David Saltz

A movement ecology paradigm for unifying organismal movement research.

Movement of individual organisms is fundamental to life, quilting our planet in a rich tapestry of phenomena with diverse implications for ecosystems and humans. Movement research is both plentiful and insightful, and recent methodological advances facilitate obtaining a detailed view of individual movement. Yet, we lack a general unifying paradigm, derived from first principles, which can place movement studies within a common context and advance the development of a mature scientific discipline. This introductory article to the Movement Ecology Special Feature proposes a paradigm that integrates conceptual, theoretical, methodological, and empirical frameworks for studying movement of all organisms, from microbes to trees to elephants. We introduce a conceptual framework depicting the interplay among four basic mechanistic components of organismal movement: the internal state (why move?), motion (how to move?), and navigation (when and where to move?) capacities of the individual and the external factors affecting movement. We demonstrate how the proposed framework aids the study of various taxa and movement types; promotes the formulation of hypotheses about movement; and complements existing biomechanical, cognitive, random, and optimality paradigms of movement. The proposed framework integrates eclectic research on movement into a structured paradigm and aims at providing a basis for hypothesis generation and a vehicle facilitating the understanding of the causes, mechanisms, and spatiotemporal patterns of movement and their role in various ecological and evolutionary processes. ”Now we must consider in general the common reason for moving with any movement whatever.“ (Aristotle, De Motu Animalium, 4th century B.C.)

Multi‐scale patterns and bush encroachment in an arid savanna with a shallow soil layer

Abstract Question: Bush encroachment (i.e. an increase in density of woody plants often unpalatable to domestic livestock) is a serious problem in many savannas and threatens the livelihood of many pastoralists. Can we derive a better understanding of the factors causing bush encroachment by investigating the scale dependency of patterns and processes in savannas? Location: An arid savanna in the Khomas Hochland, Namibia. Methods: Patterns of bush, grass, and soil nutrient distribution were surveyed on several scales along a rainfall gradient, with emphasis on intraspecific interactions within the dominant woody species, Acacia reficiens. Results: Savannas can be interpreted as patch-dynamic systems where landscapes are composed of many patches (a few ha in size) in different states of transition between grassy and woody dominance. Conclusions: In arid savannas, this patchiness is driven both by rainfall that is highly variable in space and time and by inter-tree competition. Within the paradigm of patch-dynamic savannas, bush encroachment is part of a cyclical succession between open savanna and woody dominance. The conversion from a patch of open savanna to a bush-encroached area is initiated by the spatial and temporal overlap of several (localized) rainfall events sufficient for Acacia germination and establishment. With time, growth and self-thinning will transform the bush-encroached area into a mature Acacia stand and eventually into open savanna again. Patchiness is sustained due to the local rarity (and patchiness) of rainfall sufficient for germination of woody plants as well as by plant-soil interactions. Nomenclature: Dyer (1975, 1976); Ross (1979); Gibbs-Russell et al. (1991).

Ecological Trap for Desert Lizards Caused by Anthropogenic Changes in Habitat Structure that Favor Predator Activity

Anthropogenic habitat perturbation is a major cause of population decline. A standard practice managers use to protect populations is to leave portions of natural habitat intact. We describe a case study in which, despite the use of this practice, the critically endangered lizard Acanthodactylus beershebensis was locally extirpated from both manipulated and natural patches within a mosaic landscape of an afforestation project. We hypothesized that increased structural complexity in planted patches favors avian predator activity and makes these patches less suitable for lizards due to a heightened risk of predation. Spatial rarity of natural perches (e.g., trees) in arid scrublands may hinder the ability of desert lizards to associate perches with low-quality habitat, turning planted patches into ecological traps for such species. We erected artificial trees in a structurally simple arid habitat (similar to the way trees were planted in the afforestation project) and compared lizard population dynamics in plots with these structures and without. Survival of lizards in the plots with artificial trees was lower than survival in plots without artificial trees. Hatchlings dispersed into plots with artificial trees in a manner that indicated they perceived the quality of these plots as similar to the surrounding, unmanipulated landscape. Our results showed that local anthropogenic changes in habitat structure that seem relatively harmless may have a considerable negative effect beyond the immediate area of the perturbation because the disturbed habitat may become an ecological trap.

