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Unit 4: Ecosystems // Section 1: Introduction

Ecology is the scientific study of relationships in the natural world. It includes relationships between organisms and their physical environments (physiological ecology); between organisms of the same species (population ecology); between organisms of different species (community ecology); and between organisms and the fluxes of matter and energy through biological systems (ecosystem ecology).

Ecologists study these interactions in order to understand the abundance and diversity of life within Earth's ecosystems—in other words, why there are so many plants and animals, and why there are so many different types of plants and animals (Fig. 1). To answer these questions they may use field measurements, such as counting and observing the behavior of species in their habitats; laboratory experiments that analyze processes such as predation rates in controlled settings; or field experiments, such as testing how plants grow in their natural setting but with different levels of light, water, and other inputs. Applied ecology uses information about these relationships to address issues such as developing effective vaccination strategies, managing fisheries without over-harvesting, designing land and marine conservation reserves for threatened species, and modeling how natural ecosystems may respond to global climate change.

Tropical ecologist Stuart Davies in the field

Figure 1. Tropical ecologist Stuart Davies in the field
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Change is a constant process in ecosystems, driven by natural forces that include climate shifts, species movement, and ecological succession. By learning how ecosystems function, we can improve our ability to predict how they will respond to changes in the environment. But since living organisms in ecosystems are connected in complex relationships, it is not always easy to anticipate how a step such as introducing a new species will affect the rest of an ecosystem.

Human actions are also becoming major drivers of ecosystem change. Important human-induced stresses on ecosystems are treated in later units of this text. Specifically, Unit 7 ("Agriculture") examines how agriculture and forestry create artificial, simplified ecosystems; Unit 9 ("Biodiversity Decline") discusses the effects of habitat loss and the spread of invasive species; and Unit 12 ("Earth's Changing Climate") considers how climate change is affecting natural ecosystems.

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