Environmental Engineering Reference
In-Depth Information
In stark contrast are the small 'opportunists' (or
r
-selected species), such as clupeids
(e.g. sard ines) and smelts (e.g.
Thaleichthys pacifi cus
). And inter mediate between the
K
- and
r
-species are a group of medium-sized fi sh, including the rockfi shes and
mudfi shes, that are long-lived and slow growing, but with high fecundity. Some
general management principles could be developed on the basis of the properties of
these groupings, for use even in new fi sheries where the details of species' ecologies
are unknown. (I deal with harvest management in Chapter 7.) Certainly, species at
the
K
end of the continuum can clearly only withstand modest harvest levels.
Other human impacts may also be felt by different species in different ways
because of contrasts in their life-history traits. Prinzing et al. (2002), for example,
exposed forest soil to a single application of an insecticide used to control gypsy
moth (
Lymantr ia dispar
) and other forest pests. Difl ubenzuron interferes with chitin
synthesis, disturbing the maturation and reproduction of insects, crustaceans, and
also the soil mites that are the focus of Prinzing's study. The mite taxa that are least
affected by the disturbance are those with shorter generation times and therefore
able to recover more quickly. Cultivation activities also have impacts in freshwater
bodies adjacent to the land. Doledec et al. (2006) studied the relationships between
species traits of stream invertebrates and intensity of agricultural development in
the stream's catchment area. Traits associated with population resilience, including
short generation time (many generations per year, known as plurivoltine) and
hermaphroditic reproduction, become more prevalent with more intense land use
(Figure 3.8), refl ecting increases in the intensity and frequency of disturbances to
stream chemistry. This pattern mirrors that of the forest mites. There is also a shift
away from laying unattached eggs at the water surface and a decrease in gill respira-
tion, refl ecting the increasing likelihood of smothering by sediment introduced to
the stream as a result of grazing animals disturbing the soil and banks.
Plurivoltine
Short lives
Hermaphroditism
Surface eggs
Gill
respiration
40
40
35
60
20
60
30
15
30
50
20
40
10
25
40
20
10
5
20
15
30
0
0
Fig. 3.8
Relationships between selected species traits of stream invertebrates and agricultural intensity in the stream's
catchment area. There were four levels of land-use intensity: none, ungrazed native tussock grassland; low, grazed tussock
grassland; medium, extensively grazed pasture; high, dairy or deer farming. Key: plurivoltine, % individuals in the
community with more than one generation per year; life duration, % individuals with short lives (10-30 days); hermaphro-
ditism, % individuals reproducing hermaphroditically (one individual possessing both sexes); surface, % individuals laying
unattached eggs at the stream surface; gill respiration, % individuals with external gills. Boxes show the 25%, 50% and 75%
distribution values. (From Doledec et al., 2006).
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