Even though the trees in tropical rainforests can tower over 100 feet in the air, some tropical rainforests have incredibly poor soil, and the plants that live there rapidly deplete nutrients in the soil. The lack of available nutrients is often discussed in the context of anthropogenic changes to rainforests, meaning that the productivity and effectiveness of rainforests as CO2 sinks can potentially be limited or constrained by the availability of nutrients in soil (Sayer et al., 2012). Tropical rainforests are important in maintaining the health of the planet, yet they are being cut down faster than they can be regrown. Reforestation is often challenged by the lack of available nutrients. When trees have been removed, there is little litterfall to be decomposed into soil and nutrients (Parsons et al., 2014). Because nutrient limitation is such an issue in tropical forests, it is important to understand factors that determine what nutrients are recycled from leaves and incorporated into the soil (Sayer et al., 2012). Understanding what determines nutrient makeup and soil quality in rainforests could contribute important knowledge for reforestation efforts.

The availability of nutrients in the soil of tropical rainforests is often attributed to litterfall, and the volume and chemical makeup thereof (Parsons et al., 2014). Litterfall is essentially the fallen leaves that are decomposed on the forest floor to create a layer of soil that contains nutrients to be used by various plants (Parsons et al., 2014). It is affected by multiple variables, including seasonality, which determines the temporal distribution and how decomposable the litterfall is (Parsons et al., 2014). Decomposability of leaves may also depend on herbivory of canopy leaves (Schweitzer et al., 2005). Herbivory occurs when herbivores eat aboveground primary producers (Schweitzer et al., 2005). Many rainforest plants develop defenses to combat herbivory, such as growing tougher or thicker leaves, or creating chemical compound to deter herbivory (Cárdenas et al., 2015). These changes to the makeup of leaves may then go on to change the composition of soil and nutrient availability (Schowalter et al., 2011). Herbivory may also change litterfall decomposability because the pieces of leaves that fall to the forest floor may be smaller of more fragmented after being partially eaten (Schowalter, Fonte, & Wang, 2011). These changes in the makeup of litterfall all contribute to soil composition.

It therefore makes sense to assume that understanding the roles that herbivores play in rainforest soils could be a valuable tool for reforestation efforts. The role of herbivores in soil composition is poorly understood and many studies in this area of tropical biology disagree on the scope and magnitude of the effects of herbivory. However, most research suggests that soil composition is affected in some way by herbivory. This paper will summarize and synthesize papers that relate to this issue in order to determine what effects herbivores have on rainforest soil and how important these effects are.

There are multiple ways in which herbivory can change the makeup of leaves and therefore litterfall. The relationship between plant palatability and herbivory dictates that high nitrogen content makes plants more likely to be the targets of herbivory (Cárdenas et al., 2014). This means that plants with high leaf-nitrogen levels will be consumed much faster than plants with low nitrogen (Kurokawa & Nakashizuka, 2008). However, nitrogen is an essential nutrient in soil, so leaves must contain enough nitrogen to support the soil levels and it appears that nitrogen levels significantly affect the rate of decomposition (Kurokawa & Nakashizuka, 2008). Nitrogen fixation is the most common source of nitrogen in the tropical ecosystem, and it appears to be that soil has the highest levels of available nitrogen (Reed, Cleveland, & Townsend, 2008). It has also been discovered that tree species in tropical areas affect the level of nitrogen availability in the ecosystems by controlling the amount of nitrogen in the leaves they lose (Reed, Cleveland, &Townsend, 2008). In essence, herbivores can change the composition of soil and the rate of decomposition by causing plants to change the chemical contents of their leaves. The decomposition of the leaves creates the soil from which trees receive nutrients. This is significant because not only does the soil composition affect the trees, it affects the entire ecosystem of the rainforest.

While herbivory can change soil composition by affecting the levels of specific nutrients, it can also affect soil by changing physical properties of leaves and soil. In this vein, Fleury et al. (2015) discovered that just the mere presence of herbivores in tropical forests can change soil composition. This is because the trampling of soil by herbivores leads to soil anoxia, and the leaching of nutrients can result in acidic, infertile soils (Fleury et al., 2015). These infertile soils limited the survival of seedlings growing in these areas and were heavily impacted by herbivory by the lack of nutrients in the soil. This ultimately limited the net productivity of the rainforest (Alvarez-Clare et al., 2014). Similarly, it appears that not all nutrients needed by plants are absorbed from the soil. It has been shown that trees reabsorb nutrients from mature leaves before the leaves are lost (Metcalfe et al., 2013). However, herbivores tend to prefer younger, softer leaves, so the reabsorption from older leaves would not change the loss of nutrient significantly (Metcalfe et al., 2013). This would mean that in ecosystems such as rainforests, the fecal deposits of herbivores would have high concentrations of essential nutrients and could contribute significantly to the nutrient levels in soil (Metcalfe et al., 2013). Schowalter et al. (2011) added feces collected from herbivores to a tropical forest plot and found that the deposits of herbivores do influence nutrient fluxes in soil but also decrease rates of litterfall decomposition.

Along with this, Metcalfe et al. (2013) found that many tropical trees produce low quality litter that decomposes rather slowly. This may be due to an active strategy of plants to nutrient-starve the other main competitors in nutrient absorption, the microbes. This would mean that herbivory does contribute to soil composition, but does so in a way that helps soil microbes and is potentially detrimental to trees. This is because herbivores exacerbate the nutrient limitation of trees through eating the most nutrient-rich leaves and depositing those nutrients for microbes to consume before the nutrients reach the soil (Metcalfe et al., 2013). The low quality, slow decomposing litter necessitated by herbivory has negative effects on the flora of rainforests.

