HERBIVORY: Ecosystem and Community Effects
Herbivores are taxonomically and ecologically diverse, ranging in size from microscopic zooplankton to large terrestrial vertebrates; they not only consume resources, but are also resources for other consumers. That is, they must be flexible in their behaviour to balance fitness gains from foraging with fitness loss from predation. Fluxes in herbivore populations can therefore have tremendous consequences for community dynamics up and down trophic levels, as well as ecosystem functions such as net primary production and nutrient cycling.
What is "OVERGRAZING"?
When a grazer (a herbivore) undergoes changes in abundance or physiology, it can have a negative effect on the ecosystem and the communities that reside within: a process we refer to as "overgrazing". By giving specific examples from marine and terrestrial overgrazing events, we can explain this concept in detail.
Recommended reading and why it was chosen:
Milchunas and Lauenroth. 1993. Quantitative Effects of Grazing on Vegetation and Soils Over a Global Range of Environments. Ecological Monographs. 63(4): 327-366.
The paper by Milchunas and Lauenroth was chosen because it was deemed to be very influential for the topic being researched: herbivore grazing and its effects on community and ecosystems. The paper has been cited over 837 times in all databases according to Web of Science, and is published in Ecological Monographs which is published by the Ecological Society of America (ESA). The journal is characterized by lengthy papers that document major discoveries or developments in the given field and are later used as a base for future research. This particular paper’s objective was to assess the sensitivity of ecosystems to overgrazing. They compared control (ungrazed) and grazed plots, species composition, and above ground net primary production (ANPP). Species composition refers to the identity of all the living things in an ecosystem. Primary Productivity refers to the ability of the plant to photosynthesize in the presence of carbon dioxide and produce organic compounds; ANPP refers to the rate at which a plant can increase its above ground biomass in an ecosystem.
Conclusions of the study
For above ground net primary production (ANPP), the general trend showed an increase in ANPP with areas that had high precipitation, and this result was observed in both grasslands and shrublands (Milchunas and Lauenroth 1993). For species composition, the general trend showed that more species were dissimilar in grasslands and shrublands than initially expected; also, less species were found to be dominant in grasslands and shrublands in which the effects were more severe for shrublands (Milchunas and Lauenroth 1993).
This particular video summarizes the general concepts behind overgrazing in Yellowstone National Park and how the introduction of Wolves changed the entire ecosystem.
Sea urchins have been known to be sea grass grazers, but before sea urchins became the dominant grazers, more aggressive grazers such as turtles and other larger herbivores possessed that title. In those times, sea grass evolved in such a way that it could withstand aggressive overgrazing events by increasing growth rates, and managing resources more efficiently. The paper on sea urchins chose to define overgrazing as “direct herbivory exceeding absolute growth of a plant” (Eklof et al. 2008).
So what factors affect overgrazing?
The type of factors affecting overgrazing are different for all species but in terms of the herbivore and plant relationship, these general factors may be applied to many situations:
FACTORS AFFECTING HERBIVORES:
- Type of selection (K or R), Predation, Competition, Food availability and dispersion
If a predator of sea urchins is introduced into its grazing environment, it can reduce the population of sea urchins and therefore improve the growth rate of sea grass that is under pressure from overgrazing events.
FACTORS AFFECTING THE PLANTS:
- Growth rate, Ability to relocate nutrients, Resistance
- For plants, maximizing Above Ground Net Primary Production (ANPP) is a priority
So how does overgrazing affect the marine ecosystem?
In Florida Bay a few years ago, they found that 84% of the seagrass was overgrazed! This had several affects on the wildlife and economics of the area.
1. Increased "turbidity" --> water became more hazy and dirty due to soil erosion: this meant that less light would reach the seagrass below, and therefore reduce their overall growth.
2. Population decline in crabs (crustaceans) --> since less sea grass was available, crabs had less food and therefore their population declined --> had severe economic implications for the area!
Herbivore-mediated ecosystem effects
"When we try to pick out anything by itself, we find it hitched to everything else in the Universe." - John Muir
The Green World Hypothesis, introduced by Hariston et al. (1960), was a new way of thinking about the dynamics of ecological systems by combining trophic and population ecology. Thus, in order to consider the effects herbivores may have on ecosystems it is important to take into account the abiotic factors of the system as well as the direct and indirect effects that may occur within a multi-trophic system.
