Human Disturbance On Seed Dispersal By Animals
Imagine a tropical forest that has recently been disturbed. Maybe we’ve begun hunting there, or cutting down trees, or even building a highway straight through it. What happens to that forest, its wildlife, and its seeds?
According to this study, that forest will see many changes. It will have fewer large animals, though its small animals will remain. Far fewer of its seeds will be dispersed to new locations, as large animals will no longer eat fruit and take that fruit’s seeds to grow elsewhere. Ultimately, larger animals and the large, fruit-bearing plants who depend on them will die out.
Who cares about seed dispersal?
90% of woody plants in tropical forests produce fruit, which carry seeds. When animals eat these fruit and then deposit excrement elsewhere, the seeds within the fruit are dispersed, allowing new plants to grow far from their parents.
For seeds, it’s much better to be dispersed far and wide than to grow next to a parent plant. Dispersal allows young plants to escape the dangerous pathogens, fungi, and herbivores that tend to cluster around fully-grown plants. It also brings seeds to new, unexplored places that could someday support lots of plants. Seed dispersal is therefore essential for the biodiversity and sustainability of entire ecosystems.
Yet humans can disturb this process in a number of ways. This study examines three of the most prominent human disturbances: forest fragmentation, selective logging, and hunting.
Forest fragmentation occurs when humans divide a big, interconnected wildlife area into several smaller sections that can’t interact with one another. Maybe we build a highway through the middle of a rainforest, dividing it in half; or we build up a city, leaving only scattered patches of parks, isolated from each other. By dividing forests into small, isolated parts, fragmentation can drastically change how an ecosystem functions.
Selective logging refers to the practice of cutting down only some trees within a forest, leaving the forest mostly intact. Some celebrate selective logging as better for wildlife than typical logging, where entire sections of forest are cut down. But selective logging brings heavy machinery and humans to the forest, and it still does cut down plenty of trees, so it absolutely has the potential to severely disrupt an ecosystem.
Hunting is one of the most common side-effects of selective logging: when loggers come to a forest, hunting, with its detrimental effects on wildlife, is never far behind.
This study performs a meta-analysis of 35 other case-studies, comparing 83 instances of disturbed and undisturbed tropical forests. It seeks to measure the impact of (a) forest fragmentation, where the new fragment is less that 20% of the original whole, and (b) selective logging and hunting. (Selective logging and hunting nearly always occur at the same time, so the study could not examine them separately, but rather considered the two together.) The study measured the impact of these disturbances on three environmental factors: how often animals consumed fruit, the total number of seeds dispersed, and the average distance of seed dispersal.
The key findings of the study are:
- Forest fragmentation doesn’t significantly affect any of the three measured environmental factors. This is somewhat surprising and encouraging: even when humans break a forest up into small pieces, seed dispersal mechanisms survive.
- Hunting and selective logging together massively decrease the total number of seeds dispersed. They do not, however, significantly affect how often animals consume fruit or the average distance of dispersal.
- Large seeds (>1cm diameter) and the large animals that consume their fruits were harmed much more by logging and hunting than small seeds and animals were. Over time, forests with logging and hunting saw their seeds and animals grow smaller and more uniform.
This study thoroughly demonstrates the severe effects that human disturbance can have on forests and their seeds. It raises questions about how environmentally-friendly selective logging truly is and brings to light how different sized plants and animals can respond in different ways to human disturbance.
Yet some crucial factors require more study by researchers. First, the authors note that their forest fragmentation analysis was not as complex as it could have been, lacking examination of exactly how isolated these fragments are from each other. Second, seed dispersal distance in this study was only measured up to thirty meters. More data is necessary to measure the impact of human disturbance on long distance dispersal. Third and finally, researchers must disentangle logging and hunting: do they both decrease seed dispersal, or is one the culprit? The policy implications of these questions are important and deserve further research.