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AN
INTRODUCTION
TO
TERRESTRIAL
ECOLOGY
by D. A. E n g l i s h and R. H. Ward.
During the l a s t f i f t y years ecology has advanced from a merely
observational study, i n t o a science that incorporates a l l that i s basic
to b i o l o g y .
The e x t e n t of ecology becomes apparent when a s i m p l e
d e f i n i t i o n i s considered: Ecology is the study of animals and plants
in their natural habitats,
and the investigation
of their
inter-relationships, and their dependence on their
environment.
In o r d e r t o study any organism i n i t s n a t u r a l h a b i t a t i t must be
c o r r e c t l y i d e n t i f i e d , which i n v o l v e s a knowledge of taxonomy.
The
dependence on the environment i s r e f l e c t e d i n the organism's e x t e r n a l
and i n t e r n a l s p e c i a l i s a t i o n .
This can o n l y be assessed when one i s
f a m i l i a r w i t h morphology and anatomy.
Furthermore t o understand the
processes u n d e r l y i n g the s p e c i a l i s a t i o n brought about by environmental
and h e r e d i t a r y f a c t o r s a groundwork i n physiology
and genetics i s
necessary.
The i n t e r - r e l a t i o n s h i p s of organisms comprise one of the
most e x c i t i n g f i e l d s of e c o l o g i c a l s t u d y .
As a r e s u l t o f s p e c i a l i s a t i o n
i n zoology and botany the importance of these i n t e r - r e l a t i o n s h i p s i s not
realised.
These i n t e r - r e l a t i o n s h i p s are used i n the f i e l d of a p p l i e d
e c o l o g y , which i s becoming an ever more important s c i e n c e .
The prime
example of t h i s i s b i o l o g i c a l c o n t r o l .
Moreover the dependence of
organisms on t h e i r environment i n v o l v e s both p h y s i c a l and b i o t i c f a c t o r s
of the environment.
With the r e a l i s a t i o n of the above f a c t s , ecology becomes more than
c a s u a l o b s e r v a t i o n and p l a y s an ever i n c r e a s i n g r o l e i n the b i o l o g i c a l
sciences.
SOME
ECOLOGICAL
CONCEPTS
Ecology b e i n g a s c i e n c e must base i t s study around a framework of
concepts.
The concepts of environment, h a b i t a t and ecosystem can be
reg rded as b a s i c a l l y i m p o r t a n t .
The
environment
The environment is a continuously
changing complex of factors
which are interacting
directly
and
indirectly
with the organism.
These environmental f a c t o r s can be d e s c r i b e d as those t h a t a f f e c t the
f e r t i l i t y , l o n g e v i t y , growth and s p a t i a l p o s i t i o n of an organism.
Environmental f a c t o r s may be grouped as f o l l o w s (Browning 1963): 1.
2.
3.
4.
5.
Climate: - e. g., L i g h t , h u m i d i t y , temperature, wind, r a i n , e t c .
Resources: - e. g., M i n e r a l s , w a t e r , f o o d , s u b s t r a t e , gases, s h e l t e r ,
etc.
Members of the same species: - C o m p e t i t i o n , g r e g a r i o u s n e s s , e t c .
Members of other species: - e. g., P r e d a t o r s , p a r a s i t e s , c o m p e t i t i o n ,
symbiosis, e t c .
Hazards: - e. g., F l o o d s , f i r e , e r u p t i o n s , e t c
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A l l e n v i r o n m e n t a l f a c t o r s , however, do not n e c e s s a r i l y comprise the
environment of an organism as o n l y some of them e n t e r i n t o the r e a c t i o n
system.
Environmental f a c t o r s i n d i r e c t c o n t a c t w i t h t h e organism are
known as d i r e c t f a c t o r s , some of w h i c h , termed l i m i t i n g f a c t o r s , r e s t r i c t
growth and d i s t r i b u t i o n of an organism.
L i m i t i n g f a c t o r s o p e r a t e by
exceeding the t o l e r a n c e of the organism t o t h a t f a c t o r .
P r o b a b l y the
b e s t example of a l i m i t i n g f a c t o r i s water.
When t h e r e i s i n s u f f i c i e n t
w a t e r , p l a n t s and animals t h a t cannot t o l e r a t e droughts may d i e , w h i l e i f
t h e r e i s too much w a t e r , organisms may p e r i s h due t o w a t e r l o g g i n g o r
drowning.
F a c t o r s t h a t are not i n d i r e c t c o n t a c t w i t h the organism, and
a r e thus no p a r t of the d i r e c t environment, may a f f e c t those t h a t a r e ,
e. g., f a c t o r s c a u s i n g s h a d i n g .