Schmitz (2008) examined herbivore-mediated direct and indirect effects on ecosystem functioning, such as net primary production, decomposition, and nutrient cycling via three herbivore individual-scale properties: 1) constraints associated with resource limitations; 2) resource selection and feeding mode; and 3) herbivore adaptive trade-offs required to balance resource intake and predator avoidance.
Differences in the mechanisms of herbivore resource limitation determines wither a predator effects propagate down the trophic chains to affect plants. Relative resource limitation refers to the ability of the organism to gather and ingest food resources; that is if consumers are limited by relative food shortages, then per capita resource consumption depends on daily feeding times and the rates at which food resources can be harvested. While, absolute resource limitation refers to the total resource availability; that is if consumers are limited by absolute resource supply then per capita consumption depends on the fixed amount of resources available at that location. Consequently, relative resource limitations lead to the emergent capacity of predators to control trophic structure, while absolute resource limitations lead to the decoupling of predator top-down effects.
Chase (1996) experimentally tested predator top-down effects on plant biomass by manipulating a number of trophic levels. The experiment consisted of grasses, generalist grasshopper, and a hunting spider. The experiment looked at how alterations in an abiotic factor, such as amount of light, influenced trophic cascades within the system. Chase (1996) found that the shaded treatments reduced grasshopper daily feeding time by nearly 50% compared to the unshaded treatments. Interestingly, the changes in grasshopper feeding time in the shaded treatment matched predictions of relative resource limitation and presence of the hunting spider increased plant biomass by nearly 50%. However, the unshaded treatment created conditions similar to absolute resource limitation, where the predator had no effect on plant biomass. Neither resource abundance nor nature of predator effect changed between the different treatments, instead, abiotic changes altered how herbivore consumed resources which lead to abrupt emergence of indirect top-down effects by the predator. For more information, read the full article here.
Resource selection and nutrient cycling
Herbivores can be grouped into two broad categories: grazers and sap feeders. Within these feeding modes there are differences in the degree to which herbivores are specialized or generalized in their use of the plant resources.
Direct herbivore effects on nutrient cycling refers to readily decomposable organic matter excreted back into the organic matter pool; this generally results in fast nutrient cycling within the system. While indirect herbivore effects on nutrient cycling occurs when selective foraging alters the plant community composition, and consequently, the chemical composition of dead plant material entering back into the organic matter pool. This is generally characterized by slow nutrient cycling, where herbivores may accelerate or decelerate cycling depending on the nature of their resource choice.
A study by Tracy and Frank (1998) examined the effects grazing elk had on plant communities and nutrient cycling in the Yellowstone region. They found that grazer-free plots had: 35% reduction in plant diversity; 22% lower plant tissue nitrogen content, which translated in a 53% reduction in nitrogen mineralization rates. As a result presence on elk created more diverse plant communities that were rich in nutrient quality and had higher nitrogen mineralization rates. Full paper can be found here.
A study conducted by McInnes et al. (1992) examined the effect browsing moose had on plant communities and nutrient cycles. The result were the opposite of those observed for the elk experiment. Enclosures had 1.6-fold greater tree production; 32% increase in litter nitrogen, which translated to 15-30% higher nitrogen mineralization rates. Full study found here.
The reason for the observed differences these two herbivores have on their habitats has to do with their choice of plant resources. Elk generally feed on grasses that are high in C:N, while moose preferentially feed on deciduous trees that are low in C:N which consequently reduces the abundance of deciduous trees enabling the proliferation of plant species with higher C:N, thereby lowering the amount of nitrogen going back into the system.
Predators can affect herbivore-mediated ecosystems effects in one of two ways: change herbivore foraging behaviour to avoid predation, or directly limiting herbivore density. As a result, predators can have tremendous indirect effects on ecosystem functions via top-down cascades.
An old field experiment conducted by Schmitz (2003), examined the effects predation risk had on a grasshopper. In the absence of predators, the grasshopper preferentially fed on Poa pratensis grass. This allowed a competitive Solidago rugosa to dominate the plant community, resulting in reduced plant diversity. However, in the presence of predation pressures, the grasshopper changed its feeding to the Solidago spp. due to its ability to provide better cover and thereby increase chances of survival. Consequently, the change in feeding selection, resulted in enhanced plant diversity. Full article here.
What are the potential effects of overgrazing on the ecosystem?
