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ecosystem is considered. Thus, in North America the brown-headed cowbird

(Molothrus ater) is a nest parasite of many small passerines. Its population is increas-

ing and it has caused the decline of at least two species, Least Bell’s vireo (Vireo bel-

lii) and the black-capped vireo (V.atricapillus) (Smith et al. 2000). The ultimate cause

is due to events that date back to the 1800s (Rothstein 1994). This cowbird’s ori-

ginal range lies in forests of the American northeast where it prefers open areas for

foraging. Expansion of open land through agriculture across North America has allowed

this species to spread into new areas and parasitize species that have few adaptive

traits to counter it. Both large-scale and long-term events underlie the spread of this

nest parasite.

Large-scale temporal patterns are also important in ecosystem dynamics. The classic

snowshoe hare (Lepus americanus) cycle of North America is synchronized spatially

by decadal weather events (Stenseth et al. 2002). This cycle of numbers then

influences the rest of the ecosystem (Krebs et al. 2001b). Synchrony is enhanced

by environmental correlation across sites and reduced by dispersal between sites

(Kendall et al. 2000). There is increasing evidence that complex ecosystem processes

are influenced by multi-year fluctuations in climate such as the North Atlantic

Oscillation and the southern and northern Pacific oscillations (Post and Stenseth 1998;

Coulson et al. 2001a). In the context of long-term conservation, spatial and temporal

synchronies are of particular concern when a species is rare because of its increased

probability of extinction.

Human-induced changes in climate can be considered very long-term, persistent

disturbance. Global climate change is now having measurable effects on ecosystems,

altering community composition by shifting species ranges differentially towards

the poles, higher in altitude (especially in the tropics; Pounds et al. 1999), and away

from the tropics (Schneider and Root 2002; Parmesan and Yohe 2003). In Britain,

birds and other groups are breeding earlier. Changes in community structure mean

that species will experience changes in food supply and predation rates (Crick et al.

1997).

In general, ecosystem processes at different spatial and temporal scales, including

disturbance and long-term trends, must be considered as part of any conservation

strategy for the ecosystem in which a threatened species exists. Such processes oper-

ate at larger scales than we have traditionally planned for. Moreover, conservation is

no longer a short-term exercise: we have to consider time scales of 100 years or more.

Biodiversity is defined as the complement of living organisms in an ecosystem. In

terrestrial systems there is a general tendency for animal groups to be more species

diverse in tropical latitudes (MacArthur 1972). Figure 21.2 shows such a distribu-

tion for birds. There are local anomalies in this pattern, for example for reptiles in

Australia (Schall and Pianka 1978).

There have been many hypotheses offered to explain this latitudinal gradient in

biodiversity from the tropics to the poles. Some of these are:

1 Structural heterogeneity. Tropical vegetation is more complex structurally, provid-

ing more niches for animals compared with arctic tundra or even boreal forest.

2 Community age. Ice caps covered large areas of northern North America and

Eurasia, and these melted only 10,000 years ago. There has been insufficient time to

reinvade these higher latitudes and evolve new species. This process of eradication

ECOSYSTEM MANAGEMENT AND CONSERVATION 377

21.11.2Management
at long time scales

21.12 Biodiversity

21.12.1Determinants
of diversity