These f a c t o r s are known as m o d i f y i n g
f a c t o r s , and modify the environment.
Even i f one can d i v i d e the environment i n t o a number o f c a t e g o r i e s ,
i t i s . extremely important t o remember t h a t the environment i s a v a s t
complex of f a c t o r s which do not a c t s e p a r a t e l y but as a whole.
The H a b i t a t
direct
The habitat i s the space occupied by an organism
environmental
factors
at any particular
time.
containing
the
The term habitat may be employed i n a number of d i f f e r e n t ways. I t
can be a p p l i e d w i t h r e s p e c t t o a s i n g l e organism o r a group of t h e same
species i n a p a r t i c u l a r l o c a t i o n .
T h i s i s sometimes c a l l e d the m i c r o habitat.
A S p e c i e s may occupy many d i f f e r i n g m i c r o h a b i t a t s depending
on i t s t o l e r a n c e t o l i m i t i n g f a c t o r s .
E. g., the p l a n t , Hebe
striata
o r koromiko, which has a wide range of t o l e r a n c e t o l i m i t i n g f a c t o r s ,
can be found i n a v a r i e d range o f m i c r o h a b i t a t s i n such w i d e l y d i f f e r i n g
l o c a l i t i e s as t u s s o c k g r a s s l a n d s and c o a s t a l f o r e s t .
The combination of the t o t a l range of m i c r o h a b i t a t s c o n s t i t u t e s
the m a c r o h a b i t a t f o r the organism.
Hence koromiko has o n l y one macroh a b i t a t that i n c o r p o r a t e s a l l the c h a r a c t e r i s t i c s of i t s d i f f e r i n g m i c r o habitats.
The term habitat may a l s o be a p p l i e d t o a community (an a g g r e g a t i o n
of organisms c h a r a c t e r i s e d by i t s dominant s p e c i e s ) .
A community of
organisms, may be spoken of as l i v i n g i n a l e a f l i t t e r , s o i l o r even
forest habitat.
The Ecosystem
The Ecosystem is the functioning
together of the community and the
environment and forms the basic unit of ecology.
It is the sum total
of
all the interactions
between the environmental
factors,
animals and
plants.
For example a s i m p l e ecosystem may be a r o t t i n g l o g o r , more
complex, one r e g i o n of f o r e s t .
One person's study i s u s u a l l y l i m i t e d t o i s o l a t e d p a r t s of the
ecosystem, but the system as a whole may o n l y be understood w i t h the
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3
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s y n t h e s i s of many such s t u d i e s .
The concept of the ecosystem i s a broad
one and s h o u l d s e r v e t o emphasise the i n t r i c a t e r e l a t i o n s h i p s and i n t e r dependences t h a t e x i s t w i t h i n i t .
THE
ACTION
OF
SOME
ENVIRONMENTAL
FACTORS
Environmental f a c t o r s may be grouped under s e p a r a t e headings f o r
convenience (see above), but such a c l a s s i f i c a t i o n has o n l y l i m i t e d u s e f u l ness.
To assess the a c t i o n and importance of these f a c t o r s w i t h r e s p e c t
to the organism each f a c t o r must f i r s t be s t u d i e d s i n g l y .
This i s a
necessary stage on the way t o understanding the c o m p l e x i t i e s of the e n v i r o n ment.
A number of examples have been s e l e c t e d t o show the e f f e c t and mode
of a c t i o n of some of these f a c t o r s .
I t must be r e a l i s e d t h a t these separate e f f e c t s are o n l y p a r t of a f a r g r e a t e r and more complex system.
Of the c l i m a t i c f a c t o r s light i s a v i t a l and l i m i t i n g one, f o r
w i t h o u t t h i s form of energy p l a n t s , the primary p r o d u c e r s , would not be
a b l e t o e x i s t and w i t h o u t these no o t h e r form of l i f e c o u l d s u r v i v e .
In
the t e r r e s t r i a l environment intensity
and duration of l i g h t can be i m p o r t ant l i m i t i n g f a c t o r s , but t h i s i s not n e c e s s a r i l y so f o r the q u a l i t y (wave
length).
At h i g h and low i n t e n s i t i e s l i g h t can r e s t r i c t the r a t e of
p h o t o s y n t h e s i s l i m i t i n g growth and d i s t r i b u t i o n of p l a n t s and hence the
d i s t r i b u t i o n of a n i m a l s .
The d u r a t i o n of l i g h t or day length i s now
r e a l i s e d t o be a major f a c t o r c o n t r o l l i n g the rhythms of organisms.
