Species diversity loss is expected to alter ecosystem function, but previous work has demonstrated inconsistent relationships between these two factors. Productivity is the most common measure of ecosystem function, but given the difficulty in measuring productivity, standing biomass or change in biomass are frequently used as proxy measures. A review of the recent ecosystem-function literature revealed that 93% of studies measure productivity as biomass, thereby assuming a strong positive relationship between these two variables. We tested this assumption by measuring biomass and productivity in seagrass beds in the Gulf of Mexico. We found that the relationship between standing biomass and productivity could be positive or negative, depending on site. Change in biomass over months inconsistently underestimated short-term productivity. The relationship between biomass and productivity may depend on plant age, successional stage, or site-specific rates of tissue loss to herbivory, senescence, or disturbance. Our results suggest that if biomass continues to be used as a measure of productivity without justification, highly productive communities that typically show little change in biomass, such as healthy climax communities, will not be interpreted as such. The conflicting results of previous studies investigating the relationship between diversity and productivity may be due to differences in the inherently variable relationship between biomass and productivity at different sites and scales.
To clarify the relationship between the detritivore fiddler crab, Uca thayeri, and sediment bacteria, we quantified morphotype richness, abundance and evenness of these microorganisms inside the crabs’ guts and in mangrove-associated sediments in Sisal, México (21° 9′ N, 90° 1′ W) from July to September 2008. Increased bacterial richness and abundance were observed in mesocosm experiments when nutrients were added to the sediment or in the absence of fiddler crabs. Thus, crab disturbance seems to play a role in shaping the bacterial assemblage by reducing richness and abundance just as nutrient limitation does. Crabs can also play a second role by harboring a subset of bacterial morphotypes inside their gut. We exposed sterile sediment to fiddler crabs and found that viable cells were expelled from the crab’s gut and proliferated in previously sterile substratum. The bacterial community is thus structured by the foraging behavior of fiddler crabs since it benefits some bacteria and restricts others. By agar plating we have obtained conservative results, yet the data suggest that the crab influences the bacterial assemblage in two ways by allowing inoculation of the sand from the gut and reducing bacteria diversity through disturbance when foraging on sediments.
Authors:H.L. Buckley, J.H. Burns, J.M. Kneitel, E.L. Walters, P. Munguia, and E. Miller
We examined the environmental factors associated with community structure in the inquiline communities of the purple pitcher plant (Sarracenia purpurea L.). We sampled all 141 communities in a 10- x 20-m grid and recorded their spatial relationships to determine the relative influence of environmental and spatial factors on community structure. Environmental and spatial factors contributed equally to the variance in community composition (species identity and abundance) among pitchers. The species richness of communities was influenced by both spatial and environmental variables, particularly environmental variables related to community size. In addition, our study suggests a number of hypotheses about factors influencing community structure (e.g. predation) that could be tested experimentally.