The ecological consequences of past and projected
changes in the above five categories are as varied as
the categories themselves. The only things that can be
said with certainty are that the ecological responses
will be pervasive, affecting every aspect of the environment,
and that they will be played out in ways and
at scales that will be difficult to predict. Unless there
are sustained improvements in how issues are addressed,
how information is processed into knowledge,
and how wisdom is inserted into policy decisions, one
can expect that variation and uncertainty will be the
norms (Walters 1997, Naiman et al. 1998a).
So what are some of the major ecological consequences
that one may expect to see in the next two to
three decades? As Rapport and Whitford (1999) have
stated so clearly, projected changes to aquatic ecosystems
will be manifested by a ‘‘distress syndrome’’ indicated
by reduced biodiversity, altered primary and
secondary productivity, increased prevalence of disease,
reduced nutrient cycling efficiency, increased
dominance of exotic species, and increased dominance
by smaller, shorter lived opportunistic species. For ease
of presentation, we provide seven wide-ranging examples
(land use change, disturbance regimes, ultraviolet
[UV] radiation, life history phenology, pollution,
exotic invasions, and cumulative effects) of how human
activities may affect freshwater ecosystems.
Land use change.—We use the phrase ‘‘land use’’
in the sense of Turner et al. (1998) to accommodate
changes in land cover and use. The ecological consequences
of changing land use are complex because
changing land use practices also influence ecosystemscale
structure and processes that, in turn, influence
habitat and communities (Fig. 3). Embedded at each
scale are concomitant alterations to water, sediment,
energy, and nutrient balances, to community structureand species demography, and to overall aquatic system
integrity (Innis et al. 2000).
Land use influences freshwater systems by directly
altering the composition and structure of the natural
flora and fauna, changing disturbance regimes, fragmenting
the land into smaller and more diverse parcels,
and changing the juxtaposition between parcel types.
Collectively, these changes result in alterations in the
relative abundance of habitat types, a reduction of native
biodiversity, changes in the natural patterns of environmental
variation that are needed to sustain a variety
of species, and augmentation of nitrogen and
phosphorus loads (Turner et al. 1999). Perhaps the latter
point is the best documented. There are numerous
studies either relating questionable land uses to increasing
nutrient loads or ecologically proper land uses
to the maintenance of acceptable water quality (Peterjohn
and Correll 1984, Cole et al. 1993, Hunsaker and
Levine 1995, Bolstad and Swank 1997, Johnson et al.
1997).
Disturbance regimes.—The type, intensity, duration,
timing, and spatial pattern of disturbance shape the
characteristics of populations, communities, and ecosystems.
Disturbances may be natural (e.g., wildfire,
storms, floods, grazing) or induced by human actions
(e.g., fire suppression, flow regulation, fragmentation).
However, the human-induced regimes differ so radically
from natural regimes that all levels of system
organization are affected (Poff et al. 1997, Dale et al.
2000). Consider fire suppression. Average fire size and
severity have increased in a variety of temperate ecosystems,
due in part to the legacy of increased fuel
loads and the encroachment of shade-tolerant late-successional
species. These effects of fire suppression are
pronounced in ecosystems that would typically experience
frequent fires of low intensity, such as the pine
and scrub communities in the southeastern coastal
plain, chaparral along the Pacific coast, and the pine
forests of the Southern Rocky Mountains. Fire suppression
during the past century has lengthened the fire
return interval and altered successional pathways (e.g.,
Glitzenstein et al. 1995, Linder et al. 1997). Where fire
suppression was effective, the resulting increase in the
amount and connectivity of fuel, both vertically and
horizontally, has made these systems conducive to larger
and more intense fires. The effects of these more
severe fires include greater plant mortality, and shifts
in species composition and vegetation structure may
result. The implications for freshwater systems are significant.
Fire suppression results in changing land cover
and soil properties, pulsed erosion, altered biogeochemical
cycles and nutrient retention, and severely
degraded habitat for aquatic organisms.
Altered flow regimes are another pervasive type of
artificial disturbance (Poff et al. 1997). Whereas, historically,
there were regular patterns (or at least identifiable
periods with increased probabilities) for ex