For
example i t has been shown t h a t the r e p r o d u c t i v e c y c l e s of both animals
and p l a n t s are c o n t r o l l e d by day length.
Temperature,
an e a s i l y measured f a c t o r , can l i m i t growth and d i s t r i b u t i o n of organisms and cause t h e i r z o n a t i o n and s t r a t i f i c a t i o n .
For example
f r o s t tender s p e c i e s , (such as the New Zealand mangrove which w i l l not
grow i n areas which have 4 degrees of f r o s t ) are c o n f i n e d t o areas above
t h e i r lower temperature l i m i t s .
Precipitation
a f f e c t s the environment i n two ways.
Firstly i t
det
mines the humidity and thus the e v a p o r a t i n g power of the a i r .
The
l a t t e r a f f e c t s the t r a n s p i r a t i o n of p l a n t s and d e h y d r a t i o n i n a n i m a l s .
Secondly i t i s a source of available
water (a r e s o u r c e ) , which i s a b a s i c
necessity for l i f e .
Water on the s u r f a c e can be imbibed by animals w h i l e
p l a n t s u t i l i s e s o i l water which a l s o d i s s o l v e s and t r a n s p o r t s s o l u t e s .
The i n t e r a c t i o n of temperature and water is- a common one and shows
how the e f f e c t o f one f a c t o r may i n f l u e n c e another.
A v a i l a b l e water
becomes s c a r c e a t extreme temperatures ( d e s e r t s and a r c t i c r e g i o n s ) , w h i l e
the e f f e c t of temperature i s g r e a t e s t when t h e r e i s a d e f i c i t of water.
Gases c o n s t i t u t e another r e s o u r c e .
Under normal c o n d i t i o n s gases
are not l i m i t i n g as they e x i s t i n constant p r o p o r t i o n s i n the atmosphere.
However, i f the atmospheric e q u i l i b r i u m were d i s t u r b e d n e i t h e r photos y n t h e s i s o r r e s p i r a t i o n c o u l d proceed e f f i c i e n t l y and t h i s would have a
severe e f f e c t on growth of l i v i n g organisms.
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A
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Carbon d i o x i d e and oxygen are u t i l i s e d by p l a n t s t o manufacture
n u t r i e n t s (e. g., s u g a r s ) .
These are combined w i t h inorganic
salts d e r i v e d
from the available
sources i n the substratum to form a l l the c l a s s e s of
compounds found i n p l a n t c e l l s .
On the o t h e r hand animals d e r i v e t h e i r
nutriment from the breakdown of n u t r i e n t compounds p r e v i o u s l y formed i n
o t h e r organisms
{food).
Besides o r g a n i c compounds, i n o r g a n i c compounds are a l s o necessary
to s u s t a i n l i f e , the most important being N, P, K, Ca, S, Mg.
In a d d i t i o n
s m a l l q u a n t i t i e s of Fe, Mn, Cu, Zn, B, Na, Mo, C I , V and Co, are e s s e n t i a l
f o r h e a l t h y growth.
With a l l the above f a c t o r s i t i s the
availability
that i s important.
I f s u i t a b l e nutriment i s not p r e s e n t , death o r
migration w i l l occur, while i f i t i s a v a i l a b l e i n i n s u f f i c i e n t quantity
growth d e f i c i e n c i e s w i l l r e s u l t .
F u r t h e r environmental f a c t o r s , c l a s s e d as r e s o u r c e s , are
substrate
and shelter*
Most t r e e s and shrubs need a s t a b l e substratum t o become
e s t a b l i s h e d , and hence are not a b l e to c o l o n i s e many rocky h i l l s i d e s (an
important f a c t o r i n the u n s t a b l e New Zealand h i l l c o u n t r y ) .
Shelter i s a
c r i t i c a l f a c t o r f o r animals when they are i n a d e f e n c e l e s s stage of t h e i r
l i f e c y c l e , e. g., young mammals and pupating i n s e c t s .
So f a r o n l y p h y s i c a l ( a b i o t i c ) f a c t o r s have been d i s c u s s e d ;
these
are u s u a l l y easy to measure and d e l i n e a t e , but i t must be emphasised t h a t
b i o t i c f a c t o r s are j u s t as i m p o r t a n t .
The b i o t i c environment can be due
to the presence and a c t i o n upon an organism of members of i t s own s p e c i e s
o r of members of o t h e r s p e c i e s .
