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Kaley Beins
Professor Fox
Ecology
11/14/12
Data Interpretation 4: Invasions and Extinctions
1) These functional feeding groups are split into 4 different levels. Trophic level one
includes the primary producers such as vascular plants, algae and phytoplankton. Trophic
level two includes herbivores, deposit feeders, detritivores, zooplankton, and
macroplanktivores. Trophic level three contains consumers that feed on one or more
species in the lower trophic levels (and sometimes even other members of trophic level
three). Finally, trophic level four contains predators (that eat consumers or omnivores)
and parasites.
The researchers assigned species to these functional groups using other literature
published on the subject and by observing their feeding habits such as primary food
source and modes of feeding (for example, cnidarians were considered macroplanktivores
as a result of their feeding biology). If neither previous literature nor feeding biology could
be used in species classification the researchers employed the knowledge they had of the
trophic level of the lowest taxonomic group to which the species could belong. They also
included a few species in more than one category to further inform their functional group
analyses. Although they acknowledge potential discrepancies in their categorization (with
particular regard to benthic and pelagic planktivores), they maintain that any changes
made would not significantly alter their results.
2) The authors estimate the rate of species introductions and extinctions by comparing
lists of marine species and how both introductions and extinctions affected the trophic
levels. In order to determine species introductions the authors examined lists for the San
Francisco Bay (n = 166 species), Australia (n = 153 species), the Wadden Sea (n = 34), and
the Gulf of the Farallones National Marine Sanctuary in Northern California (n = 141
species). In order to determine species extinctions the authors examined the list of
documented local and global marine extinctions from Dulvy (n = 33).
The researchers could look at lists such as these and subtract invasive species and add
extinct species in order to calculate an original (baseline) list for each region. Then they
could compare this list to the current state of these regions in order to examine which
trophic levels were more affected by invasion and extinction. Specifically, the authors
looked to the previous Wadden Sea functional group abundance and compared it to their
baseline or “predisturbance” list of species (n = 716), which, as previously explained, was
created by subtracting invasive species and adding regionally extinct species.
3) Only one trophic level (secondary consumers or trophic level 3) separates herbivores
(tropic level 2) from predators (trophic level 3).
4) We expect extinctions to occur with greater frequency at trophic levels 3 and 4 because
extinctions are more likely at higher trophic levels. This pattern may result from
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anthropogenic effects such as overfishing, hunting, and climate change. Fishermen and
hunters tend to kill the largest organisms, which in turn tend to be the predators (level 4)
and secondary consumers (level 3). Therefore, these human activities apply selective
pressure to the third and fourth trophic levels, which could, if the fishing and hunting
occur often enough, lead to extinctions in these levels.
Likewise, although climate change affects all organisms and their environments, it is likely
to affect the third and fourth trophic levels more intensely because not only are these
predators and consumers affected by climate change, their food sources are also.
Therefore, following a similar principle as the biomagnification of chemicals in higher
trophic levels, higher trophic levels will be more greatly affected by global climate change
because each level before them will have been affected. This would likely result in limited
resources for the higher trophic levels and therefore, greater extinction. Additionally,
according to the 10% Rule, higher trophic levels need to consume 10x more organisms
than herbivores in order to gain the same amount of energy because the energy efficiency
of the food source decreases as trophic level increases (for example, say eating ten
caterpillars gives a bird as much energy as eating one leaf gave the catepillars).
Consumption of more organisms will likely not be possible if the organisms from lower
trophic levels are affected by climate change.
5) Overall, according to Figure 2, trophic level 3 suffers the greatest extinction rate
(consumer extinction rate = 37.6% and omnivorous consumer extinction rate = 8.3%),
closely followed by trophic level 4 (predator extinction rate = 24.1%).
Together, the top two trophic levels, 3 and 4 (secondary consumers and predators)
suffered the largest percentage of extinctions; 70% of the 133 recorded instances of global
and regional marine species extinction occurred in these top two levels.
6) Overall, trophic level 2 was most commonly introduced; about 70% of non-endemic
species were suspension feeders, deposit feeders, or detrivores. According to Figure 2, the
largest invasion in each area was the invasion of macroplanktivores (Australia = 44.4%,
The Gulf of the Farallones = 49.7%, San Francisco Bay = 39.6%), which are part of trophic
level 2.
7) Historically the most common trophic level in the Wadden Sea is trophic level 1
(according to Figure 4A). Most extinctions occurred in the third level (Figure 4B) while
most introductions occurred in trophic level two (Figure 4C). Therefore, although the
Wadden Sea underwent changes in taxonomic abundance and distribution, these changes
ultimately reflected the conclusions of Brynes et al.
8) Based on this study one would expect trophic level 1 to increase due to introduction of
new species. Trophic level 2 will likely increase more than trophic level 1, as
macroplanktivores (a second tiered organism) are some of the most common invasive
species. Trophic levels 3 and 4 will decrease due to extinction. However, trophic level 3
may in fact decrease at a slower rate than trophic level 4 because level 3 organisms feed
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on levels 1 and 2 organisms (increasing in number), while level 4 organisms feed on level
3 organisms (decreasing in number). Also, because the higher trophic levels are
decreasing, the lower trophic levels will likely increase in number due to decreased
predation in addition to invasion.
These results are consistent with the current and projected results shown in Figure 4E.
Level 4 is -14.0% and -65.1% respectively. Level 3 is -5.4% and -24.6%. Level 2 is +8.6%
and +50.0%. Finally, level 1 is -0.1% and -3.3%.