Reconstructing landscapes of ungulate parturition and predation using vegetation phenology

Abstract

Enhanced vegetation index (EVI) data can be used to identify and define the space in which ungulates practice parturition and encounter predation. This study explores the use of EVI data to identify landscapes linked to ungulate parturition and predation events across space, time, and environmental conditions. As a case study, we used the moose population (Alces alces) of northern Minnesota in the USA. Using remotely sensed EVI data rasters and global positioning system collar data, we quantified how vegetation phenology and moose movement shaped the births and predation of 52 moose calves from 2013 to 2020 on or adjacent to the Grand Portage Indian Reservation. The known sources of predation were American black bears (Ursus americanus, n = 22) and gray wolves (Canis lupus, n = 28). Satellite-derived data summarizing seasonal landscape features at the local level revealed that landscape heterogeneity use by moose can help to quantitatively identify landscapes of parturition and predation in space and time across large areas. Vegetation phenology proved to be differentiable between adult moose ranges, sites of cow parturition, and sites of calf predation. Landscape characteristics of each moose group were consistent and tractable based on environment, suggesting that sites of parturition and predation of moose are predictable in space and time. It is possible that moose selected specific landscapes for parturition despite risk of increased predation of their calves, which could be an example of an "ecological trap." This analytical framework can be employed to identify areas for future ungulate research on the impacts of landscape on parturition and predation dynamics.

Keywords: EVI; bear; moose; parturition; predation; prey; vegetation phenology.

Figure 1.

Landscape information in the study area. (A) Map of the study area showing landscape heterogeneity. Landscape heterogeneity is shown by the first three principal components from the principal component analysis of monthly EVI rasters. Colors represent the distribution of each principal component one (red), two (green), and three (blue) across the landscape. Heterogeneous landscape conditions were present across the study area, and are displayed as changes in color intensity and combination. (B) Distribution of adult moose (blue crosses) and sites of parturition (red points) resampled to one report per pixel (see Materials and Methods section). Adult moose reports cover a large geographic area within the study area. Monthly average EVI values are displayed in the background, ranging from high (dark green) to low (light green), to represent seasonal variation in primary productivity (i.e., EVI value) during the year. (C) Sites of calf predation by bears (black silhouettes), wolves (blue silhouettes), and sites of unknown calf mortality cause (red question marks). There was a dispersed distribution and overlap in sites of attacks.

Figure 2.

Moose distribution in environmental space. A principal component analysis was conducted on monthly EVI data from the study area from 2013 to 2020. Axis of each pane: X=Principal component one, Y=Principal component two, and Z=Principal component three. (A) Distribution of adult moose (blue polyhedron) and cloud of landscape conditions (i.e., monthly EVI data) summarized in the first 3 principal components. (B) Landscape conditions of sites of parturitions (yellow polyhedron) showing that calving occurs in a subset of landscape environmental conditions used by the adult population. (C) Sites of calf predation displayed by environment (pink polyhedron) showing that predation occurs in a subset of the landscape conditions where parturitions occur. (D) The environmental overlap of conditions where adults, parturitions, and predation attacks occur as shown by minimal volume ellipsoids of each phenomena. Here, predation nested in a portion of the conditions where parturition occurs, and parturition nested in a portion of the species realized niche.

Figure 3.

Average monthly EVI at locations of moose predation. The mean EVI value of each month across the study period (February 2013 to May 2020) at sites of predation by bears (n = 22) (red) and wolves (n = 28) (blue) was calculated across years. Peaks of primary productivity occurred in the month of May. There was also a disparity between overall EVI values between predator species, with bears predating moose in areas with lower average EVI values when compared with wolves (t₂₆₆₉ = 7.46, P = 0.001).