(2) Biodiversity in Ecosystems Flashcards
Describe the ecosystem services: climatic regulation, hydrological regulation, and pollination.
Climatic regulation. Destruction of tropical forests account for at least one quarter of all anthropogenic carbon emissions. Up to 3 billion tonnes per year. Forests also produce biogenic volatile organic compounds (BVOCs) that reduce net positive radiative forcing (they reduce effectiveness of CO2 and CH4 in increasing temperatures). Combining effected, global deforestation could cause 0.8K warming after 100 years.
Hydrological regulation. In tropics, evapotranspiration maintains atmospheric moisture in air passing over extensive vegetation (rainforests).Air that has passed over extensive vegetation in preceding few days produces at least twice as much rainfall. Amazonian deforestation predicted to reduce precipitation by up to 21% by 2050, due to less efficient moisture recycling.
Pollination. About two thirds of food crops benefit from pollination. Pollination services worth up to £500 million each year in UK. Global annual value: US$200 billion. This is about 10% of total agricultural food value, Recognised in UK by Insect Pollinators Initiative.
What are the 3 possible relationships between increasing species richness and ecosystem function?
Redundancy – initial increase but plateaus out.
Linear
Idiosyncratic – some adverse interactions.
Which of the possible relationships between increasing species richness and ecosystem function does data support?
Plus EXTRA READING by Chisholm?
Data support positive relationship, with some evidence of redundancy. Redundancy has been noted in total plant cover, net primary production, and shoot biomass. Positive relationships have been noted in microbial biomass, and CO2 flux (plant respiration).
EXTRA READING - Research by Chisholm et al. (2012) suggested that consistent (positive) relationships between species richness and ecosystem functioning were observed at small spatial scales for forest plots, but the relationship was less more variable at large scales. It is likely at larger scales that environmental covariates are leading to difference between sites. But further research is need to identify these covariates.
What are some possible mechanisms for the positive relationship between species richness and ecosystem functioning?
Sampling. Species differ in their contributions. A large number of species is more likely to contain one or more species that contributes a lot.
Species complementarity. Species differ in their resource use. More species means resources are used more thoroughly.
Positive interactions. More species leads to more mutualistic interactions.
Cardinale et al. (2007) suggested main effects are sampling and complementarity.
Redundancy suggests spare species in terms of ecosystem functioning. What are some issues/ things to be considered?
Current data are incomplete. Don’t reflect full range of species richness e.g. can’t fully exclude rivet-popping.
We need insurance. A pool of species that can buffer a system against environmental uncertainties.
We need a longer view. Consider not only short-term relationships but also long-term consequences.
Describe 3 examples of trophic cascades and co-extinctions.
Barro Colorado Islands. Small Island in Gatun Lake, Panama: formed with River Chagres dammed to provide water for Panama Canal. At time of isolation, had several large predators – pumas, jaguars, harpy eagles – but these all disappeared soon after. Resulted in 2-10 fold increase in medium-sized predators and omnivores – cotimundis, pacas, agoutis, monkeys, pacaries. By 1970s, 45 species of bird had disappeared, including all ground-nesting species – wood-quail, ground-cuckoos, ant-thrushes. Probably due to insupportable high nest predation by inflated populations of medium-sized predators.
Lago Guri. Set of islands created by hydroelectric impoundment in Caroni Valley, Venezuela. Small islands (<= 12 ha) had virtually no predators of vertebrates. Densities of seed –predators and herbivores (rodents, howler monkeys, iguanas, leaf-cutter ants) 10-100 time greater than on nearby mainland. Densities of seedlings and saplings of canopy severely reduced. This was termed “ecological meltdown”
Alaska. Over-hunting of sea otters Enhydra lutris reduced once complex kelp forests to a two trophic layer system of sea urchins grazing on algae. Relaxation of hunting pressure restored the system in some places but not others. Problem is predation by Orcas: normally prey on much larger seals and sea lions, but they have declined due to decline in fish stocks due to impacts of climate change on primary production. So system is mainly top-down control but with increasing bottom-up effects.
What are the two opposing views, termed May’s Paradox?
Species rich communities are more stable. More complex food webs with more cross-links enables losses to be absorbed with less impact on remaining species. Prevalent view up until 1970s.
Species rich communities are less stable. With more species, each species is less abundant, so more at risk from extinction processes. Consequences then propagate more widely through multiple trophic connection. Prevalent view during 1970s-1900s.
What is the portfolio and evenness effect?
The more stocks and the more even their value (abundance), the smaller is the decrease in overall value or abundance. So higher species richness and evenness may lead to greater stability.
Describe the role of weak and/or intraguild trophic links in stabilising a food web.
Stabilising effect of weak tropic interactions. With just strong interactions populations of all three species oscillate continuously (Lotka-Volterra cycles). In practice, a weak intra-guild trophic link between the two predators stabilises all three populations: if muscles start to decline, gulls switch diet. Mussels recover without decline in gulls and whelks decline less than before.
More species leads to more but weaker trophic links. Removal of predator has cascading effects in one food web (fewer stronger links) but not in the other (more weaker links).
What is possible solution of May’s Paradox? Example?
A possible solution of May’s paradox. Species rich communities may have lower constancy. They may be more likely to lose species over time (e.g. trophic cascades, “relaxation” in species richness following isolation and habitat fragments). But they may have higher resilience. They may be more likely to continue functioning normally despite lower constancy.
Evidence from experimental grasslands. Tilman et al (2006) found plots with higher species richness had not only higher average productivity but also less variable productivity over a period of 10 years. At the same time, species-rich plots varied much more in terms of the relative abundance of species. Stable biomass output was maintained not because of steady populations but despite a more unstable dynamic system when there were more species present.
EXTRA READING.
What is a criticism of Tilman’s experimental grasslands?
Experiments manipulating diversity have been criticised because of their small spatial and time scales. Bai et al (2004) conducted a 24 year study of natural grassland in Mongolia in relation to growing season precipitation. It was found that while the abundance of individual species fluctuated, species within particular functional groups tended to respond differently where a decrease in the abundance of one species resulted in the increase in the abundance of another species in that functional group, hence stabilising the whole productivity of the community in the fluctuating environment. Thus, this show that local species richness confers stability on ecosystem processes in natural communities.
EXTRA READING.
Give an example of a paper regarding ecosystem functioning and species richness in an aquatic ecosystem.
Schindler (1990) investigated species diversity and ecosystem function in aquatic ecosystems. It was shown that acidification of a lake in Canada originally resulted in reduced species diversity. Overtime, the species composition changed but ecosystem function was maintained. This suggests that given sufficient time and appropriate dispersal mechanisms, new species can colonize communities from the regional species pool and compensate for those species that are locally lost (Fischer et al. 2001). This observation emphasizes the importance of maintaining connectivity among natural habitats as they experience environmental changes.
