Asynchrony, Fragmentation, and Scale Determine Benefits of Landscape Heterogeneity to Mobile Herbivores

Authors: SEARLE, K. - Colorado State University; HOBBS, N. - Colorado State University; Jaronski, Stefan

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/13/2010
Publication Date: 6/17/2010
Citation: Searle, K.R., Hobbs, N.T., Jaronski, S. 2010. Asynchrony, Fragmentation, and Scale Determine Benefits of Landscape Heterogeneity to Mobile Herbivores. Oecologia. 163(3): 815-824.

Interpretive Summary: Habitat fragmentation threatens biodiversity and ecosystem function worldwide. Despite a large investment of effort in understanding the biotic effects of such fragmentation, predicting how a species will respond remains difficult. Predicting effects of habitat fragmentation depends on identifying mechanisms that link species, community, and ecosystem responses to changes in habitat pattern. Many models of interactions between consumers and resources rely on the simplifying assumption that resources are distributed homogeneously in space and time. It is clear however, that this is usually not the case—these resources often come in pulses, appearing then disappearing as time unfolds, occurring at some locations but not others. For herbivores, resource pulses result from flushes of rapidly growing, nutritious plant tissue. We explored how a generalist herbivore can exploit a fragmented food resource, patchy in space and time, using a model system of the Migratory Grasshopper feeding on 4.5 m2 plots of young wheat and protein-rich wheat bran. We examined six landscape types differing in the grain of spatial and temporal heterogeneity in forage, as well as the spatial extent of fragmentation in these mesocosm landscapes. Landscapes offered uniformly distributed low quality resources and variable pulses of high quality resources. The total amount of each resource was the same on all experimental landscapes, but the number of resources pulses, their timing of appearance, and their size differed among landscapes. Individuals with access to asynchronous pulses of wheat as food, but in a homogenous habitat, had increased growth of 13%. When the grasshopper habitat was fragmented and patchy, however, grasshopper growth rate declined by 15% compared to controls. Our research demonstrates that a fragmented, patchy habitat can adversely affect an herbivore even though the quality and overall quantity of food remains unchanged.

Technical Abstract: Fragmentation of landscapes into spatially isolated parts is a prevailing source of environmental change worldwide. However, predicting the consequences of fragmentation for populations remains problematic, in large measure because the mechanisms translating landscape change into population performance remain poorly understood. We explore how fragmentation in variable habitats influences the ability of mobile consumers to exploit asynchronous resource pulses. We propose that the performance of consumers is strongly influenced by an individual’s access to resource pulses offering asynchronous peaks in resource quantity or quality. We offer a simple model of resource-use efficiency that explains why habitat fragmentation can depress performance of consumer populations even when the total amount of resources available to consumers remains unchanged. We tested hypotheses motivated by our model using a mesocosm experiment consisting of the migratory grasshopper, Melanoplus sanguinipes, feeding on young wheat and protein-rich bran. We demonstrated access to asynchronous resource pulses enhanced resource use efficiency and increased growth of individuals by 13%. Disruption of this mechanism when landscapes were fragmented lowered resource use efficiency and caused growth of individuals to decline by 15%. However, these negative effects of fragmentation on consumer performance were mitigated when increasing severity of fragmentation was matched by increasingly fine-grained heterogeneity in resources. Our results demonstrate that fragmenting spatially heterogeneous landscapes can harm consumers even when the total amount of habitat and the quality and quantity of resources available to them remains unchanged. The magnitude of these effects depends on the scale of fragmentation relative to the grain of distribution of resources. Reduced resource use efficiency due to restricted access to asynchronous resource pulses is an important mechanism for the formation of robust predictions of species response to habitat fragmentation.