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!

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Can nature compensate for human impacts?

Algal turfs dominating under acidified conditions at cold-water (temperate) CO2 seeps, which we use at "natural experiments". You can just see the fronds of a solitary kelp plant in the right of the photo, otherwise they are rare at the site (when they should be 8 - 10 plants per metre!).

Algal turfs dominating under acidified conditions at cold-water (temperate) CO2 vents, which we use at “natural experiments” to try and understand the effects of carbon emissions on our oceans. You can just see the fronds of a solitary kelp plant in the right of the photo, otherwise they are rare at the site (when they should be 8 – 10 plants per metre!). This is a system that has been pushed past its ability to resist or compensate for human activities.

One thing that humans are really good at is having an impact on the environment through their activities. The problem is that we generally don’t realise that we’re having an impact until something changes in a drastic way. We talk about things called phase-shifts, where the environment changes from one “phase” to another. Good (and unfortunately common) examples are the loss of kelp forests for bare reef, seagrass meadows for bare sand, or coral reefs for algal habitats. In all of these cases, the environment has been degraded to the point where it no longer functions as it should, meaning that biodiversity and productivity are massively reduced.

There are two questions to ask here, (1) why don’t we see these phase-shifts coming, and (2) does nature have any resistance to them? A new paper by one of my PhD students, Giulia Ghedini, shows that nature may actually try to resist human-caused stressors (such as increased nutrient pollution, ocean acidification, warming) by increasing the strength of compensation. In this case, Giulia found that the compounding effects of multiple disturbances increasingly promoted the expansion of weedy algal turfs (which replace kelp forests), but that this response was countered by a proportional increase in grazing of those same turfs by gastropods. This is a natural compensatory mechanism, but it has limits.

What does this mean for our understanding of phase-shifts? First, it means that nature is stronger at resisting than we realised. BUT, because it is extremely difficult to either see or quantify this resistance we generally don’t realise it is happening…. until it stops. Then, once we push the systems past their ability to compensate for the increased pressure we place on them we see a sudden shift. It’s like watching a duck on a river – it may look extremely calm on the surface, seemingly stationary, but underneath it is paddling extremely hard. At some point the current strengthens too much and it can’t paddle harder and so, seemingly suddenly, the duck begins to float down the river.

Unfortunately, when put together, this means that more systems may be more stressed than we realise, and the only way to stop detrimental phase-shifts is to take the conservative approach and start to reduce our impacts on these systems. For example, we know that nutrient pollution, carbon emissions, overfishing and many other activities have damaged marine ecosystems, why not begin to reduce our impacts before we add more systems to the list of those we didn’t realise were at breaking point?

Mediation of global change by local biotic and abiotic interactions

Dr Laura FalkenbergThis post is basically a short synopsis of the work done by one of my (now ex-) Ph.D. students, Dr Laura Falkenberg. Laura’s work has turned much of what we thought we knew about the effect of increased CO2 and nutrients on its head; we found synergies where we didn’t expect them (reviewed in a book chapter) and system resilience and resistance to change beyond what we hoped (via strong competitive interaction and trophic links; published in Oecologia, PLoS One and Marine Ecology Progress Series). Laura has certainly helped us look at things in new ways and given us hope that in marine systems where synergies between stressors exist that management of local conditions could potentially buy us some time in mitigating climate change (e.g. reducing nutrient flows into the marine environment, in Journal of Applied Ecology).

Ph.D. thesis: Mediation of global change by local biotic and abiotic interactions
by Dr Laura Falkenberg.

Throughout my Ph.D., I assessed the conceptual model that while cross-scale abiotic stressors can combine to synergistically favour shifts in marine habitats from kelp forests to mats of turfing algae, management of local conditions can counter this change. My experimental manipulations found broad support for the hypotheses that; 1) cross-scale factors (i.e. local and global) can have interactive effects which increase the probability of expansion of turfs but not kelp and, 2) management of local conditions (e.g. maintaining intact forests, limiting nutrient enrichment) can dampen the effects of global change (e.g. forecasted carbon dioxide). I published the results from my thesis in four papers. In the first, I showed that experimental enrichment of CO2 and nutrients influence the biomass accumulation of turf and kelp differently, with turf responding positively to enrichment of both resources while kelp responded to enrichment of nutrients but not CO2. Given that such direct responses could be mediated by interactions with other taxa, in the second paper I considered a key competitive interaction and revealed that the presence of kelp can inhibit the synergistic positive effect of resource enrichment (i.e. CO2 and nutrients) on their turf competitors. Similarly, in the third paper I highlighted the importance of herbivory by showing that under enriched CO2 conditions rates of this process were increased to counter the expansion of turfs. Finally, in the fourth paper, I considered a scenario in which these biotic controls were absent and identified that where multiple resources had been enriched and prompted a synergistic response (i.e. the expansion of turf where CO2 and nutrients are modified), subsequent reduction of the locally-determined factor alone (i.e. nutrients) substantially slowed further expansion of turf algae, but that the legacy of nutrient enrichment was not entirely eradicated. Together, these results represent progress in ecological tests of hypotheses regarding global climate change as they incorporate comprehensive sets of abiotic and biotic community drivers.

You can access all of Laura’s publications from the University of Adelaide’s digital library, or email her for a copy.