As well as chemically altering the composition of litterfall in tropical forests, herbivores mechanically change the litterfall through mastication and fragmentation of leaves before they reach the forest floor. This grinding of plant matter speeds up decomposition of the litterfall (Cárdenas & Dangles, 2012). This is because fragmentation increases the surface area of leaves, so less time is needed for the fragment to decompose than is needed for the whole leaf to decompose. Cárdenas and Dangles (2012) discovered that mechanically affected litterfall decomposed faster than undamaged litterfall because of the availability of leaf edges to microbes that are responsible for the decomposition of leaves into soil. Understanding the relationship of this geometric manipulation of leaves with the decomposability of leaves offers insight into other ways that herbivores contribute to rainforest soil. Just as trees have ways to limit the chemical effects of herbivory on leaves; they can change their physical properties in order to constrain mechanical effects. Trees often do this by increasing the toughness of their leaves, which decreases the herbivory rate experienced by trees (Kurikawa & Nakashizuka, 2008). Leaf toughness is an effective defense against herbivory and is negatively correlated with litter decomposition (Kurikawa & Nakashizuka, 2008). This shows that there is a tradeoff between defenses against herbivory and soil quality because as leaves get tougher to deter herbivores, they are converted more slowly into soil.

There are many ways for herbivores to affect the soil composition of tropical rainforests. While there is some dissent to the nature and scope of these effects, there is general agreement that herbivory does change amount of nutrients available to trees. One of the ways that herbivores do this is by changing the rate of decomposition of litterfall by fragmenting the leaves that fall to the forest floor. Another way herbivores affect soil composition is by depositing high concentrations of nutrients that are used by the soil microbes and decomposers, creating nutrient fluxes in soil. Herbivores also trample the forest floor, making it easier for nutrient run off to occur. Lastly, and perhaps most important, is the fact that herbivory is a driving cause for the nutrient composition of plants themselves to change, so fewer nutrients are lost with leaf fall, limiting the absorbable nutrients in rainforest soil. Understanding the relationship between herbivores and soil nutrients is an essential part of understanding the delicate balance that rainforests need to maintain in order to grow and be healthy, which is an important step in reforestation efforts.

References

Alvarez-Clare, S., Mack, M. C., & Brooks, M. (2013). A direct test of nitrogen and phosphorus limitation to net primary productivity in a lowland tropical wet forest. Ecology, 94(7), 1540-1551.

Cardenas, R. E., & Dangles, O. (2012). Do canopy herbivores mechanically facilitate subsequent litter decomposition in soil? A pilot study from a Neotropical cloud forest. Ecological research, 27(5), 975-981.

Cárdenas, R. E., Hättenschwiler, S., Valencia, R., Argoti, A., & Dangles, O. (2015). Plant herbivory responses through changes in leaf quality have no effect on subsequent leaf‐litter decomposition in a neotropical rain forest tree community. New Phytologist, 207(3), 817-829.

Cardenas, R. E., Valencia, R., Kraft, N. J., Argoti, A., & Dangles, O. (2014). Plant traits predict inter‐and intraspecific variation in susceptibility to herbivory in a hyperdiverse Neotropical rain forest tree community. Journal of Ecology, 102(4), 939-952.

Fleury, M., Silla, F., Rodrigues, R. R., do Couto, H. T., & Galetti, M. (2015). Seedling fate across different habitats: the effects of herbivory and soil fertility. Basic and Applied Ecology, 16(2), 141-151.

Metcalfe, D. B., Asner, G. P., Martin, R. E., Silva Espejo, J. E., Huasco, W. H., Farfán Amézquita, F. F., … & Quispe, H. (2014). Herbivory makes major contributions to ecosystem carbon and nutrient cycling in tropical forests. Ecology letters, 17(3), 324-332.

Parsons, S. A., Valdez-Ramirez, V., Congdon, R. A., & Williams, S. E. (2014). Contrasting patterns of litterfall seasonality and seasonal changes in litter decomposability in a tropical rainforest region. Biogeosciences, 11(18), 5047-5056.

Sayer, E. J., Wright, S. J., Tanner, E. V., Yavitt, J. B., Harms, K. E., Powers, J. S., … & Turner, B. L. (2012). Variable responses of lowland tropical forest nutrient status to fertilization and litter manipulation. Ecosystems, 15(3), 387-400.

Schowalter, T. D., Fonte, S. J., Geaghan, J., & Wang, J. (2011). Effects of manipulated herbivore inputs on nutrient flux and decomposition in a tropical rainforest in Puerto Rico. Oecologia, 167(4), 1141-1149.

Schweitzer, J. A., Bailey, J. K., Hart, S. C., Wimp, G. M., Chapman, S. K., & Whitham, T. G. (2005). The interaction of plant genotype and herbivory decelerate leaf litter decomposition and alter nutrient dynamics. Oikos, 110(1), 133-145.

Silfver, T., Paaso, U., Rasehorn, M., Rousi, M., & Mikola, J. (2015). Genotype. Herbivore effect on leaf litter decomposition in Betula Pendula Saplings: ecological and evolutionary consequences and the role of secondary metabolites. PloS one,10(1), doi:10.1371/journal.pone.0116806

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