Freshwater Ecosystems: River Systems and Lakes

     Freshwater is scarce. While oceans cover approximately 70% of Earth's surface, only about 0.65% of all freshwater in found on the continents. The majority of this continental freshwater (0.62%) is groundwater, with only about 0.0001% located in streams and 0.017% located in lakes (Sand-Jensen, 2013). As freshwater is such a rare and crucial commodity, it is critical that we have a thorough understanding of its ecological processes.

     Higher trophic levels tend to be uncommon within freshwater ecosystems and predominantly consist of herbivorous zooplankton and benthic invertebrates (invertebrates that live on/in the bottom substrate). The next trophic level generally consists of fauna in the pelagic zone, such as predatory zooplankton and small fishes, followed by medium-sized piscivorous fishes and finally, large predatory piscivorous fishes (Wetzel, 2001).

Figure 1: Trophic levels in a typical freshwater river (image obtained via http://pie-lter.ecosystems.mbl.edu/)

     An interesting example of an indicator species being used to determine ecosystem trophic condition is a study that was conducted by Ristau and Traunsperger in 2011. This study focused on the relationship of nematode communities to the trophic state of southern Swedish lakes. Trophic level was found to strongly influence nematode species richness, as oligotrophic and mesotrophic lakes supported the greatest species numbers (Ristau et al. 2011).

     The introduction of Nile perch in Lake Victoria is a classic example of an invasive species altering nutrient cycles by shifting the relative proportion of each trophic level. Nile perch have been linked to dramatic food web alterations and major limnological changes throughout Lake Victoria (Hall et al. 2000).

Figure 2: The influence of Nile perch on the food web in Lake Victoria (Ligtvoet and Witte, 1991).

     Lake Victoria is Earth's largest tropical lake and the birthplace of almost 400 native fish species, the majority of which are endemic haplochromine cichlid species (Coucherousset et al. 2011). Nile perch were first introduced to the lake in the early 1960s, in hopes of boosting the local fishery economy. As Nile perch are very efficient predators, they quickly increased in number as they began to consume the smaller native fish species. Conservative estimates hold that approximately 60% of the endemic fish species have become extinct (Witte et al. 1992).

Animal Planet's Jeremy Wade posing with one of his river monsters: a Nile Perch (http://www.animalplanet.com/tv-shows/river-monsters/photos/killer-fish-nile-perch-pictures.htm).

References Cited:

Coucherousset, J. and J. D. Olden. 2011. Ecological impacts of nonnative freshwater fishes. Fisheries 36:215-230.

 Hall, S. R. and E.L. Mills. 2000. Exotic species in large lakes of the world. Aquatic Ecosystem Health & Management 3:105-135.

Ristau, K. and W. Traunsperger. 2011. Relation between nematode communities and trophic state in southern Swedish lakes. Hydrobiologia 663:121-133.

Sand-Jensen, K.. 2013. Freshwater ecosystems, human impact on. Encyclopedia of Biodiversity 3:570-586.

Wetzel, R. G.. 2001. Freshwater ecosystems. Encyclopedia of Biodiversity 3:560-569.

Witte, F., T. Goldschmidt, P. C. Goudswaard, W. Ligtvoet, M. J. P. van Oijen, and J. H. Wanink. 1992. Species extinction and concomitant ecological changes in Lake Victoria. Netherlands Journal of Zoology 42:214-232.

Food Web and Trophic Dynamics Overview

 Figure 1: A typical freshwater aquatic and terrestrial food web (Thompsma, 2010. Wikimedia Commons).

     While most of us have heard the term “food web”, many people are not fully aware of how food webs and trophic dynamics affect ecological functioning on an ecosystems scale. This blog will aim to provide a general overview of how trophic processes relate to ecosystem functionality with a primary focus being on invasive species, indicator species, and some of the effects that human impacts may cause. These topics will be examined over a variety of different ecosystem types, ranging from tropical forests to polar regions, with noteworthy (and hopefully interesting) examples provided for each. 

