Is an empty forest a doomed forest? A study of how seed mortality is affected by density and type of cover

Abstract

Numerous papers and studies have shown that the rainforests are experiencing a decline in biodiversity. The rainforests are getting depleted of larger primates and this poses a problem for the trees as many of them rely on primates and other seed dispersers to distribute their seeds. These empty forests are spreading, but does that mean that everything is hopeless? This study aims to investigate the seed mortality in an experimental design with peanuts (Arachis hypogaea) as a test-seed placed on the forest floor in different densities and with different types of cover. Bush mango (Irvingia gabonensis) and guniea corns (Sorghum bicolor) were used to examine potential seed preference in relation to the peanuts. The study was performed in Southeast Nigeria, in Cross river county. The seed predation was measured overnight and camera traps were used to identify which species used the plots. Many of the species were rodents, and most of them were nocturnal species. Survival of the seeds was affected by density, but not in the way expected. As the density in the plot went up, so did the survival. However, if the collective density in nearby plots went up the survival of the individual seed went down. Differences in survival did not depend on seed, at least not if compared between bush mango and peanuts. Although, when comparing peanuts and guniea corns the difference was significant and the latter proved higher in survival.

Popular Abstract

Is an empty forest a doomed forest?

Seed dispersers are animal species which aid trees in their regeneration by feeding on the fruits of the trees without harming the seeds. What would happen if these disappeared? This study wants to examine what happens to the seeds on the ground. Do they get eaten? At what rate do they get eaten? Are there some influencing factors that affect at which rate the seeds are consumed?

Rainforests are some of the most intricate ecosystems on our planet. Humans have been manipulating the forest for as long has history have been recorded. But in later years the degree of manipulation has increased to such an extent that the forests are being severely damaged. In a healthy rainforest there are several animal species which aid in the regeneration of the trees. But when humans hunt these species they disappear quickly and leave a void in the ecosystem. When the seeds start falling directly to the ground below, the seed density becomes so high that the survival of the seeds are affected. This is called the Janzen-Connell hypothesis. When the density becomes too high pathogens and seed predators get an easy meal and the seeds die.

The idea with this study is that with the seed dispersers consuming most of the fruits and seeds up in the trees there should not be enough recourses falling to the ground to support seed predators. But now that the primates have declined in numbers, have the seed predators become more numerous? By placing peanuts (Arachis hypogaea) on the ground, in different densities and in different types of cover, the purpose of the study was to estimate the seed survival. How many survived and which designs or density was favourable for the seeds? In some designs, the mortality of peanuts was compared to ginea corns (Sorghum bicolor) and bush mango (Irvingia gabonensis).
The more the merrier! And size does matter!
The density of seeds played a part in the seed mortality, depending on distance. The individual seed had a higher risk of getting eaten if the plot had fewer seeds, but also if the area had a higher density of seeds. This could be explained with higher seed density the chance of that individual seed being eaten is less, but with more seeds in the area the predators will most likely stay for a longer period of time and eat from more patches. When testing survival between different seeds the tests showed no significant difference between bush mango and peanut. While guinea corns had a significantly higher survival rate than peanuts. This was probably due to the smaller size and lower nutritional value and thus not being worth the effort of searching for them.

This study did not find any significant relationship between seed mortality and the type of cover. Most probably due to that many of the animals visiting the plots being larger than a mouse and therefore they might consider all plots as covered. The results might have been different if the open plots had been placed out on the trails or in other more open areas where larger animals possibly could have experienced the area as more exposed.

So, is an empty forest a doomed forest? Who knows? The results from this study could mean that if the seed dispersers were to vanish completely, then the trees which rely on them will have a hard time spreading their seeds and the regeneration of the forest will be affected. The rain forest will most probably experience a change, but not necessarily be doomed.

Master’s Degree Project in Biology 45 credits 2017
Department of Biology, Lund University

Advisor: Ola Olsson
Biodiversity unit

• author: Morein, Lina
• supervisor: Ola Olsson
• organization: Degree Projects in Biology
• course: BION01 20152
• year: 2017
• type: H2 - Master's Degree (Two Years)
• subject: Biology and Life Sciences
• language: English
• id: 8927168
• date added to LUP: 2017-10-11 14:32:28
• date last changed: 2017-10-11 14:32:28