Date of Award
Doctor of Philosophy
Swamp rabbit and eastern cottontail populations have declined substantially in Illinois within the last half-century. Habitat loss and fragmentation, particularly of bottomland hardwood (BLH) forest, have left swamp rabbit populations patchily distributed along major rivers in the southern portion of the state. In addition, the decline of early-successional upland habitats due to changes in farming techniques have led to as much as 90% declines in cottontail populations in Illinois. Managers need information on basic vital rates and habitat use to conserve both species in southern Illinois; however, many questions regarding demography and behavior have not been investigated. My research examined the importance of several factors that may influence survival and habitat use among swamp rabbits and eastern cottontails. My specific research objectives were to 1) estimate seasonal and annual survival rates and identify primary mortality agents, 2) examine the influence of intrinsic factors and habitat variables on annual and seasonal 50% core area (CA) and 95% home range (HR) sizes, 3) assess differences in space use and habitat use by season, and 4) evaluate differences in space and habitat use between species for swamp rabbits and cottontails in BLH forests in southern Illinois. During December-March 2009-2013, I live-trapped and radiocollared adult swamp rabbits (>1.9 kg) and cottontails (>1.0 kg) at 7 BLH sites along the Cache River and Cypress Creek within the Cypress Creek National Wildlife Refuge in southern Illinois. I monitored rabbits for survival every 24-48 hr and estimated radiolocations by triangulation more than twice weekly during morning (0500-0900 hr), daytime (0900-1700 hr), and evening (1700-2400 hr) time periods on a rotating schedule. I delineated annual and seasonal 50% CA and 95% HR isopleths using kernel density estimators, comparing winter-spring (W/S; 21 Dec-19 Jun) and summer-fall (S/F; 20 Jun-21 Dec) seasons. During May-August 2012, I sampled microhabitat in 0.02-ha circular plots (3.5 plots/ha) randomly placed throughout home ranges of rabbits at each site. In ArcGIS, I classified macrohabitat patches into 1 of 4 cover types: agriculture, early-successional BLH (EBLH), mature BLH (MBLH), or upland. I also measured the distance from all radiolocations to the nearest patch of each cover type and the nearest river or creek. I modeled the influence of species, sex, and season on annual survival using parametric survival regression models. Then, I used the best model (based on AICc) to examine the influence of habitat covariates on survival. I also modeled the influence of habitat on annual and seasonal CA and HR sizes using generalized linear and generalized linear mixed effects models with AIC model selection. Finally, I estimated conspecific and heterospecific CA and HR overlap for swamp rabbits and cottontails, and examined differences between species in space and habitat use. I documented causes of mortality and estimated survival for 129 swamp rabbits and 75 cottontails during the period of study. Predation (71%) was the primary mortality agent for both species, followed by weather (9%) and hunter harvest (6%). Models with survival rates differing by species and season received the most support; swamp rabbits had higher estimated annual survival (0.37 0.05) than did cottontails (0.20 0.05), and survival for both species was lowest during the W/S season (βW/S=-2.28 0.46). None of the habitat covariates that I measured apparently influenced survival. Core areas and HRs were estimated for 60 swamp rabbits (SR; 34 M, 26 F) and 21 cottontails (CT; 10 M, 11 F) during the W/S season; of these, 57 swamp rabbits and 11 cottontails lived long enough to estimate S/F seasonal and annual home ranges. The average annual CA and HR for swamp rabbits were 2.49 1.42 ha and 11.60 5.81 ha, respectively. Cottontails had an average annual CA and HR size of 2.48 1.26 ha and 13.54 7.24 ha, respectively. Core areas and HRs for both species during the were larger during W/S than S/F (CA: βW/S=0.59 0.11, wi=1.0; HR: βW/S=0.53 0.11, wi=1.0). Seasonal CA sizes increased with decreasing proportions of woody ground cover within CAs (SR: βShrubs=-2.75 0.50, wi=1.0; CT: βShrubs=-2.30 0.74, wi=0.91). Few macrohabitat variables influenced space use for either species. The coefficient of variation in patch size within 1 km of study sites was positively associated with space use for both swamp rabbits (CA; β=0.01 0.004, wi=0.81; HR: β=0.01 0.003, wi=0.95) and cottontails (CA; β=0.02 0.01, wi=0.29; HR: β=0.02 0.01, wi=0.23); I did not detect significant differences between species in pairwise conspecific overlap within CAs or pairwise conspecific volume of intersection (VI) over the entire utilization distribution. Median conspecific CA overlap was higher during W/S than S/F, with mean CA overlap proportions of 0.20 ± 0.21 (range: 0−0.87) and 0.10 ± 0.18 (range: 0−0.73) respectively. Median conspecific VIs also differed between seasons, with mean pairwise VIs of 0.32 ± 0.19 (range: 0−0.77) during winter-spring and 0.19 ± 0.18 (range: 0−0.73) during S/F. Heterospecific CA overlap was 48% and 46% lower than conspecific overlap during W/S and S/F, respectively. Differences in habitat use between species were apparent. Swamp rabbits had HRs and CAs in areas with higher basal area compared to cottontails during both seasons (all U≥483, Z≥2.83, p≤0.005). Multivariate tests for both CAs and HRs indicated a significant effect of species on habitat use (CA: T2=0.58, F3, 71=13.70, p<0.001; HR: T2=0.24, F3, 71=5.80, p=0.001). During W/S, swamp rabbits had CAs and HRs composed of significantly higher proportions of EBLH (CA: F1, 73=16.46, p<0.001; HR: F1, 73=8.55, p=0.005) and MBLH (CA: F1, 73=17.99, p<0.001; HR: F1, 73=7.78, p<0.007). Swamp rabbits were located significantly closer to a permanent watercourse (F1, 79=24.18, p<0.001) than cottontails. Indeed, 95% of all swamp rabbit radiolocations were ≤332.0 m away from a permanent watercourse (mean=169.0 100.0 m; range=1.0−571.0 m; Figure 18), whereas 95% of cottontail radiolocations were ≤536.0 m away from a permanent watercourse (mean=289.0 142.0 m; range=1.7−670.0 m; Figure 18). Swamp rabbits also were significantly closer to MBLH patches (F1, 79=9.05, p=0.003) and farther from agriculture (F1, 79=12.36, p=0.001) than cottontails. My study represents the most complete record to date on survival and habitat use by swamp rabbits. Rabbit survival was positively associated with basal area so management actions that provide woody cover for concealment and thermoregulation may benefit both species. Although cottontails in my study used early-successional BLH, patterns of space and habitat use described here demonstrate that cottontails remained on the periphery of bottomlands. My study confirms the utility of early-successional BLH to both species; however, stands that are located too far from a permanent water sources are unlikely to be used by swamp rabbits, and may be less suitable for other BLH specialists as well. Allowing grasslands and crop fields to succeed into old fields containing bushes, vines, and other woody species will benefit both cottontails and swamp rabbits. Within BLH forests, canopy gaps can be created to promote tree regeneration and woody ground vegetation such as vines and shrubs. Finally, upland early-successional habitats that border bottomland forests are especially important as refugia for swamp rabbits during flooding.
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