Spatial Ecology and Conservation

If you take a close look, you’ll notice that landscapes are patchy. Palm oases are splashes of green in desert landscapes where water rests close to the surface (Figure 1). Light gaps reveal breaks in tropical forest canopies where trees have blown down during windstorms. Spatial and temporal variation in the distribution and abundance of vital resources, as well as in geological and ecological processes, results in landscape spatial heterogeneity, often called habitat patchiness.

In many terrestrial systems, patchiness involves spatial variation in topography, bedrock, soils, nutrients, or water that affects distribution of plant species, which at least partially determines animal distributions. For example, cattails only grow in places where the soil is saturated, such as along the margins of freshwater wetlands. In marine or freshwater systems, patchiness in species distributions may be the result of variation in substrate type, water depth, or period of inundation, for example. Natural disturbances, including fires, floods, disease outbreaks, wind storms, and wave action also create patchiness, by altering the structure of populations, communities, and ecosystems and causing changes in resource availability or the physical environment. And species may generate their own patches, independent of any underlying environmental heterogeneity, by their clumped dispersal patterns (often referred to as spatial pattern formation).

Ecologists have a long history of observing and studying the underlying causes of patchiness. Concepts and methods from landscape ecology, which focus specifically on causes and consequences of spatial heterogeneity for ecological processes, have converged with research efforts in population and community ecology, to create the recognized area of study called spatial ecology. Of relevance to biological conservation, most contemporary studies of habitat patchiness emphasize patterns and outcomes of human-caused landscape heterogeneity (Figure 2) rather than naturally occurring patterns and processes. It is important to recognize that in most, if not all landscapes, natural patchiness and human activities interact to create the spatial patterns that we see.

In 1994, the well-known ecologist Peter Kareiva introduced a special feature in the journal Ecology with the provocative title, “Space: the final frontier for ecological theory.” This was clearly homage to the great science fiction writer Gene Rodenberry, creator of the now-classic TV series, Star Trek. But was Kareiva really talking about space, as in the domain of galaxies, nebulae and black holes? No. Ecologists weren’t ready to colonize space just yet. Instead, Kareiva’s title was a call to ecologists to recognize and incorporate understanding of spatial patterns and processes into ecological research. This explicit emphasis on spatial ecology emerged from an active convergence of research efforts in landscape ecology and population and community ecology in recent decades.

Spatial ecology centers on how specific spatial arrangements of organisms, populations, and landscapes influences ecological dynamics. In particular, much of spatial ecology is closely related to conservation biology because it emphasizes the study of habitat loss and fragmentation caused by human activities (Figure 2). The study of habitat loss and fragmentation provides an important synthesis for the entire field of ecology, because it unites fundamental ecological theories and concepts with an extensive body of empirical literature. It also provides important links to environmental applications, including conservation, restoration, and planning. There are literally thousands of papers that have been published in the past 40 years on the ecology of fragmented landscapes.

Several key ecological theories contribute to spatial ecology and guide empirical investigations in the field and laboratory. Ecologists have used these theories to develop and refine our understanding of the implications of spatial variation for ecological processes. For example, the basic notion of species-area curves reveals that as the size of a natural area increases, the number of species in that area increases as well. Because species richness is so tightly linked to habitat area, it is not surprising that the major reason for species decline in the past fifty years is because rates of habitat loss are unprecedented. The theory of island biogeography predicts that larger and less isolated islands will contain more species than smaller, more isolated islands. Metapopulation theory recognizes that local populations of organisms undergo periodic colonization and extinction, but that these local populations are linked to other populations nearby by migration. Hence, the collection of local populations, termed the metapopulation, can persist indefinitely if rates of local population extinction are balanced by rates of colonization from surrounding populations.

Recent elaborations of metacommunity theory are particularly relevant to understanding species interactions in fragmented landscapes. Metacommunity ecology is essentially an extension of metapopulation theory — but instead of single species populations blinking in and out of scattered habitat patches, we now have multiple species that occur and interact across patchy landscapes. A metacommunity is broadly defined as a collection of communities connected by dispersal. Studies in metacommunity ecology seek to understand how habitat patchiness structures species interactions.

