Herbivores compensate for nutrient pollution

Photo courtesy of the NOAA photo library (www.photolib.noaa.gov) Photographer: Paige Gill.

Photo courtesy of the NOAA photo library (www.photolib.noaa.gov)
Photographer: Paige Gill.

This post is a guest blog by one of my (now ex-) Ph.D. students, Dr Chloe McSkimming. Chloe has been working on what drives decline in seagrass systems and how we may be able to help stop the decline. The work described below is from one of her recent papers in the Journal of Experimental Marine Biology and Ecology.

Human activities continue to challenge the capability of ecosystems to absorb disturbances, yet many systems that face substantial human pressure remain stable, resisting change. Over the past several decades, ecologists have extensively studied resilience – the ability of an ecosystem to bounce-back – but we really don’t understand resistance, or the ability to not change. There is recent evidence that in marine systems this resistance may be in part due to biological compensatory mechanisms – basically processes that can counter disturbances which cause change. Therefore, understanding how these mechanisms allow systems to resist change to a degraded state is essential for improving our current knowledge of habitat stability.

In coastal systems, change in resource availability, particularly the increase in available nutrients via land-based runoff, favours the growth of weedy species which can displace highly productive, slower-growing species. In seagrass systems, nutrient inputs increase the growth of algae that overgrow seagrass, significantly reducing the amount of light available to the seagrass themselves, and ultimately leading to seagrass death. Small herbivores are important consumers which may provide a compensatory mechanism in these systems by eating these fast-growing algae, potentially increasing the survival of seagrass under nutrient pollution (also demonstrated for other benthic systems by myself and one of my colleagues Laura Falkenberg).

In this present study, we increased nutrient concentrations and altered herbivore abundance in a seagrass meadow to test whether this compensation does exist; that is, does nutrient pollution stimulate herbivores to increase feeding to counter the increased growth of algae?

As expected, nutrients increased the growth of algae so that they started to smother the seagrass. However, this effect was only present when herbivore abundance was reduced. When the herbivores were present, however, they reduced the effects of nutrient pollution by reducing the amount of algae on seagrass leaves. Interestingly, the abundance of herbivores did not increase, meaning that the increased consumption of algae was due to an increase in how much individuals were eating.

We still have a long way to go to understand compensatory effects in ecosystems. However, these results suggest that in some situations natural populations of herbivores may help to reduce the effects of nutrient pollution in seagrass systems by consuming the additional growth of weedy species. Herbivores are therefore an important component in the management of nutrient addition in coastal systems. BUT, it is also essential to remember that there is still a global decline of seagrasses driven by nutrient pollution, meaning that such compensatory mechanisms do have limits and ultimately the only way to stop the negative effects of nutrient pollution is to stop nutrient pollution!