Does interspecific competition drive patterns of habitat use in desert bat communities

Bodies of water are a key foraging habitat for insectivorous bats. Since water is a scarce and limiting resource in arid environments, bodies of open water may have a structuring effect on desert bat communities, resulting in temporal or spatial partitioning of bat activity. Using acoustic monitoring, we studied the spatial and temporal activity patterns of insectivorous bats over desert ponds, and hypothesised that sympatric bat species partition the foraging space above ponds based on interspecific competitive interactions. We used indirect measures of competition (niche overlap and competition coefficients from the regression method) and tested for differences in pond habitat selection and peak activity time over ponds. We examined the effect of changes in the activity of bat species on their potential competitors. We found that interspecific competition affects bat community structure and activity patterns. Competing species partitioned their use of ponds spatially, whereby each species was associated with different pond size and hydroperiod (the number of months a pond holds water) categories, as well as temporally, whereby their activity peaked at different hours of the night. The drying out of temporary ponds increased temporal partitioning over permanent ponds. Differences in the activity of species over ponds in response to the presence or absence of their competitors lend further support to the role of interspecific competition in structuring desert bat communities. We suggest that habitat use and night activity pattern of insectivorous bats in arid environments reflect the trade-offs between selection of preferred pond type or activity time and constraints posed by competitive interactions.

Response to topography in a hilltopping butterfly and implications for modelling nonrandom dispersal

Animal movement between habitat patches is often considered a random process. However, responses to landscape heterogeneity can direct the movement of animals and affect connectivity patterns. Topographical heterogeneity is a major source of habitat heterogeneity, which often directs animal movements and yet is scarcely studied in the context of dispersal. We investigated the mechanisms of response to topography and movement rules, using hilltopping as a behavioural case study. Hilltopping is a mate-searching strategy where males and virgin or multiple-mating females seek a topographical summit on which to mate. Mated females descend from the summits thereafter to search for host plants. We investigated the behavioural rules of hilltopping in males and virgin females of the butterfly Melitaea trivia, and female postmating movements. We released butterflies in different topographical formations, in a landscape that contained no larval host plants. We followed them individually, mapped the flight routes, and analysed them with respect to the surrounding landscape, using a Digital Elevation Model. Males and virgin females initiated hilltopping behaviour only in the absence of other individuals. After an initial orientation phase, butterflies flew towards the maximal inclination available. However, some downward movements interrupted the upward flight. When arriving at or released on a summit, males strongly adhered to it. After copulating, females showed little response to topography. Males and virgin females responded to topographical cues within about 50xa0m of their location. Our results show that nonrandom movements, such as hilltopping, are based on simple and predictable decision rules. We discuss the relation between hilltopping and dispersal, and the implications for modelling dispersal.

The effectiveness of various rabies spatial vaccination patterns in a simulated host population with clumped distribution

Abstract In Israels Negev Desert, periodic episodes of rabies epidemics erupt every 5–7 years despite low densities of the main reservoir species of rabies in this area. Canids in this desert are spread non-uniformly over space, with high densities in and around human related waste sites and low densities (presumably below the disease threshold) in the rest of the region. The persistence of the disease in the Negev desert is due likely to the high host density patches around human waste sites. We used an individual-based spatially explicit model to test whether a non-uniform spread of oral rabies vaccination could be more effective in eradicating the disease than the commonly used uniform distribution. We evaluated two models based on a 20×20 grid: the first with 17 high-density patches distributed evenly over the grid and the second with 16 high-density patches clumped in four groups of four. We tested six patterns of spreading the oral vaccination under four different host dispersal ranges. Immunizing only the high-density patches was ineffective in all cases. Immunizing only low host-density patches eradicated the disease within 5 years, on average. Immunizing only the areas surrounding the high-density patches was ineffective except when high-density patches were not clumped and dispersal distance of the host was short. We conclude that in desert environments, where host densities vary over space, non-uniform spreading of oral rabies vaccination may, under certain circumstances, be more effective than the commonly used uniform spread. However, this requires good knowledge of the hosts dispersal patterns and its distribution over space.