PEROTTO-BALDIVIESO, H. L.

SANTOS, S. A.

YOUNG, D.

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Tropical Grasslands – Forrajes Tropicales (2014) Volume 2, 138−139 www.tropicalgrasslands.info Resilience and degradation in a tropical wetland overgrazed by cattle   SANDRA A. SANTOS1, HUMBERTO L. PEROTTO-BALDIVIESO2 AND DYLAN YOUNG2  1Empresa Brasileira de Pesquisa Agropecuária, Embrapa Pantanal, Corumbá, MS, Brazil. www.cpap.embrapa.br      2Cranfield University, Cranfield, UK. www.cranfield.ac.uk     Keywords: Natural pastures, state and transition model, Pantanal, vegetation monitoring, pasture degradation indica-tors, pasture resilience.   Introduction  The Pantanal, one of the largest wetlands in the world, has a great diversity of flora and fauna. The dynamic hydrological regime, combined with heterogeneous to-pography, has resulted in a mosaic of diverse habitat types in terms of species and physical structure. Due to the abundance of forage resources, the Pantanal flood-plains are important for beef cattle production. Cattle prefer grazing near water bodies because these areas have high quality forage as a result of flooding regimes (Santos et al. 2002). Many wetlands go through a wet/dry cycle that is essential to maintain their produc-tivity and function. In drier years, wetland drawdown provides optimal conditions for a diverse range of forage species, and cattle can graze continuously, leading to pasture degradation. It is therefore essential to under-stand the spatial and temporal dynamics of forage production and consumption.  In this study we assessed and monitored vegetation at the edge of one of the pond habitats intensively grazed by cattle, in order to evaluate degradation and quantify indicators of resilience (Briske et al. 2006).  Methods   To measure changes in vegetation during these periods, 25 plots of 0.25 m2 were randomly selected at 10-m in-tervals along a permanent transect of 250 m situated on the edge of a wetland pond at a constant elevation. Measurements were taken at the end of the wet season     ___________ Correspondence: Sandra A. Santos, Embrapa Pantanal, Rua 21 de Setembro, 1880, Bairro Nossa Senhora de Fátima, Caixa Postal 109, Corumbá CEP 79331-090, MS, Brazil.  Email: sandra.santos@embrapa.br  (March) and dry season (September) in each plot during 3 hydrological years (2007−2010). Variables measured within each plot were: number of plant species (rich-ness); visual estimate of percent live plant species cover; visual estimate of percent soil covered with plants, litter and gravel; mean plant height; and total dry matter yield. We also developed a state and transition model using plant functional groups (Figure 1). Plant cover was clas-sified as: C3 and C4 grasses; invasive forbs; macrophyte forbs; exotic forages; and shrub cover. To determine the loss of grassland resilience, we estimated a threshold between forage cover (functional indicator) and invasive forbs cover (degradation and structural indicator) (López et al. 2011), using piecewise regression (Muggeo 2008). Temporary exclosure cages (1 m2) were allocated along a transect in order to examine recovery of forage cover for at least one complete growth cycle, and permanent exclosure cages to monitor long-term effects.                      Figure 1.  State and transition model for wetland grazing areas using plant functional groups. 139         S.A. Santos, H.L. Perotto-Baldivieso and D. Young  Figure 2.  Relationship between forage cover (%) and inva-sive forbs cover (%) in a wetland grassland, showing the position of the functional threshold identified by piecewise regression.   Results   During the study period we identified and monitored 40 plant species that decreased, while invasive forbs in-creased. Piecewise regression of forage cover and invasive forbs cover identified a breakpoint at ~50% in-vasive forbs cover, indicating a loss of function in relation to forage production (Figure 2).  This loss of resilience for forage production was demonstrated in an exclosure cage set up at a site with invasive forbs cover over 70% and around 10% forage cover. The values for forage cover after 6 months, 1 year and 2 years of rest were 5, 3 and 2%, respectively. Per-manent exclosure also presented predominance of invasive forbs after 3 years indicating that long rest peri-ods did not ensure the return of desirable species.  Conclusion   During the observations, invasive forbs replaced peren-nial grasses. This is probably a result of a combination of below average rainfall and the associated overgrazing. Therefore, the main challenge in relation to sustainable management of these wetland areas is to identify grazing thresholds to avoid severe disturbances and subsequent loss of resilience, as well as to define optimal timing, intensity and frequency of grazing. As such, invasive forbs cover can be used as an early warning indicator of habitat degradation.   References   Briske DD; Fuhlendorf SD; Smeins FE. 2006. A unified framework for assessment and application of ecological thresholds. Rangeland Ecology and Management 59:225−236. López DR; Cavallero L; Brizuela MA; Aguiar MR. 2011. Ecosystemic structural approach of the state and transition model. Applied Vegetation Science 14:6−16. Muggeo VMR. 2008. Segmented: An R package to fit regres-sion models with broken-line relationships. R News 8/1:20−25. Santos SA; Costa C; Souza GS; Moraes AS; Arrigoni MB. 2002. Qualidade da dieta selecionada por bovinos no Pan-tanal da sub-região da Nhecolândia. Revista Brasileira de Zootecnia 31:1663−1673.    © 2014   Tropical Grasslands−Forrajes Tropicales is an open-access journal published by Centro Internacional de Agricultura Tropical (CIAT). This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/      Santos SA; Perotto-Baldivieso HL; Young D. 2014. Resilience and degradation in a tropical wetland overgrazed by cattle. Tropical Grasslands – Forrajes Tropicales 2:138–139.  DOI: 10.17138/TGFT(2)138-139          This paper was presented at the 22nd International Grassland Congress, Sydney, Australia, 15−19 September 2013. Its publication in Tropical Grasslands – Forrajes Tropicales is the result of a co-publication agreement with the  IGC Continuing Committee. Except for adjustments to the journal’s style and format, the text is essentially the same as that published in: Michalk LD; Millar GD; Badgery WB; Broadfoot KM, eds. 2013. Revitalising Grasslands to Sustain our Communities. Proceedings of the 22nd International Grassland Congress, Sydney, Australia, 2013. New South Wales Department of Primary Industries, Orange, NSW, Australia. p. 881–882.                            

Resilience and degradation in a tropical wetland overgrazed by cattle.

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