Ungulate Reproductive Parameters Track Satellite Observations of Plant Phenology across Latitude and Climatological Regimes

Abstract

The effect of climatically-driven plant phenology on mammalian reproduction is one key to predicting species-specific demographic responses to climate change. Large ungulates face their greatest energetic demands from the later stages of pregnancy through weaning, and so in seasonal environments parturition dates should match periods of high primary productivity. Interannual variation in weather influences the quality and timing of forage availability, which can influence neonatal survival. Here, we evaluated macro-scale patterns in reproductive performance of a widely distributed ungulate (mule deer, Odocoileus hemionus) across contrasting climatological regimes using satellite-derived indices of primary productivity and plant phenology over eight degrees of latitude (890 km) in the American Southwest. The dataset comprised > 180,000 animal observations taken from 54 populations over eight years (2004-2011). Regionally, both the start and peak of growing season ("Start" and "Peak", respectively) are negatively and significantly correlated with latitude, an unusual pattern stemming from a change in the dominance of spring snowmelt in the north to the influence of the North American Monsoon in the south. Corresponding to the timing and variation in both the Start and Peak, mule deer reproduction was latest, lowest, and most variable at lower latitudes where plant phenology is timed to the onset of monsoonal moisture. Parturition dates closely tracked the growing season across space, lagging behind the Start and preceding the Peak by 27 and 23 days, respectively. Mean juvenile production increased, and variation decreased, with increasing latitude. Temporally, juvenile production was best predicted by primary productivity during summer, which encompassed late pregnancy, parturition, and early lactation. Our findings offer a parsimonious explanation of two key reproductive parameters in ungulate demography, timing of parturition and mean annual production, across latitude and changing climatological regimes. Practically, this demonstrates the potential for broad-scale modeling of couplings between climate, plant phenology, and animal populations using space-borne observations.

Fig 1. The study region spans portions of the Great Basin, Colorado Plateau, and Chihuahuan Desert ecoregions in southwestern North America (ca. 34–42° N).

White polygons represent sampling units for all demographic and plant phenology data. Sampling units are based on the intersection of state-defined wildlife management units and high elevation summer mule deer habitat.

Fig 2

Panel A. Polygons symbolize high elevation summer mule deer habitats in southwestern North America (ca. 34–42° N). Map illustrates the latitudinal shift in the seasonality of moisture across the mountains of the Great Basin, Colorado Plateau, and northern Chihuahuan Desert ecoregions. Colors represent the percentage of total annual precipitation that comes in the form of thunderstorms during July-September (color key: blue ≤ 17%; beige = 18–49%; green ≥ 50%). Panel B. Growing season phenological curves for high elevation mule deer summer ranges in southwestern North America (2004–2011; 33–42° N). From top to bottom, curves represent a transition from ecosystems where plant phenology is driven by spring snowmelt and dry summers, to those in which phenology is timed to the onset of summer (monsoonal) rains. Thick bands represent approximate mule deer parturition dates at select latitudes.

Fig 3. Estimated mule deer fawning dates (Birthdate), start of growing season (Start), and peak of growing season (Peak) as a function of latitude in southwestern North America, 2004–2011.

Fig 4. The proportion of annual precipitation occurring from July-September (monsoonal moisture) and the associated standard deviation (SD) in the start (Start) and peak (Peak) of season as a function of latitude in southwestern North America.

Fig 5. Values for Spearman’s rank correlation coefficient (r) between fawn counts and monthly NDVI values among migratory mule deer populations (2004–2011; 34–42° N).

June and July are the primary months of parturition in this region. Asterisks indicate a statistically significant relationship between fawn counts and NDVI for that month.

Fig 6. Mean autumn fawn counts and associated coefficient of variation (CV) for migratory mule deer populations in southwestern North America, 2004–2011.