Our contemporary fragmented landscapes are spatially complex patchworks, in which populations and species of plants, insects, birds, mammals, reptiles, and fish have shifted significantly throughout the globe. For example, many native species have decreased in abundance as a consequence of habitat loss and fragmentation. Others, such as crops and invasive species, have increased due to human manipulations. Moreover, functional aspects of ecosystems, such as pollination, seed dispersal, the decomposition of dead organic matter, carbon sequestration, and water filtration, have also been altered. In the past few decades, innovative theoretical and empirical research in spatial ecology has clarified the important roles of landscape spatial configuration for species interactions and ecosystem processes. Combining ecological and evolutionary theory with empirical evidence on a range of issues crucial to biological conservation, research has focused on gene flow and local adaptation, population demography and genetic variation, interspecific interactions, landscape heterogeneity, conservation planning, and ecological restoration.

Research in spatial ecology reveals that populations and communities often suffer declines or changes in composition as a result of reductions in fragment area and connectivity. This is frequently because in fragmented landscapes, species often encounter obstacles in their attempts to travel safely from one suitable patch to another. Because some species avoid certain landscape features, such as parking lots or cornfields, habitat loss and fragmentation may force species to navigate through complex patchworks of suitable habitat, ultimately reducing their success.

Habitat loss and fragmentation constitute the most serious threats to Earth’s biological diversity, and understanding their consequences for persistence of native populations and communities remains a daunting challenge for conservation biologists. Two particularly relevant species interactions that may be altered as a result of habitat loss and fragmentation serve to illustrate critical links between spatial ecology and conservation. In both of these examples, human manipulation of the spatial arrangement of land cover and land uses has affected the ways in which species interact with one another.

First, a vital ecological interaction for plants is the mutualistic relationship between plants and their pollinators (Figure 3), which may also be affected by a variety of mechanisms that stem from changes in habitat spatial characteristics. Studies of agricultural landscapes in California, for example, have shown that the abundance and diversity of native pollinators, and their contribution to pollination, is strongly influenced by spatial characteristics of the landscape. Specifically, farms that are surrounded by a relatively high proportion of natural habitat (30% or more cover the native habitat within 1 km of the farm) enjoy higher pollen deposition on watermelon flowers than farms surrounded by a low cover of native habitat (less than 1% within 1 km of the farm). The patches of native habitat provide critical nesting and foraging habitat for native bees, so the abundance and diversity of bees is higher at these farms.

Second, the transmission and spread of infectious diseases is largely a function of interactions within and among species within ecological communities. Recent research reveals that losses of biological diversity caused by habitat loss and fragmentation may increase the risk of several well-known, vector-borne diseases that are shared between humans and wildlife. For example, recent increases in human incidence of malaria, Lyme disease, and West Nile virus are all associated with declines in species richness caused by landscape change. In these cases, habitat loss and fragmentation have reduced the abundance and richness of predators and competitors from ecological communities, causing an increased abundance of competent host species and thus increased pathogen prevalence. There is a clear warning that, as we reduce biological diversity, we are compromising the ability of natural systems to dilute pathogens, as well as increasing our own exposure to infectious diseases.

The general theme that emerges from research in spatial ecology so far is that the spatial structure of landscapes may have profound impacts on populations of species, their interactions within ecological communities, and the function of ecosystems. These impacts may depend upon the species under consideration, the character of the landscape interspersed with the preferred habitat, and the particular spatial configuration of the habitat, to name a few.

The study of spatial ecology provides a critical venue for students and practitioners to apply this knowledge toward efforts to protect and re-assemble natural ecosystems, thereby maintaining both species and ecosystem services. By merging our understanding of broad-scale habitat modification with responses of populations, species, and communities to these landscape changes, spatial ecologists will be better positioned to predict and resolve the consequences of landscape change for native biological diversity. The field of spatial ecology is rich and rewarding, with exciting opportunities to link rigorous ecological theory with applications to conservation of biological diversity and ecosystem services.