Plant growth and herbivory in a mangrove system - article review
Onuf, C. P., J. M. Teal, and I. Valiela. 1977. Interactions of Nutrients, Plant Growth and Herbivory in a Mangrove Ecosystem. Ecology 58:514–526.
The authors of this paper set out to evaluate the effects of different nutrient dynamics on communities of red mangroves (Rhizophora mangle) in Florida. The state’s increasing human population, along with more intensive agricultural practices and thus increases in nutrient runoff to coastal estuaries, serve as the general motivation for assessing mangrove responses to various nutrient regimes. Considering the ecosystem services that mangroves provide; including protection from erosion, water filtration, and nursery habitat for fish; the authors also chose to evaluate ecosystem functions of red mangroves. The authors aimed to evaluate tree growth, nutrient levels, and herbivory impacts across two similar mangrove islands that differed primarily in the nutrient inputs that they naturally received.
The authors refer to their two chosen study sites as a “natural” experiment in which the presence and absence of piscivorous birds provide a high-nutrient and low-nutrient study condition, respectively. A bird census was conducted to determine the temporal variation of bird presence in the high-nutrient site with the bird rookery. The low-nutrient site did not host any bird aggregations. Nutrient samples were taken at each site to assess the effects of bird presence and guano excretion on environmental nutrient concentrations. Main mangrove stems were arbitrarily selected and tagged at each site, and the leaves, internodes, and reproductive structures of these branches were measured. Biomass production estimates were made based on these measurements. Carbon and nitrogen content of vegetative structures from mangrove trees were evaluated to assess for differences in the nutritional value of plant tissue. Additionally, herbivorous impacts by six species of insects were observed and quantified when possible.
The bird census revealed a high seasonal abundance of bird species across every month besides September through November in the high-nutrient, or bird rookery, site. Water samples taken at the high-nutrient site contained 5 times the concentration of ammonium and phosphate compared to samples from the low-nutrient site. Nutrient levels of interstitial water revealed a similar pattern. During the months when birds were not present at the high-nutrient site these differences did not occur, demonstrating a correlation between bird presence and high environmental nutrient levels. Tagged branches demonstrated similar patterns of leaf production across both sites, however, new branch production and flowering showed periods of significantly higher growth at the high-nutrient site. Flowering occurred earlier at the high-nutrient site and was significantly greater than at the low-nutrient site in early summer and fall. Variations in estimates of leaf biomass production were not statistically significant across sites, but overall production per main stem for all plant parts (leaves, branches, stems, flowers, and fruits) was significantly greater in the high-nutrient site. Woody biomass production and reproductive biomass production were predominantly greater in the high-nutrient site as well. Leaves and fruit at the high-nutrient site had higher nitrogen content percentages than those at the low-nutrient site, however, differences across other plant parts were not statistically significant. Due to the wide variety of herbivory factors measured in this experiment, only the total herbivory losses will be noted. This measurement was significantly greater in the high-nutrient site with a p-value less than 0.001.
The methods used in this paper appear to have been carried out effectively with keen attention to detail and awareness of shortcomings. Confounding factors, such as timing of the bird surveys and environmental traits of the two study sites, were mentioned and taken into consideration. Limitations to the capabilities of the herbivory sampling methods were noted. It seemed appropriate to discuss other papers that evaluated nutrient enrichment in different systems and point out that additional variables may have played a critical role across these studies. It also appears that careful consideration was made in selecting the study sites. Although lengthy, the paper was well-written with the objectives clearly laid out.
While well-meaning in their motives for this study, the authors may have been somewhat misguided in associating bird guano with nutrient pollution emitted by anthropogenic activities. Aside from the obvious differences in how these nutrient sources spatially relate to red mangrove forests, the timing and influx of these nutrients vary drastically. Artificial fertilization of mangroves may have been a more relevant method than relying on natural nutrient sources, especially considering the lack of a published nutrient budget on mangrove ecosystems at the time of this study (Odum et al, 1984). Whereas influxes of bird guano are determined by ecological variables like seasonal bird migration, nutrient-rich wastewater fluxes into estuaries based on anthropogenic and hydrological factors like timing of fertilizer application, river flow, and precipitation. Also, fast-growing primary producers, such as algae, seem far more likely to uptake nutrient-enriched water than mangroves, which have likely adapted to using bird guano over thousands of years and have only faced major increases in anthropogenic nutrient inputs recently. The two sample sites were relatively small and may have differed considerably in terms of hydrology and tidal pulse based on aerial imagery. The high-nutrient site was located along the main channel of the Intracoastal Waterway, whereas the low-nutrient site is tucked inside a shallow cove. While the structure and depth of this cove may have changed in the years since this study was conducted, these differences should have at least been mentioned. Additionally, in the discussion section the authors imply that herbivore responses to nutrient enrichment may hamper the effectiveness of red mangroves as a nutrient uptake tool by pointing to the potential increase in biomass at the high-nutrient site if herbivory had not occurred. Considering the limitations that the authors discussed regarding their herbivory assessment it does not seem appropriate to make this implication. Similarly, the authors make several statements that seem to imply a causal relationship based on correlation. For example, when they discuss the nonsignificant differences in ammonium and phosphate measurements between sites that occurred only when the birds were not present, they seem to attribute this change to the bird guano secreted when the rookery was present. Although a causal relationship may be responsible, it would be wise to use caution in implying it when only correlational data is presented.
Based on review of similar literature from the time period of this paper as well as contemporary knowledge of mangrove systems, this study appears to hold some value for estuarine scientists. The methodological limitations acknowledged by the authors could help provide guidance for researchers looking to perform studies involving mangrove growth or herbivory. Although the authors may have serious weaknesses as far as the relevance of their study to their main objective, their data still shows a relationship between mangrove productivity, the presence of bird colonies, and herbivory rates. As with many outdoor ecological studies, confounding variables render it difficult to draw definite conclusions. In order to address the authors’ motives in a follow-up experiment it would make sense to test mangrove growth across various nutrient wastewater regimes in a controlled greenhouse environment.
Works Cited
E. Odum, William & Mcivor, Carole & Smith III, Thomas J.. (1984). The ecology of the mangroves of South Florida: A community profile.
