3. Meeting nature
It is vital to provide a value when assessing impacts on nature with a view to protecting them by long-term management .  The Darnell system of evaluation is based on the idea that impacts arising from when humans meet nature can be broken down into twelve generalizations, or 'principles of environmental impact'. These could be regarded as a set of working rules which would be the first step in a conservation management.  Some of these generalizations are self- evident, but they are none the less important for that. They are paraphrased as follows.
A Natural values and the nature of the impact
1 Natural ecosystems are of great value to society and should not be modified without good cause.
2 Most human intrusions into natural ecosystems are likely to be deleterious to that ecosystem.
3 Ecological impacts always come in groups, i.e. natural phenomena are interrelated and impacts are transmitted via species interactions and system connectivity.
B Impact as a function of construction type and location
4 Each general type  of intrusion results in  a characteristic  suite  of environmental effects.
5 Each specific type of intrusion induces a specific set of environmental impacts.
6 There is a regional dimension to all impacts, i.e. mangroves are not salt marshes.
7 Each  specific   location  exhibits   site-related  values  and  site- related response characteristics which need checking, i.e. field inspection.
C Impact predictability and environmental response
8 Effects of intrusion are in part predictable, but in part unpredictable.
9 Different types of intrusion often produce the same type of response in a system.
10 Severe or prolonged intrusion into a natural system produces systemic stress, i.e. total breakdown.
11 Ecosystem responses to severe intrusion take two forms: a generalized stress response somewhat unrelated to the nature of the stress agent, and a specific response which is particular to the nature of the stress agent.
12 Ecosystem responses to stress agents of various types are not well understood, and therefore further study is needed if we are to increase our predictive capability and eventually to minimize the ecological effects of intrusion.
However, such a procedural progression of considerations still begs a number of important questions, such as the following. What techniques are available for minimizing the intrusion/development effects? How far is society prepared to go in ameliorating the impacts or creating new environments? Is society prepared to forgo immediate monetary gains for future benefits? What institutional mechanisms, which, of course, are different in every country and even transcend national boundaries required to achieve the protection desired? The difficult and vexing questions of establishing markers for the non-consumptive value of scenic, heritage and historical issues that are still sidestepped.
With respect to the latter aspect, seven marker categories are presented based on the different cultural routes taken to evaluate nature. The non-consumptive benefits of include scenic, recreational, educational, aesthetic, archaeological, scientific, heritage and historical benefits that are difficult to define, let alone quantify. These overlap and intertwine, and consequently a precise definition is not possible or, perhaps, even desirable.
Non-consumptive values have usually been considered of secondary importance compared with the direct consumptive and economic products of environment and the physical, chemical and biological services that they provide.  However, they are accepted and funded by ordinary people. The range of value categories reflects the fact that the values are intangible and can be highly personal and subjective. Often values are placed in society through the media of literature and art, or through scenic or visual- cultural assessments, and a positive attitude to conservation may be activated through these doors and windows. Additionally, because there is no way of measuring these values objectively, it is difficult to compare non-consumptive with consumptive benefits, or one type of non-consumptive type with another non- consumptive type in the same kind of environment. The one non- consumptive benefit that can sometimes be quantified is that of recreation, as the number of people participating, and what they are prepared to pay for the privilege of that participation in the form of licence fees, equipment or travel, is a measure of sorts. Thus it is estimated in 1980-1 that 83.2 million Americans spent $14.8 billion on observing and photographing fish and wildlife in 1980-1.