  

     Trophic dynamics govern the movement of carbon, energy, and nutrients among organisms in an ecosystem (Chapin, et al. 2002). Food webs are formed by all of the combined energy transfers for a given ecosystem and the alteration or removal of a single species can result in a cascading disruption of the whole system. Energy flow through an ecosystem can be controlled by both bottom-up and top-down controls. Bottom-up controls can limit primary production and food/nutrient availability at the lowest end of a food web, while top-down controls can also drastically alter food web dynamics from the highest end of the food web. The disruption or removal of a key player in a food web can lead to a trophic cascade, which can cause inverse abundance or biomass patterns across more than one trophic link in a food web (Pace, 2013). This may occur when predators reduce certain prey numbers, leading to other species, lower in the food web, increasing in population size. Alternatively, when top predators are removed from an ecosystem, prey numbers can increase down the line. This can lead to many adverse effects such as overgrazing and other stresses that may negatively impact trophic dynamics on a whole ecosystem scale.

Figure 2:

Effect of food chain length on primary producer biomass in situations where trophic cascades operate. Plant biomass is abundant where there are odd numbers of trophic levels (1, 3, 5, etc.) because these have a low biomass of herbivores; plant biomass is reduced where there are even numbers of trophic levels (2, 4, 6, etc.) because these have a large biomass of herbivores (Chapin, et al., 2002).

     Trophic cascades may lead to impacts both within and across ecosystems. An example of trophic cascades causing impact across ecosystem boundaries can be illustrated by examining the relationship between fish and terrestrial plant production. Fish can indirectly facilitate terrestrial plant production by reducing larval dragonfly numbers. This leads to a decrease in adult dragonflies which feed on insect pollinators, therefore indirectly increasing terrestrial plant pollination near ponds containing fish (Knight, et al. 2005).

Figure 3: Interaction web showing the pathway by which fish facilitate plant reproduction. (White and Stierwalt, 2005).

Figure 4: Food web diagram contrasting energy flow pathways to consumers under (a) historical, fishless conditions and (b) as influenced by introduced trout. The size of arrows indicates the relative strength of trophic connections between food web components. Darker arrows indicate important changes in the flow of energy to fish in lakes where trout have been introduced compared to fishless lakes (Finlay et al. 2007).

     

     A keystone species can be defined as consumers that have a disproportionately large effect, relative to their abundance, on communities and ecosystems (Menge, et al. 2013). Keystone species can range from tiny pollinators to apex predators and the removal of these critical species can disrupt food webs and effect entire ecosystem trophic dynamics. An indicator species can be defined as a species or group of species that reflect the biotic or abiotic state of an environment, reveal evidence for and impacts of environmental change, or indicate diversity of other species, taxa, or entire communities of a given area (Lawton et al. 2001). Amphibians are ideal as an indicator species, as their permeable skins are highly susceptible to environmental contaminants.
   
     Invasive species versus introduced species: what’s the difference? An introduced species can be defined as any species living outside of its native area by way of human impact. These introductions can either be deliberate (e.g. kudzu) or accidental (e.g. brown marmorated stink bug). An invasive species is also an introduced species, but the criteria for a species being classified as invasive also includes the tendency to invade and spread, leading towards ecological and/or environmental detriment.There are numerous examples of food web disruption by invasive species, several of which will be examined in later posts.

References Cited:

Chapin III, F. S., P. A. Matson, and H. A. Mooney. 2002. Principles of terrestrial ecosystem ecology. 297-320.

Finlay, J. C., and V. T. Vredenburg. 2007. Introduced trout sever trophic connections in watersheds: consequences for a declining amphibian. Ecology 88:2187-2198.

 Knight, T. M., M. W. McCoy, J. M. Chase, K. A. McCoy, and R. D. Holt. 2005. Trophic cascades across ecosystems. Nature 437:880-883.

Lawton, J. H., and K. J. Gaston. 2001. Indicator species. Encyclopedia of Biodiversity 4:253-263. 

Menge, B. A., A. C. Iles, and T. L. Freidenburg. 2013. Keystone species. Encyclopedia of Biodiversity 4:442-457.

Pace, M. L. 2013. Trophic cascades. Encyclopedia of Biodiversity 7:258-263.