Human Habitats Are Affecting Animal Migration Patterns


The fact that humans are having an impact on how animals behave may not be news to many, but as a recent City Lab article shows,ut to visualize this phenomenon on a global scale is truly unique. This topic is what some of the chapters in the book, Where Animals Go, take a look at. Essentially, the book focuses on the visualization of movements of more than 35 animals, using various tracking technologies.

The Data of White Stork Migration
The main reason for animals to migrate is to forage (eat) or to mate. White storks migrate from across Europe, certain parts of Central Asia, and Africa to other areas with lush vegetation and ,thus, food. The supply of food rises and drops with seasons, so warmer seasons may have greater supply than winters. When it is winter in the northern hemisphere, it is summer in the southern hemisphere. Therefore, storks may move around from north to south and south to north following the seasons in search of food.

However, there is a cost to migration. Since energy is spent on flying, storks must make a decision on how far they are willing to fly to get food as if they don’t get enough food along the route they are following, they might die. This is especially prominent among younger juvenile storks, as they are still growing.
If food is easily available, one can hypothesize that it is better to not fly so far at all. This is what is being observed in the migrations of certain groups of storks. Researchers mapped 70 juvenile storks using Global Positioning System (GPS) as they migrated in one season. Of these, eight storks died before they could migrate. All of the storks being tracked were from eight different geographic locations shown below.

Fig. 1: Migratory movement of tracked individuals from eight different populations. (A) Migration paths (B) Departure dates of individual storks. Color scale indicates departure dates (white indicating no departure). (C) Departure date as a function of maximum distance reached (each color represents one population). Dots in the light gray–shaded area represent individuals that left, but survived for less than 150 days. Source: Flack et al. (2016).

The researchers observed that individuals from Poland, Russia, and Greece followed the traditional eastern route via the Sahel Region of Africa up to South Africa. This can be interpreted from Fig. 1 (C), showing the departure date as a function of distance covered. The storks from these regions covered the longest distances. Similarly, the ones from Spain and Tunisia also followed the routes to vegetated areas of Africa, while some from Spain also did the same. However, four out of six individuals from Southwest Germany (orange dots) ended up spending at least five months on the landfills of Morocco. Similar behavior was hypothesized for the storks from Uzbekistan as they did not move to their traditional grounds of Afghanistan and Pakistan. Some storks who eventually migrated may also have been delayed in their migration by the opportunity to feed by just flying a shorter distance. The researchers also note that feeding on landfills has been observed for storks predominantly in Spain and sometimes in Europe.

Fig. 2: Activity of storks (as a measure for energy expenditure) in relation to human population density and vegetation cover. Daily activity as a function of human population density (B) and vegetation cover of shrubs and low-lying bushes (hence “low” vegetation cover) (D). Blue and light brown correspond to birds that overwintered, or stayed at a location after winter, north and south of 33°N, respectively. Plots on the top and the side are the density histograms of population density (A), vegetation cover (C), and Activity (E).

From the above figures we see that most storks were moving around, or were active, in areas of low population density. It can be observed that storks generally prefer areas of low population density and higher vegetation cover. However, some storks to the north of 33°N, i.e. in the northern hemisphere in Europe, North Africa and Central Asia, stayed in areas of high population density. They are represented by the blue cluster of points to the right in Fig. 2 (A). It is these storks that were feeding on the landfills. We can also observe that they had a lower activity or energy expenditure than the ones who preferred areas of higher vegetation cover.

Other interesting examples noted in the book are the effects of highways on cougars in California and the movement of baboons while they try to avoid human villages. It is important to understand animals movements, as each species plays a role in the ecosystems in which it exists or is part of, even if for a short time. Animals that migrate affect the ecology, pest behavior and populations, pollination, dynamics of infectious diseases, and a number of different factors. Therefore, understanding migration patterns and their implications can allow a better understanding of the behavior of ecosystems, especially the ones which we depend upon.

Sophia Kim