Animal Depopulation Due to Armed Conflict Can Drastically Alter Ecosystems For Decades

Article: Stalmans ME, Massad TJ, Peel MJS, Tarnita CE, Pringle RM (2019) War-induced collapse and asymmetric recovery of large-mammal populations in Gorongosa National Park, Mozambique. PLOS ONE 14(3): e0212864. https://doi.org/10.1371/journal.pone.0212864

Background:

Anthropogenic, or human-generated, harm to the environment is well-known, be it climate change, deforestation, pollution, or urban sprawl. Another method of environmental impact is armed conflict – war, uprisings, and the like.

Armed conflicts occur in irregular intervals, for a variety of reasons, and can theoretically occur anywhere. They can be massive in geographic and population scales and involve nation-states and multi-national alliances. They can also be localized and involve small groups of fighters. They can be relatively brief or last decades. They can involve a variety of weapons, from small arms to airborne bombs and missiles to chemical, biological and nuclear weapons. They can occur in heavily populated areas, or in remote areas of wilderness, or anywhere in between.

Anthropogenic, or human-generated, harm to the environment is well-known, be it climate change, deforestation, pollution, or urban sprawl. Another method of environmental impact is armed conflict – war, uprisings, and the like.

Armed conflicts occur in irregular intervals, for a variety of reasons, and can theoretically occur anywhere. They can be massive in geographic and population scales and involve nation-states and multi-national alliances. They can also be localized and involve small groups of fighters. They can be relatively brief or last decades. They can involve a variety of weapons, from small arms to airborne bombs and missiles to chemical, biological and nuclear weapons. They can occur in heavily populated areas, or in remote areas of wilderness, or anywhere in between.

The effects of war and conflict have not yet been as heavily studies as other anthropogenic processes, partly due to such huge potential variability. This is particularly true about the effects of war and conflict on animal populations.

A paper by Marc E. Stalmans and colleagues recently published in PLOS One investigated the effects of war on animal populations in Mozambique, in southeastern Africa.

Methodology:

The researchers focused on Gorongosa National Park (as shown below in Figure 1 from the article). The park is at the southern end of the highly biodiverse and evolutionarily important Great Rift Valley. This park, and this valley as a whole is teeming with biodiversity and is home to many species found nowhere else on earth.

This national park was established in 1960, when Mozambique was still a colony of Portugal. It has been no stranger to war in the decades since. Frist was the Mozambican War of Independence (1964-1974). Then came the brutal Mozambican Civil War (1977-1992), which was centered on and around this park. According to the paper, during the latter conflict, much of the wildlife within the park was hunted for food and to generate money for the warring sides (selling elephant ivory, for example).

Gorongosa’s relatively long history meant that there were records of wildlife population surveys from as early as 1968. The researchers compared six pre-war aerial surveys from 1968 – 1972 with twelve post-war aerial surveys from 1994 – 2018. Most of the early surveys were conducted by airplane, and most of the later surveys were conducted by helicopter. The surveys varied somewhat in pattern, duration, and time of year. However, they were similar enough and yielded enough data points to allow comparison. For simplicity, the researchers focused on nine species of herbivores, or plant-eating animals. Data on the population of these species were available in all the surveys used in the comparison.

Findings:

The post-war surveys of the early 1990s showed that many animal species had declined by over 95% compared to the pre-war surveys. Such a drastic decline showed how acutely the war had destroyed the parks wildlife population. Thankfully, by 2018, the numbers of these species had recovered to more than 95% of the pre-war numbers, as illustrated below in Figure 3 from the article.

Each of the nine species the researchers focused on is shown in a separate graph along with the average (or mean) number of each animal per square kilometer during various time periods. The time period that each bar represents is notated below the bottom row of graphs.

Of particular note is that different species recovered to their previous numbers at different rates, and some didn’t really recover (hippo, buffalo, and wildebeest for example).

Overall the nine species of interest were divided into megaherbivores (the largest animals – the top row of Figure 3), and mesoherbivores (medium-sized animals – the bottom two rows of the graph). When grouped in this manner, the significant change in animal population before and after the war becomes very clear, as shown below in Figure 4 from the article.

The measure used to compare the two groups of animals in this figure is mean biomass density per square kilometer. This is essentially a measure of the numbers of each group per square kilometer. Before the war, as shown in the ‘1969-1972’ bars (leftmost), megaherbivores (blue bar) far outnumbered mesoherbivores (yellow bar).

This was true immediately after the war, as well – even though numbers of both groups were lower than before the war. By 2004-2012, and especially by 2014-2018, mesoherbivores far outnumbered megaherbivores.

Drawing Conclusions:

The reasons for this aren’t necessarily clear. One reason could be the absence of predators for the mesoherbivores. Megaherbivores, being very large, are not nearly as easy to kill and have far fewer natural predators. Less predators for mesoherbivores means that mesoherbivore populations grow faster. This increase in mesoherbivores may mean increased competition for the same food sources that megaherbivores depend on. Less food for megaherbivores leads to fewer megaherbivores in total.

Another reason could be the targeted hunting of elephants (a megaharbivore) for ivory during the war. While numbers of elephants have increased somewhat, they are not at pre-war levels. This does not explain the decline in the population of the two other megaherbivores.

Overall, this study shows the drastic and devastating effect war can have on animal populations. Even after the war ends, and animal numbers increase, the populations of animals can look very different. Secondarily, this study highlights the importance of conducting and carefully maintaining the records of regular animal population surveys for this type of research.