Separating the influences of environment and species interactions on patterns of distribution and abundance: competition between large herbivores

Abstract

Summary 1.Much recent research has focused on the use of species distribution models to explore the influence(s) of environment (predominantly climate) on species' distributions. A weakness of this approach is that it typically does not consider effects of biotic interactions, including competition, on species' distributions. 2.Here we identify and quantify the contribution of environmental factors relative to biotic factors (interspecific competition) to the distribution and abundance of three large, wide-ranging herbivores, the antilopine wallaroo ( Macropus antilopinus ), common wallaroo ( Macropus robustus ) and eastern grey kangaroo ( Macropus giganteus ), across an extensive zone of sympatry in tropical northern Australia. 3.To assess the importance of competition relative to habitat features, we constructed models of abundance for each species incorporating habitat only and habitat + the abundance of the other species, and compared their respective likelihoods using Akaike's information criterion. We further assessed the importance of variables predicting abundance across models for each species. 4. The best-supported models of antilopine wallaroo and eastern grey kangaroo abundance included both habitat and the abundance of the other species, providing evidence of interspecific competition. Contrastingly, models of common wallaroo abundance were largely influenced by climate and not the abundance of other species. The abundance of antilopine wallaroos was most influenced by water availability, eastern grey kangaroo abundance and the frequency of late season fires. The abundance of eastern grey kangaroos was most influenced by aspects of climate, antilopine wallaroo abundance and a measure of cattle abundance. 5. Our study demonstrates that where census and habitat data are available, it is possible to reveal species' interactions (and measure their relative strength and direction) between large, mobile and/ or widely-distributed species for which competition is difficult to demonstrate experimentally. This allows discrimination of the influences of environmental factors and species interactions on species' distributions, and should therefore improve the predictive power of species distribution models.

Occurrence and abundance models of threatened plant species: Applications to mitigate the impact of hydroelectric power dams

“…In addition, this effect likely plays a role in the relationship between occurrence and abundance, obscuring the true relationship between occupied area and abundance. Ritchie et al (2009) confirmed this idea demonstrating that the predicted abundance of wallaroos and kangaroos was improved with the addition of the occurrence and abundance of interspecific competitors into the models. However, obtaining these data is difficult and model practitioners must continue to use mostly abiotic factors Elith and Leathwick, 2009).…”

“…Austin et al (1990) and Guisan and Thuiller (2005), for example, suggested that the response curve of a species along an environmental gradient can be seriously constrained by interaction with biotic factors. This hypothesis was recently tested by Heikkinen et al (2007) and Ritchie et al (2009), using different organisms, and they both found similar results. When data related to interspecific competitors were incorporated into models, species predictions were significantly improved.…”

Evidence for large-scale effects of competition: niche displacement in Canada lynx and bobcat

“…Although the role of species interactions in influencing fine-scale patterns in species distribution or abundance is well documented and accepted, their role at larger spatial scales remains unresolved [3,40]. Previous work on this subject focused on more indirect measures of biotic interactions at large scales, such as incorporating competitor or predator distribution or abundance as covariates within models [16,40], identifying areas of overlap or high favourability in the niches of potentially competing species [41], examining differences in abiotic variables in zones of allopatry and sympatry [42], or assessing the degree of range filling in the presence of competitors [4]. Our study builds on such work by showing that the presence of a competitor can alter range-wide climatic niche space occupied by a species in zones of contact.…”

Genetic inference as a method for modelling occurrence: A viable alternative to visual surveys

“…To fully understand the drivers of species distributions, biotic factors such as species interactions would need to be considered and potentially incorporated into ecological models of occurrence (Fordham et al 2013b). Ritchie et al (2009) found statistical support for competition between M. antilopinus and M. giganteus, which affects occurrence and abundance.…”

“…A candidate set of environmental predictors of macropod presence/absence was selected using published literature (Ritchie & Bolitho 2008;Ritchie et al , 2009) and expert advice. To avoid co-linearity among predictors, variables with a Spearman's rank ≥ 0.7 were excluded from the analysis.Where possible, we favoured retaining variables with the strongest ecological support (see Appendix S1).…”