figure 2.3 Comparison of proportion of plant species consumed versus available within hare enclosures time 2 27 Figure 2.4 Mean crude protein content of leaf tissue 36 Figure 2.5 Standardized selection ratios as a function of protein content time 1 39 Figure 2.6 Mean energy content of plant leaf tissue 44 Figure 2.7 Standardized selection ratios as a function of energy content time 1 48 Figure 2.8 Correlation of protein and energy content of leaf tissues at both times... .50 Figure2.9 Mean fibre content of leaf tissue 52 Figure2.10 Mean water content of leaf tissues 55 Figure3.1 Correlations of fecal pellet number and distance to cover for three study sites 83 Figure3.2 Regression of composite food variables against distance to cover for each study site 88 Figure 4.1 Fecal index of hare daily activity Ill Figure 4.2 Optimization results from new diet selection model at time 1 125 Figure 4.3 Optimization results from new diet selection model at time 2 128 Figure 4.4 Optimization results from new model at time 2. Adjustments applied to secondary compounds of Betula glandulosa and Shepherdia canadensis 131 Figure 4.5 Comparison of time minimized, energy maximized and simultaneous maximization goals with the observed diet 134 vii Acknowledgements This work was funded by NSTP, a graduate fellowship from NSERC and an NSERC operating grant for Dr. Turkington. I would like to thank those who helped me in the field: Patrick Carrier, Sarah Davidson, Leo Frid, Elvira Harms, Chris Lortie, Liz Gillis, Jes Logher, and Chris Wulff but most particularly Katie Breen and Jaroslav Welz who had to suffer me daily. This work could not have been completed without the lab support was provided by Tony Sinclair, Gilles Galzi and Elaine Humphrey. Summer field support came from the Arctic Institute of North America, particularly Jan, Sian and Andy Williams. A special thank you goes to Karen Hodges was supportive and helpful in directing my ideas in the early stages. An enormous thank you to Clive Welham who patiently convinced me that modeling and animal behaviour was not so bad; without his guidance this thesis would not have taken the same direction. A special thank you to Roy who guided me, supported me and patiently dealt with my difficulties in reaching the end of this. Finally I have to thank Matthew Evans who was involved in every step of the way whether he wanted to or not. viii CHAPTER 1 DIET SELECTION IN THE SNOWSHOE HARE CYCLE INTRODUCTION Populations of snowshoe hares (Lepus americanus) fluctuate in numbers with population peaks occurring at periods of 8 to 11 years throughout most of their range in the northern boreal forest (Keith 1963, Krebs etal. 1986). Many long-term studies have investigated this periodicity in hare numbers (e.g. Keith and Windberg 1978, Krebs et al. 1995) and several hypotheses have been generated concerning the causes of these fluctuations. The two main hypotheses for this cause are predation, and over-winter food shortage (Krebs et al. 1995). As a result, research on the hare cycle has concentrated on investigating the interactions between the hares, their predators and winter food supply and consequently there has been relatively little research effort focused on the summer food supply. In this thesis I will examine summer diet selection by snowshoe hares and attempt to elucidate how plant qualities such as spatial distribution, nutritional content and defensive compounds affect hare food choice. The cycle in hare population numbers is more or less synchronous throughout most of their range (Sinclair et al. 1993 and Sinclair and Goslinel997) and when the hare population reaches peak numbers it is the dominant herbivore in the boreal forest system (Boutin etal. 1995). Changes in hare population numbers effect the whole boreal ecosystem. Concurrent changes (with a one-year lag) in population numbers have been documented in the predators of hares, (Keith et al. 1977, Keith 1990, Boutin et al. 1995), 1 both avian (e.g. great horned owls and goshawks) and mammalian (e.g. lynx and coyotes). Woody plant species, which comprise the majority of the winter diet of the hares, suffer heavy browsing in peak population years (Pease et al. 1979, Smith et al. 1988, Keith 1990) and subsequently undergo a period of regrowth during the time of low hare numbers. Many of these plant species contain chemicals which act as feeding deterrents (Bryant and Kuropat 1980, Sinclair and Smith 1984, Bryant et al.