The e f f e c t of b i o t i c f a c t o r s can be
n e g a t i v e , e. g., c o m p e t i t i o n , o r p o s i t i v e , e. g., a g g r e g a t i o n and
gregariousness.
s p e c i e s , as each member r e q u i r e s s i m i l a r environmental c o n d i t i o n s .
When
the p o p u l a t i o n d e n s i t y i s such that c o m p e t i t i o n becomes i n t e n s e then o n l y
the i n d i v i d u a l s b e s t s u i t e d f o r c o m p e t i t i o n w i l l s u r v i v e .
However both
p l a n t s and animals can e x p e r i e n c e advantages by grouping t o g e t h e r w i t h
members of t h e i r own k i n d o r w i t h o t h e r s .
The a g g r e g a t i o n of t u s s o c k
p l a n t s i n t o clumps a i d s e s t a b l i s h m e n t and prevents i n d i v i d u a l s s u f f e r i n g
from e r o s i o n , w h i l e the g r e g a r i o u s n e s s of herd animals such as the
Himalayan Thar a f f o r d s some p r o t e c t i o n to the i n d i v i d u a l .
Thus the
p o p u l a t i o n d e n s i t y of any s p e c i e s i s c o n t r o l l e d by the e q u i l i b r i u m
between c o m p e t i t i o n and a g g r e g a t i o n .
C o m p e t i t i o n between members of d i f f e r e n t s p e c i e s i s demonstrated by
the New Zealand Rata (Metrosideros
robusta).
By competing f o r l i g h t ,
space, n u t r i e n t s , e t c , i t e v e n t u a l l y k i l l s the host p l a n t .
Or the
S h i n i n g Cuckoo, which d e p o s i t s i t s eggs i n another b i r d ' s nest and as a
n e s t l i n g competes f o r food and space w i t h the young o f o t h e r b i r d s .
Besides c o m p e t i t i o n t h e r e are many v a r i e d ways by which members of
d i f f e r e n t s p e c i e s may i n t e r a c t :
1.
Parasites
may be a v i t a l f a c t o r l i m i t i n g the growth and d i s t r i b u t i o n
of the h o s t organism.
The fungus Phytophthora
cinamani.
a s e r i o u s pathogen of many p l a n t s , e s p e c i a l l y Pinus radiata,
can cause complete d e v a s t a t i o n
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of whole stands of the h o s t .
The h e a l t h o f the K i w i and o f sheep and other
i n t r o d u c e d mammals s u f f e r s w i t h heavy i n f e s t a t i o n s o f t i c k s .
2.
Symbiosis o r Che l i v i n g together o f two organisms t o t h e i r mutual
b e n e f i t , can be s i g n i f i c a n t i n the l i f e o f many animals and p l a n t s .
The
l i c h e n , a combination o f an a l g a and a fungus, i s a c l e a r example o f t h i s .
Rumen micro-organisms l i v i n g i n the caecum o f cows and o t h e r h e r b i v o r e s ,
a s s i s t i n n u t r i t i o n , w h i l e the micro-organism r e c e i v e s s h e l t e r i n r e t u r n .
I n both these cases the c o o p e r a t i o n o f each member i s e s s e n t i a l t o the
wellbeing of both.
3.
Predation
i s important t o many s m a l l a n i m a l s , an u n l e s s they have
some form o f p r o t e c t i o n , may r e s u l t i n t h e i r e l i m i n a t i o n from the a r e a .
Thus the p o p u l a t i o n o f s m a l l b i r d s and r a b b i t s can be s e v e r e l y a f f e c t e d by
the depredations o f hawks.
G r a z i n g (a form o f p r e d a t i o n ) may l i k e w i s e
deplete populations of e d i b l e plant species.
4.
Another example i s oommensalism, where one organism d e r i v e s b e n e f i t
from another w i t h n e g l i g i b l e e f f e c t on i t .
The t u a t a r a uses the burrows
of p e t r e l s f o r s h e l t e r w i t h o u t s e r i o u s l y d i s r u p t i n g the l i f e o f the p e t r e l ,
w h i l e e p i p h y t e s a r e another example, t y p i c a l o f New Zealand v e g e t a t i o n .
Important environmental f a c t o r s n o t y e t mentioned a r e those grouped
under hazards.
The e f f e c t of f l o o d s , f i r e , e r u p t i o n s , e t c . a r e obvious
and do n o t need t o be e l a b o r a t e d .
The d i s c u s s i o n has i n c l u d e d j u s t a few o f the many and v a r i e d examples
demonstrating the a c t i o n o f i n d i v i d u a l environmental f a c t o r s .
However i t
must be r e a l i s e d t h a t every organism i s s u b j e c t t o a complex o f environmenta l f a c t o r s forming a r e a c t i o n system which c o n t a i n s the organism.
