Evaluating patterns of plant phenological progression and pronghorn movement behaviors across diverse landscapes

A variety of metrics based on the remotely sensed normalized difference vegetation index (NDVI) are commonly used as proxies for nutritional conditions of landscapes for ungulates, and these proxies are used to explain variation in animals’ vital rates and movements. One common application of NDVI data is to derive the instantaneous rate of green-up (IRG) to represent the rate of plant phenological progression and evaluate hypotheses regarding animal movements related to the green wave. Although the green wave is generally assumed to occur along elevational gradients in mountain-valley systems, it is unknown how variable patterns of plant phenological progression occur across heterogeneous landscapes. Additionally, the consequences of spatial and temporal variability in these phenological patterns on animal movement behaviors in different ecological systems are unknown. Many ungulates worldwide exhibit migratory behaviors to track the leading edge of plant phenological progression; however, some species, such as pronghorn (Antilocapra americana), that occupy mountain-valley and prairie systems may experience variable patterns of plant phenological progression and employ variable movement strategies to exploit these systems. Within 8 pronghorn herd ranges that span broad and heterogeneous landscapes in Montana, USA, our objectives were to 1) calculate and interpret phenology metrics, 2) evaluate spatial and temporal variability in plant phenology patterns, 3) characterize the patterns of plant phenological progression, 4) relate the variability and patterns of plant phenological progression to pronghorn migratory movement behaviors, and 5) evaluate potential nutritional trade-offs between employing a migratory or resident migratory behavior in different landscapes. The study area included the ranges of 3 pronghorn herds in western Montana characterized by an elevational gradient from lower elevation grasslands to higher elevation forests and the ranges of 5 pronghorn herds in eastern Montana characterized by mixed-grass prairie and sagebrush steppe. Across these 8 herds, we collected global positioning system (GPS) collar location data from 586 female pronghorn during 912 animal-years from 2019 to 2021. We processed 12 years of phenology data collected during 2010–2021. Spatial and temporal patterns and predictability of plant phenological progression varied across different pronghorn ranges. In general, the 3 western Montana herd ranges had less annual variation but greater intra-annual spatial variation in values of phenology metrics, as compared to the eastern Montana herd ranges, and greater predictability across years. The estimated green-up order, which represented the strength of the green wave, suggested that a defined and strong wavelike pattern of phenological progression (i.e., a green wave) was not common across pronghorn ranges in Montana. Most ranges in most years experienced a truncated green wave or synchronous greening across the landscape rather than a green wave. Of 817 animal-years of spring migration behavior, we classified 193 as migrant and 624 as resident. The probability of being classified as a migrant increased as landscape productivity increased. We sampled migrants’ and residents’ potential exposure to phenology values along spring movement paths and summer ranges and found that within herds and years, the resident and migrant movement strategies conferred similar exposure to indices of nutrition. Collectively, these results highlight the variability in patterns of plant phenological progression across pronghorn ranges in Montana and indicate that migration behavior is not the dominant movement strategy of the pronghorn sampled or a strategy that resulted in higher exposure to indices of nutrition. Pronghorn employing either resident or migrant movement behaviors may have similar exposure to indices of nutrition, which suggests that factors other than tracking indices of nutrition may influence pronghorn movement behaviors or that remotely sensed nutrition indices may not adequately quantify nutrition-related reasons for migration. Based on these results confirming residency as a behavior employed by the majority of animals, we recommend that habitat conservation strategies should focus efforts to increase landscape permeability not only within movement corridors used by migrants but also within ranges where non-migratory animals are moving.