Step 1: Review the general pattern of ecological succession.
Ecological succession usually starts with species that can spread quickly and establish themselves easily in new or disturbed areas.
As time passes, these early colonisers are gradually replaced by species that are better at competing for resources within the habitat.
Step 2: Examine option (A).
At the beginning of succession, resources like space, light, and nutrients are generally plentiful rather than scarce.
Limitations in resources typically become more pronounced in later stages as plant and animal biomass increases.
Therefore, option (A) does not correctly explain the replacement pattern.
Step 3: Examine option (B).
This option refers more to the role of keystone species or food-web interactions rather than the shift from colonisers to competitors.
As such, it is not directly related to the mechanism driving successional replacement.
Hence, option (B) is incorrect.
Step 4: Examine option (C).
In many terrestrial ecosystems, trees dominate the later stages of succession.
These species typically show strong competitive ability but relatively limited dispersal capacity.
Since the statement reverses these characteristics, option (C) is incorrect.
Step 5: Examine option (D).
A key concept in ecology is the tradeoff between dispersal ability and competitive strength.
Early successional species disperse efficiently but compete poorly, while late-successional species disperse less effectively but outcompete others for resources.
This tradeoff clearly explains why good colonisers are replaced by good competitors over time.
Therefore, option (D) is correct.
Step 6: Final conclusion.
The observed replacement of early colonisers by strong competitors during succession is best explained by:
\[ \boxed{(D)} \]