This
r e a c t i o n system i n c l u d e s the m o d i f i c a t i o n o f the environment by the organism.
To show t h i s , an example o f t h e a c t i o n of a known complex o f e n v i r o n mental f a c t o r s can be seen i n the accompanying diagram.
I t must be s t r e s s ed t h a t i n nature a l l these f a c t o r s w i l l be i n t e r a c t i n g .
E c o l o g i c a l s t u d i e s have an important economic v a l u e .
Man can
achieve a h i g h e r standard i n growth and h e a l t h f o r h i s crops and f l o c k s
by changing and m o d i f y i n g t h e i r environment.
T h i s i s t y p i f i e d i n New
Zealand w i t h the development o f t o p d r e s s i n g t o r e p l e n i s h n u t r i e n t s i n the
s o i l s l e a d i n g t o good pasture and h e a l t h y l i v e s t o c k .
A s o p h i s t i c a t e d example of environmental m o d i f i c a t i o n i s t h a t o f
b i o l o g i c a l c o n t r o l where pests may be r e g u l a t e d by the i n t r o d u c t i o n o f
biotic factors.
T h i s i s e x e m p l i f i e d i n t h e measures taken t o p r o t e c t
New Zealand's e x o t i c f o r e s t s a g a i n s t the depredations o f Sirex
noctilo,
Sirex, a wood wasp, d e p o s i t s i t s eggs i n h o l e s d r i l l e d i n the t r e e by the
female.
Each egg i s i n f e c t e d by a fungus (Amylostereum) which causes white
rot d i s e a s e .
This fungus p r e d i g e s t s the timber f o r t h e l a r v a e , and renders
the wood u s e l e s s f o r p r o d u c t i o n .
Sirex was kept i n c o n t r o l by i t s n a t u r a l
enemies i n i t s n a t i v e Europe, b u t i n New Zealand where i t had none, i t
SUN
Tha action of scrrw known environmental factors on an individual Kauri (Agathis australis).
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spread r a p i d l y t h r e a t e n i n g the country's timber economy.
I n order t o
combat t h i s menace Rhyssa persuasoria
was i n t r o d u c e d .
T h i s wasp seeks
out Sirex l a r v a e w i t h i t s l o n g o v i p o s i t o r and l a y s h e r eggs which develop
at the expense of t h e l a r v a e .
I n t h i s way c o n t r o l o f Sirex i s
achieved.
However the i n d i s c r i m i n a t e i n t r o d u c t i o n o f b i o t i c f a c t o r s may very
r a p i d l y upset the f i n e balance o f n a t u r e .
A s t r i k i n g example was t h e
u n c o n t r o l l e d d e v a s t a t i o n of manuka (Leptospermum
scoparium
manuka
blight.
Manuka, a pest t o p a s t o r a l farmers, became known as a weed i n
New Zealand.
I n 1942 i t was n o t i c e d t h a t Eriococcus an i n t r o d u c e d mealy
bug a t t a c k e d manuka and a l l o w e d i n f e c t i o n by Capnodium spp. a sooty mould
fungus.
The i n f e s t a t i o n was allowed t o proceed u n t i l manuka was t h r e a t ened w i t h e x t i n c t i o n .
However w i t h the d e p l e t i o n o f manuka there was a
r a p i d i n c r e a s e i n e r o s i o n and subsequent damage t o p l a n t communities,
which became a more s e r i o u s t h r e a t than manuka i t s e l f .
L a t e r i t was
n o t i c e d t h a t another fungus (Myrangium spp. ) a t t a c k e d the mealy bug and
rendered i t innocuous, thus r e s t o r i n g the balance o f n a t u r e .
Man by sound knowledge of ecology may, w i t h c a r e , c o n t r o l i n t e r a c t i o n s between organisms and thus o b t a i n a u s e f u l and potent f o r c e i n
the m o d i f i c a t i o n o f the environment.
The f u t u r e i n c r e a s e i n man's
primary p r o d u c t i o n w i l l depend on h i s a b i l i t y t o manipulate the b i o t i c
and a b i o t i c f a c t o r s o f the environment.
Although t h i s a r t i c l e c o n t a i n s the minimum of concepts and examples
the c o m p l e x i t i e s o f t h e environment have been c o n t i n u a l l y s t r e s s e d .
The
complex of i n t e r a c t i n g f a c t o r s must be observed and comprehended i n the
f i e l d b e f o r e an i n s i g h t can be gained i n t o the b i o l o g i c a l processes present
i n nature.
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