Why do rare species benefit from a higher survival rate than common species? A look into the viability of trees in tropical regions.
by Krystal Jennings

Northeastern State University

In the tropical regions of the world, many different species of trees can survive and flourish amongst the more common species. It has been estimated that in a typical tropical forest, there can be anywhere between 150-300 different species of trees per hectare (Kageyama, 2013, p. 3)(G., 2001); this number is quite amazing when you think about it. Consequently, with a diverse group of species there is a greater chance that one is more widespread than the other. This is the concern with rare and common tree species in the tropical rainforests. With a high degree of variation, does a rare species have a higher survival rate than a more common species? To gain a better understanding of the problem we will look at the difference between a rare and common species, why rare species have a higher survival rate than common species, and how we might be able to create a state of equilibrium between the different species groups.
Beach cacti surrounded by different sizes and types of trees in Costa Rica. Taken by Dr. Erik Terdal
Beach cacti surrounded by different sizes and types of trees in Costa Rica. Taken by Dr. Erik Terdal

What is meant by the word "rare"?

First off, we need to establish the fact that tropical rain forests are voluminous, high in biomass, and structurally elaborate (Kricher, 2011, p. 79). Thus, rainforests have the ability to harbor many different life forms that can evolve and increase in numbers (2011, p. 79). When a certain species is rare it has sustained itself in one way or the other. For example, an article about different species of trees in the forest reads, “…a species had to begin with some advantage: they must have exploited a "niche opportunity" by using some resource or resisting herbivores and pathogens more effectively or economically than their competitors… (Leigh Jr., et al., 2008)”. Wills et al. also confirmed this hypothesis by adding, “The niche hypothesis posits that species differ in the sub-environments or resources they exploit, and as a result, individuals compete more intensively with conspecifics than with individuals of other species (Wills, et al., 2006, p. 527)”. The latter makes sense according to the Janzen-Connell model which predicts that diversity should increase as a group of individual’s age, because more common species are selectively removed by pathogens and predators (Wills, et al., 2006). This information proves how interesting the impasse between rare and common species in the tropical rainforest really is. At first thought, one would think that the common species would have a greater chance of surviving or increasing their numbers but the real issue is not with how many of a species there is but with the fact that rare species have a greater chance to evolve, become more diverse, and increase in numbers. The word “rare” in this context is not an issue of numbers but an issue with how many different kind of species there are and how they have sustained themselves.
What makes a species rare?
There are different ways in which a species of trees is termed “rare”. One way is by seedlings traveling far from their parent tree. If this is the method in which a tree becomes rare it has a major advantage over its more common neighbor. To gain a better understanding of how rare species benefit from being introduced to an unfamiliar territory I would like to define the Janzen-Connell hypothesis: “According to their hypothesis, there is a density- or distance-dependent factor in recruitment of seedlings from adults of tropical tree species due to host-specific predators or pathogens (Carson, 2008)." The seedlings that travel further from their parent tree have an advantage over their uncommon counterparts. They do not suffer, to the same degree, from species-specific predators like those that are common to the region (Fangliang, et al., 1997). This not only helps the viability of the rare species but also has major implications for conservation of near-extinct trees. I believe that if a species of tree were near extinction, their seedlings could be manually placed in other areas away from species-specific predators, or at least this is the idea, thus prolonging their life and increasing their diversity and numbers. However, this is hypothetical and the conservation of trees is far more difficult than this simple hypothesis. A second determing factor of tree rareness was determined by two scientists, Hubbel and Foster. They stated that for a tree to be considered "rare", the species whose densities [were] equal to or less than one tree per hectare [were to] be considered rare (Fangliang et al., 1997). For example, if scientists were studying an area that consisted of 60 hectares (ha), then a species of trees that was numbered at 60 members or below would be considered "rare".

The more common species.

On the other hand there is the common species. A common species is one that is endemic to its respective area and has gained an advantage over its competitor(s) (Leigh Jr., et al., 2008). A common species of tree experiences more danger than its rare, non-common counterpart. For example, because more common species are selectively removed by pathogens and predators (Leigh Jr., et al., 2008) they have a higher mortality rate than a rare species in the same local area. Also as a group of individual’s age, the more common species experiences higher mortality rate while the opposite occurs with a rare species. The fact that common species are more likely to be close together they increase their chance of disease and mortality because predators can easily find a new host if they are spaced closely together (Clark, 1984). The rare species may benefit from the fact that they are one in a few in a particular area. As a rare species ages, they have a higher chance of surviving, thus increasing their viability. Unfortunately, common species do not have many advantages. The most obvious advantage for common species is the fact that they are common; they are more abundant in number than a rare species and their local areas cater to their needs and demands. But, this is also where they encounter trouble because the local predators are also what kill them.

As you can recall from before, a rare species is one that has grown far from its parent tree. They can experience a lower mortality rate because they are not subject to the same diseases and predators compared to the common species of tree in their local area. Therefore, the rare species can take advantage of the distance from their parent tree. Since they are too far from their parent tree, they are not affected by the same predators that common trees encounter at the local area. This is the demise of the common tree; a high mortality rate due to diseases and predation that is common in its local area. A common species of tree does not undergo the same evolution and diversification as the rare species and this is the main reason common species have low diversity. The advantage of rare species is that they are “one-in-a-million” in their local area and have the opportunity to evolve and become more diverse. This helps the rare species experience a higher survival rate then its close neighbor, the common species. The video below is a good example of the fast differences in a tropical forest. There are many, many differences in the types of trees that are reside in a tropical forest. You can see that there are different sizes, different colors, and different types of plants and tree species existing together in one area. Because of a rare species relative distance from their parent tree, they are able to experience a relative higher survival rate when compared to a common species in the local area.

Carson, Walter A. and Stefan A. Schnitzer. Tropical Forest Community Ecology. Blackwell Publishing: Hoboken, NJ, 2008.

Clark, D. A., & Clark, D. B. (1984). Spacing dynamics of a tropical rain forest tree: evaluation of the Janzen-Connell model. American Naturalist, 769-788.

Fangliang, H., Legendre, P., & LaFrankie, J. V. (1997). Distribution patterns of tree species in a Malaysian tropical rain forest. Journal of Vegetation Science, 8(1), 105-114.

G., M. (2001). Tropical Rainforest. Retrieved 06 21, 2013, from Blue Planet Biomes: http://www.blueplanetbiomes.org/rainforest.htm

Kageyama, P. (2013, 03 26). Rare and Common Tropical Tree Species: Management and Conservation. Retrieved 06 20, 2013, from Global Timber Tracking Network.

Kricher, J. (2011). Tropical Ecology. Princeton and Oxford: Princeton University Press.

Leigh, E. G., Davidar, P., Dick, C. W., Terborgh, J., Puyravaud, J. P., Steege, H., & Wright, S. J. (2004). Why do some tropical forests have so many species of trees?. Biotropica, 36(4), 447-473.

Wills, C., Harms, K. E., Condit, R., King, D., Thompson, J., He, F., et al. (2006). Nonrandom Processes Maintain Diversity in Tropical Forests. Science , 311 (5760), 527-531.