Overgrazing has been researched to cause loss in Ecosystem Functions on many different levels. As ecosystems are being transformed into managed systems such as livestock or croplands, these areas end up containing fewer dominant species. This eventually yields to lower levels of biodiversity leading to a loss of the fundamental bases of the ecosystem functions. To elaborate, overgrazing reduces plant and animal richness, alters soil composition and horizon structure, shifts water and nutrients flows as well as affects key ecosystem processes which results in landscape degradation. By declining the species richness in the surrounding ecosystem, we decline the levels of ecosystem functioning. But lets not forget that there is a need of balance within the herbivore-plant interface in order to reach ecosystem sustainability. So there is a need to regulate herbivory in the affected ecosystems in question. by regulating herbivory, we in turn regulate biodiversity and thus monitor and tweak the fundamental ecosystem functions. Another point to question would be on how can we monitor such complex interactions sometimes covering large areas? What are the potential solutions that we could adopt to manage sustainability in ecosystems and biodiversity?
Tracking changes in Ecosystems using Remote Sensing and GIS
Remote Sensing and GIS could be labelled as recently new tools as they have only been around since the advent of the satellite. Remote sensing offers a great deal of options in oder to monitor different aspects of our earth ranging from analyzing vegetation health and composition to analyzing tectonic activity and plate interactions. For the context of our topic, remote sensing is a useful tool in analyzing vegetation cover change in herbivory impacted areas. These satellites are equipped with cameras and sensors with resolution that can go as close as a few cm on the ground. The spatial resolution of the research (Asner et al. 2009) that analysed the effect of herbivory on the ecosystem in Kruger National Park had a spatial resolution of 26cm. Remote sensing offers to see things that our eye can not process such as wavelengh bands that go beyond the visible spectrum. This is useful as many Vegetation indexes such as NDVI uses wavelength in the near infrared.
Remote Sensing tracking ecosystem shifts caused by herbivory in Kruger National Park
As previously discussed, herbivores are both a major agent of disturbance and a core focus for conservation. Too many or too few herbivores can lead to the loss of ecological functioning through alterations in vegetation composition and structure. By intergrading various state of the art technology such as HiFIS (high frequency imaging spectroscopy), LiDAR (Light detection and ranging), vegetation indexes (NDVI) and a high resolution camera, it is possible to map out areas which are currently undergoing change. First it is important to visualize the concept of 'layers'. In GIS, much of environmental analysis and delimiting is through the interaction of layers. For the case study in analyzing herbivory in the Savanna ecosystem at Kruger National Park, a first layer delimiting substrate was computed with ground sampling data. On top of this layer it is possible to locate areas which are prone to herbivory and overgrazing by looking at topography (Plateaus vs Steep Mountains). The next layer could be the vegetation layer mapping areas where vegetation is present. With the addition of a temporal resolution, it possible to see areas that have already gone through shifts in vegetation composition and model other prone areas. The data yields visual results on where herbivory has affected the surrounding ecosystem.
Check out this map that will show you changes that have occurred in the world over the past 30 years. Don't forget to click on "Explore the World" so that you can search the places you wish
Benefits and Consequences of Herbivore Overgrazing
- Soil degradation
- Soil erosion
- Decrease in plant density
- Loss of biodiversity
- Altered water and nutrient flows
- Increased fitness
- Offset loss of biodiversity
- Increased growth rate
- Increased seed production
BUT some factors are both beneficial and detrimental depending on how much or little there is.
Elephant Overgrazing in Africa
Elephant impacts are a powerful tool of habitat change in African savanna ecosystems. Impacts may reduce local biodiversity of plants, species richness, and plant and volume height.
The two papers looked at were The flora of the Addo Elephant National Park, South Africa: are threatened species vulnerable to elephant damage by Johnson, Cowling, and Phillipson and Woodland loss and restoration in a savanna park: a 20-year experiment by Western and Maitumo.
These papers provide examples of consequences and benefits of herbivore overgrazing.
The first paper concludes that there was a decrease in plant density and threat to extinction due to the impacts of elephant overgrazing.
The second paper looks at the benefits and consequences of herbivore overgrazing. It was concluded that overgrazing by large herbivores except elephants was beneficial as it promoted seed growth, but overgrazing by elephants was detrimental as they plucked the whole root of the seedling out therefore resulting in seedling mortality.
* Please consult the following papers for in depth information regarding points made: *
Johnson, C.F., Cowling, R.M., & Phillipson, P.B. (1999). The flora of the Addo Elephant National Park, South Africa: are threatened species vulnerable to elephant damage? Biodiversity and Conservation, 8: 1447-1456.
Western, D. & Maitumo, D. (2004). Woodland loss and restoration in a savanna park: a 20-year experiment. African Journal of Ecology, 42: 111-121.